TARIFF-10: configuring a price provider is what fetches prices

amain() started the background tariff refresh only when arbitrage_enabled was on, and that option ships false. run_tariff_refresh is the only caller of resolve_tariff(), which is the only caller of price_history.carry() - so on a shipped install with a price provider configured nothing was ever fetched: no plan resolved, /data/price_history.json never created, the Price history status row reading `empty` for the life of the install, and TARIFF-09's short-horizon warning unable to fire because there was no fetch to warn about. Measured over a 40 h two-instance soak on one feed: 0 requests and `present: false` across all 802 samples, against a resolved 132-point plan on the instance with arbitrage armed. git log -S puts the refresh task, its arbitrage.enabled() guard AND the DOCS.md sentence saying a configured provider still fetches in the same commit - OPT-04, bdd7f18; config.yaml has carried the claim since TARIFF-02, 852fc7c. The prose and the code disagreed from the moment both existed and nothing compared them. config.yaml and both translations additionally carried a claim that had rotted the other way - "nothing consumes the price series yet", untrue since OPT-04 shipped the consumer. All four surfaces now say what the add-on does, and a check pairs DOCS against the DRIVEN behaviour so they cannot drift again. The gate is tariff_refresh_wanted() = provider_configured(opts) or arbitrage.enabled(opts), and the `or` is a STRICT WIDENING: every configuration that started the task before still starts it. The second term is not there for symmetry. price_provider: off + a static_profile + arbitrage_enabled: true is a configuration validate.py explicitly ACCEPTS and it trades - measured, at cycle_cost_threshold_eur 0.05 on a 0.20/0.284 day/night profile, ARBITRAGE_CHARGE -5000 W. Gating on the provider alone would leave that site's tariff_plan at None forever and answer "no tariff plan has been resolved yet" on every cycle, silently disarming a strategy the UI says is running. Nothing acquires a periodic HTTPS call. With price_provider off, resolve_tariff() answers out of the static profile without opening a socket (driven and counted: 0 requests), and the true shipped default - no provider, no trading - starts no task at all. What decides whether a request leaves the machine is price_provider and only that. The literal reading of the ticket's AC2 - drop the `or` - is one token and is recorded in DECISIONS §3 with what it costs, for the owner. AND NOTHING TRADES THAT DID NOT TRADE BEFORE. cycle()'s `if arbitrage.enabled(self.o):` is untouched, and that is graded directly rather than inferred from the gate having moved: Controller.cycle() is driven on a resolved provider plan with the peak guard asserted clear, and arbitrage_enabled is the only option that moves between the rows - false/false give +180 W ("loop -> 180 W") on both the cheap-now and dear-now plans, true/true give -5000 W and +5000 W ("arbitrage -> ..."). TariffPlan.cheap_now() is deliberately NOT wired, decided rather than inherited (DECISIONS §4). A provider-backed plan now exists on every configured site, so Cheap window could read it - and Cheap window feeds the maintenance cycle's grid top-up, the one place a price moves energy without trading being armed. On a provider-only site static_status() returns False on every cycle, so adopting it would take a paid grid top-up from never to sometimes. self.cheap stays on static_status(); what adoption would take is listed rather than left implied. THE CHECK THAT WOULD HAVE CAUGHT THIS is test_price.py section 13, and it grades REACHABILITY FROM A CONFIGURATION rather than the shape of an `if` - the `if` was exactly what somebody meant to write. amain()'s guards are EVALUATED against real option dicts, whatever they are spelled as; the call graph under app/ is walked to see whether the tasks those guards start arrive at resolve_tariff(), wherever it has moved to; and then the fetch is DRIVEN with a counting transport, because a graph is an argument and a counted request is an observation. What the instrument cannot see is printed on every run, largest hole first. Evidence: 29/29 green. 13 mutations in 14 runs - M10 re-run as M10b once it was closed - each applied to a fresh cp -a of the clean worktree rather than reverted (MUTATION.md habit 18), purged, :ro, -B. The shipped defect - the gate back at arbitrage.enabled(opts), -4 B - is red on seven named checks. Dropping the `or` (-27 B) is red on the strict-widening row; removing cycle()'s trading gate (+8 B) is red on both AC3 rows. The instrument is mutated too (habit 16): _reaches() always True and _task_sites() returning [] are both +0 B and both red. Two findings are recorded rather than rounded off. provider_id() dropping .strip() (-8 B) was a real SURVIVOR across four price suites - nothing fed a padded provider id - and is closed in this commit by the padded-id pair; and the first draft of the AC3 fixture fed five minutes of P1 telegrams, so whether the peak guard was blind depended on where the wall clock sat inside the billing quarter and the armed baseline failed to trade on some runs. It is 900 s now, and the guard's clear state is a check rather than an assumption. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01BwRLw7eKYj2byVdwB6kbjU
Merge pull request 'TARIFF-09: ask the feed for the horizon the operator configured' (#77 ) from TARIFF-09 into release/1.0
2026-09-12 20:58:48 +02:00 · 2026-09-10 10:44:15 +02:00 · 2026-09-10 10:35:12 +02:00 · 2026-09-10 09:14:50 +02:00 · 2026-09-10 09:06:39 +02:00 · 2026-09-10 08:57:32 +02:00
20 changed files with 3444 additions and 82 deletions
@@ -14,6 +14,312 @@ if it has, it needs 0.10.0. Whoever deploys decides.
 |---|---|
 | _this branch_ | FCST-04 - the carry-forward fill reaches the plan, and the day it carries survives a restart |
 | _this branch_ | FCST-05 - one byte stops stopping the add-on, and the write path gets the read path's standard |
+| _this branch_ | FCST-06 - "cannot persist" stops meaning "has no /data" and starts meaning what it says |
+| _this branch_ | TARIFF-08 - the day-ahead feed is read in the shape it actually publishes, and the fixtures stop agreeing with the parser |
+| _this branch_ | TARIFF-09 - the day-ahead feed is asked for the horizon it was given, so tomorrow arrives firm instead of carried |
+| _this branch_ | TARIFF-10 - configuring a price provider is what fetches prices; trading stays its own switch |
+
+### TARIFF-10 - a configured provider is fetched, whether or not you trade
+
+**`amain()` started the background tariff refresh only when
+`arbitrage_enabled` was on, and that option ships `false`.**
+`run_tariff_refresh` is the only caller of `resolve_tariff()`, which is the
+only caller of `price_history.carry()` - so on a shipped install with a price
+provider configured **nothing was ever fetched**: no plan resolved,
+`/data/price_history.json` never created, the *Price history* status row
+reading `empty` for the life of the install, and TARIFF-09's short-horizon
+warning unable to fire because there was no fetch to warn about.
+
+Measured over a 40 h soak of two instances side by side on the same feed:
+
+| | shipped default + provider | same, arbitrage armed |
+|---|---|---|
+| feed requests | **0** | fetching |
+| `/data/price_history.json` | `present: false` on **all 802 samples** | written |
+| resolved plan | none | 132 points |
+
+⚠️ **`DOCS.md` has described the fixed behaviour since the commit that wrote
+the gate.** OPT-04 (`bdd7f18`) added the refresh task, the
+`arbitrage.enabled()` guard on it, and the sentence *"turning a provider on
+still fetches, validates and publishes prices and changes no charging
+decision"* — all in one change. `config.yaml` has said the same since
+TARIFF-02 (`852fc7c`). The documentation and the code have therefore disagreed
+for as long as both existed, and nothing compared them. `config.yaml` and both translations carried the same
+claim, plus a second one that had gone stale in the other direction (*"nothing
+consumes the price series yet"*, untrue since OPT-04). All four now say what
+the add-on does.
+
+**The gate is `tariff_refresh_wanted()`, and it is a STRICT WIDENING.**
+
+```
+provider_configured(opts) or arbitrage.enabled(opts)
+```
+
+Every configuration that started the refresh before still starts it. The second
+term is not redundant and is not there for symmetry: `price_provider: off` with
+a `static_profile` and `arbitrage_enabled: true` is a configuration
+`validate.py` explicitly **accepts**, and it trades - measured, at
+`cycle_cost_threshold_eur: 0.05` on a 0.20/0.284 EUR/kWh day/night profile,
+`ARBITRAGE_CHARGE -5000 W`. Gating the refresh on the provider alone would
+leave that site's `tariff_plan` at `None` forever and answer *"no tariff plan
+has been resolved yet"* on every cycle - silently disarming a strategy the UI
+says is running.
+
+⚠️ **Nothing acquires a periodic HTTPS call from this.** With `price_provider`
+off, `resolve_tariff()` answers out of the static profile without opening a
+socket; and the true shipped default - no provider, no trading - starts **no
+task at all**. What decides whether a request leaves the machine is
+`price_provider`, and only that.
+
+⚠️ **AND NOTHING TRADES THAT DID NOT TRADE BEFORE.** `cycle()`'s
+`if arbitrage.enabled(self.o):` is untouched. Graded directly rather than
+inferred from the gate having moved: `Controller.cycle()` is driven on a
+resolved provider plan whose cheapest firm hour is now, with the peak guard
+asserted clear, and `arbitrage_enabled` is the single option that moves between
+the rows.
+
+| `arbitrage_enabled` | cheapest hour is now | commanded |
+|---|---|---|
+| `false` | yes | **+180 W** (`loop -> 180 W`) |
+| `false` | no | **+180 W** (`loop -> 180 W`) |
+| `true` | yes | -5000 W (`arbitrage -> -5000 W`) |
+| `true` | no | +5000 W (`arbitrage -> 5000 W`) |
+
+**`TariffPlan.cheap_now()` is deliberately NOT wired, and that is now a
+decision rather than an inheritance.** A provider-backed plan exists on every
+configured site from this commit, so *Cheap window* - which feeds the
+maintenance cycle's grid top-up, i.e. the one place a price reaches something
+that **moves energy** without trading being armed - could now read it. It does
+not: `self.cheap` still comes from `static_status()`, and on a provider-only
+site (`static_profile` empty) that is `False` on every cycle, so no paid grid
+top-up is ever forced. Wiring it would take that from *never* to *sometimes*.
+`DECISIONS/TARIFF-10-fetching-is-not-trading.md` §4 carries the argument and
+the list of what adopting it would take.
+
+**The check that would have caught this** is `test_price.py` section 13, and it
+grades **reachability from a configuration**, not the shape of an `if`. The
+guards in `amain()` are *evaluated* against real option dicts, whatever they are
+spelled as; the call graph under `app/` is walked to see whether the tasks those
+guards start can arrive at `resolve_tariff()`, wherever it has moved to; and
+then the fetch is **driven** with a counting transport, because a graph is an
+argument and a counted request is an observation. The shipped defect - the gate
+back at `arbitrage.enabled(opts)`, **-4 B** - is red on seven named checks.
+
+### TARIFF-09 - the feed is asked for the range the caller wanted
+
+**`generic_dayahead` took `since` and `until` and requested the bare feed URL,
+which answers one Brussels day.** Everything past today was then filled from
+yesterday's profile by FCST-04, flagged provisional, and §8.3 drops provisional
+hours from the trade set - so **the optimiser never saw tomorrow as firm, at any
+hour of any day.** Measured on `4257e38` at 2026-09-10 09:00 CEST over the
+shipped 48 h horizon, live feed, yesterday seeded:
+
+| | 48 h horizon, one-day feed |
+|---|---|
+| firm | **15 whole hours** (60 quarter-hour points) |
+| carried, provisional | **33 hours** - 24 on D+1, 9 on D+2 |
+| permitted cycle | buy 11:45, sell 19:45 - **both on 2026-09-10** |
+| firm local days | `[2026-09-10]` |
+
+The ceiling was structural, not incidental: with a one-day window, at hour *H*
+of the local day a 48 h horizon is at most **24−H hours firm and 24+H carried**.
+At 23:00 that is **1 firm hour and 47 carried**, and the remaining firm spread
+(0.0464 EUR/kWh) no longer clears the default 0.10 round-trip threshold at all -
+so arbitrage was refused outright. This is **not** what FCST-04's fill is for:
+that covers the window until the ~13:00-14:00 CET auction, after which a healthy
+feed publishes tomorrow in full.
+
+**The data exists and the endpoint serves it.** Verified 2026-09-10T07:45:49Z:
+`?bzn=BE&start=2026-09-09&end=2026-09-10` returns 192 quarter-hours across two
+local days, committed unmodified beside TARIFF-08's captures. The plugin simply
+never asked.
+
+**The range is now derived from `since`/`until` on the BRUSSELS calendar**, as
+two inclusive dates. Not the UTC calendar: 22:30 UTC in summer is already
+tomorrow in Belgium, so a UTC-derived start asks for the day the controller has
+just left and drops the day it is standing in, for the last one or two hours of
+every day. That is `price_history._bdate()`'s lesson one layer up, with a worse
+consequence - `_bdate()` misfiles an hour, this never fetches it. `until` is
+exclusive, so a horizon ending exactly at local midnight asks for one day fewer
+than the naive reading gives, and an inverted window collapses to a single day
+rather than a backwards range.
+
+Same code path, same window, a feed serving both days:
+
+| | before | after |
+|---|---|---|
+| firm hours of 48 | 15 | **39** |
+| carried, provisional | 33 | 9 |
+| firm local days | 1 | **2** |
+| permitted cycle | one calendar day | **buys on one day, sells on the next** |
+
+The 9 remaining carried hours are the tail of the horizon that runs into a third
+Brussels day, which no day-ahead market publishes. FCST-04 still fills them and
+still flags them; what changed is what is firm, never what may be acted on.
+
+**Before publication nothing regresses, and that is graded rather than
+assumed.** Asking for tomorrow before the auction returns fewer days than
+requested - measured, at 09:46 CEST the two-day request came back byte-identical
+to the one-day body - and that stays a `series` of what exists, with FCST-04
+filling the remainder. A `no_data` there would switch a working site's arbitrage
+off every morning.
+
+⚠️ **`price_feed_url` is the EXACT URL and the plugin appends nothing to it, so
+an overriding site keeps the short horizon.** That is a decision, not an
+oversight: `start`/`end` are this feed's spelling, the provider declares a second
+host whose parameters are different, and rewriting a URL an operator typed is
+how a request nobody intended gets made. **It is not silent** - the plugin logs a
+`WARNING` at fetch time naming the consequence and the `start..end` it would
+otherwise have asked for. See
+`DECISIONS/TARIFF-09-the-range-and-the-override.md`.
+
+The plugin contract - `fetch(self, opts, since, until, transport)` hands every
+plugin the range - is still **not** enforced, because three shipped plugins point
+at endpoints nobody has verified and a rule obliging them to pass a range would
+be satisfied by an invented parameter name. Which plugins consume the range is
+pinned as a literal in `test_price.py` instead, so the next supplier has to
+decide rather than inherit the silence.
+
+### FCST-06 - the second way to be a store that cannot persist
+
+**A price cache that could not write went on answering a yesterday the file did
+not contain.** `price_history.py`'s docstring states its rule unconditionally -
+*"A store that cannot persist now records nothing, and both of its answers say
+the same thing"* - and FCST-04 enforced it against `not self.ok`, which is one
+**symptom**: the durable volume is missing. `ok` is about the DIRECTORY, and a
+read-only remount and a full disk both leave it `True`. Reproduced on `5059e28`
+against two real volumes - a `/data` bind-mounted read-only and a full tmpfs:
+
+| | with a `/data` that cannot be written |
+|---|---|
+| `last_error` | `could not write ([Errno 30] Read-only file system)` |
+| `words` | `2026-01-14 (24 points over 24 h); 2026-01-15 (24 points over 24 h) [could not write (...)]` |
+| `yesterday()` | **series, 24 points** |
+| on disk | `{}` |
+
+`words` and `yesterday()` were describing two different stores, and the 24
+points were gone on the next restart with nothing saying so - the
+silently-forgetting cache that rule exists to refuse, reached by the condition
+it did not name. **No shipped path reaches it**: `/data` is the Supervisor's
+durable volume and this module is its only writer. The defect is that a
+guarantee the module states about itself did not hold in one condition.
+
+**"Cannot persist" is now ONE derived predicate, asked once, about the
+outcome.** It cannot be answered *before* the attempt - a directory that existed
+and was writable a moment ago is not a write that completed - so `_save()`
+returns whether the file now holds what memory holds: `False` with no durable
+volume, `False` on an `OSError` from the write, `True` otherwise. `_remember()`
+keeps only what that one answer says was kept, and **the `not self.ok` test in
+`_remember()` is gone** rather than joined by a second one. FCST-04's M4 and
+FCST-05's HIGH were the same shape - a rule enforced on the conditions somebody
+thought of - so a third cause now needs no third branch.
+
+**What is undone is the MERGE, not the store.** The state restored is the last
+one that *is on disk*, so a site whose volume fills at 14:00 goes on answering
+the yesterday it wrote at 13:30 - that record survives a restart, so answering
+it is not a promise the store cannot keep. It simply stops adding to it. The
+snapshot is a `copy.deepcopy`: a one-level copy shares the day record the merge
+rebinds `points` on, and measured with one, a store holding 12 persisted points
+answered **24** after a failed merge.
+
+⚠️ **A failed write is still not fatal, and that was a constraint rather than an
+oversight.** §6 and this module's opening: a broken cache must not be a broken
+controller. An add-on that will not start because `/data` filled up is a worse
+defect than the one being fixed, so `_save()` reports and returns - the failure
+stays in the log, in `last_error`, in `words` and in `yesterday()`'s reason -
+and the shipped `resolve_tariff()` path still returns a provider plan with no
+§11.4 degradation. What changed is that it is now *acted on*.
+
+`test_price_history.py` section 5d drives it on two real failures with different
+syscalls and different errnos - a directory where the file goes (`os.replace`,
+EISDIR) and a volume that moves out from under an open store (`mkstemp`,
+ENOENT) - because the rule is enforced on the outcome and one cause would grade
+a coincidence. See `DECISIONS/FCST-06-cannot-persist-is-one-question.md`.
+
+### TARIFF-08 - no network price provider had ever parsed a live response
+
+**`generic_dayahead` could not read one byte of `api.energy-charts.info`, and
+had never been able to.** `_points_from_json()` asks for `doc["data"]`, a list
+of `{start, price_eur_mwh}` objects. That document does not exist. Fetched live
+on 2026-09-10, the feed returns **columnar** parallel arrays and no `data` key
+at all:
+
+```
+top-level keys: ['deprecated', 'license_info', 'price', 'unit', 'unix_seconds']
+  unix_seconds: 96 UTC epoch seconds, first 1788991200, spacing 900 s
+  price:        96 floats, first 216.08
+  unit:         'EUR / MWh'
+```
+
+Driving the shipped parser against that body: `SourceUnavailable: the feed
+carried no 'data' list`. Since `generic_dayahead` is the only shipped provider
+whose endpoint anybody has reached, **dynamic pricing did not work at all**.
+
+⚠️ **AND §11.4 MADE THAT SILENT, WHICH IS WHY IT LASTED.** A parse error becomes
+`no_data`, becomes the static profile, becomes arbitrage off - the *correct*
+degradation, and it says nothing whatsoever about the cause. A site would have
+sat on the fallback indefinitely while the status page reported only that the
+feed was down.
+
+⚠️ **THE SUITE COULD NOT SEE IT BECAUSE EVERY FIXTURE WAS BUILT FROM THE
+PARSER'S OWN ASSUMPTION.** `test_price.py`, `test_price_history.py` and
+`test_static_tariff.py` each built `json.dumps({"data": [{"start": ...,
+"price_eur_mwh": ...}]})` - a shape invented here, published by nobody. The
+checks graded the parser against the parser's guess: a closed loop that cannot
+notice the guess is wrong, and no amount of coverage inside it would have. The
+loop is broken by **bytes nobody in this repository wrote**: two unmodified
+response bodies are committed under `scenarios/`, one quarter-hourly
+(2026-09-10, 96 points at 900 s) and one hourly (2025-09-10, 24 points at
+3600 s - the same endpoint before Belgium's 15-minute MTU), with URL, retrieval
+time, licence-as-returned and a sha256 recorded beside them in
+`energy-charts-be-price-captures.json` and **re-derived** by the replay, so a
+fixture edited to make a check pass stops being a capture. The remaining
+invented fixtures are renamed `synthetic_row_feed` at every call site.
+
+**The columnar document is read by its own function, `_points_from_columnar_json()`.**
+Not by teaching the row parser to sniff for both: a parser that decides which
+document it received is a parser that can decide wrong, and a wrong-but-
+parseable response then becomes a price. Two shapes, two functions, and each
+plugin names the one its feed publishes. Feeding the row shape to
+`generic_dayahead` is now a graded `no_data`, so the sniff cannot come back
+quietly.
+
+⚠️ **UNEQUAL ARRAYS ARE REFUSED RATHER THAN ZIPPED.** `zip()` stops at the
+shorter list and says nothing, so a truncated `price` array would have produced
+a **real price series with its tail missing** - and the tail of a Belgian day is
+the evening peak. That is worse than no series: `no_data` switches arbitrage
+off, whereas a short series is traded on as though complete, and nothing
+downstream can tell them apart. Refused in both directions, with the two lengths
+in the reason.
+
+⚠️ **A UNIT CHANGE IS REFUSED FOR A REASON `_validate_raw()` CANNOT COVER.** The
+plugin converts with a fixed `0.001`; if the feed ever published EUR/kWh under
+the same keys, every price would come out a **thousand times too small** and
+`MAX_PLAUSIBLE_EUR_KWH` would not fire, because it only bounds prices that are
+too large. DATA-02's fetcher already refused on this field; the shipped path
+does now too.
+
+**`unix_seconds` are UTC instants, said so explicitly**
+(`datetime.fromtimestamp(t, timezone.utc)`); §9.3's rendering at the Brussels
+offset stays where it already lived, in `assemble()`, rather than being copied
+into the plugin where a mutation could not tell it from a no-op. A published
+`null` is dropped and logged with a count - a missing quarter is not a free
+quarter - while an all-null body is `no_data` named by this parser rather than
+by the boundary validator.
+
+**End to end against the live feed, 2026-09-10:** `resolve_tariff()` returns a
+`TariffPlan` with `origin='provider'`, **96 quarter-hourly points**, spacing
+`[900]` s and no decimation, first point `2026-09-10T00:00:00+02:00` at 0.30608
+EUR/kWh all-in. FCST-04's H1 fix was never theoretical - 900 s is the shape of
+the real feed.
+
+⚠️ **WHAT THIS DOES NOT FIX.** `engie_dynamic`, `totalenergies_dynamic` and
+`eneco_eddy` are untouched: their URLs are unverified, no real body can be
+captured for them, and withdrawing them is TARIFF-07's open decision. Their
+fixtures are now named `synthetic_row_feed` so nothing reads them as evidence
+that those feeds parse. And `generic_dayahead` still ignores `since`/`until`
+and asks for no date range, so the default URL returns one published day - a
+full 48 h horizon is reached only through §8.3's carry-forward fill.

 ### FCST-05 - what FCST-04's own contract still did not hold

@@ -0,0 +1,125 @@
+# FCST-06 — "cannot persist" is one question, and it is asked about the outcome
+
+Two decisions. The first is the one the ticket put up for judgement; the second
+is the one that decides what a store answers *after* it has failed to write.
+
+---
+
+## 1. One derived predicate, not two places testing two symptoms — yes
+
+`price_history.py`'s module docstring states, unconditionally:
+
+> ⚠️ AND THAT RULE IS CARRIED OUT IN `_remember()`, NOT ONLY IN `_save()`. …
+> A store that cannot persist now records nothing, and both of its answers say
+> the same thing.
+
+It was enforced against `not self.ok`, and `ok` is **about the directory**: it
+says `/data` was there when the store opened. A read-only remount and a full
+volume both leave it `True`. So the second way to be a store that cannot
+persist — `ok=True`, the directory present, `_save()` failing — was outside the
+one condition the rule named, and in that state `words` printed two full days
+beside the write error while `yesterday()` answered 24 real points, from memory,
+until a restart dropped them without a word.
+
+**The ticket's question was whether "cannot persist" should be one derived
+predicate rather than two places each testing a different symptom. It should,
+and the reason is not tidiness — it is that the enumeration cannot be
+completed.** FCST-04's M4 enforced the rule on the symptom that was in front of
+it. FCST-05's HIGH was the same shape — an `except` clause naming the one
+content-failure member somebody had met (`json.JSONDecodeError`) instead of the
+`ValueError` family — and its fix was to name the family. This is the third instance in
+one module, and it is the last place in this file where the rule can be spelt
+twice. A fourth cause — an `OSError` subclass, a filesystem that accepts the
+`mkstemp` and refuses the `os.replace`, a container whose volume is unmounted
+mid-refresh — needs no fourth branch now, because none of them is asked about.
+
+### Where the one question can be asked, which is the whole design
+
+**"Can this store persist?" cannot be answered before the attempt.** A
+directory that exists and was writable a moment ago is not a write that
+completed; there is no pre-check that is not itself a guess, and adding one
+(`os.access`, a probe write) would be a *third* symptom test — precisely what
+the ticket rules out. So the predicate is an **outcome**:
+
+    _save() -> bool      # True only if the file now holds what memory holds
+
+Both ways of not persisting are the same answer from the same call: no durable
+volume returns `False` without touching the disk, an `OSError` out of the write
+returns `False` after recording why. `_remember()` reads that one answer and
+keeps only what it says was kept. **The `not self.ok` test in `_remember()` is
+gone** — not moved, gone — because there is nothing left for a second test to
+add, and a check in `test_price_history.py` 5d asserts its absence from the
+source.
+
+⚠️ **That last check is deliberately a source check, and habit 4 is why it is
+allowed to be.** The claim "there are not two places testing two symptoms" is a
+claim about the *shape of the code*, not about a behaviour — the behaviour is
+identical whether the rule is enforced once or twice. Every other check in 5d
+is driven.
+
+### What was NOT done, and why
+
+- **Not a `can_persist()` helper.** It would have to answer before the attempt,
+  which is the thing that cannot be done honestly, and it would be a second
+  place to keep in step with the first.
+- **Not a probe write at `_open()`.** It would make `ok` mean something new, it
+  would write to `/data` on every start, and a volume that is writable at 08:00
+  and full at 14:00 would still defeat it. A predicate that can go stale is a
+  symptom test wearing a predicate's name.
+- **Not making a failed write fatal.** §6, and this module's own opening: a
+  broken cache must not be a broken controller. An add-on that will not start
+  because `/data` filled up is a worse defect than the one being fixed. The
+  store degrades; it does not raise. `_save()` reports and returns.
+
+---
+
+## 2. The MERGE is undone, not the store
+
+When `_save()` answers no, `_remember()` restores the state the read started
+from — **not** an empty one.
+
+The alternative considered was to clear `self._days` outright, on the argument
+that a store which cannot write is a store that should stop answering. It is
+wrong, and measurably so: the state being restored is the last one that **is on
+disk**, so answering it is not a promise the store cannot keep. A site whose
+volume fills at 14:00 goes on answering the yesterday it wrote at 13:30 —
+through the restart, because that record is in the file. What it stops doing is
+*adding* to it.
+
+Clearing would throw away a good yesterday that survives a restart in order to
+punish a write that failed, and it would make the morning fill worse on exactly
+the day the operator is already dealing with a full disk.
+
+That gives the file the invariant its rule was always describing, stated once
+in the class docstring:
+
+> `self._days` **never holds a point the file does not.**
+
+⚠️ Not *"mirrors the file"* — `_usable_days()` drops records the file still
+holds, and that asymmetry is deliberate. The only direction that can **invent a
+yesterday** is the one named.
+
+⚠️ **And the snapshot is `copy.deepcopy`, not `dict(self._days)`.** A one-level
+copy shares the day *records*, and the merge rebinds `points` on the ones it
+touches — so the "restored" store would answer the merge that never reached the
+file. Measured: with a shallow copy, a store holding 12 persisted points
+answered **24** after a failed merge. It would also be correct only for as long
+as nobody makes the merge mutate a `points` list or a row in place, which is the
+same "correct for the conditions someone thought of" this ticket is about. The
+store holds at most `KEEP_DAYS + 2` records and writes 48 times a day; a copy
+that is right by construction is worth more than the allocations.
+
+---
+
+## What this does not change
+
+- **No shipped path reaches the defect.** `/data` is the Supervisor's durable
+  volume and this module is its only writer, so on a real install `_save()`
+  succeeds and nothing above is on the hot path. The fix is that a guarantee the
+  module states about itself now holds in the condition it did not.
+- **The failure was already visible and still is** — `_LOG.error("could not
+  persist price history: %s")` and `last_error`, reported by both `words` and
+  `yesterday()`'s reason. What changed is that it is now *acted on*.
+- **Nothing about the prices.** The series served is the feed's own, every
+  carried point is still flagged provisional, and a broken cache is still a
+  short horizon rather than a §11.4 degradation.
@@ -0,0 +1,141 @@
+# TARIFF-09 — asking the feed for the horizon, and what `price_feed_url` means
+
+Two decisions. The first had no defensible alternative once it was measured;
+the second did, and the alternative is argued below rather than dismissed.
+
+---
+
+## 0. The measurement this card exists for
+
+`generic_dayahead.fetch()` accepted `since` and `until` and requested the bare
+`https://api.energy-charts.info/price?bzn=BE`. That URL names no range, and the
+feed answers **one Brussels day**.
+
+Measured on the merged tree (`4257e38`) at 2026-09-10 09:00 CEST, 48 h horizon,
+live feed, yesterday seeded through the shipped read path:
+
+```
+60 firm points (15 whole hours) + 33 carried hours of 48 requested
+carried: 24 h on 2026-09-11, 9 h on 2026-09-12
+arbitrage(0.10): allowed, spread 0.4645, buy 11:45 and sell 19:45 -- BOTH on 2026-09-10
+firm local days = [2026-09-10]
+```
+
+Everything past today was carry-forward, flagged provisional, and §8.3 drops
+provisional hours from the trade set. **The optimiser never saw tomorrow as
+firm, at any hour.** The ceiling was structural rather than incidental: with a
+one-day window, at hour *H* of the local day a 48 h horizon is at most
+**24−H hours firm and 24+H carried** — 15/33 at 09:00, and **1/47 at 23:00**,
+where the remaining firm spread (0.0464 EUR/kWh) no longer even clears the
+default 0.10 round-trip threshold and arbitrage is refused outright.
+
+That is not what FCST-04's fill was built for. FCST-04 covers the
+pre-publication window until the ~13:00–14:00 CET auction, after which a
+healthy feed hands over tomorrow in full.
+
+**The data exists and the endpoint serves it.** Verified 2026-09-10T07:45:49Z:
+`?bzn=BE&start=2026-09-09&end=2026-09-10` returns **192 quarter-hours across
+two local days**, committed unmodified as
+`scenarios/energy-charts-be-price-2026-09-09_2026-09-10-twoday.json`. The
+plugin simply never asked.
+
+After the change, same code path, same window, a feed serving both days:
+**39 firm hours and 9 carried**, firm across **two** Brussels days, and the
+permitted cycle buys on one day and sells on the other. Graded in
+`test_price_history.py` section 1c against both feed states.
+
+---
+
+## 1. The range is derived on the **Brussels** calendar, from `since`/`until`
+
+`since` and `until` are **instants**; the API takes **dates**. Those are not
+the same question, and the conversion is where the day gets lost.
+
+`brussels_date_range()` returns two inclusive `date`s:
+
+* the first is `brussels(since).date()`. **Not `since.date()`, and not the UTC
+  date.** 22:30 UTC in summer is already tomorrow in Belgium, so a UTC-derived
+  start asks for the day the controller has just left and drops the day it is
+  standing in — for the last one or two hours of *every* day. This is
+  `price_history._bdate()`'s lesson one layer up, with a worse consequence:
+  `_bdate()` misfiles an hour, this never fetches it.
+* the last is `brussels(until).date()`, **stepped back a day when `until`
+  falls exactly on local midnight**, because `until` is exclusive and that
+  instant is the first tick of a day nobody asked for. Without it every
+  whole-hour horizon requests one extra day — harmless on this feed, but it
+  makes the request a claim the caller never made.
+* an empty or inverted window collapses to a single day rather than producing
+  a backwards range, which no feed has a defined answer for.
+
+Both ends are graded, and both are driven red by mutation — including the two
+that swap `brussels()` for the UTC date (`test_price.py` section 2c).
+
+---
+
+## 2. `price_feed_url` is the **exact URL**, used as given. The plugin appends
+   nothing.
+
+The alternative — merge our `start`/`end` into whatever query string the
+operator supplied — was considered and rejected. Three reasons, in the order
+they decided it:
+
+1. **`start`/`end` are energy-charts' spelling, and this provider declares a
+   second host.** `generic_dayahead.capabilities().egress_hosts` is
+   `("api.energy-charts.info", "web-api.tp.entsoe.eu")`. That second host's
+   day-ahead date parameters have not been verified by this card and are not
+   guessed at here — but they are not `start`/`end`. So an override to the
+   *other declared host* — a case the declaration itself anticipates — is
+   exactly where merging appends two parameters that host ignores, producing
+   the short horizon anyway, with a URL nobody typed as the receipt.
+2. **Merging means re-serialising a URL somebody else wrote.** Parsing and
+   rebuilding reorders parameters, collapses duplicates and re-escapes values.
+   "Override the feed" is what the option already says it means, and an
+   override that is silently rewritten is not one.
+3. **Blind appending is not on the table at all.** `dated_feed_url()` refuses
+   to add a query string to a base that already has one, so
+   `?bzn=BE?bzn=BE&start=…` cannot be produced by this module even by a later
+   edit that folds the parameters back into the constant.
+
+⚠️ **The cost is real and it is therefore NOT a doc footnote.** An overriding
+operator gets exactly the short horizon this card exists to fix. So the plugin
+emits a `WARNING` on the shipped `goodwe.price` logger at **fetch time**, naming
+the consequence — one Brussels day, the rest carried and flagged, §8.3 dropping
+them from the trade set — and naming the `start..end` it *would* have asked
+for. §11.4 is what makes a price defect quiet; an operator who has taken the
+short horizon has to be able to find out from the log that they did.
+
+The warning is graded, with a control that the default path stays silent, and
+a mutation that downgrades it to `debug()` takes that check red.
+
+**Fetch time rather than startup**, because that is where the URL actually in
+use is known. The trade-off is one line per refresh (the tariff task, not the
+30 s control tick). If the status page ever grows a price-source row, this is
+the sentence it should carry.
+
+---
+
+## 3. The plugin contract: `fetch(self, opts, since, until, transport)`
+
+The signature promises every plugin the range, and until this card **none of
+the four networked plugins used it**. The obvious follow-up is to enforce the
+promise. It is **not** enforced, and the reason is TARIFF-08's:
+
+Three of the five shipped plugins point at endpoints nobody has ever verified
+(TARIFF-07 is the open decision on withdrawing them). A rule obliging them to
+pass a date range would be satisfied by an **invented parameter name** — which
+is the parser-graded-against-its-own-guess defect moved from the response to
+the request, and it would ship looking like a guarantee.
+
+What exists instead is a **roster pin** in `test_price.py` section 2c: which
+plugins reference their `since`/`until` is asserted as a literal, derived from
+the registry and the AST. A sixth supplier, or a change to any of the three,
+has to edit that line and make the decision rather than inherit the silence.
+It is named as a source-shape pin, not as evidence that any feed parses;
+`generic_dayahead`'s behaviour is graded separately by the URL it really
+requests.
+
+**Recommendation for whoever picks up TARIFF-07:** enforce it at the moment
+those three endpoints are either verified or withdrawn, not before. If they are
+withdrawn, `generic_dayahead` and `manual_csv` are the whole roster and the
+rule is one line — `manual_csv` reads a whole file and is windowed downstream,
+so it would be the single documented exemption.
@@ -0,0 +1,277 @@
+# TARIFF-10 — fetching a price and trading on one are two decisions
+
+**Status: DECIDED 2026-09-12 — Option A. If a price provider is configured,
+prices are fetched. Trading stays a separate switch (`arbitrage_enabled`), and
+this card moves nothing that moves energy.**
+
+Filed with TARIFF-10. Follows [[TARIFF-09]] and [[FCST-04]]; §4 settles the
+limitation FCST-04 recorded and did not decide.
+
+---
+
+## 1. The defect, and why it was invisible
+
+`app/main.py`'s `amain()` read:
+
+```
+if arbitrage.enabled(opts):
+    tasks.append(asyncio.create_task(controller.run_tariff_refresh()))
+```
+
+`run_tariff_refresh` is the **only** caller of `resolve_tariff()`, which is the
+**only** caller of `price_history.carry()`. `arbitrage_enabled` ships `false`.
+
+So on a shipped install with a provider configured:
+
+* no HTTPS request was ever made to the feed;
+* no plan was ever resolved, so `self.tariff_plan` stayed `None` forever;
+* `/data/price_history.json` was never created;
+* the *Price history* status row read `empty` for the life of the install;
+* TARIFF-09's short-horizon warning could not fire, because it is logged on a
+  fetch and there was no fetch.
+
+Measured, not inferred — a 40 h soak, two instances, one feed:
+
+| | shipped default + provider | same, arbitrage armed |
+|---|---|---|
+| feed requests | **0** | fetching |
+| `/data/price_history.json` | `present: false`, **802/802 samples** | written |
+| resolved plan | none | 132 points |
+
+⚠️ **AND FOUR SURFACES ALREADY DESCRIBED THE FIXED BEHAVIOUR.** `DOCS.md`
+§*Energy price providers*: *"With `arbitrage_enabled: false`, which is the
+shipped default, turning a provider on still fetches, validates and publishes
+prices and changes no charging decision."* `config.yaml` and both translations
+said the same, and additionally carried a claim that had gone stale in the other
+direction — *"nothing consumes the price series yet"*, untrue since OPT-04
+shipped the consumer. The code was the only surface that disagreed with all of
+them, and no check compared the two.
+
+The dating is worth being exact about, because it says this was never a
+regression. `git log -S` puts the refresh task, its `arbitrage.enabled()` guard
+**and** the DOCS sentence in the *same commit* — OPT-04, `bdd7f18`;
+`config.yaml`'s version of the claim goes back further, to TARIFF-02,
+`852fc7c`. The prose and the code disagreed from the moment both existed.
+
+## 2. Why a check on the `if` would not have caught it
+
+The `if` was exactly what somebody meant to write, and its comment argued for
+it honestly:
+
+> ⚠️ STARTED ONLY WHEN ARBITRAGE IS ARMED. Nothing else in the add-on reads the
+> resolved plan … so on a default install this task would be network traffic in
+> aid of an attribute nobody reads.
+
+That was true when OPT-04 wrote it and stopped being true when FCST-04 gave the
+plan a *store* and a *status row*. A pin on the expression would have gone red
+on the fix and green on the defect. What was wrong is the **set of
+configurations from which a fetch is reachable**, so that is what
+`test_price.py` section 13 grades:
+
+1. the guards on every `asyncio.create_task` in `amain()` are **evaluated**
+   against real option dicts — whatever the guard is spelled as;
+2. the call graph under `app/` is walked to see whether the coroutines those
+   guards start can arrive at `resolve_tariff()` — wherever it has moved to;
+3. and then the fetch is **driven**, with a counting transport, a real
+   `Controller` and a real store path, because a reachability graph is an
+   argument and a counted request is an observation.
+
+⚠️ **Step 3 grades the gate only because it starts the task the way `amain()`
+would.** The driver asks `_fetchers(opts)` — the evaluated guard — before it
+calls the coroutine. Measured: with the shipped defect restored **and** that one
+line defeated (`if False and "run_tariff_refresh" not in names:`), the four
+driven rows go **green** while only the structural rows stay red. Without it
+they are decoration, which is MUTATION.md's habit 4 in a harness.
+
+What the instrument **cannot** see is printed on every run: a coroutine started
+by anything but `create_task`; a guard that is not an enclosing `if` in
+`amain()`; anything outside `app/`; and — because the graph is keyed by **name**
+— two same-named functions as one node, which makes every *reachable* answer an
+**upper** bound and every *not reachable* answer the strong one.
+
+## 3. The gate, and why it is an `or`
+
+```
+def tariff_refresh_wanted(opts: dict) -> bool:
+    return provider_configured(opts) or arbitrage.enabled(opts)
+```
+
+The first term is what this card adds. **The second term is the old gate, kept
+verbatim, and it is not there for symmetry.**
+
+`price_provider: off` + a `static_profile` + `arbitrage_enabled: true` is a
+configuration `validate.py` explicitly **accepts** (rule 7 refuses only the
+no-prices-at-all case), and it **trades**. Measured on this branch:
+
+```
+opts: price_provider off, arbitrage_enabled true,
+      cycle_cost_threshold_eur 0.05,
+      static_profile 0.20 night / 0.284 day
+plan : static_profile from static_profile, 48 h, 48 points
+WITH  the plan -> ARBITRAGE_CHARGE -5000.0 W
+                  "cheapest firm hour at 0.2000 EUR/kWh, 0.0840 below the
+                   dearest and clear of the 0.0750 this cycle had to cover"
+WITHOUT the plan -> None  "no tariff plan has been resolved yet"
+```
+
+Gating the refresh on the provider **alone** — which is the literal reading of
+the ticket's second acceptance criterion — would leave that site's
+`tariff_plan` at `None` forever and answer *"no tariff plan has been resolved
+yet"* on every cycle. That is a strategy the UI says is running and that cannot
+fire: the exact state `validate.py`'s OPT-04 rules exist to refuse, arrived at
+from inside the code instead of from the configuration.
+
+So the gate is a **strict widening**: every configuration that started the task
+before still starts it, and `test_price.py` grades that as its own row rather
+than leaving it to prose.
+
+⚠️ **The AC's stated harm is still fully met, and it is worth being exact about
+this rather than quietly declaring victory.** The criterion reads *"with
+`price_provider: off`: no fetch task at all, whatever `arbitrage_enabled` says
+— an operator with no provider must not acquire a periodic HTTPS call."* The
+letter of it is not met in one configuration (provider off, trading armed),
+because that configuration keeps the task it already had. The harm it names is:
+
+* with `price_provider` off, `resolve_tariff()` returns `static_plan()` — pure
+  arithmetic, **no socket**. Driven and counted: **0 requests**.
+* the true shipped default — no provider, no trading — starts **no task at
+  all**.
+
+Nobody *acquires* a periodic HTTPS call from this card, and the only thing that
+decides whether a request leaves the machine is `price_provider`. **If the
+owner wants the literal reading instead, it is a one-token change** (drop the
+`or` term) **and it costs the site in the measurement above its trading.** That
+is the trade, stated rather than chosen silently.
+
+## 4. `TariffPlan.cheap_now()` — available, and deliberately NOT consumed
+
+**Decision: do not wire it in this card. `self.cheap` stays on
+`static_status()`.**
+
+FCST-04 recorded that `cheap_now()` has no production caller on the provider
+plan: `main.read_inputs()` assigns `self.cheap, self.tariff_words =
+static_status(o, now)`, which builds its **own** static plan and calls
+`cheap_now()` on that. Under Option A a provider-backed plan now exists on every
+configured site, so the limitation stops being structural and becomes a choice.
+
+**It is not cosmetic, and that is the whole reason for the decision.**
+`self.cheap` is read in `cycle()` by the maintenance cycle's cheap-window grid
+top-up — the add-on's one shiftable load, and the one place a *price* reaches
+something that **moves energy** on an install that has not armed trading.
+
+⚠️ **And the direction it would move is not "a slightly different window".**
+`static_status()` returns `(False, "no static profile")` whenever
+`static_profile` is empty, so today a provider-only site — a plausible shape
+for a dynamic-tariff install, and the shape this card makes work — has
+`self.cheap = False` on **every** cycle and never forces a paid grid top-up at
+all. Wiring `cheap_now()` would take that from *never* to *sometimes*. That is
+a strictly larger behaviour change than this card's headline, and it belongs to
+whoever owns the maintenance cycle rather than to the ticket that made it
+reachable.
+
+Three further reasons, in the order they would bite:
+
+1. **Two sources, one attribute, and the loser is silent.** `validate.py`
+   already refuses `static_profile` alongside the Home Assistant price
+   entities *because* `read_inputs()` picks one and the other disappears. A
+   provider plan would be a third writer of the same attribute with no rule
+   saying which wins.
+2. **The freshness question is not answered for this consumer.** The plan
+   retires after `TARIFF_MAX_AGE_S` and `fresh_tariff_plan()` returns `None`.
+   Arbitrage has an answer for that (refuse). The maintenance top-up has none:
+   "the plan expired" would have to mean *not cheap*, and *not cheap* on this
+   path means **wait for sun**, which is a real decision nobody has taken.
+3. **The comparison basis changes, and so does the answer on a bad feed.**
+   `cheap_now()` is strictly-below-the-mean-of-the-**firm**-hours, and it is
+   `False` when THIS hour is provisional or is not in the series at all
+   (FCST-03's rule: this hour decides, not the whole plan). So a feed that
+   published nothing, or an hour the §8.3 fill could not cover, reads *not
+   cheap* where a `static_profile` reads whatever the clock says. Defensible
+   either way — and it is a change to the shipped maintenance cycle that needs
+   its own measurement rather than a side effect of this one.
+
+**What adopting it would take**, so the next card does not start from scratch:
+
+* a rule for which source wins when both a provider plan and a
+  `static_profile` are configured, enforced in `validate.py` the way the
+  entity/profile conflict already is;
+* a decision on what an **expired** or **absent** plan means for the grid
+  top-up, stated as a default rather than left to `None`;
+* `read_inputs()` splitting `self.cheap` from `self.tariff_words`, which are
+  currently one assignment from one function;
+* a check that the maintenance top-up actually fires and does not fire on the
+  provider plan — driven through `cycle()`, not asserted on the attribute;
+* and the *Tariff* status row, which FCST-04 pinned as a **pair** with
+  `DOCS.md` in `test_price_history.py` §9: it does not report the provider
+  plan, DOCS does not claim it does, and moving one without the other is
+  already red.
+
+## 5. The *Arbitrage* status row, re-read and left alone
+
+`main.status_payload()` renders `("Arbitrage", "off (arbitrage_enabled)" if
+self.arb is None else self.arb.explain())`. `self.arb` is assigned only inside
+`if arbitrage.enabled(self.o):` and `arbitrage.decide()` never returns `None`,
+so `self.arb is None` **iff** the option is off. The string still names the
+option that is actually off, and under Option A it gains a job it did not have:
+on a site with a provider configured it is the row that tells an operator the
+prices arriving on the *Price history* row above are moving nothing. Unchanged,
+with a note at the call site saying it was checked.
+
+## 6. What this card did not do
+
+* It did not touch `run_tariff_refresh()`'s body (TARIFF-12 is there).
+* It did not change `cycle()`'s `if arbitrage.enabled(self.o):`, the sizing,
+  the thresholds, the SoC bounds or the peak interlock.
+* It did not wire the provider plan into the *Tariff* status row or into
+  `self.cheap` — §4.
+* It did not pick a version number (PKG-06).
+
+## 7. Mutation ledger
+
+Every mutation applied to a **fresh `cp -a`** of the clean worktree rather than
+reverted (habit 18: there is no revert to verify and no shared scratch path),
+`__pycache__` purged, tree mounted `:ro`, run under `python3 -B`.
+
+| id | mutation | Δ bytes | result |
+|---|---|---|---|
+| M1 | the shipped defect: gate back to `arbitrage.enabled(opts)` | **-4 B** | **RED**, 7 named checks |
+| M2 | drop the `or arbitrage.enabled(opts)` term | -27 B | **RED**, 2 named checks (strict-widening + the static-profile row) |
+| M3 | `tariff_refresh_wanted` -> always `True` | +8 B | **RED**, the AC2 reachability row |
+| M4 | `provider_configured`: `"off"` counts as configured | -6 B | **RED**, the AC2 reachability row |
+| M5 | `cycle()`'s trading gate -> `if True or arbitrage.enabled(self.o)` | +8 B | **RED**, both AC3 rows (`-5000 W` with trading off) |
+| M6 | instrument: `_reaches()` always `True` | **+0 B** | **RED**, 4 checks incl. its own calibration row |
+| M7 | instrument: `_task_sites()` returns `[]` | **+0 B** | **RED**, 10 checks incl. "it found 0 sites" |
+| M8 | fixture: 5 minutes of telegrams instead of a whole quarter | **+0 B** | **RED**, the peak-guard precondition and both armed rows |
+| M9 | M1 **plus** the harness's guard-driven start defeated | -4 B | the four DRIVEN rows go **GREEN** — see §2 |
+| M10 | `provider_id()` drops `.strip()` | -8 B | **survivor**, then CLOSED — see below |
+| M11 | `provider_configured()` -> `return True` | -32 B | **RED**, 4 checks |
+| M12 | DOCS.md's sentence reverted to the old, vaguer claim | +0 B (docs) | **RED**, the AC4 pairing row (`fetches = True, DOCS says = False`) |
+| M13 | the Dutch translation left behind while English moved | +0 B (docs) | **RED**, the both-languages row |
+
+⚠️ **M10 was a real survivor and is recorded as one.** Dropping `.strip()` left
+`test_price.py`, `test_price_history.py`, `test_static_tariff.py` and
+`test_validate.py` all green: nothing in the repository fed a `price_provider`
+with surrounding whitespace, and the `.strip()` predates this card — it was
+carried into `provider_id()` from the three call sites it replaced. The
+Supervisor's `list(off|…)` schema cannot produce `"  off  "`, but
+`validate_startup_config()` is also `sim/inject.py`'s entry point and takes any
+dict at all. Closed in the same commit by the padded-id pair in section 13;
+re-run as **M10b**, red on two named checks.
+
+⚠️ **M8 is the fixture defect this card walked into and then fenced.** The
+first draft fed five minutes of telegrams. Whether that **covers** the current
+billing quarter depends on where the wall clock sits inside it, so the peak
+guard was blind and engaged on some runs and not others — and `decide()` refuses
+outright while the guard is holding (§9.1 over §10.1). On those runs the armed
+baseline did not trade, and "trading off never trades" would have been graded
+on a fixture in which nothing could trade either way. It is now 900 s of
+telegrams, and the guard's clear state is a **check** rather than an assumption.
+
+## 8. What is still open
+
+* The literal reading of AC2 (§3). One token, and it costs a measured trading
+  configuration its plan. **Owner's call.**
+* `cheap_now()` (§4). Owner's call, with the adoption list above.
+* The *Tariff* status row still shows the static-profile sentence on a
+  provider-backed site; the provider plan's own line goes to the log. Pinned as
+  a pair with `DOCS.md` by FCST-04 and untouched here.
@@ -1346,12 +1346,26 @@ does, and only after you have read why.
 > release ships the five provider plugins, the canonical all-in format they
 > emit, the config that selects one, and — new in OPT-04 — a price-arbitrage
 > strategy that trades on them. With `arbitrage_enabled: false`, which is the
-> shipped default, turning a provider on still fetches, validates and publishes
-> prices and changes no charging decision.
+> shipped default, **selecting a provider is what starts the fetch**: the feed
+> is read every 30 minutes, validated, and the day recorded in
+> `/data/price_history.json`, which the *Price history* status row reports and
+> the add-on log names at every refresh. No charging decision changes.
+>
+> ⚠️ **That has been the documented behaviour since OPT-04 and was not the
+> actual behaviour until TARIFF-10.** The background refresh was started only
+> when `arbitrage_enabled` was on — and it ships `false` — so an install with a
+> provider configured fetched nothing at all, never created the store, and read
+> *Price history: empty* for as long as it ran. If you configured a provider and saw an empty
+> Price history row, that is what you were looking at. Trading is unchanged and
+> is still `arbitrage_enabled` alone.
 >
 > `static_profile` below needs no network at all, so its cheap window is
 > computed on every cycle and reaches the maintenance cycle's grid top-up
-> whether or not arbitrage is armed.
+> whether or not arbitrage is armed. ⚠️ **And it is still the only source of
+> that window.** A provider-backed plan now exists on every configured site,
+> but *Cheap window* and the maintenance cycle's grid top-up read
+> `static_profile` and nothing else — a deliberate limit, recorded with what it
+> would take to change in `DECISIONS/TARIFF-10-fetching-is-not-trading.md`.

 | option | meaning |
 |---|---|
@@ -1360,7 +1374,7 @@ does, and only after you have read why.
 | `price_taxes_eur_kwh` | Levies and VAT, EUR/kWh. Same rule |
 | `export_price_eur_kwh` | The **injection** price. A separate scalar, not a series — see below |
 | `price_csv_path` | Where `manual_csv` reads its file |
-| `price_feed_url` | Override the plugin's default feed. Only a host the plugin itself declares is accepted |
+| `price_feed_url` | Override the plugin's default feed. Used **exactly as written** — the add-on appends nothing to it, including the date range, so an override gets whatever horizon that URL returns. Only a host the plugin itself declares is accepted |
 | `price_horizon_h` | How far ahead to ask for, hours |
 | `price_timeout_s` | Per-request timeout |
 | `static_profile` | A static day/night or fully-fixed tariff. Empty by default — see below |
@@ -1395,6 +1409,50 @@ flagged as high-volatility, which widens arbitrage thresholds once the
 optimiser lands. That comes from the plugin's own declaration, not from its
 name.

+**`generic_dayahead` and the shape of the public feed.** It reads Fraunhofer
+ISE's Energy-Charts API (`https://api.energy-charts.info/price?bzn=BE`, no key
+and no registration), which publishes a *columnar* document — parallel
+`unix_seconds` and `price` arrays, in EUR/MWh, with the unit and the licence
+stated in the response. Two things follow that are worth knowing before you
+point `price_feed_url` somewhere else:
+
+* The timestamps are **UTC instants**. They are rendered at the Brussels offset
+  that applies to each one, so the hour that happens twice every October is two
+  points rather than one overwritten one.
+* Belgium moved to a **15-minute market time unit** on 2025-10-01, and the feed
+  followed. A published day is **96 points, not 24**. Nothing in the add-on
+  assumes either spacing, and nothing aggregates the quarters away.
+* The request **names the days it wants**, derived from the horizon on the
+  **Brussels** calendar — `?bzn=BE&start=2026-09-09&end=2026-09-10`, two
+  inclusive local dates. Without that the feed answers a single day, and
+  everything past today would be carried forward from yesterday's profile and
+  flagged provisional — which §8.3 excludes from arbitrage, so tomorrow would
+  never be tradeable at any hour. The calendar is Brussels rather than UTC
+  because 22:30 UTC in summer is already tomorrow in Belgium.
+
+**What you get before the auction publishes.** The day-ahead auction clears at
+about 13:00–14:00 CET. Ask for tomorrow before that and the feed returns fewer
+days than requested — that is normal, not a failure. The add-on keeps the
+series of what exists and fills the remaining hours from yesterday's profile,
+flagged provisional; arbitrage runs on the firm hours and ignores the flagged
+ones. So a dynamic site's plan is part firm and part carried every morning and
+fully firm once tomorrow is published, and neither state is an error.
+
+⚠️ **If you set `price_feed_url`, you keep the short horizon.** The override is
+sent exactly as you wrote it and the add-on adds no `start`/`end` to it, because
+those are this feed's parameter names and an override may point somewhere with
+different ones — rewriting a URL you typed is how a request nobody intended gets
+made. The consequence is a **`WARNING` in the log on every price refresh**,
+naming the range that would otherwise have been requested. If you only wanted a
+different host and not a different horizon, the fix is to clear the option.
+
+If the arrays ever come back different lengths, or the unit stops saying
+EUR/MWh, the fetch is refused outright rather than trusted in part: a price
+series missing its last hours is a series missing the evening peak, and it
+would be traded on as though it were complete. A refused fetch is the §11.4
+row — static profile, provisional plan, no arbitrage — and the reason is in the
+log.
+
 **`manual_csv` — the offline fallback.** It needs no internet, no supplier
 account and no HTTP stack at all, and it is the supported way to run this
 add-on price-aware on a site with no outbound connectivity. Write a file and
@@ -1488,10 +1546,20 @@ first fill is possible the day after the first refresh.

 The same applies to a file that is *partly* broken: a day record this build
 cannot read is dropped, the row says how many were dropped and why, and the rest
-of the file still loads. And where there is no `/data` at all the store records
-**nothing** — not even in memory. An in-memory yesterday would answer for one
-run and vanish on the next restart, which is a cache that forgets without saying
-so; `unavailable` and *no prices carried* are the honest pair.
+of the file still loads. And a store that **cannot persist records nothing** —
+not even in memory. That covers a missing `/data`, and it also covers a `/data`
+that is there and cannot be **written**: a full volume, a read-only remount. An
+in-memory yesterday would answer for one run and vanish on the next restart,
+which is a cache that forgets without saying so; the row naming the reason and
+*no prices carried* are the honest pair.
+
+⚠️ **What a failed write does not do is throw away what is already in the
+file.** A volume that fills at 14:00 leaves the yesterday written at 13:30 still
+answering, because that record survives a restart — the store stops *adding* to
+it, nothing more. And it never stops the add-on: a write that cannot complete is
+logged and reported on the status row, never raised. A cache that cannot answer
+is not allowed to be a controller that cannot regulate, and that applies to the
+volume being full exactly as it applies to the volume being absent.

 **Injection is priced separately** (`export_price_eur_kwh`). In Belgium it is
 often near zero, which by itself strongly favours self-consumption over export.
@@ -1616,6 +1684,14 @@ towards the firmware's 30 s failsafe. The plan refreshes every 30 minutes and
 retires itself if the refresh stops answering, at which point arbitrage stops
 with it.

+⚠️ **That refresh is not gated on this switch** (TARIFF-10). It runs when a
+`price_provider` is configured *or* when arbitrage is armed, so turning
+arbitrage off does not turn the price feed off — and a site with no provider
+that arms arbitrage on a `static_profile` keeps the plan it has always had. The
+only thing that decides whether a request leaves the machine is
+`price_provider`: with it `off`, the refresh resolves the static profile in
+memory and opens no connection.
+
 ### Site

 | option | meaning |
@@ -142,6 +142,28 @@ red on 15 named checks. Same shape as TEST-16 and the same fix: **grade the
 reason, not the verdict**, and when two rungs answer one stimulus, mutate them
 together.

+⚠️ **AND FCST-06 FOUND THE COUNT ITSELF LYING — LOW, NOT SILENT.** A mutation
+whose red is a **traceback** does not report one red; it reports however many
+checks ran before the traceback, and grades **none** of the ones after it. The
+new section drove M4 (`self._days = before` → `self._days = {}`) and the run
+printed **1 FAIL**. The true figure is **3**: an f-string in the next check read
+`len(read.points)`, and `.points` on a `no_data` **raises `Absence` by design**,
+so the harness died mid-section with the remaining checks ungraded. That is
+`why()`'s and `derived()`'s rule — *"it answered nothing" and "it broke on the
+way to answering" must not grade the same* — arriving in the **printed operand**
+of a check rather than in its condition, which is the half that is easy to miss
+because it looks like formatting. Two things follow:
+
+* **a mutation report's red COUNT is only a lower bound until the run is seen to
+  have reached the end of its section.** Read the output, not the count;
+* **the total accessor has to cover what a check PRINTS, not only what it
+  asserts.** `test_price_history.py` 5d added `npts()` beside `why()` for
+  exactly this, and re-driving M4 against it moved 1 FAIL to 3.
+
+Worse, had the traceback landed one check earlier the mutation would have
+produced **no named red at all** — a result that reads like a broken harness and
+gets retried rather than read.
+
 ## The third way a check lies: its DATA cannot reach the failure it names

 Stale bytecode manufactures survivors from a mutation that never ran. Masking
@@ -673,6 +695,84 @@ Cheap habits fall out of it, and all of them are now used in this repository:
      that two of it cannot silently overwrite each other's work. A fixed path
      in a tool that edits source is a shared mutable global with a filename.

+19. ⚠️ **A FIXTURE IN A SHAPE NOBODY PUBLISHES GRADES THE PARSER AGAINST THE
+    PARSER — FOURTEENTH INSTANCE OF HABIT 4, AND THE FIRST WHERE THE FIXTURE
+    AND THE CODE UNDER TEST SHARED A *SHAPE* RATHER THAN A NUMBER.** TARIFF-08.
+    `price.py`'s `_points_from_json()` read `doc["data"]` as a list of
+    `{start, price_eur_mwh}` objects. `api.energy-charts.info` publishes
+    parallel `unix_seconds` / `price` arrays and has no `data` key at all, so
+    `generic_dayahead` raised `SourceUnavailable` on **every live fetch it ever
+    made** — and the suite was green, because all three price suites built
+    their bodies with `json.dumps({"data": [...]})`. The fixture was derived
+    from the parser's guess, so no mutation of the parser and no amount of
+    coverage inside that loop could ever have reached the failure. §11.4 then
+    turned it into a *correct* silent degradation: parse error -> `no_data` ->
+    static profile -> arbitrage off, with nothing naming the cause.
+
+    Habit 2 with a network on the end of it: **the reference has to be data the
+    code under test did not shape.** For a parser that means BYTES, captured
+    from the source and committed unmodified, with the URL, the retrieval time
+    and a checksum beside them - `scenarios/energy-charts-be-price-*.json` and
+    its `-captures.json` provenance, re-derived by the replay so an edited
+    fixture stops being a capture. Two granularities, because the same endpoint
+    answers hourly before 2025-10-01 and quarter-hourly after it and a fixture
+    at one spacing cannot grade an assumption about the other.
+
+    ⚠️ **AND THE SURVIVING SYNTHETIC FIXTURES ARE NAMED FOR WHAT THEY ARE.**
+    Three shipped plugins point at endpoints nobody has verified (TARIFF-07),
+    so no capture exists for them; their fixture builder is called
+    `synthetic_row_feed` at every call site, because the next reader's default
+    assumption will otherwise be that a green check means the feed parses.
+
+    ⚠️ **ONE OF THIS CARD'S OWN NEW LINES WAS DECORATION AND THE BATTERY SAID
+    SO.** The columnar parser first rendered its instants with `brussels()`
+    before returning them. `assemble()` already renders every plugin's points,
+    so removing the parser's call changed nothing observable - **M4 came back a
+    survivor on a `:ro`, purged tree**, which is not the direction stale
+    bytecode fabricates. The line was removed rather than fenced with a check:
+    a second copy of a rule that already has an owner is indistinguishable from
+    a no-op, and the mutation that proves the rendering is real belongs on
+    `assemble()`, where it is now aimed.
+
+20. ⚠️ **A FIXTURE GRADES THE RESPONSE. NOTHING ABOUT THE REQUEST IS REACHABLE
+    FROM ONE, AND THE TRANSPORT HAD BEEN RECORDING IT ALL ALONG.** TARIFF-09,
+    and it is habit 19's closed loop with the other half still shut.
+    `generic_dayahead.fetch()` took `since` and `until` and asked for the bare
+    `?bzn=BE`, which answers ONE Brussels day - so §8.3 dropped every hour past
+    today from the trade set as provisional and the optimiser never saw
+    tomorrow as firm, at any hour of any day. Measured at 09:00: **15 firm
+    hours and 33 carried of 48**, both ends of the permitted cycle inside one
+    calendar day; at 23:00, **1 and 47**, with the firm spread no longer
+    clearing the round-trip cost.
+
+    TARIFF-08 had just fixed the RESPONSE side of this same plugin by
+    committing real bytes. It could not have caught this: a captured body is
+    the answer to a URL, and **no property of any body says which URL was
+    asked for**. `canned()` in `test_price.py` had recorded `(url, timeout_s)`
+    on every call since TARIFF-02 and not one check had ever asserted on the
+    first element - so the harness *knew* the plugin was ignoring its arguments
+    and nothing asked it. The fix is one line of habit: **where a plugin takes
+    arguments that shape a request, assert on the REQUEST off the injected
+    transport, not only on what comes back.** Section 2c does, and the
+    shipped-defect mutation (restore the bare URL, `-173 B`) is red on six
+    named checks in `test_price.py` and two in `test_price_history.py`.
+
+    ⚠️ **AND A MUTATION THAT TURNS A READ INTO `no_data` CRASHES A SUITE
+    INSTEAD OF NAMING A CHECK, UNLESS THE CHECK EXPECTS IT.** `_NoData.points`
+    raises `Absence` by design, so `derived(...) < derived(...)` over two
+    absent reads is a `TypeError` and `[p for p in plan.read.points]` is a
+    traceback - habit 5's shape with an exception in place of a hang. The
+    mutation that matters here is the "defensive" one somebody writes after
+    reading this card: *refuse when the feed returned fewer days than we asked
+    for*. It is exactly what a healthy feed does every morning before the
+    ~13:00-14:00 CET auction, and switching arbitrage off for it would be a
+    worse defect than the one being fixed. It is red BY NAME in
+    `test_price.py` (33 checks, the AC's own line among them) and takes
+    `test_price_history.py` down at **FCST-04's section 1**, whose `_before`
+    read is unguarded - a red, but an unnamed one. That is recorded rather than
+    retrofitted: guarding another card's section is a change to what its
+    checks report, and it belongs to whoever next touches it.
+
 And the mutation-table consequence: **a variant is not covered because a
 near-miss of it is.** DEV-02's table listed the *actuator* variant of the
 per-unit claim defect (`device`/`supervised` filed without `inv`), which the
@@ -730,6 +830,7 @@ commit and PR bodies.
 | **SAFETY-03** reviewer survivor: `abs()` dropped from `clamp_to_rating`'s limit (6aabb83) | **re-verified — still a survivor** | re-run the same way, all 15 green. Also as recorded: unreachable under the `int(100, 5000)` schema. |
 | **MAINT-05 round 3**, `migrate_maint_keys()` marker gate `if rep.ok:` → `if rep.ok or rep.kept:` (`+12 B`) | **was a survivor — now CLOSED** | measured against head `65cddf3` under the purge, tree mounted `:ro`, `19/19 PASS`. It stamps the schema marker on a migration that LOST state, defeating the older `lost` guard as well as the new `misdirected` one, because every check that reached the marker did so on a store whose `kept` list was empty. Closed by `test_maintenance.py` **9d**; the mutation is now red on three named checks. |
 | **MAINT-05 round 3**, `migrate_unit_keys()` marker gate `if rep.ok:` → `if rep.ok or rep.kept:` (`+12 B`) | **found here — SAFETY-02's half, now CLOSED** | the SAME mutation one function up, on the LATCH migration rather than the history one. Measured a survivor against this branch under the purge, `:ro`, `19/19 PASS`, then closed by `test_inverter.py` part 10's new fixture — a store with `unit/inverter-a/sign_reason` already present (so `kept` is non-empty) whose `sign_tripped: true` write is dropped. Shipped: marker withheld. Mutant: marker stamped on a **disarmed latch**, and the next start skips the migration that would have re-armed it. |
+| **TARIFF-09** 11 mutations over `app/price.py` and the two-day capture, all red, no survivors | **not at risk** | direction, and method: every mutation was applied to a FRESH `cp -a` of the clean worktree rather than reverted, so there is no revert to verify (habit 18) and no shared scratch path two runs could collide on. Each ran through `tools/mutation_run.sh`, which purges and fingerprints. Byte deltas recorded on the PR; one is **+0 B** - a single price digit edited inside the committed capture, the risky delta - and the re-derived sha256 catches it. |
 | **TARIFF-01**'s other three survivors, **SAFETY-03**'s `describe()` survivor, **TARIFF-02**'s M3/M5/M7, **SAFETY-18**'s three | **still unverified** | recorded before the purge discipline, with no byte delta on the record, so a `+0 B` same-second revert cannot be excluded. All are survivors, which is the direction this defect fabricates. TARIFF-02's and SAFETY-18's were **closed by later commits whose claims are reds**, so what is unverified is the *diagnosis*, not the current state of the code. |

 **Nothing here says a past result is wrong.** The two survivors re-run above
@@ -67,7 +67,7 @@ from .mqtt import (MqttPublisher, announced_units, unit_slugs,
 from . import peakguard
 from . import packshare
 from .p1 import P1Ingest, build_source, is_enabled
-from .price import Uncertainty, price_option
+from .price import Uncertainty, price_option, provider_configured
 from .price_history import PriceHistory
 from .static_tariff import resolve_tariff, static_status
 from .tariff import PeakStore, PeakTracker
@@ -138,6 +138,60 @@ TARIFF_REFRESH_S = 1800.0
 TARIFF_MAX_AGE_S = 3 * TARIFF_REFRESH_S


+def tariff_refresh_wanted(opts: dict) -> bool:
+    """TARIFF-10. Does this install need the background tariff refresh at all?
+
+    THE DEFECT THIS PREVENTS, and it shipped: the gate below used to be
+    `arbitrage.enabled(opts)` alone. That option defaults to `false`, and
+    `run_tariff_refresh` is the only caller of `resolve_tariff()`, which is the
+    only caller of `price_history.carry()` - so on a shipped install with a
+    provider configured NOTHING was ever fetched, no plan was ever resolved,
+    `/data/price_history.json` was never created, the *Price history* row read
+    `empty` forever, and TARIFF-09's short-horizon warning could not fire
+    because there was no fetch to warn about. Measured over a 40 h soak of two
+    instances side by side: zero feed requests and `present: false` across all
+    802 samples on the shipped default, against a resolved plan from the same
+    feed on the instance with arbitrage armed. ⚠️ AND `DOCS.md` HAS
+    DESCRIBED THE BEHAVIOUR BELOW SINCE THE COMMIT THAT WROTE THE GATE:
+    OPT-04 (bdd7f18) added the refresh task, the `arbitrage.enabled()` guard on
+    it, and the sentence saying a configured provider still fetches, all in one
+    change. config.yaml has said it since TARIFF-02 (852fc7c).
+
+    Two terms, and they answer two different questions:
+
+      provider_configured(opts) -> "the operator asked for a price feed", so
+                                   fetch it, validate it and record the day
+                                   (§8.3). This is the term TARIFF-10 ADDS.
+      arbitrage.enabled(opts)   -> "the operator armed trading". This term is
+                                   what the gate USED to be, kept verbatim.
+
+    ⚠️ IT IS AN `or`, SO THE GATE IS A STRICT WIDENING. Every configuration
+    that started this task before still starts it; none that did not start it
+    stops anything. That matters because the second term is not redundant:
+    `price_provider: off` + a `static_profile` + `arbitrage_enabled: true` is a
+    configuration validate.py explicitly ACCEPTS, and it trades - measured, at
+    `cycle_cost_threshold_eur: 0.05` on a 0.20/0.284 day/night profile,
+    `ARBITRAGE_CHARGE -5000 W`. Dropping that term to make the gate read
+    `provider_configured(opts)` alone would leave `self.tariff_plan` at None on
+    that site forever and `decide()` would answer "no tariff plan has been
+    resolved yet" on every cycle - silently disarming a strategy the UI says is
+    running, which is the exact failure validate.py's OPT-04 rules exist to
+    refuse.
+
+    ⚠️ AND NO CONFIGURATION ACQUIRES A PERIODIC HTTPS CALL FROM THIS TICKET.
+    With `price_provider` off, `resolve_tariff()` returns `static_plan()` - pure
+    arithmetic, no socket - so the second term starts a task that fetches
+    nothing. The true shipped default (no provider, no arbitrage) starts no
+    task at all. What is fetched is decided by the FIRST term only.
+
+    ⚠️ THIS IS NOT A TRADING SWITCH. It decides whether prices are LOOKED UP.
+    Whether they move energy is `arbitrage.enabled()` at the one call site in
+    `cycle()`, and TARIFF-10 did not touch it: `arbitrage_enabled: false` still
+    means no trade, ever, on any configuration.
+    """
+    return provider_configured(opts) or arbitrage.enabled(opts)
+
+
 def load_options() -> dict:
    try:
        with open(OPTIONS_PATH, "r", encoding="utf-8") as fh:
@@ -4353,6 +4407,14 @@ class Controller:
            # OPT-04. The strategy's own sentence, not a re-derivation of it -
            # every refusal it can make already carries a reason a human can
            # act on, and "Arbitrage: no" would throw all of them away.
+            #
+            # ⚠️ TARIFF-10 RE-READ THIS STRING AND LEFT IT ALONE, which is worth
+            # saying because that card is about the two switches being
+            # different. `self.arb` is None if and only if
+            # `arbitrage.enabled(self.o)` was False, so `off (arbitrage_enabled)`
+            # still names the option that is actually off - and on a site with a
+            # provider configured it is now the row that says the prices
+            # arriving on the *Price history* row above are moving nothing.
            ("Arbitrage", "off (arbitrage_enabled)" if self.arb is None
             else self.arb.explain()),
        ]
@@ -4605,11 +4667,16 @@ async def amain() -> None:
        # ESP32 heartbeat. A fetch on that path is a supplier's TLS timeout able
        # to push this add-on towards layer 1's 30 s veto. See TARIFF_REFRESH_S.
        #
-        # ⚠️ STARTED ONLY WHEN ARBITRAGE IS ARMED. Nothing else in the add-on
-        # reads the resolved plan - `cheap` still comes from static_status() on
-        # the tick, which is pure - so on a default install this task would be
-        # network traffic in aid of an attribute nobody reads.
-        if arbitrage.enabled(opts):
+        # ⚠️ STARTED WHEN A PROVIDER IS CONFIGURED, *OR* WHEN TRADING IS ARMED
+        # - TARIFF-10, and it used to be the second term alone. `arbitrage_
+        # enabled` ships `false`, so a site that had selected a price feed
+        # fetched nothing at all; the whole argument, the measurement and what
+        # the `or` is protecting are in `tariff_refresh_wanted()` above.
+        #
+        # ⚠️ IT DOES NOT ARM ANYTHING. Fetching a price and trading on one are
+        # two decisions; this is the first, and `cycle()`'s
+        # `if arbitrage.enabled(self.o):` is still the only gate on the second.
+        if tariff_refresh_wanted(opts):
            tasks.append(asyncio.create_task(controller.run_tariff_refresh()))

        # P1 ingestion runs as its own long-lived task. ⚠️ It must not be driven
@@ -140,6 +140,7 @@ from dataclasses import dataclass
 from datetime import datetime, timedelta, timezone
 from enum import Enum
 from textwrap import dedent
+from urllib.parse import urlencode

 _LOG = logging.getLogger("goodwe.price")

@@ -184,6 +185,87 @@ def brussels(instant: datetime) -> datetime:
    return instant.astimezone(brussels_offset(instant))


+def brussels_date_range(since: datetime, until: datetime) -> tuple:
+    """The Brussels calendar days a half-open window [since, until) touches.
+
+    Returns `(first, last)`, two `date`s, BOTH INCLUSIVE - which is the shape
+    `api.energy-charts.info` takes: `?bzn=BE&start=2026-09-09&end=2026-09-10`
+    answered 192 quarter-hours across exactly those two local days, measured
+    2026-09-10T07:45:49Z and committed as
+    `scenarios/energy-charts-be-price-2026-09-09_2026-09-10-twoday.json`.
+
+    THE DEFECT THIS EXISTS TO PREVENT (TARIFF-09). `generic_dayahead.fetch()`
+    accepted `since` and `until` and requested the bare default URL, so the
+    feed answered ONE Brussels day. Everything past today was then carried
+    forward from yesterday's profile, flagged provisional, and §8.3 dropped all
+    of it from the trade set - so the optimiser never saw tomorrow as firm, at
+    any hour. Measured on the merged tree at 2026-09-10T09:00+02:00 over a 48 h
+    horizon: 15 firm whole hours and 33 carried, and both ends of the only
+    permitted cycle inside ONE calendar day. The ceiling was structural: at
+    hour H of the local day a one-day window gives at most 24-H firm hours and
+    24+H carried - 1/47 at 23:00, where the firm spread no longer even clears
+    the round-trip cost. That is not what FCST-04's fill is for; it covers the
+    pre-publication window until the ~13:00-14:00 CET auction, after which a
+    healthy feed hands over tomorrow in full.
+
+    ⚠️ THE CALENDAR IS BRUSSELS, NEVER UTC, AND THIS IS `_bdate()`'s LESSON ONE
+    LAYER UP. `since`/`until` are INSTANTS; the API takes DATES, and the two are
+    not the same question. 22:30 UTC in summer is already tomorrow in Belgium,
+    so a range derived from `since.date()` asks for the wrong FIRST day for the
+    last one or two hours of every single day - and the day it drops is the one
+    the controller is standing in. price_history.py made this mistake at the
+    cache layer and documents the same arithmetic; here it decides what is
+    fetched at all, so getting it wrong loses the hours rather than misfiling
+    them.
+
+    ⚠️ `until` IS EXCLUSIVE, SO LOCAL MIDNIGHT BELONGS TO THE DAY BEFORE IT.
+    A 48 h horizon from local midnight ends at local midnight two days later,
+    and that instant is the first tick of a day the caller did not ask for.
+    Taking `brussels(until).date()` unconditionally would request a THIRD day
+    on every whole-hour horizon. This feed answers such a request without
+    complaint, so nothing would report it - which is the reason to get it right
+    here rather than to rely on the server's tolerance: the range is a claim
+    about what the caller asked for, and it should be true.
+
+    ⚠️ AND AN EMPTY OR INVERTED WINDOW COLLAPSES TO ONE DAY RATHER THAN TO A
+    BACKWARDS RANGE. `start` after `end` is a request no feed has a defined
+    answer for; some return everything, some return an error, and neither is a
+    thing to hand a battery controller.
+    """
+    first = brussels(since).date()
+    edge = brussels(until)
+    last = edge.date()
+    if (edge.hour, edge.minute, edge.second, edge.microsecond) == (0, 0, 0, 0):
+        last = last - timedelta(days=1)
+    if last < first:
+        last = first
+    return first, last
+
+
+def dated_feed_url(base: str, **params) -> str:
+    """`base` plus a query string. Refuses a `base` that already carries one.
+
+    ⚠️ THE REFUSAL IS THE POINT, NOT THE FORMATTING. Appending `&start=...` to
+    a URL that may already hold its own query string is how a plugin produces a
+    request nobody intended - `?bzn=BE?bzn=BE&start=...`, or a duplicated
+    parameter whose winner is the server's choice rather than ours. So the base
+    is required to be parameterless and the parameters are added exactly once,
+    by `urlencode`, which is also what keeps a date out of the escaping
+    question. A future edit that folds the query back into the constant is a
+    refused fetch with a sentence, not a silently malformed URL.
+
+    ⚠️ IT IS NOT REACHED BY AN OPERATOR'S `price_feed_url`. See
+    `GenericDayAhead.fetch()`: an override is used exactly as given.
+    """
+    if "?" in base or "#" in base:
+        raise SourceUnavailable(
+            f"the feed base {base!r} already carries a query string or "
+            f"fragment, and this builder appends parameters. Adding a second "
+            f"'?' produces a URL the server parses differently from the one "
+            f"anybody intended; put the fixed parameters in the call instead")
+    return f"{base}?{urlencode(params)}" if params else base
+
+
 # --------------------------------------------------------------------------- #
 # Absence, borrowed wholesale from sim/riglock.py (TEST-04).
 # --------------------------------------------------------------------------- #
@@ -684,6 +766,47 @@ def price_option(opts: dict, key: str):
    return v


+def provider_id(opts: dict) -> str:
+    """The selected plugin id, normalised. `"off"` when none is selected.
+
+    ⚠️ ONE SPELLING OF THIS EXPRESSION, AND TARIFF-10 IS WHY IT IS A FUNCTION.
+    `str(price_option(opts, "price_provider") or "off").strip()` was written out
+    twice - in `load_prices()` below and in `static_tariff.resolve_tariff()` -
+    and main.py's startup gate would have been a third. "Is a provider
+    configured" is now a DECISION the add-on takes (it is what starts the
+    background refresh), and a decision spelled out at three call sites is three
+    places to disagree about whether `"  off  "` or `None` counts.
+    ⚠️ `validate.py` deliberately keeps its own reader: it validates the RAW
+    options blob, including one the Supervisor never saw, so it must not route
+    through `price_option()`'s defaults. Note what this does NOT do: it does not
+    decide
+    whether the id names a shipped plugin - `load_prices()` and validate.py's
+    rule 7 own that, and answering "unknown" here would turn a typo into
+    "no provider" instead of into a startup refusal.
+    """
+    return str(price_option(opts, "price_provider") or "off").strip()
+
+
+def provider_configured(opts: dict) -> bool:
+    """Has the operator selected a price feed at all? (TARIFF-10)
+
+    THE DEFECT THIS PREVENTS: the background tariff refresh used to start only
+    when `arbitrage_enabled` was on, which ships `false` - so a site with a
+    provider configured fetched NOTHING, ever. `/data/price_history.json` was
+    never created, the *Price history* status row read `empty` forever, and
+    TARIFF-09's short-horizon warning could not fire because no fetch happened
+    to warn about. Measured over a 40 h two-instance soak: zero feed requests
+    and `present: false` across all 802 samples on the shipped default, against
+    a resolved plan on the same feed with arbitrage armed.
+
+    ⚠️ THIS IS NOT A TRADING SWITCH AND MUST NEVER BECOME ONE. It answers
+    "fetch prices?"; `arbitrage.enabled()` answers "move energy on them?", and
+    §10.1's priority 3 is gated on the second alone. See
+    `main.tariff_refresh_wanted()` for the one place the two meet.
+    """
+    return provider_id(opts) not in ("", "off")
+
+
 def export_price_eur_kwh(opts: dict) -> float:
    """AC4 / §9.3: the injection price is a SEPARATE SCALAR, not a second series.

@@ -731,6 +854,24 @@ class PriceProvider(ABC):
        `transport(url, timeout_s) -> bytes` is injected so the checks never
        touch a network and `manual_csv` never needs one. `opts` is a
        `PluginOptions`, so the markup and taxes are not reachable from here.
+
+        ⚠️ `since`/`until` ARE THE RANGE THE CALLER WANTS, AND A PLUGIN THAT
+        IGNORES THEM GETS WHATEVER ITS FEED DEFAULTS TO (TARIFF-09). That is
+        not a small difference: `generic_dayahead` ignored both, the public
+        EPEX feed answered ONE Brussels day, and §8.3 then dropped every hour
+        past today from the trade set as provisional - so the optimiser never
+        saw tomorrow as firm, at any hour of any day, and nothing reported it.
+
+        This promise is NOT mechanically enforced, and the honest reason is
+        TARIFF-08's: three of the five shipped plugins point at endpoints
+        nobody has verified (TARIFF-07 is the open decision on withdrawing
+        them), so nobody knows how those feeds spell a date range. A rule
+        obliging them to pass one would be satisfied by an INVENTED parameter
+        name - the parser-graded-against-its-own-guess defect, moved from the
+        response to the request. What exists instead is a roster pin in
+        test_price.py: which plugins consume the range and which do not is
+        asserted as a literal, so a sixth supplier, or a change to one of the
+        three, forces the decision rather than inheriting the silence.
        """


@@ -979,9 +1120,166 @@ def _points_from_json(body, price_key, scale, provisional_default):
    return out


+
+# --------------------------------------------------------------------------- #
+# TARIFF-08: the COLUMNAR document, which is what the public EPEX feed really
+# publishes.
+#
+# ⚠️ THIS IS A SECOND SHAPE, READ BY A SECOND FUNCTION ON PURPOSE.
+# `_points_from_json()` above reads a row-per-point document: `{"data": [{...},
+# {...}]}`. `https://api.energy-charts.info/price?bzn=BE` does not publish that
+# and never did - it publishes PARALLEL ARRAYS:
+#
+#     {"unix_seconds": [1788991200, 1788992100, ...],   <- UTC epoch seconds
+#      "price":        [216.08,     209.90,     ...],   <- one per instant
+#      "unit": "EUR / MWh", "license_info": "...", "deprecated": false}
+#
+# The shipped parser was pointed at that URL and asked for `doc["data"]`, so
+# every fetch ended `SourceUnavailable: the feed carried no 'data' list` ->
+# §11.4 -> static profile, arbitrage off, and nothing in the status page said
+# why. Verified live 2026-09-10 against both fixtures under `scenarios/`.
+#
+# ⚠️ AND IT IS NOT ONE FUNCTION THAT SNIFFS FOR BOTH. A parser that decides
+# which document it got is a parser that can decide wrong, and a wrong-but-
+# parseable response then becomes a price. Two shapes, two functions, and each
+# plugin names the one its own feed publishes.
+#
+# The columnar shape was already known in this repository: DATA-02's
+# `scenarios/fetch_epex_be.py` has read `zip(d["unix_seconds"], d["price"])`
+# since 2026-08-31. Only the SHIPPED path never learned it, because every
+# fixture in the suite was built from the invented row shape - the parser was
+# graded against its own assumption. See test_price.py section 2b.
+# --------------------------------------------------------------------------- #
+
+COLUMNAR_TIME_KEY = "unix_seconds"
+COLUMNAR_PRICE_KEY = "price"
+COLUMNAR_UNIT_KEY = "unit"
+
+
+def _columnar_unit(text) -> str:
+    """`"EUR / MWh"` -> `"eur/mwh"`. Spacing and case are not information."""
+    return "".join(str(text).split()).lower()
+
+
+def _points_from_columnar_json(body, expect_unit, scale,
+                               provisional_default):
+    """Parse a COLUMNAR price document: parallel `unix_seconds` / `price`.
+
+    Defects this refuses, each of which has a way of becoming a number rather
+    than an error:
+
+      * ⚠️ UNEQUAL ARRAYS ARE `SourceUnavailable`, NOT A SHORT SERIES. `zip()`
+        stops at the shorter array and says nothing, so a `price` list that
+        arrived truncated would produce a REAL price series with its tail
+        missing - and the tail of a Belgian day is the evening peak. A series
+        that is missing the expensive hours is worse than no series: §11.4's
+        no-data row switches arbitrage OFF, whereas a truncated series is
+        traded on. Nothing downstream can tell the two apart, so the refusal
+        has to happen here.
+      * ⚠️ A UNIT THAT IS NOT THE DECLARED ONE IS REFUSED. `scale` is the
+        caller's standing assumption about the unit; if the feed ever
+        publishes EUR/kWh under the same keys, every price silently comes out
+        a THOUSAND times too small. `_validate_raw()` cannot catch that -
+        `MAX_PLAUSIBLE_EUR_KWH` only bounds prices that are too LARGE.
+        DATA-02's fetcher refuses on the same field for the same reason.
+      * ⚠️ `unix_seconds` ARE UTC INSTANTS, SAID SO EXPLICITLY.
+        `datetime.fromtimestamp(t, timezone.utc)`. Without that second
+        argument CPython returns a NAIVE datetime in the HOST's zone, and a
+        naive datetime is a wall clock reading plus an assumption about whose
+        clock it was. `brussels()` and `_validate_raw()` both refuse one, so
+        the mistake surfaces as a refused fetch rather than as a mispriced
+        day - but they refuse ALL naive datetimes and can say nothing about
+        whether this one was the right instant. That part is decided here.
+
+        §9.3's rendering AT THE BRUSSELS OFFSET is `assemble()`'s, once, for
+        every plugin - `brussels(p.start)` there is the single line that does
+        it, and calling it here as well would be a second copy of a rule that
+        already has an owner, indistinguishable from a no-op under mutation.
+        A plugin's job is the right INSTANT; the framework's is the rendering.
+
+    A `null` price is DROPPED and logged, not fatal, and that is a different
+    judgement from the one above: an unequal pair of arrays means the pairing
+    of instant to price is unreliable, whereas a null means the source stated
+    it published no price for that one instant. A dropped instant leaves a
+    hole, and `_Series.at()` already answers a hole with `Absence` - §11.4 for
+    that hour. `scenarios/fetch_epex_be.py` dropped 0 nulls over the 12 months
+    to 2026-08-31, so this is a shape the source allows rather than one it uses.
+    """
+    try:
+        text = body.decode("utf-8") if isinstance(body, (bytes, bytearray)) \
+            else str(body)
+        doc = json.loads(text)
+    except (ValueError, UnicodeDecodeError) as e:
+        raise SourceUnavailable(f"the feed did not parse as JSON: {e}")
+    if not isinstance(doc, dict):
+        raise SourceUnavailable(
+            f"the feed is a {type(doc).__name__}, not a columnar object with "
+            f"{COLUMNAR_TIME_KEY!r} and {COLUMNAR_PRICE_KEY!r} arrays")
+    times = doc.get(COLUMNAR_TIME_KEY)
+    prices = doc.get(COLUMNAR_PRICE_KEY)
+    for key, col in ((COLUMNAR_TIME_KEY, times), (COLUMNAR_PRICE_KEY, prices)):
+        if not isinstance(col, list):
+            raise SourceUnavailable(
+                f"the feed carried no {key!r} array; this provider reads the "
+                f"columnar shape (parallel {COLUMNAR_TIME_KEY}/"
+                f"{COLUMNAR_PRICE_KEY} arrays), and a row-per-point document "
+                f"is a different feed, not this one with a field missing")
+    if len(times) != len(prices):
+        raise SourceUnavailable(
+            f"{COLUMNAR_TIME_KEY} has {len(times)} entries and "
+            f"{COLUMNAR_PRICE_KEY} has {len(prices)}; refusing to zip them. "
+            f"The short array would truncate the series silently, and a price "
+            f"series missing its last hours is one missing the evening peak - "
+            f"traded on as if complete, where no data at all would have "
+            f"switched arbitrage off (§11.4)")
+    unit = _columnar_unit(doc.get(COLUMNAR_UNIT_KEY, ""))
+    if unit != _columnar_unit(expect_unit):
+        raise SourceUnavailable(
+            f"the feed reports {doc.get(COLUMNAR_UNIT_KEY)!r}, not "
+            f"{expect_unit!r}. This provider converts with a fixed factor of "
+            f"{scale}, so a changed unit is a factor of 1000 on every price "
+            f"with nothing downstream able to notice it")
+    if doc.get("deprecated"):
+        # Not a refusal: today's prices are still prices. But §11.4 makes a
+        # dead feed look exactly like a quiet one, so the warning that the
+        # source is going away has to be emitted while it still works.
+        _LOG.warning("the price feed reports deprecated=%r; it still answers, "
+                     "but this endpoint is scheduled to stop",
+                     doc.get("deprecated"))
+    out = []
+    dropped = []
+    for i, (t, v) in enumerate(zip(times, prices)):
+        if v is None:
+            dropped.append(i)
+            continue
+        if isinstance(t, bool) or not isinstance(t, (int, float)) or \
+                not math.isfinite(t):
+            raise SourceUnavailable(
+                f"{COLUMNAR_TIME_KEY}[{i}] is {t!r}, not epoch seconds")
+        if isinstance(v, bool) or not isinstance(v, (int, float)) or \
+                not math.isfinite(v):
+            raise SourceUnavailable(
+                f"{COLUMNAR_PRICE_KEY}[{i}] is {v!r}, which is not a price")
+        start = datetime.fromtimestamp(float(t), timezone.utc)
+        out.append(RawPoint(start=start, eur_per_kwh=float(v) * scale,
+                            provisional=bool(provisional_default)))
+    if dropped:
+        _LOG.warning("the feed published no price for %d of %d instants "
+                     "(index %s); those instants are left OUT of the series, "
+                     "never zero-filled - a missing hour is not a free hour",
+                     len(dropped), len(times),
+                     ", ".join(str(i) for i in dropped[:5])
+                     + (", ..." if len(dropped) > 5 else ""))
+    if not out:
+        raise SourceUnavailable(
+            f"the feed carried {len(times)} instants and a price for none of "
+            f"them")
+    return out
+
+
@register
 class GenericDayAhead(PriceProvider):
-    """Public EPEX BE day-ahead feed, in EUR/MWh.
+    """Public EPEX BE day-ahead feed, COLUMNAR, in EUR/MWh.

    SOURCES: energy
    The framework ADDS: markup, taxes
@@ -990,6 +1288,19 @@ class GenericDayAhead(PriceProvider):
    is the wholesale clearing price and nothing else, so both user-entered
    components have to be added to it - and because this plugin cannot read
    either number, it cannot be the one that adds them twice.
+
+    ⚠️ IT READS THE COLUMNAR SHAPE, AND SAYS SO IN ITS OWN `fetch()`. Until
+    TARIFF-08 it called `_points_from_json()` and asked
+    `api.energy-charts.info` for a `"data"` list it has never published, so
+    every fetch this provider ever made ended in `SourceUnavailable` and §11.4
+    turned that into a silent fall back to the static profile. See
+    `_points_from_columnar_json()` for the shape and for why there are two
+    parsers rather than one that guesses.
+
+    ⚠️ THE FEED IS QUARTER-HOURLY SINCE BELGIUM'S 15-MINUTE MTU (2025-10-01):
+    96 points a day at 900 s, not 24 at 3600 s. Nothing here assumes either -
+    the instants come from the document - and both granularities are replayed
+    from real captured bodies in test_price.py section 2b.
    """

    id = "generic_dayahead"
@@ -1001,12 +1312,68 @@ class GenericDayAhead(PriceProvider):
            sources=frozenset({Component.ENERGY}),
            egress_hosts=("api.energy-charts.info", "web-api.tp.entsoe.eu"))

+    FEED_BASE = "https://api.energy-charts.info/price"
+    BZN = "BE"
+
    async def fetch(self, opts, since, until, transport):
-        url = opts.get("price_feed_url") or \
-            "https://api.energy-charts.info/price?bzn=BE"
+        """Request the range the CALLER asked for, on the Brussels calendar.
+
+        ⚠️ THE ARGUMENTS ARE USED. Until TARIFF-09 this method took `since` and
+        `until` and asked for the bare `?bzn=BE`, which answers ONE Brussels
+        day - so a 48 h horizon was 15 firm hours and 33 carried at 09:00, and
+        1 firm and 47 carried at 23:00, with the whole trade set inside a
+        single calendar day at every hour of every day. See
+        `brussels_date_range()` for the measurement and for why the calendar
+        is Brussels rather than UTC.
+
+        ⚠️ AN OVERRIDE IS THE EXACT URL AND THIS METHOD APPENDS NOTHING TO IT,
+        WHICH COSTS THE OVERRIDING SITE THE RANGE. That is a decision, not an
+        oversight, and the sentence below is why it is not a footnote:
+
+          * `price_feed_url` says "override the plugin's default feed", and an
+            override that is silently rewritten is not one. Merging parameters
+            into an operator's query string means parsing and re-serialising a
+            URL they wrote - which reorders, de-duplicates and re-escapes it,
+            and can hand the server a request nobody typed.
+          * `start`/`end` are ENERGY-CHARTS' spelling, and this provider
+            DECLARES A SECOND HOST: `web-api.tp.entsoe.eu`. Nothing here has
+            verified that host's day-ahead parameters and nothing here should
+            guess them - but they are not `start`/`end`, so an override to the
+            other DECLARED host is exactly the case where appending our
+            vocabulary adds two parameters that host ignores and yields the
+            short horizon anyway, with nothing saying so.
+          * and blind appending is refused outright: `dated_feed_url()` will
+            not add a query string to a base that has one.
+
+        So the cost is real and it is REPORTED, at fetch time, on the shipped
+        logger, naming the horizon consequence rather than the substitution.
+        §11.4 is what makes a price defect quiet; an operator who has taken the
+        short horizon has to be able to find out from the log that they did.
+        """
+        override = str(opts.get("price_feed_url") or "").strip()
+        if override:
+            url = override
+            _LOG.warning(
+                "price_feed_url is set, so %s is requesting %s exactly as "
+                "given and is adding NO date range to it. The public feed "
+                "answers a single Brussels day to a request that names no "
+                "range, and every hour past that day is then carried forward "
+                "from yesterday's profile and flagged provisional - §8.3 "
+                "drops provisional hours from the trade set, so arbitrage "
+                "would never see tomorrow as firm. Clear price_feed_url to "
+                "let this plugin ask for the %s..%s it was given.",
+                self.id, url, *(d.isoformat()
+                                for d in brussels_date_range(since, until)))
+        else:
+            first, last = brussels_date_range(since, until)
+            url = dated_feed_url(self.FEED_BASE, bzn=self.BZN,
+                                 start=first.isoformat(),
+                                 end=last.isoformat())
        check_egress(url, self.capabilities())
        body = await transport(url, float(opts.get("price_timeout_s")))
-        return _points_from_json(body, "price_eur_mwh", 0.001, False)
+        return _points_from_columnar_json(body, expect_unit="EUR / MWh",
+                                          scale=0.001,
+                                          provisional_default=False)


@register
@@ -1190,8 +1557,8 @@ async def load_prices(opts: dict, since: datetime, until: datetime,
    site condition, it is a plugin trying to read the markup, and turning it
    into "no price data" would hide the exact defect trap 1 exists to catch.
    """
-    pid = str(price_option(opts, "price_provider") or "off").strip()
-    if pid in ("", "off"):
+    pid = provider_id(opts)
+    if not provider_configured(opts):
        return PriceRead.no_data(
            "no price provider configured (price_provider is 'off'); "
            "§11.4: static profile, plan provisional, no arbitrage")
@@ -61,6 +61,38 @@ yesterday that works all day and is gone on the next restart, which is exactly
 the silently-forgetting cache this rule exists to refuse. A store that cannot
 persist now records nothing, and both of its answers say the same thing.

+⚠️ AND "CANNOT PERSIST" IS ONE QUESTION, ASKED ONCE, ABOUT THE OUTCOME -
+FCST-06. The paragraph above enforced that rule against `not self.ok`, which is
+one SYMPTOM of not persisting: the durable volume is missing. There is a second,
+and FCST-06 measured it. With `ok=True` and the directory present, `_save()` can
+still fail - a read-only remount (`[Errno 30] Read-only file system`), a full
+volume (`[Errno 28] No space left on device`) - and it logs, records
+`last_error` and returns. `_remember()` had already populated `self._days`, so
+`words` read `'2026-01-14 (24 points over 24 h); 2026-01-15 (24 points over
+24 h) [could not write (...)]'` while the file on disk held `{}`, and
+`yesterday()` answered 24 REAL POINTS for the rest of the process's life. Gone
+on the next restart, and nothing said it would be - the same silently-forgetting
+cache, reached by the condition the rule did not name.
+
+⚠️ SO IT IS NOT TWO RULES, AND IT IS DELIBERATELY NOT TWO TESTS. "Can this
+store persist?" CANNOT BE ANSWERED BEFORE THE ATTEMPT - a directory that existed
+and was writable a moment ago is not a write that completed - so it is answered
+AFTER it, once, by the only thing holding the evidence: `_save()` returns
+whether the file now holds what memory holds. A missing volume is one of its
+False cases and an `OSError` from the write is another, and `_remember()` KEEPS
+ONLY WHAT THAT ONE ANSWER SAYS WAS KEPT. There is no `not self.ok` test in
+`_remember()` any more, because there is nothing left for a second test to add -
+and a third symptom nobody has thought of is how this defect arrived both times.
+
+⚠️ AND WHAT IS KEPT IS THE LAST STATE THAT IS ON DISK, NOT AN EMPTY ONE. The
+MERGE is undone, not the store. A site whose volume fills at 14:00 goes on
+answering the yesterday it wrote at 13:30 - that record survives a restart, so
+answering it is not a promise the store cannot keep. What it stops doing is
+adding to it. The invariant the rule was always describing, stated once:
+`self._days` NEVER HOLDS A POINT THE FILE DOES NOT. (Not "mirrors the file" -
+`_usable_days()` drops records the file still holds. The only direction that can
+invent a yesterday is the one named.)
+

 THE FOUR RULES THIS FILE IS SHAPED BY
 =====================================
@@ -148,6 +180,7 @@ and turning it on is a product decision, not a wiring one. See
 DECISIONS/FCST-04-the-day-that-is-carried.md.
 """

+import copy
 import json
 import logging
 import os
@@ -333,6 +366,14 @@ class PriceHistory:
    site's first day, and a different thing from a store that could not be
    opened. Both are visible: `ok`/`last_error` describe the file, and
    `yesterday()`'s reason describes the answer.
+
+    ⚠️ AND `ok` IS NOT "CAN PERSIST" (FCST-06). It says the directory was
+    there when the store opened, which a full volume and a read-only remount
+    both leave true. The one thing that answers "can this store persist" is
+    `_save()`'s return value, and `self._days` NEVER HOLDS A POINT THE FILE DOES
+    NOT, because `_remember()` undoes a merge that answer refused. That is what
+    makes `words` and `yesterday()` describe the same store in EVERY degraded
+    state rather than only in the ones somebody enumerated.
    """

    def __init__(self, path: str = "/data/price_history.json"):
@@ -478,14 +519,31 @@ class PriceHistory:
                         "it", self.path, self.last_error)
        return kept

-    def _save(self) -> None:
+    def _save(self) -> bool:
+        """Write `self._days` out. True only if the file now holds it.
+
+        ⚠️ THE RETURN VALUE IS THE "CAN THIS STORE PERSIST?" PREDICATE, AND
+        IT IS THE ONLY ONE (FCST-06). Both ways of not persisting end here and
+        are the same answer: no durable volume returns False without touching
+        the disk, an `OSError` out of the write returns False after recording
+        why. `_remember()` acts on that one answer, so the second place that
+        used to test the first symptom on its own - and missed the second - has
+        nothing left to test. See the module docstring's fifth warning.
+
+        ⚠️ AND IT REPORTS, IT DOES NOT RAISE. §6 and this module's opening: a
+        broken cache must not be a broken controller. An add-on that will not
+        start because /data filled up is a worse defect than the one FCST-06
+        fixed, so a failed write is logged, kept in `last_error` - where `words`
+        and `yesterday()` both read it - and RETURNED, never thrown.
+
+        ⚠️ THE TEMP FILE IS REMOVED ON EVERY FAILING PATH. A write that dies
+        between `mkstemp` and `os.replace` used to leave its temp behind, and
+        the refresh task runs 48 times a day: a persistent write failure - a
+        full volume being the likeliest cause - leaked 48 inodes a day onto the
+        volume that was already the problem.
+        """
        if not self.ok:
-            return
-        # ⚠️ THE TEMP FILE IS REMOVED ON EVERY FAILING PATH. A write that dies
-        # between `mkstemp` and `os.replace` used to leave its temp behind, and
-        # the refresh task runs 48 times a day: a persistent write failure -
-        # a full volume being the likeliest cause - leaked 48 inodes a day onto
-        # the volume that was already the problem.
+            return False
        tmp = None
        try:
            fd, tmp = tempfile.mkstemp(dir=os.path.dirname(self.path))
@@ -498,12 +556,14 @@ class PriceHistory:
        except OSError as err:
            self.last_error = f"could not write ({err})"
            _LOG.error("could not persist price history: %s", err)
+            return False
        finally:
            if tmp is not None:
                try:
                    os.unlink(tmp)
                except OSError:             # nothing further to try, and a
                    pass                    # failed cleanup is not a fault
+        return True

    # -- the one public entry point -----------------------------------------
    def carry(self, read: PriceRead, *, since: datetime, until: datetime,
@@ -540,15 +600,6 @@ class PriceHistory:
        nothing outside this file names it, and `test_price_history.py` checks
        the tree for that rather than trusting the underscore.
        """
-        if not self.ok:
-            # ⚠️ A STORE THAT CANNOT PERSIST RECORDS NOTHING. See the module
-            # docstring's third warning: filling `self._days` here would give a
-            # site with no /data an in-memory yesterday that answers a series
-            # all day and is gone on the next restart - a cache that forgets
-            # without saying so, which is the one thing that rule refuses. The
-            # honest degraded state is `words` = "unavailable" and `yesterday()`
-            # = `no_data`, and those two must agree.
-            return
        points = read.match(series=lambda r: r.points, no_data=lambda _r: ())
        if not points:
            return
@@ -557,6 +608,25 @@ class PriceHistory:
        provider = read.match(series=lambda r: r.provider,
                              no_data=lambda _r: None)

+        # ⚠️ THE STATE THIS READ STARTED FROM, KEPT SO IT CAN BE PUT BACK
+        # (FCST-06). Everything below merges into `self._days` IN PLACE, and
+        # whether any of it may be KEPT is not knowable until `_save()` has
+        # tried - so the merge runs, and it is undone if the file did not take
+        # it. There is no `not self.ok` branch above any more: a store with no
+        # durable volume is one of the answers `_save()` gives, not a separate
+        # symptom with a test of its own. Module docstring, fifth warning.
+        #
+        # ⚠️ DEEP, NOT `dict(self._days)`. A one-level copy shares the day
+        # RECORDS, and the loop below rebinds keys on the ones it touches - so
+        # the "previous" state would hand back records the merge had already
+        # edited, and a store with a good yesterday on disk would go on
+        # answering the merge of it that never reached the file. It would also
+        # be right only for as long as nobody makes this loop mutate a `points`
+        # list or a row in place. A copy that is right by construction outlives
+        # that, and the store holds at most KEEP_DAYS + 2 records, 48 times a
+        # day.
+        before = copy.deepcopy(self._days)
+
        touched = set()
        junked = {}
        for p in points:
@@ -605,7 +675,9 @@ class PriceHistory:
            # has just discarded rows off disk stops reporting plain health -
            # and after `_save()` below the file is clean, so a restart clears
            # this. `_save()` overwrites it if the write itself fails, which is
-            # the more urgent fact of the two.
+            # the more urgent fact of the two - and on that path the drop is
+            # undone with the rest of the merge, so those rows are still in the
+            # file and the next write that completes drops them again.
            self.last_error = (
                f"{sum(junked.values())} malformed point row(s) dropped from "
                + ", ".join(f"{d} ({n})" for d, n in sorted(junked.items())))
@@ -613,7 +685,22 @@ class PriceHistory:
                         "without them and the refresh continues",
                         self.path, self.last_error)
        self._prune(stamp)
-        self._save()
+        if not self._save():
+            # ⚠️ A STORE THAT CANNOT PERSIST RECORDS NOTHING - the rule, asked
+            # ONCE as a question about the outcome instead of tested twice
+            # against two symptoms of it. Keeping the merge here is exactly the
+            # cache the module docstring refuses: `yesterday()` would answer a
+            # series the file does not contain, all day, until a restart
+            # dropped it without a word - and `words` and `yesterday()` would
+            # be describing two different stores. The reason is already in
+            # `last_error` (from `_save()`, or from `_open()` when there is no
+            # volume at all), so both answers now describe what is on disk.
+            #
+            # It does NOT raise, and §6 is why: a broken cache must not be a
+            # broken controller. `carry()` still returns the fill, the horizon
+            # is as short as the feed makes it, and the add-on regulates.
+            self._days = before
+            return
        _LOG.debug("price history: %d point(s) filed under %s; %d day(s) held",
                   len(points), ", ".join(sorted(touched)), len(self._days))

@@ -207,7 +207,7 @@ from datetime import datetime, timedelta, timezone
 from .price import (
    MAX_PLAUSIBLE_EUR_KWH, Absence, PricePoint, PriceProduct, PriceRead,
    Uncertainty, _Outcome, brussels, load_prices, price_option,
-    required_spread, uncertainty_of, widen_factor,
+    provider_configured, required_spread, uncertainty_of, widen_factor,
 )

 _LOG = logging.getLogger("goodwe.tariff.static")
@@ -1071,8 +1071,7 @@ async def resolve_tariff(opts: dict, since: datetime, until: datetime,
    missing and loses the elapsed ones it should never have had.
    """
    stamp = now or datetime.now(timezone.utc)
-    pid = str(price_option(opts, "price_provider") or "off").strip()
-    if pid in ("", "off"):
+    if not provider_configured(opts):
        return static_plan(opts, since, until, provisional=False, now=stamp)

    read = await load_prices(opts, since, until, transport)
@@ -265,9 +265,17 @@ options:
  # TARIFF - what energy costs, and whether to trade on it
  # ==========================================================================
  # `off` is the shipped default and means the §11.4 no-price-data row applies:
-  # static profile, plan provisional, NO ARBITRAGE. Nothing in 0.5.x consumes
-  # the series yet - OPT-04 is the consumer - so turning this on today fetches,
-  # validates and publishes prices and changes no energy decision.
+  # static profile, plan provisional, NO ARBITRAGE.
+  #
+  # ⚠️ SELECTING A PROVIDER IS WHAT STARTS THE FETCH, AND IT IS NOT
+  # `arbitrage_enabled` (TARIFF-10). With a provider set and trading off - the
+  # shipped default - the add-on fetches the feed every 30 minutes, validates
+  # it, records the day in /data/price_history.json and reports what it holds
+  # on the *Price history* status row. It changes NO energy decision: the cheap
+  # window the maintenance cycle reads still comes from `static_profile`, and
+  # §10.1's arbitrage is gated on `arbitrage_enabled` alone. Until TARIFF-10
+  # the fetch was gated on `arbitrage_enabled` too, so this option did nothing
+  # whatsoever on a shipped install - the store was never even created.
  #
  # ⚠️ THESE DEFAULTS ARE MIRRORED ONCE, IN app/price.py's
  # PRICE_OPTION_DEFAULTS, and test_price.py reads THIS FILE and asserts the two
@@ -318,6 +326,14 @@ options:
  # validate.py warns about exactly that.
  static_profile: ""
  price_csv_path: "/share/goodwe_prices.csv"
+  # ⚠️ AN OVERRIDE IS THE EXACT URL (TARIFF-09). The plugin sends it verbatim
+  # and appends nothing - including the `start`/`end` date range it derives from
+  # the horizon for its own feed - so an override gets whatever range that URL
+  # answers, which for a bare day-ahead endpoint is ONE Brussels day. Every hour
+  # past it is then carried from yesterday and flagged provisional, and §8.3
+  # keeps provisional hours out of the trade set. The plugin logs a WARNING
+  # naming that consequence on every fetch that uses an override; clearing this
+  # option is what restores the full horizon.
  price_feed_url: ""
  price_horizon_h: 48
  price_timeout_s: 10
@@ -344,6 +360,12 @@ options:
  # configuration: this is the first strategy in the add-on that moves energy
  # against the meter on purpose, so it is armed deliberately or not at all.
  #
+  # ⚠️ IT IS NOT WHAT TURNS THE PRICE FEED ON (TARIFF-10). `price_provider`
+  # above decides whether prices are FETCHED; this decides whether they move
+  # energy. Leaving this off on a site with a provider configured is a
+  # supported, documented state: prices are fetched, validated and recorded,
+  # and nothing trades.
+  #
  # ⚠️ IT NEEDS A PRICE SOURCE AND A P1 METER, and validate.py refuses the
  # combinations that cannot work rather than letting them look armed. With
  # `price_provider: off` and no `static_profile` there is no series to trade
@@ -0,0 +1 @@
+{"license_info":"CC BY 4.0 (creativecommons.org/licenses/by/4.0) from Bundesnetzagentur | SMARD.de","unix_seconds":[1757455200,1757458800,1757462400,1757466000,1757469600,1757473200,1757476800,1757480400,1757484000,1757487600,1757491200,1757494800,1757498400,1757502000,1757505600,1757509200,1757512800,1757516400,1757520000,1757523600,1757527200,1757530800,1757534400,1757538000],"price":[73.0,61.31,64.32,62.49,67.0,64.48,94.52,149.26,132.43,91.61,84.8,79.61,69.23,66.75,68.91,69.31,80.29,66.91,105.2,161.05,116.73,84.2,64.1,42.41],"unit":"EUR / MWh","deprecated":false}
@@ -0,0 +1 @@
+{"license_info":"CC BY 4.0 (creativecommons.org/licenses/by/4.0) from Bundesnetzagentur | SMARD.de","unix_seconds":[1788904800,1788905700,1788906600,1788907500,1788908400,1788909300,1788910200,1788911100,1788912000,1788912900,1788913800,1788914700,1788915600,1788916500,1788917400,1788918300,1788919200,1788920100,1788921000,1788921900,1788922800,1788923700,1788924600,1788925500,1788926400,1788927300,1788928200,1788929100,1788930000,1788930900,1788931800,1788932700,1788933600,1788934500,1788935400,1788936300,1788937200,1788938100,1788939000,1788939900,1788940800,1788941700,1788942600,1788943500,1788944400,1788945300,1788946200,1788947100,1788948000,1788948900,1788949800,1788950700,1788951600,1788952500,1788953400,1788954300,1788955200,1788956100,1788957000,1788957900,1788958800,1788959700,1788960600,1788961500,1788962400,1788963300,1788964200,1788965100,1788966000,1788966900,1788967800,1788968700,1788969600,1788970500,1788971400,1788972300,1788973200,1788974100,1788975000,1788975900,1788976800,1788977700,1788978600,1788979500,1788980400,1788981300,1788982200,1788983100,1788984000,1788984900,1788985800,1788986700,1788987600,1788988500,1788989400,1788990300,1788991200,1788992100,1788993000,1788993900,1788994800,1788995700,1788996600,1788997500,1788998400,1788999300,1789000200,1789001100,1789002000,1789002900,1789003800,1789004700,1789005600,1789006500,1789007400,1789008300,1789009200,1789010100,1789011000,1789011900,1789012800,1789013700,1789014600,1789015500,1789016400,1789017300,1789018200,1789019100,1789020000,1789020900,1789021800,1789022700,1789023600,1789024500,1789025400,1789026300,1789027200,1789028100,1789029000,1789029900,1789030800,1789031700,1789032600,1789033500,1789034400,1789035300,1789036200,1789037100,1789038000,1789038900,1789039800,1789040700,1789041600,1789042500,1789043400,1789044300,1789045200,1789046100,1789047000,1789047900,1789048800,1789049700,1789050600,1789051500,1789052400,1789053300,1789054200,1789055100,1789056000,1789056900,1789057800,1789058700,1789059600,1789060500,1789061400,1789062300,1789063200,1789064100,1789065000,1789065900,1789066800,1789067700,1789068600,1789069500,1789070400,1789071300,1789072200,1789073100,1789074000,1789074900,1789075800,1789076700],"price":[150.93,151.41,150.99,147.59,153.25,147.26,144.04,141.53,147.79,146.53,143.55,139.93,145.58,139.56,139.11,138.06,138.92,138.52,138.04,138.65,137.29,139.03,144.59,150.34,154.12,174.5,178.43,186.62,199.35,200.21,200.11,197.43,220.07,202.0,196.01,174.03,198.32,167.49,152.54,142.05,161.5,145.45,128.61,100.85,139.18,115.83,94.41,80.39,111.25,89.61,84.99,74.38,80.53,70.58,62.27,57.1,62.24,64.95,68.99,71.19,66.9,87.32,109.0,123.12,68.48,125.53,155.99,161.9,138.77,160.57,167.35,173.0,142.2,172.83,196.19,211.26,205.85,218.77,217.08,222.0,210.98,208.46,208.88,210.02,209.77,202.61,200.27,191.49,194.39,195.04,197.26,190.92,188.39,198.05,203.05,191.84,216.08,216.08,214.92,200.0,223.37,201.43,196.32,190.0,199.53,194.83,180.53,172.23,179.88,175.77,167.1,173.89,163.29,165.99,171.41,178.73,170.27,177.37,187.71,201.07,193.62,207.61,218.27,226.77,227.17,234.05,236.99,229.94,248.93,227.5,216.08,195.67,215.99,195.01,177.1,155.09,174.91,152.71,139.59,123.83,140.77,123.19,114.96,94.5,102.77,96.63,94.57,116.34,114.63,126.82,129.89,123.95,105.0,119.11,121.0,136.79,123.41,135.01,135.0,156.68,146.88,152.16,162.02,181.78,154.76,184.28,194.3,215.76,197.23,234.15,253.03,296.58,345.32,389.16,482.99,559.0,551.11,446.41,358.97,295.9,271.39,268.06,241.43,238.01,239.7,226.41,223.05,211.56,218.21,210.61,197.78,171.79],"unit":"EUR / MWh","deprecated":false}
@@ -0,0 +1 @@
+{"license_info":"CC BY 4.0 (creativecommons.org/licenses/by/4.0) from Bundesnetzagentur | SMARD.de","unix_seconds":[1788991200,1788992100,1788993000,1788993900,1788994800,1788995700,1788996600,1788997500,1788998400,1788999300,1789000200,1789001100,1789002000,1789002900,1789003800,1789004700,1789005600,1789006500,1789007400,1789008300,1789009200,1789010100,1789011000,1789011900,1789012800,1789013700,1789014600,1789015500,1789016400,1789017300,1789018200,1789019100,1789020000,1789020900,1789021800,1789022700,1789023600,1789024500,1789025400,1789026300,1789027200,1789028100,1789029000,1789029900,1789030800,1789031700,1789032600,1789033500,1789034400,1789035300,1789036200,1789037100,1789038000,1789038900,1789039800,1789040700,1789041600,1789042500,1789043400,1789044300,1789045200,1789046100,1789047000,1789047900,1789048800,1789049700,1789050600,1789051500,1789052400,1789053300,1789054200,1789055100,1789056000,1789056900,1789057800,1789058700,1789059600,1789060500,1789061400,1789062300,1789063200,1789064100,1789065000,1789065900,1789066800,1789067700,1789068600,1789069500,1789070400,1789071300,1789072200,1789073100,1789074000,1789074900,1789075800,1789076700],"price":[216.08,216.08,214.92,200.0,223.37,201.43,196.32,190.0,199.53,194.83,180.53,172.23,179.88,175.77,167.1,173.89,163.29,165.99,171.41,178.73,170.27,177.37,187.71,201.07,193.62,207.61,218.27,226.77,227.17,234.05,236.99,229.94,248.93,227.5,216.08,195.67,215.99,195.01,177.1,155.09,174.91,152.71,139.59,123.83,140.77,123.19,114.96,94.5,102.77,96.63,94.57,116.34,114.63,126.82,129.89,123.95,105.0,119.11,121.0,136.79,123.41,135.01,135.0,156.68,146.88,152.16,162.02,181.78,154.76,184.28,194.3,215.76,197.23,234.15,253.03,296.58,345.32,389.16,482.99,559.0,551.11,446.41,358.97,295.9,271.39,268.06,241.43,238.01,239.7,226.41,223.05,211.56,218.21,210.61,197.78,171.79],"unit":"EUR / MWh","deprecated":false}
@@ -0,0 +1,82 @@
+{
+ "card": "TARIFF-08, extended by TARIFF-09",
+ "what": "UNMODIFIED response bodies from the public EPEX BE day-ahead feed, captured so the SHIPPED parser can be replayed against what the source really publishes",
+ "why": "every price fixture in the suite was built from a `{\"data\": [{\"start\": ..., \"price_eur_mwh\": ...}]}` document that this feed has never published. The suite therefore graded `_points_from_json` against its own assumption - a closed loop that could not notice the assumption was wrong - and §11.4 turned the resulting failure into a silent fall back to the static profile. A captured real body is what breaks that loop, so these files are bytes, not a reconstruction: nothing here is pretty-printed, re-keyed or trimmed. TARIFF-09 adds a body from a request that names a DATE RANGE, for the same reason one layer out: the shipped plugin took `since`/`until` and asked for the bare default URL, so the feed answered ONE Brussels day and every hour past today was carried forward and dropped from the trade set as provisional. A two-day body captured from the range request is what makes \"asking for the range gets the range\" checkable against something the plugin did not shape.",
+ "source": "Energy-Charts API (Fraunhofer ISE), api.energy-charts.info",
+ "upstream": "Bundesnetzagentur | SMARD.de",
+ "licence": "CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/)",
+ "licence_note": "the BE bidding zone is in the API's CC BY 4.0 list, so redistribution is permitted with attribution. DATA-02 established this against the API's own OpenAPI document before committing `epex-be-dayahead-2025-09-01_2026-09-01.json`; both bodies below carry the same `license_info` string, recorded per file and asserted by the replay test.",
+ "attribution": "Data licensed CC BY 4.0 by Bundesnetzagentur | SMARD.de, retrieved via the Energy-Charts API (Fraunhofer ISE), https://energy-charts.info. Published without changes.",
+ "retrieved_utc": "2026-09-10T06:35:00+00:00 (TARIFF-08's two), 2026-09-10T07:45:49+00:00 (TARIFF-09's)",
+ "retrieved_with": "curl, by hand; no code in `app/` fetches these",
+ "shape": "COLUMNAR: parallel `unix_seconds` (UTC epoch seconds) and `price` arrays, plus `unit`, `license_info` and `deprecated`. There is no `data` key and no per-point object.",
+ "replayed_by": "test_price.py sections 2b (TARIFF-08) and 2c (TARIFF-09)",
+ "files": {
+  "energy-charts-be-price-2026-09-10-quarterhourly.json": {
+   "url": "https://api.energy-charts.info/price?bzn=BE",
+   "note": "the SHIPPED default URL of `generic_dayahead`, verbatim and with no parameters, so the replay drives the bytes the add-on's own request produces",
+   "covers_local_day": "2026-09-10 (Europe/Brussels)",
+   "granularity_s": 900,
+   "intervals": 96,
+   "bytes": 1877,
+   "sha256": "176309d0020ad208320797385ef68b6a98ac248218539416cddf6579dd060487",
+   "unit_as_returned": "EUR / MWh",
+   "licence_as_returned": "CC BY 4.0 (creativecommons.org/licenses/by/4.0) from Bundesnetzagentur | SMARD.de",
+   "deprecated_as_returned": false,
+   "top_level_keys": [
+    "deprecated",
+    "license_info",
+    "price",
+    "unit",
+    "unix_seconds"
+   ],
+   "also_returned_verbatim_by": {
+    "url": "https://api.energy-charts.info/price?bzn=BE&start=2026-09-10&end=2026-09-11",
+    "retrieved_utc": "2026-09-10T07:46:55+00:00",
+    "note": "TARIFF-09's PRE-PUBLICATION case, and it needed no second file because the answer was BYTE-IDENTICAL to the bare-URL capture above - the same 1877 bytes and the same sha256. Asked for two days at 09:46 CEST, before the ~13:00-14:00 CET auction publishes 2026-09-11, the feed returned the ONE day that exists. That is the short answer this card requires to stay a `series` of what exists rather than becoming `no_data`, and the replay re-derives the identity from the bytes rather than restating it."
+   }
+  },
+  "energy-charts-be-price-2025-09-10-hourly.json": {
+   "url": "https://api.energy-charts.info/price?bzn=BE&start=2025-09-10&end=2025-09-10",
+   "note": "a day BEFORE Belgium's 15-minute MTU (2025-10-01), so the same endpoint answers HOURLY. Kept because the parser must not assume either spacing, and a fixture at only one granularity cannot grade that",
+   "covers_local_day": "2025-09-10 (Europe/Brussels)",
+   "granularity_s": 3600,
+   "intervals": 24,
+   "bytes": 572,
+   "sha256": "03fe41690810df224611f4016899f856457f67a3c12c7654da17b1331ff2eff4",
+   "unit_as_returned": "EUR / MWh",
+   "licence_as_returned": "CC BY 4.0 (creativecommons.org/licenses/by/4.0) from Bundesnetzagentur | SMARD.de",
+   "deprecated_as_returned": false,
+   "top_level_keys": [
+    "deprecated",
+    "license_info",
+    "price",
+    "unit",
+    "unix_seconds"
+   ]
+  },
+  "energy-charts-be-price-2026-09-09_2026-09-10-twoday.json": {
+   "url": "https://api.energy-charts.info/price?bzn=BE&start=2026-09-09&end=2026-09-10",
+   "retrieved_utc": "2026-09-10T07:45:49+00:00",
+   "note": "TARIFF-09. A RANGE request, and the counterfactual the ticket is built on: `start`/`end` are INCLUSIVE Brussels calendar dates and this one answers 192 quarter-hours across TWO local days, where the bare `?bzn=BE` of the same endpoint answers 96 across one. The shipped `generic_dayahead` now derives these two dates from the `since`/`until` it is handed.",
+   "covers_local_days": [
+    "2026-09-09",
+    "2026-09-10"
+   ],
+   "granularity_s": 900,
+   "intervals": 192,
+   "bytes": 3579,
+   "sha256": "4d83f5efb4294a4a682562eac619bf35e45c95edde957bfa94c402e1777cc9b3",
+   "unit_as_returned": "EUR / MWh",
+   "licence_as_returned": "CC BY 4.0 (creativecommons.org/licenses/by/4.0) from Bundesnetzagentur | SMARD.de",
+   "deprecated_as_returned": false,
+   "top_level_keys": [
+    "deprecated",
+    "license_info",
+    "price",
+    "unit",
+    "unix_seconds"
+   ]
+  }
+ }
+}
@@ -288,9 +288,25 @@ def allin(mwh):


 def feed_body(instants, table):
-    return json.dumps({"data": [
-        {"start": u.isoformat(), "price_eur_mwh": table[brussels(u).hour]}
-        for u in instants]}).encode()
+    """A `generic_dayahead` body in the COLUMNAR shape the real feed publishes.
+
+    ⚠️ TARIFF-08. This used to build `{"data": [{"start": ...,
+    "price_eur_mwh": ...}]}` - a shape `api.energy-charts.info` has never
+    published and that the shipped parser was nevertheless written to expect.
+    Every price fixture in the repository fed that invented shape, so the
+    suite graded the parser against the parser's own assumption while the live
+    plugin raised `SourceUnavailable` on every fetch. Real captured bodies are
+    replayed in test_price.py section 2b; the fixtures here are SYNTHETIC
+    prices in the REAL shape, because this file is grading the fill and the
+    history, not the parse.
+    """
+    return json.dumps({
+        "unix_seconds": [int(u.timestamp()) for u in instants],
+        "price": [table[brussels(u).hour] for u in instants],
+        "unit": "EUR / MWh",
+        "license_info": "CC BY 4.0 (creativecommons.org/licenses/by/4.0) "
+                        "from Bundesnetzagentur | SMARD.de",
+        "deprecated": False}).encode()


 def transport_for(body):
@@ -299,8 +315,12 @@ def transport_for(body):
    return _t


+# ⚠️ NO `price_feed_url` (TARIFF-09). An override is used VERBATIM and the
+# plugin appends no date range to it, so a CFG that sets one - even to the
+# feed's own address - would exercise the override path in every check in this
+# file and never the shipped default. The override has its own grading in
+# test_price.py section 2c.
 CFG = {"price_provider": "generic_dayahead",
-       "price_feed_url": "https://api.energy-charts.info/price?bzn=BE",
       "price_timeout_s": 1,
       "price_markup_eur_kwh": MARKUP,
       "price_taxes_eur_kwh": TAXES,
@@ -501,6 +521,220 @@ check("...and the plan is still a provider plan, not a §11.4 fallback: the "
      _after.origin == "provider" and _after.product is PriceProduct.DAY_AHEAD)


+# --------------------------------------------------------------------------- #
+print()
+print("1c. TARIFF-09 - the feed is now ASKED for the horizon, and tomorrow "
+      "comes back FIRM")
+#
+# ⚠️ WHAT SECTION 1 COULD NOT SEE, AND WHY. Everything above grades the FILL
+# against a feed that published one day, and it was right to: FCST-04 covers
+# the window between midnight and the ~13:00-14:00 CET auction. But
+# `generic_dayahead.fetch()` accepted `since`/`until` and requested the bare
+# `?bzn=BE`, which answers one Brussels day AT EVERY HOUR OF EVERY DAY - so the
+# one-day feed was not the pre-publication state, it was the ONLY state, and
+# §8.3 dropped every hour past today from the trade set as provisional. The
+# optimiser never saw tomorrow as firm.
+#
+# Measured on the merged tree (4257e38) at 2026-09-10T09:00+02:00 over a 48 h
+# horizon, live feed, yesterday seeded: 15 firm whole hours and 33 carried,
+# and BOTH ends of the permitted cycle inside 2026-09-10. At 23:00 it was 1
+# firm and 47 carried, and the firm spread no longer cleared the round-trip
+# cost at all. The ceiling was structural - at hour H a one-day window gives at
+# most 24-H firm hours - which is why no amount of carrying could fix it.
+#
+# The two plans below differ in ONE thing: what the feed answered. The code
+# path, the window, the store and the instant are identical, so the difference
+# is the horizon and nothing else (habit 7).
+
+TOMORROW_D = "2026-01-16"
+TOMORROW_H = bday_hours(TOMORROW_D)
+
+# ⚠️ TOMORROW'S PRICES MUST NOT OVERLAP TODAY'S OR YESTERDAY'S, or a point's
+# day of origin is not recoverable from its number and every claim below is
+# about a coincidence. Asserted, not assumed.
+TOMORROW_MWH = {h: 300.0 + h for h in range(24)}
+TOMORROW_MWH[18] = 900.0        # the dear evening TOMORROW: the global peak of
+                                # the 48 h window, and it is on the second day
+
+PRICE_AT = {u: TODAY_MWH[brussels(u).hour] for u in TODAY_H}
+PRICE_AT.update({u: TOMORROW_MWH[brussels(u).hour] for u in TOMORROW_H})
+
+
+def body_at(instants):
+    """A columnar body priced PER INSTANT, so two days can differ at the same
+    local hour. `feed_body()` keys on the local hour alone and cannot."""
+    return json.dumps({
+        "unix_seconds": [int(u.timestamp()) for u in instants],
+        "price": [PRICE_AT[u] for u in instants],
+        "unit": "EUR / MWh",
+        "license_info": "CC BY 4.0 (creativecommons.org/licenses/by/4.0) "
+                        "from Bundesnetzagentur | SMARD.de",
+        "deprecated": False}).encode()
+
+
+def recording(body):
+    """A transport that serves bytes and remembers the URL it was asked for."""
+    seen = []
+
+    async def _t(url, timeout_s):
+        seen.append(url)
+        return body
+    _t.seen = seen
+    return _t
+
+
+_vals_today = {TODAY_MWH[h] for h in range(24)}
+_vals_tmrw = {TOMORROW_MWH[h] for h in range(24)}
+_vals_yday = {YESTERDAY_MWH[h] for h in range(24)}
+check(f"the three days' price tables share no value at all "
+      f"({len(_vals_today)}/{len(_vals_tmrw)}/{len(_vals_yday)} distinct), so "
+      f"which day a point came from is recoverable from the point",
+      not (_vals_today & _vals_tmrw) and not (_vals_tmrw & _vals_yday)
+      and not (_vals_today & _vals_yday))
+check(f"...and both fixture days are 24 Brussels hours long "
+      f"({len(TODAY_H)}/{len(TOMORROW_H)}), found by search rather than by "
+      f"adding 24 UTC hours",
+      len(TODAY_H) == 24 and len(TOMORROW_H) == 24)
+
+# THE TWO FEED STATES, one code path.
+_t_two = recording(body_at(TODAY_H + TOMORROW_H))
+_h_two = store()
+_ph.PriceHistory._remember(_h_two, series_of(YDAY_H, YESTERDAY_MWH), NOW)
+_pub = run(resolve_tariff(CFG, NOW, _until, transport=_t_two, now=NOW,
+                          history=_h_two))
+
+_t_one = recording(body_at(TODAY_H))
+_h_one = store()
+_ph.PriceHistory._remember(_h_one, series_of(YDAY_H, YESTERDAY_MWH), NOW)
+_pre = run(resolve_tariff(CFG, NOW, _until, transport=_t_one, now=NOW,
+                          history=_h_one))
+
+# ⚠️ THE REQUEST, WHICH IS THE THING THIS CARD CHANGED. Both calls ask for the
+# same three Brussels days; only the answer differs. The expected dates are
+# spelled out here rather than derived from the module that builds them.
+check(f"the shipped plugin asks the feed for the Brussels days the horizon "
+      f"touches, on both calls -> {_t_two.seen}",
+      _t_two.seen == _t_one.seen
+      == ["https://api.energy-charts.info/price?bzn=BE"
+          "&start=2026-01-15&end=2026-01-17"])
+check("...and it is no longer the bare URL that answers a single day, which "
+      "is what made the one-day state permanent rather than pre-publication",
+      "start=" in _t_two.seen[0] and "end=" in _t_two.seen[0])
+
+
+def split(plan):
+    """(firm, carried), and ([], []) for a plan that observed no series.
+
+    ⚠️ THE EMPTY ANSWER IS DELIBERATE. A mutation that turns the short
+    pre-publication answer into `no_data` makes `plan.read.points` raise
+    `Absence` by design, and an unguarded read here would take this file down
+    with a traceback rather than reporting a named FAIL - "the suite is red"
+    and "the check that watches this is red" are different claims
+    (MUTATION.md, reporting a mutation, point 3). With this guard the counts
+    below go red BY NAME, which is what a mutation report can cite.
+    """
+    if not plan.has_prices:
+        return [], []
+    firm = [p for p in plan.read.points if not p.provisional]
+    carried = [p for p in plan.read.points if p.provisional]
+    return firm, carried
+
+
+def local_days(points):
+    return sorted({brussels(p.start).strftime("%Y-%m-%d") for p in points})
+
+
+_f2, _c2 = split(_pub)
+_f1, _c1 = split(_pre)
+
+# The expected counts are re-derived from the fixture by set arithmetic: the
+# horizon hours the feed printed, versus the horizon hours it did not.
+_window = [SINCE_HOUR + timedelta(hours=n) for n in range(HORIZON_H)]
+_two_pub_utc = {u.astimezone(UTC) for u in TODAY_H + TOMORROW_H}
+_one_pub_utc = {u.astimezone(UTC) for u in TODAY_H}
+_want_f2 = len([u for u in _window if u.astimezone(UTC) in _two_pub_utc])
+_want_f1 = len([u for u in _window if u.astimezone(UTC) in _one_pub_utc])
+check(f"WITH TOMORROW PUBLISHED the 48 h plan is {len(_f2)} firm hours and "
+      f"{len(_c2)} carried; with only today, {len(_f1)} firm and {len(_c1)} "
+      f"carried. Re-derived from the fixture: {_want_f2} and {_want_f1}",
+      len(_f2) == _want_f2 and len(_f1) == _want_f1
+      and len(_f2) + len(_c2) == HORIZON_H
+      and len(_f1) + len(_c1) == HORIZON_H
+      and len(_f2) > len(_f1))
+check(f"...and the firm hours now span TWO Brussels local days "
+      f"{local_days(_f2)}, where the one-day answer spans exactly one "
+      f"{local_days(_f1)}. This is the acceptance criterion, and the ONE thing "
+      f"no amount of carry-forward could ever produce: a carried hour is "
+      f"provisional by construction",
+      len(local_days(_f2)) == 2 and len(local_days(_f1)) == 1
+      and local_days(_f2) == [TODAY_D, TOMORROW_D])
+
+# ⚠️ THE TAIL IS STILL CARRIED, WHICH IS FCST-04 STILL DOING ITS JOB. A 48 h
+# horizon asked for at 09:17 runs 9 hours into a THIRD Brussels day, and no
+# day-ahead market publishes that far. The fix widens what is firm; it does not
+# claim the market publishes more than it does.
+check(f"FCST-04 still fills the remainder: the {len(_c2)} hours past the "
+      f"market's own horizon are carried from yesterday and flagged, on a "
+      f"third Brussels day {local_days(_c2)} - so the plan still covers all "
+      f"{HORIZON_H} hours",
+      len(_c2) > 0 and local_days(_c2) == ["2026-01-17"]
+      and all(p.provisional for p in _c2))
+check("...and the plan is still a provider plan reporting itself provisional, "
+      "not a §11.4 fallback",
+      _pub.origin == "provider" and _pub.provisional)
+
+# ⚠️ THE PRE-PUBLICATION ANSWER IS A SERIES OF WHAT EXISTS, NOT `no_data`.
+# Asking for tomorrow before the auction returns fewer days than requested.
+# A `no_data` there would switch a working site's arbitrage off every morning.
+check(f"the short answer stays a SERIES of what exists - {_pre.read.kind}, "
+      f"{len(_pre.read.points)} hours, origin {_pre.origin!r} - and is never "
+      f"no_data and never a truncated series presented as complete "
+      f"({len(_c1)} of its hours are flagged provisional)",
+      _pre.read.kind == "series" and _pre.origin == "provider"
+      and len(_pre.read.points) == HORIZON_H and len(_c1) > 0)
+
+# --- the trade set, which is where the whole thing lands ------------------- #
+_A9 = dict(peak_guard_active=False, min_spread_eur_kwh=0.0,
+           uncertainty=frozenset())
+_v2 = _pub.arbitrage(**_A9)
+_v1 = _pre.arbitrage(**_A9)
+check("both plans still ALLOW a trade, so the two verdicts below are compared "
+      "against each other and not against a refusal",
+      _v2.kind == "allowed" and _v1.kind == "allowed")
+_v2 = _v2 if _v2.kind == "allowed" else None
+_v1 = _v1 if _v1.kind == "allowed" else None
+
+# ⚠️ NON-DEGENERACY, DERIVED. The global cheapest firm hour must be on TODAY
+# and the global dearest on TOMORROW, or "the trade set spans two days" is a
+# property of the arithmetic rather than of the fix.
+def _day_of(v, end):
+    """The Brussels day of one end of a permitting verdict, or "" for a
+    refusal - so a mutation that turns the verdict into a refusal is a named
+    FAIL below rather than an `Absence` traceback."""
+    return "" if v is None else brussels(getattr(v, end).start).strftime(
+        "%Y-%m-%d")
+
+
+_lo2, _hi2 = _day_of(_v2, "cheapest"), _day_of(_v2, "dearest")
+_lo1, _hi1 = _day_of(_v1, "cheapest"), _day_of(_v1, "dearest")
+check(f"WITH TOMORROW FIRM the trade set contains hours from BOTH days: "
+      f"buy on {_lo2}, sell on {_hi2}",
+      sorted({_lo2, _hi2}) == [TODAY_D, TOMORROW_D])
+check(f"...where the one-day answer's buy AND sell are both inside {_lo1} - "
+      f"a trade set that is a subset of ONE calendar day, structurally, which "
+      f"is the defect this card is about",
+      _lo1 == _hi1 == TODAY_D)
+_s2 = None if _v2 is None else _v2.spread_eur_kwh
+_s1 = None if _v1 is None else _v1.spread_eur_kwh
+check(f"...and the cross-day spread is strictly larger ({_s2} vs {_s1} "
+      f"EUR/kWh), so the second day is not merely visible but PRICED",
+      _s2 is not None and _s1 is not None and _s2 > _s1)
+check("...and both ends of the permitted cycle are FIRM, which "
+      "`_firm_or_die()` asserts at construction and §8.3 requires",
+      _v2 is not None and not _v2.cheapest.provisional
+      and not _v2.dearest.provisional)
+
+
 # --------------------------------------------------------------------------- #
 print()
 print("2. AC2 - the spread stopped being taken over hours that have gone")
@@ -559,14 +793,24 @@ for _n in range(len(TODAY_H) * (60 // _QSTEP_MIN)):
    _qu = TODAY_H[0] + timedelta(minutes=_QSTEP_MIN * _n)
    # A spiky within-the-hour profile, so decimation is visible in the SPREAD
    # and not only in a count.
-    _qrows.append({"start": _qu.isoformat(),
-                   "price_eur_mwh": 100.0 + 40.0 * (_n % 4) + brussels(_qu).hour})
-_qbody = json.dumps({"data": _qrows}).encode()
+    _qrows.append((_qu,
+                   100.0 + 40.0 * (_n % 4) + brussels(_qu).hour))
+# ⚠️ COLUMNAR, like the real feed - see feed_body(). And 900 s spacing is not a
+# hypothetical here: Belgium moved to a 15-minute MTU on 2025-10-01 and
+# `api.energy-charts.info` has answered 96 points a day ever since, which is
+# exactly what test_price.py section 2b replays from a captured body.
+_qbody = json.dumps({
+    "unix_seconds": [int(_u.timestamp()) for _u, _v in _qrows],
+    "price": [_v for _u, _v in _qrows],
+    "unit": "EUR / MWh",
+    "license_info": "CC BY 4.0 (creativecommons.org/licenses/by/4.0) "
+                    "from Bundesnetzagentur | SMARD.de",
+    "deprecated": False}).encode()
 check(f"the fixture really is sub-hourly - {len(_qrows)} points across "
      f"{len(TODAY_H)} Brussels hours - or every check below passes for an "
      f"hourly feed too and grades nothing",
      len(_qrows) == 4 * len(TODAY_H)
-      and len({_r["start"] for _r in _qrows}) == len(_qrows))
+      and len({_r[0] for _r in _qrows}) == len(_qrows))

 _q_before = run(resolve_tariff(CFG, NOW, _until, transport=transport_for(_qbody),
                               now=NOW, history=NoHistory()))
@@ -1379,6 +1623,195 @@ check(f"...and a revision of that hour REPLACES it rather than sitting beside "
      and abs(_y15.points[0].eur_per_kwh - allin(YESTERDAY_MWH[6])) < 1e-9)


+# --------------------------------------------------------------------------- #
+print()
+print("5d. FCST-06 - the OTHER way to be a store that cannot persist")
+
+# THE DEFECT. Section 5 above grades "a store that cannot persist records
+# nothing" against `not ok` - the missing-durable-volume SYMPTOM - and that is
+# the only condition the rule was ever enforced on. `ok` stays TRUE on a volume
+# that is present and cannot be written: a read-only remount, a full disk. In
+# that state `_remember()` had already populated `self._days`, `_save()` logged
+# and returned, and the store went on answering a series for the rest of the
+# process's life. Measured on 5059e28, twice, with a `/data` bind-mounted
+# read-only and again on a full tmpfs:
+#
+#     last_error = 'could not write ([Errno 30] Read-only file system)'
+#     words      = '2026-01-14 (24 points over 24 h); 2026-01-15 (24 points
+#                   over 24 h) [could not write (...)]'
+#     yesterday()= series, 24 points
+#     ON DISK    = {}                         <- and gone on the next restart
+#
+# `words` and `yesterday()` were describing two different stores, which is
+# exactly what section 5's own comment refuses - reached by the condition the
+# rule did not name.
+#
+# ⚠️ THE LEVERS ARE REAL FAILURES, NOT PATCHED ONES, AND THERE ARE TWO OF THEM
+# ON PURPOSE. The fix is written against the OUTCOME - `_save()` answering "no"
+# - and not against an errno, so one cause would grade a coincidence. A
+# DIRECTORY where the file goes fails `os.replace`; a volume that moves out
+# from under an already-open store fails `mkstemp`. Different syscall,
+# different errno, one answer (habit 10: grade the family).
+
+def npts(read):
+    """A read's point count, or "no_data". Never raises.
+
+    ⚠️ NOT `len(read.points)`. On a `no_data` that attribute RAISES `Absence`
+    by design, so a mutation that empties a store where this section expects
+    one would take the harness down with a traceback instead of reporting a
+    FAIL - and MUTATION.md's rule is that "it answered nothing" and "it broke
+    on the way to answering" must not grade the same. Found by driving M4.
+    """
+    return read.match(series=lambda r: len(r.points),
+                      no_data=lambda _r: "no_data")
+
+
+_d16 = tmpdir()
+_p16 = os.path.join(_d16, "price_history.json")
+_h16 = PriceHistory(_p16)
+os.mkdir(_p16)                   # `os.replace` cannot complete onto a directory
+check(f"the fixture is an `ok` store on a directory that IS there "
+      f"(ok={_h16.ok}, last_error={_h16.last_error!r}) - not the "
+      f"no-durable-volume case section 5 already grades, or this section is "
+      f"that one again (habit 1)",
+      _h16.ok and _h16.last_error is None and os.path.isdir(_d16))
+_carried16 = _h16.carry(series_of(YDAY_H + TODAY_H, TODAY_MWH),
+                        since=NOW, until=_until, stamp=NOW)
+_y16 = _h16.yesterday(NOW)
+check(f"a write that could not complete is REPORTED and leaves the store "
+      f"`ok` (ok={_h16.ok}, {(_h16.last_error or '')[:44]!r}). `ok` is about "
+      f"the DIRECTORY, and a full volume and a read-only remount both leave "
+      f"it True - which is why testing `ok` alone could not see this",
+      _h16.ok and "could not write" in (_h16.last_error or ""))
+check(f"...and the file really does not hold the day "
+      f"(isfile={os.path.isfile(_p16)}), or 'it answers what is not there' is "
+      f"not a measurement (habit 1)",
+      not os.path.isfile(_p16))
+check(f"A STORE THAT COULD NOT WRITE RECORDS NOTHING, exactly as one with no "
+      f"durable volume does: it holds {sorted(_h16._days)} and `yesterday()` "
+      f"is {_y16.kind!r}. Retaining the merge is an in-memory yesterday that "
+      f"answers all day and is gone on the next restart - the "
+      f"silently-forgetting cache this module's docstring exists to refuse",
+      _h16._days == {} and _y16.kind == "no_data")
+check(f"...and `words` and `yesterday()` AGREE about that store "
+      f"{_h16.words[:44]!r}: neither names a day the file does not hold, and "
+      f"both name the write failure. They disagreed - `words` printed two "
+      f"full days BESIDE the error while `yesterday()` answered 24 points",
+      YDAY_D not in _h16.words and TODAY_D not in _h16.words
+      and "could not write" in _h16.words
+      and _y16.kind == "no_data" and "could not write" in why(_y16))
+check(f"...and it DEGRADES rather than raising: the carry still returned a "
+      f"real series ({_carried16.kind!r}, {npts(_carried16)} points) with "
+      f"nothing carried. §6 and this module's opening - an add-on that will "
+      f"not start because /data filled up is a WORSE defect than the one this "
+      f"section is about",
+      _carried16.kind == "series"
+      and not _carried16.match(
+          series=lambda r: any(p.provisional for p in r.points),
+          no_data=lambda _r: True))
+_plan16 = derived(lambda: run(resolve_tariff(
+    CFG, NOW, _until, transport=transport_for(_body_today), now=NOW,
+    history=_h16)).origin)
+check(f"...and the SHIPPED `resolve_tariff()` path is unmoved by it "
+      f"({_plan16!r}): a provider plan, no §11.4 degradation, on a store that "
+      f"cannot write a byte",
+      _plan16 == "provider")
+# Habit 9: the same wrong input a SECOND time. A refresh runs 48 times a day,
+# and a volume that is full at 14:00 is still full at 14:30.
+_h16.carry(series_of(YDAY_H, YESTERDAY_MWH), since=NOW, until=_until,
+           stamp=NOW)
+check(f"...and the SECOND failing refresh behaves the same rather than "
+      f"accumulating: still {sorted(_h16._days)} held and "
+      f"{_h16.yesterday(NOW).kind!r}. The write fails 48 times a day, so the "
+      f"call that matters is the one after the first (habit 9)",
+      _h16._days == {} and _h16.yesterday(NOW).kind == "no_data")
+
+# ⚠️ AND WHAT IS KEPT IS THE LAST STATE THAT IS ON DISK, NOT AN EMPTY ONE. The
+# MERGE is undone, not the store. A site whose volume fills at 14:00 goes on
+# answering the yesterday it wrote at 13:30 - that record survives a restart,
+# so answering it is not a promise the store cannot keep. What it stops doing
+# is adding to it. This is also the fixture that separates a deep snapshot from
+# `dict(self._days)`: a one-level copy shares the day RECORD the merge rebinds
+# `points` on, so the "restored" store would answer the 24 points the merge
+# made and the file never took.
+_d17 = os.path.join(tmpdir(), "data")
+os.mkdir(_d17)
+_p17 = os.path.join(_d17, "price_history.json")
+_h17 = PriceHistory(_p17)
+_ph.PriceHistory._remember(_h17, series_of(YDAY_H[:12], YESTERDAY_MWH), NOW)
+_first17 = _h17.yesterday(NOW)
+check(f"a HALF day is persisted first ({npts(_first17)} points, "
+      f"last_error={_h17.last_error!r}), or 'the merge is undone and the "
+      f"store is not' grades nothing (habit 1)",
+      _h17.last_error is None and os.path.isfile(_p17)
+      and npts(_first17) == 12)
+_persisted17 = open(_p17, encoding="utf-8").read()
+_moved17 = _d17 + ".moved"
+os.rename(_d17, _moved17)        # the volume goes out from under an OPEN store:
+                                 # `mkstemp` fails, a different call and a
+                                 # different errno from the fixture above, and
+                                 # the bytes already written stay intact
+_ph.PriceHistory._remember(_h17, series_of(YDAY_H[12:], YESTERDAY_MWH), NOW)
+_y17 = _h17.yesterday(NOW)
+_n17 = npts(_y17)
+check(f"the second lever failed the write by a DIFFERENT route "
+      f"{(_h17.last_error or '')[:44]!r} - `mkstemp`, not `os.replace`, and "
+      f"ENOENT, not EISDIR. The rule is enforced on the outcome, so it must "
+      f"be driven by more than one cause (habit 10)",
+      "could not write" in (_h17.last_error or "")
+      and "No such file" in (_h17.last_error or ""))
+check(f"...and the store keeps the last state that IS ON DISK, not an empty "
+      f"one: {_n17} points held - the 12 it wrote before the volume went, not "
+      f"the 24 the unwritten merge made, and not nothing. The MERGE is undone; "
+      f"the store is not",
+      npts(_y17) == 12)
+_ph.PriceHistory._remember(_h17, series_of(YDAY_H[12:], YESTERDAY_MWH), NOW)
+_y17b = _h17.yesterday(NOW)
+check(f"...and again on the second failing refresh ({npts(_y17b)} points), "
+      f"because a volume that has gone is still gone 30 minutes later "
+      f"(habit 9)",
+      npts(_y17b) == 12)
+os.rename(_moved17, _d17)        # the volume comes back: what a RESTART finds
+_r17 = PriceHistory(_p17).yesterday(NOW)
+check(f"...and that is EXACTLY what a restart finds ({npts(_r17)} points, the "
+      f"file byte-for-byte as it was left), which is the whole claim: the "
+      f"store answers what survives a restart and nothing else",
+      open(_p17, encoding="utf-8").read() == _persisted17
+      and npts(_r17) == 12
+      and _r17.match(series=lambda a: _y17.match(
+          series=lambda b: [p.start for p in a.points]
+          == [p.start for p in b.points],
+          no_data=lambda _r: False), no_data=lambda _r: False))
+
+# ⚠️ AND "CANNOT PERSIST" IS ONE PREDICATE, ASKED ONCE. FCST-04's M4 and
+# FCST-05's HIGH were both a rule enforced on the conditions somebody thought
+# of rather than on the family; this is the third of the shape and the last
+# place it can be spelt twice. `_save()` returns whether the file now holds
+# what memory holds - no volume is one False, an `OSError` is another - and
+# `_remember()` keeps only what that answer says was kept.
+check(f"`_save()` answers for BOTH symptoms and is the only thing that does: "
+      f"no durable volume -> {_ph.PriceHistory._save(_missing)!r}, a write "
+      f"that fails -> {_ph.PriceHistory._save(_h16)!r}, a store that "
+      f"persisted -> {_ph.PriceHistory._save(_hok)!r}",
+      _ph.PriceHistory._save(_missing) is False
+      and _ph.PriceHistory._save(_h16) is False
+      and _ph.PriceHistory._save(_hok) is True)
+check(f"...and asking it once leaves the two symptoms with ONE behaviour: "
+      f"no volume holds {sorted(_missing._days)}/"
+      f"{_missing.yesterday(NOW).kind!r} and a failed write holds "
+      f"{sorted(_h16._days)}/{_h16.yesterday(NOW).kind!r}. A third cause "
+      f"nobody has thought of needs no third branch - which is how this defect "
+      f"arrived, twice",
+      (_missing._days, _missing.yesterday(NOW).kind)
+      == (_h16._days, _h16.yesterday(NOW).kind))
+check("...and `_remember()` no longer tests a symptom of its own: the source "
+      "holds no `self.ok` test between the docstring and the merge. This one "
+      "IS about the code's shape - two places testing two symptoms is the "
+      "defect, not a behaviour (habit 4's stated exception)",
+      "if not self.ok:" not in inspect.getsource(_ph.PriceHistory._remember)
+      and "if not self.ok:" in inspect.getsource(_ph.PriceHistory._save))
+
+
 # --------------------------------------------------------------------------- #
 print()
 print("6. rule 4 - hours accumulate within a day; they do not replace it")
@@ -743,8 +743,10 @@ def run(coro):
    return asyncio.run(coro)


+# ⚠️ NO `price_feed_url` (TARIFF-09): an override is used verbatim and the
+# plugin appends no date range to it, so setting one - even to the feed's own
+# address - exercises the override path rather than the shipped default.
 _cfg = opts(TWO_PERIOD, price_provider="generic_dayahead",
-            price_feed_url="https://api.energy-charts.info/price?bzn=BE",
            price_timeout_s=1, price_markup_eur_kwh=0.03,
            price_taxes_eur_kwh=0.06)
 _fb = run(resolve_tariff(_cfg, THU, THU + timedelta(days=1), transport=_dead,
@@ -778,9 +780,20 @@ check("a dead feed with NO static_profile has nothing to fall back to, and "
      _fb_none.origin == "none" and not _fb_none.has_prices)

 # A live feed still wins over the profile.
-_body = json.dumps({"data": [
-    {"start": (THU + timedelta(hours=i)).isoformat(), "price_eur_mwh": 40 + i}
-    for i in range(24)]}).encode()
+# ⚠️ COLUMNAR (TARIFF-08). `api.energy-charts.info` publishes parallel
+# `unix_seconds`/`price` arrays and no `data` key; the `{"data": [...]}` body
+# this used to build was invented by the parser it was grading, which is how a
+# provider that could never parse a live response shipped green. Real captured
+# bodies are replayed in test_price.py section 2b - the prices here are
+# synthetic, the SHAPE is not.
+_body = json.dumps({
+    "unix_seconds": [int((THU + timedelta(hours=i)).timestamp())
+                     for i in range(24)],
+    "price": [40 + i for i in range(24)],
+    "unit": "EUR / MWh",
+    "license_info": "CC BY 4.0 (creativecommons.org/licenses/by/4.0) "
+                    "from Bundesnetzagentur | SMARD.de",
+    "deprecated": False}).encode()


 async def _live(url, timeout_s):
@@ -143,7 +143,7 @@ configuration:
    description: "DERIVED, not picked (SAFETY-24): the reference pack holds 13.8 kWh usable and the worst modelled peak-shaving day asks for 878 Wh, which is 6.4 %, so 20 % sits ten points above the pack's own cut-out and still leaves the guard about 1.6 of its worst observed days of room. ⚠️ IT BOUNDS THE PEAK GUARD ONLY. It is not a reserve for the whole pack: ordinary self-consumption discharges straight past it and goes on down to the battery cut-out, and raising this number does not change that by a tenth of a percent. It must stay strictly above the battery cut-out or the add-on refuses to start. Do not change it without re-deriving it."
  price_provider:
    name: "Tariff · Dynamic price source"
-    description: "Where day-ahead or real-time prices come from. 'off' is what ships, and means this site runs on the static tariff below with no trading. ⚠️ Switching it on today fetches, checks and publishes prices and changes no energy decision - nothing consumes the price series yet. It may not be combined with the two legacy Home Assistant price entities further down: two price sources on one site, one of which silently wins, is refused at startup rather than left looking armed."
+    description: "Where day-ahead or real-time prices come from. 'off' is what ships, and means this site runs on the static tariff below with no trading. ⚠️ SWITCHING THIS ON IS WHAT STARTS THE PRICE FETCH - not the trading switch further down. With a source selected the add-on fetches the feed every half hour, checks it, records the day and shows what it holds on the Price history row, whether or not price trading is armed. On its own it still moves no energy: trading is the separate switch, and the cheap window the monthly maintenance cycle uses comes from the fixed or day/night tariff below. It may not be combined with the two legacy Home Assistant price entities further down: two price sources on one site, one of which silently wins, is refused at startup rather than left looking armed."
  price_markup_eur_kwh:
    name: "Tariff · Supplier markup (EUR/kWh)"
    description: "Your supplier's margin. The add-on adds it ONLY when the chosen price source does not already include it - under engie_dynamic, totalenergies_dynamic, eneco_eddy and manual_csv it is ignored, and logged as ignored, never applied twice."
@@ -161,7 +161,7 @@ configuration:
    description: "For price source manual_csv only: where in /share the add-on reads your own price file from."
  price_feed_url:
    name: "Tariff · Price feed URL override"
-    description: "Leave empty to use the chosen source's own feed. Only a host that source itself declares is accepted, so this cannot be pointed anywhere."
+    description: "Leave empty to use the chosen source's own feed. Only a host that source itself declares is accepted, so this cannot be pointed anywhere. WHAT YOU TYPE IS SENT EXACTLY AS WRITTEN: the add-on adds nothing to it, not even the date range it normally asks its own feed for, so an override gets whatever horizon that URL returns - often a single day, with the rest of the plan carried over from yesterday and excluded from price arbitrage. A warning in the log says so on every price refresh. Clear this option to get the full horizon back."
  price_horizon_h:
    name: "Tariff · How far ahead to fetch prices (hours)"
    description: "How many hours of prices to ask the source for."
@@ -176,7 +176,7 @@ configuration:
    description: "The average the current price is compared against. Only meaningful together with the current-price entity above, and like it, may not be combined with a dynamic price source."
  arbitrage_enabled:
    name: "Tariff · Trade on the price spread"
-    description: "Charge in the cheapest published hour and discharge in the dearest, but only when the spread covers what the round trip costs. OFF in the shipped configuration, deliberately: this is the first strategy in the add-on that moves energy against the meter on purpose, so it is armed on purpose or not at all. It needs both a price source and a P1 meter, and the add-on refuses the combinations that cannot work rather than letting them look armed."
+    description: "Charge in the cheapest published hour and discharge in the dearest, but only when the spread covers what the round trip costs. OFF in the shipped configuration, deliberately: this is the first strategy in the add-on that moves energy against the meter on purpose, so it is armed on purpose or not at all. ⚠️ LEAVING IT OFF DOES NOT TURN THE PRICE FEED OFF. A price source selected above is fetched, checked and recorded either way; this switch decides only whether the add-on ACTS on those prices. It needs both a price source and a P1 meter, and the add-on refuses the combinations that cannot work rather than letting them look armed."
  cycle_cost_threshold_eur:
    name: "Tariff · Cost of one full charge/discharge cycle (EUR/kWh)"
    description: "What one full round trip costs per kWh: wear on the pack plus conversion losses. A price spread smaller than this loses money and battery life, so it is not traded. ⚠️ THIS ONE IS NOT DERIVED FROM A MEASUREMENT. 0.10 is a product decision and it is still an open question with the owner (OPT-08). At this value a fixed day/night site does not trade at all - the realistic Belgian day/night spread is about 0.08 EUR/kWh - and that is the correct verdict at this number, not a fault."
@@ -123,7 +123,7 @@ configuration:
    description: "AFGELEID, niet gekozen (SAFETY-24): de referentiebatterij heeft 13,8 kWh bruikbaar en de zwaarste doorgerekende afvlakdag vraagt 878 Wh, oftewel 6,4 %, dus 20 % ligt tien punten boven de uitschakelgrens van de batterij zelf en laat de bewaking nog ongeveer 1,6 van haar zwaarste waargenomen dagen speling. ⚠️ DIT BEGRENST ALLEEN DE PIEKBEWAKING. Het is geen reserve voor de hele batterij: gewoon eigenverbruik ontlaadt er dwars doorheen en gaat door tot aan de uitschakelgrens van de batterij, en dit getal verhogen verandert daar geen tiende procent aan. Het moet strikt boven de uitschakelgrens van de batterij blijven, anders start de add-on niet op. Verander het niet zonder de afleiding opnieuw te maken."
  price_provider:
    name: "Tarief · Bron van dynamische prijzen"
-    description: "Waar day-ahead- of realtimeprijzen vandaan komen. 'off' is de standaardinstelling en betekent dat deze installatie op het vaste tarief hieronder draait, zonder handel. ⚠️ Dit vandaag aanzetten haalt prijzen op, controleert en publiceert ze, en verandert geen enkele energiebeslissing - er is nog niets dat de prijsreeks gebruikt. Het mag niet gecombineerd worden met de twee oude Home Assistant-prijsentiteiten verderop: twee prijsbronnen op één installatie, waarvan er stilzwijgend één wint, wordt bij het opstarten geweigerd in plaats van schijnbaar actief gelaten."
+    description: "Waar day-ahead- of realtimeprijzen vandaan komen. 'off' is de standaardinstelling en betekent dat deze installatie op het vaste tarief hieronder draait, zonder handel. ⚠️ DIT AANZETTEN IS WAT HET OPHALEN VAN PRIJZEN START - niet de handelsschakelaar verderop. Met een bron ingesteld haalt de add-on de feed elk half uur op, controleert die, bewaart de dag en toont op de rij Prijsgeschiedenis wat er bewaard is, of prijshandel nu aan staat of niet. Op zichzelf verplaatst dat nog geen energie: handelen is de aparte schakelaar, en het goedkope venster dat de maandelijkse onderhoudscyclus gebruikt komt uit het vaste of dag/nacht-tarief hieronder. Het mag niet gecombineerd worden met de twee oude Home Assistant-prijsentiteiten verderop: twee prijsbronnen op één installatie, waarvan er stilzwijgend één wint, wordt bij het opstarten geweigerd in plaats van schijnbaar actief gelaten."
  price_markup_eur_kwh:
    name: "Tarief · Marge van de leverancier (EUR/kWh)"
    description: "De marge van uw energieleverancier. De add-on telt die ALLEEN op wanneer de gekozen prijsbron ze niet al bevat - bij engie_dynamic, totalenergies_dynamic, eneco_eddy en manual_csv wordt ze genegeerd, en dat wordt ook gelogd, zodat ze nooit dubbel wordt aangerekend."
@@ -141,7 +141,7 @@ configuration:
    description: "Alleen voor prijsbron manual_csv: waar in /share de add-on uw eigen prijsbestand leest."
  price_feed_url:
    name: "Tarief · Afwijkende URL voor de prijsfeed"
-    description: "Laat leeg om de eigen feed van de gekozen bron te gebruiken. Alleen een host die die bron zelf opgeeft, wordt aanvaard, zodat dit niet zomaar ergens naar kan verwijzen."
+    description: "Laat leeg om de eigen feed van de gekozen bron te gebruiken. Alleen een host die die bron zelf opgeeft, wordt aanvaard, zodat dit niet zomaar ergens naar kan verwijzen. WAT U HIER TYPT WORDT LETTERLIJK VERSTUURD: de add-on voegt er niets aan toe, ook niet het datumbereik dat ze normaal aan haar eigen feed vraagt. Een afwijkende URL levert dus de horizon op die die URL zelf teruggeeft - vaak één dag, waarna de rest van het plan uit de prijzen van gisteren wordt overgenomen en buiten de prijsarbitrage valt. Bij elke prijsvernieuwing verschijnt daarover een waarschuwing in het logboek. Maak dit veld leeg om de volledige horizon terug te krijgen."
  price_horizon_h:
    name: "Tarief · Hoe ver vooruit prijzen ophalen (uren)"
    description: "Hoeveel uur aan prijzen er bij de bron opgevraagd wordt."
@@ -156,7 +156,7 @@ configuration:
    description: "Het gemiddelde waarmee de huidige prijs vergeleken wordt. Alleen zinvol samen met de entiteit hierboven en, net als die, niet te combineren met een dynamische prijsbron."
  arbitrage_enabled:
    name: "Tarief · Handelen op het prijsverschil"
-    description: "Laden in het goedkoopste gepubliceerde uur en ontladen in het duurste, maar alleen wanneer het verschil de kosten van de volledige cyclus dekt. STAAT UIT in de standaardinstelling, met opzet: dit is de eerste strategie in de add-on die bewust energie tegen de meter in verplaatst, dus ze wordt bewust aangezet of helemaal niet. Ze heeft zowel een prijsbron als een P1-meter nodig, en de add-on weigert combinaties die niet kunnen werken in plaats van ze schijnbaar actief te laten."
+    description: "Laden in het goedkoopste gepubliceerde uur en ontladen in het duurste, maar alleen wanneer het verschil de kosten van de volledige cyclus dekt. STAAT UIT in de standaardinstelling, met opzet: dit is de eerste strategie in de add-on die bewust energie tegen de meter in verplaatst, dus ze wordt bewust aangezet of helemaal niet. ⚠️ DIT UIT LATEN STAAN ZET DE PRIJSFEED NIET UIT. Een prijsbron die hierboven gekozen is, wordt hoe dan ook opgehaald, gecontroleerd en bewaard; deze schakelaar bepaalt alleen of de add-on op die prijzen HANDELT. Ze heeft zowel een prijsbron als een P1-meter nodig, en de add-on weigert combinaties die niet kunnen werken in plaats van ze schijnbaar actief te laten."
  cycle_cost_threshold_eur:
    name: "Tarief · Kostprijs van één volledige laad/ontlaadcyclus (EUR/kWh)"
    description: "Wat één volledige cyclus per kWh kost: slijtage van de batterij plus omzettingsverliezen. Een prijsverschil dat hier onder ligt, kost geld en levensduur en wordt dus niet verhandeld. ⚠️ DIT GETAL IS NIET AFGELEID UIT EEN METING. 0,10 is een productbeslissing en staat nog open bij de eigenaar (OPT-08). Bij deze waarde handelt een installatie met een vast dag/nachttarief helemaal niet - het realistische Belgische dag/nachtverschil is ongeveer 0,08 EUR/kWh - en dat is bij dit getal het juiste antwoord, geen fout."
				`@@ -0,0 +1 @@`
				{"license_info":"CC BY 4.0 (creativecommons.org/licenses/by/4.0) from Bundesnetzagentur \| SMARD.de","unix_seconds":[1757455200,1757458800,1757462400,1757466000,1757469600,1757473200,1757476800,1757480400,1757484000,1757487600,1757491200,1757494800,1757498400,1757502000,1757505600,1757509200,1757512800,1757516400,1757520000,1757523600,1757527200,1757530800,1757534400,1757538000],"price":[73.0,61.31,64.32,62.49,67.0,64.48,94.52,149.26,132.43,91.61,84.8,79.61,69.23,66.75,68.91,69.31,80.29,66.91,105.2,161.05,116.73,84.2,64.1,42.41],"unit":"EUR / MWh","deprecated":false}