TARIFF-15: your own tariff card, as two coefficients the card itself grades

A Belgian dynamic contract prices energy as `B x index + A`, and the two
coefficients exist on exactly one artefact: the customer's own tariff card.
There is no machine-readable source for them anywhere - CREG's consumer
tariff-card pages 404, CREG Scan is a JS wizard, the annual report has the
supplier list without per-product coefficients, VREG was unreachable, and
every coefficient found in research came off a supplier's own PDF, one
image-only. So nothing can be scraped and a per-supplier plugin would have no
source for its numbers, which is why TARIFF-07 withdrew three of them. The
customer is the only party who can see their own coefficients; they type them.

New provider `custom_formula`, one new option `price_formula` (a JSON string,
the `static_profile` precedent exactly), and a new module
`app/custom_formula.py`. The index path is `generic_dayahead`'s, not a second
one: the URL/override decision is now one shared `dayahead_feed_url()` both
plugins call, same feed, same Brussels-calendar range, same columnar parser,
same declared egress hosts.

It declares {ENERGY, MARKUP}, NOT ALL_IN. A card's Tariefformule line is the
energy component alone - Eneco tabulates network tariffs and levies
separately, ENGIE states the formula excludes network, distribution,
transport, levies and excise - so ALL_IN would lie for every operator who
typed what their card says and would drop the taxes from every hour
(0.2125 -> 0.1555 EUR/kWh, measured).

THE CARD IS THE GRADER, and that is why this is safe to ship. The framework
already mechanises that a PLUGIN cannot double-count. This introduces a risk
none of those guards can see because it is config, not code: the OPERATOR
double-counting, by folding network charges into `offset` while
price_taxes_eur_kwh carries them too. So validate.py recomputes the card's own
printed price from the operator's coefficients at startup and refuses to start
unless the two agree to within 0.01, naming the discrepancy. An inflated
offset cannot pass: 15.55 becomes 28.27. The same check catches VAT omitted,
VAT doubled, a transposed digit and a unit mismatch between the two sides.

And the limit, in the same sentence as the claim: it grades the coefficients
against the card, NOT the card against reality. A consistent-but-wrong pair
passes. A second refusal covers the one unit error the first cannot see - the
one applied to both sides at once - by bounding the card's own point to
0.02-1.00 EUR/kWh. Its FLOOR is applied to the energy figure rather than the
all-in one because the shipped 0.06 of taxes is three times that floor and
would carry a hundredfold error over it alone; found by writing the mutation,
not by review (M10, +0 B).

The market's own arithmetic, reproduced through the shipped code:

  Eneco (0.102 x 133.9905 + 1) x 1.06 = 15.5471   card prints 15,55
  ENGIE (0.1   x  61.96   + 0.9) x 1.06 = 7.5218  card prints 7,522

NO EXPRESSION EVALUATION OF ANY KIND. The string is settable over HTTP through
the add-on's own ingress page, so eval on it is RCE in a root container
holding SUPERVISOR_TOKEN and driving an inverter over RS485 - but the deciding
argument is that an expression is UNVERIFIABLE: two coefficients can be graded
against the card, an arbitrary expression cannot be graded against anything,
so it would make the invisible-wrong-coefficient problem strictly worse. An
unknown field is REFUSED rather than ignored, so `"expr"` is told it does not
exist. Future shapes become named fields, never a grammar. test_price.py 15f
walks the module's AST for eval/exec/compile/__import__/literal_eval and
exercises the scanner's red direction on every run.

Refused at startup: card absent or out by >0.01; `units` absent (no default);
factor == 0 (every spread 0, arbitrage silently never fires - the message
points at static_profile); factor < 0 (inverts the market); vat_pct outside
0-30 (0 stays legitimate); non-finite or non-numeric; unknown field, index or
resolution; the plausibility band. A card older than ~6 months WARNS and does
not refuse - ENGIE freezes coefficients for a contract's duration. Negative
output prices are not refused anywhere.

A startup preview logs the next 24 h as `index EUR/MWh -> all-in EUR/kWh` with
min/median/max and the day's spread, through a new default-silent
`PriceProvider.startup_preview()` hook - only the plugin knows what went in to
its own arithmetic. Emitted once per process, and a preview that raises cannot
cost the site its prices.

`custom_formula` joins the config.yaml enum through the DERIVED set
PROVIDERS | RETIRED_PROVIDERS | {"off"}, not a hardcoded list. Both
translations describe the option and the new id. DOCS.md replaces the
"planned" sentence with the shape, a worked example from a real card, a real
startup transcript and the limit of the check stated beside the claim.

22 mutations, one per check added, no survivors - MUTATION.md carries the
ledger. M13 (the preview's index column) was a -13 B survivor of the whole
suite on the first attempt because the check graded the HEADING and not the
row; it is graded on a rendered row now. All 29 suites green.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BwRLw7eKYj2byVdwB6kbjU

goodwe_controller/DECISIONS/TARIFF-15-the-card-is-the-grader.md

@@ -0,0 +1,158 @@
+# TARIFF-15 — the operator's own tariff card, and why the card has to grade it
+
+**Status: DECIDED 2026-09-13 — ship `custom_formula` with FIXED coefficients and
+a REQUIRED `card` block whose printed price the add-on reproduces at startup or
+refuses to run. No expression evaluation, ever. Declare `{ENERGY, MARKUP}`,
+never `ALL_IN`.**
+
+Filed with TARIFF-15. Follows [[TARIFF-07]], which withdrew three per-supplier
+plugins and named a formula provider as the answer it could not give.
+
+---
+
+## 1. Why a formula rather than more plugins
+
+TARIFF-07 established the negative half: a per-supplier plugin has no source
+for its numbers. This card establishes the positive one.
+
+* **There is no machine-readable source of Belgian tariff coefficients
+  anywhere.** CREG's consumer tariff-card pages 404. CREG Scan is a
+  JavaScript-only wizard. The annual report carries the supplier list and no
+  per-product coefficients. VREG was unreachable. Every coefficient found in
+  research came from a supplier's **own PDF**, one of them image-only.
+* So there is nothing to scrape, and fourteen plugins would still have no
+  source for their numbers — they would be fourteen copies of the invention
+  TARIFF-07 deleted three of.
+* **The customer is the only party who can see their own coefficients**, and
+  the card they read them from **is the only artefact that can grade them**.
+  That second clause is the whole design; everything below follows from it.
+
+## 2. The shape is the market's published arithmetic
+
+Both suppliers whose 2026 cards were readable print three things on one line:
+the formula, the reference index it assumes, **and the resulting price**.
+
+| | computed | the card prints |
+|---|---|---|
+| Eneco `(0.102 × 133.9905 + 1) × 1.06` | **15.5471** | **15,55** |
+| ENGIE `(0.1 × 61.96 + 0.9) × 1.06` | **7.5218** | **7,522** |
+
+Two independent suppliers, reproduced to the last printed digit, through the
+shipped code (`test_price.py` 15a runs both, and then runs both again end to
+end through `load_prices()`).
+
+That third column is not a nicety. It is the only externally-published number
+in this whole feature, and it is what makes the design safe.
+
+## 3. The double-counting risk, and which half of it is new
+
+`price.py`'s trap 1 already mechanises that a **plugin** cannot double-count:
+`PluginOptions` refuses the framework keys, `code_leaks()` refuses a plugin
+whose *code* mentions them, `assemble()` adds the complement exactly once.
+
+`custom_formula` introduces a risk none of those can see, **because it is not
+in code**: the **operator** double-counting, by folding their network charges
+into `offset` while `price_taxes_eur_kwh` also carries them. `code_leaks()`
+scans code. This is config, typed by a person reading a PDF.
+
+It is a real reading of a real card, not a hypothetical. A Belgian card's
+`Tariefformule` line is the **energy component alone** — Eneco tabulates
+network tariffs in one table and levies in a second; ENGIE states in so many
+words that the formula excludes network, distribution, transport, grid levies
+and excise. An operator who wants "the price I actually pay" and adds ~12
+c€/kWh to `offset` has done something entirely reasonable and is now billed for
+network charges twice, on every hour, with §11.4 making the whole class of
+price defect invisible.
+
+**Decision: the card grades it.** At startup `validate.py` recomputes the
+card's own printed price from the operator's coefficients and refuses to start
+unless the two agree to within `0.01` — which is the last digit a card prints.
+An inflated `offset` cannot pass, because the printed estimate is by
+construction the energy component: 15.55 becomes 28.27 and the add-on stops and
+says why.
+
+**And the limit, stated in the same sentence as the claim**, exactly as
+`price.py` states trap 1's: *it grades the coefficients against the card, not
+the card against reality.* A consistent-but-wrong pair — both numbers off the
+wrong product, or off a card the contract has since rolled over from — agrees
+with itself perfectly and passes. Nothing here or anywhere else can catch that.
+
+### 3b. The one unit error the card check cannot see
+
+A factor applied to **both** sides — coefficients *and* printed price — cancels
+out of a comparison between them. That is a second quantity, so it is a second
+refusal: the card's own point must land inside 0.02–1.00 EUR/kWh.
+
+⚠️ **Its floor is applied to the ENERGY figure and not to the all-in one, and
+that asymmetry was found by a mutation rather than by reasoning.** The shipped
+`price_taxes_eur_kwh` is 0.06 — three times the floor — so a card read a
+hundred times too small comes out at 0.0016 EUR/kWh of energy, is carried over
+0.02 by the flat tax addend alone, and passes a band applied to the total. The
+hundredfold error is exactly what the band exists for. Graded from both
+directions in `test_price.py` 15e/15i and driven red as M10.
+
+## 4. Fixed coefficients. No expression evaluation of any kind
+
+**Decided: never a grammar.** Two arguments, and the second is the one that
+decides it.
+
+* **Security.** The add-on serves `POST config/save` → `POST
+  /addons/self/options` on its own ingress page, so `price_formula` is
+  **settable over HTTP**. `eval` on it is remote code execution in a root
+  container holding `SUPERVISOR_TOKEN`, with `homeassistant_api` and
+  `hassio_api`, driving an inverter over RS485. `eval(s, {"__builtins__": {}})`
+  is not a sandbox and this repository is not going to pretend otherwise.
+* **⚠️ AND THE DECIDING ARGUMENT IS NOT SECURITY: AN EXPRESSION IS
+  UNVERIFIABLE.** Two coefficients can be graded against the card's printed
+  estimate. An arbitrary expression cannot be graded against **anything** — so
+  it makes the invisible-wrong-coefficient problem *strictly worse*, which is
+  the exact failure this provider exists to prevent. An expression would buy
+  the operator freedom to be wrong in a way nothing could report.
+
+All 24 surveyed Belgian products, both injection legs and aWATTar AT fit
+`B × index + A`, so an expression buys nothing the market needs either.
+
+**Future shapes become NAMED FIELDS** — `clamp_lo`, `factor_negative`,
+`offset_bands` — each individually refusable and each individually gradeable
+against a card. `parse_price_formula()` therefore **refuses an unknown key**
+rather than ignoring it: an operator who types `"expr"` is told it does not
+exist, instead of being priced on the fields they also typed with the one they
+believed in dropped in silence.
+
+`test_price.py` 15f walks the module's AST for `eval`, `exec`, `compile`,
+`__import__` and `literal_eval`, and exercises the scanner's **red** direction
+on every run so it cannot become a scanner that scans nothing.
+
+## 5. `{ENERGY, MARKUP}`, not `ALL_IN`
+
+The contract's formula **is** the supplier's markup — that is what `B` and `A`
+are — but it is not the levies. Declaring `ALL_IN` would lie on behalf of every
+operator who typed exactly what their card says and would drop
+`price_taxes_eur_kwh` from every hour: measured at 0.2125 → 0.1555 EUR/kWh.
+`price_markup_eur_kwh` is therefore **ignored** under this provider, and logged
+as ignored, which is the framework's existing rule rather than a new one.
+
+## 6. What was reused rather than rebuilt
+
+The index path is `generic_dayahead`'s: same EPEX BE feed, same
+Brussels-calendar range, same columnar parser, same declared egress hosts. The
+override/range decision moved into one shared `dayahead_feed_url()` that both
+plugins call, because two copies of that rule would be two places for the
+short-horizon defect TARIFF-09 fixed to come back in only one of them.
+
+`resolution: hourly` averages each hour's members. ⚠️ **That is not
+`_Series.at()`'s hourly assumption returning (TARIFF-14):** it decides how many
+points are *emitted*; the window each point covers is still derived from its
+neighbours in `_cover_table()`. A feed that is already hourly averages one
+member per hour — the identity — so the option cannot silently assume the
+granularity it was handed. Graded.
+
+## 7. What this card did NOT do
+
+* **The injection leg.** Eneco uses `0,102` on consumption and `0,100` on
+  injection, and injection is VAT-exempt where consumption is at 6 % — a second
+  per-contract formula, not a second copy of this one. Separate card.
+* **The hour-varying network charge that bites Wallonia.** Separate card.
+* **Raising `price_taxes_eur_kwh` from 0.06 to ~0.12.** Separate card, and an
+  owner's decision. Note §3b: the band's floor is deliberately insensitive to
+  whatever that number becomes.