SAFETY-12 review: name the safety claim that could not fire, and fix two figures

Review fixes.

A. test_loop.py takes 73 s, not the ~35 s claimed in three places. That number
is the whole defence against a future developer deleting a slow test, so it was
understated by 2x in exactly the place it matters. Corrected - and it is now
~95 s (measured 93 s), because this commit adds the scenario below rather than
trimming windows to make an inconvenient number smaller. Both call sites carry
an instruction to re-measure if the windows change.

B. "242 checks" was test_p1's own count quoted as the total for all five
suites. The real figure is 395 across the five, plus 14 here.

And the finding this PR should have led with. stale_input_s and the
`inputs missing -> 0 W` claim both live INSIDE cycle(), so pre-fix, on a meter
whose value happened to repeat, that claim could not fire at all. Constant
meter, SoC entity dying at t=4 s, one binary:

  pre-fix   final target 220.0 W, reason 'loop -> 220 W'
  post-fix  final target   0.0 W, reason 'safety -> 0 W'

The old controller held 220 W through the whole input outage with its status
line still reading loop -> 220 W, and layer 1 does not cover that on a
`meter_source: off` install - no P1 age sensor is published, so the firmware has
nothing to veto on, while the heartbeat keeps its watchdog fed. The pre-fix
behaviour was not "holds a stale command safely"; it was a failsafe a repeating
meter switched off. Asserted in test_loop.py, both halves.

Three points of reasoning tightened, none of them wrong before:

- the re-tune is unreachable by construction, not merely by the reset: the
  combined cycle rate is max(arrival rate, 1/floor), never the sum, and 1/15 s
  is strictly slower than the ~1/5 s arrival rate. That holds only while the
  floor exceeds the meter's update interval - and the schema minimum of 5 s is
  exactly the reference meter's interval. Flagged for validate.py, not fixed
  here.
- the ~11 s lower bracket is not only a dropped poll: a 5.0 s poller against a
  4.97 s meter beats, so about every 14 minutes the poller steps over an update
  entirely. Reaching 15 s takes a beat-skip AND a failed poll.
- the "freshness is decided upstream" guarantee is transport-conditional. On
  `meter_source: off` the grid figure comes straight from hass.number() with no
  age gate. Running the full law is still right; the claim is narrower than it
  was stated.

The 15 s default is not independently justified - it rests on the firmware's
30 s ingest veto and moves if that ever does. Said beside the constant.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Du77usMj8XNKNFZGmUiWDa

HANDOVER.md

@@ -52,9 +52,18 @@ Run the checks before deploying. No framework needed:
 
 ```bash
 cd goodwe_controller
-python3 test_control.py && python3 test_maintenance.py && python3 test_arbiter.py
+python3 test_control.py && python3 test_maintenance.py && python3 test_arbiter.py \
+  && python3 test_validate.py && python3 test_p1.py && python3 test_loop.py
 ```
 
+⚠️ `test_loop.py` takes about **95 seconds** (measured 93 s) and that is not a bug. It
+drives the real control loop, which sleeps 1 s a tick, eight times over, to prove a
+wall-clock property about it (SAFETY-12); mocking the clock there would leave the
+assertion testing the mock. The other five suites together are ~395 checks and finish in
+seconds. If you shorten `test_loop.py`'s windows, re-measure and correct this figure —
+an understated runtime is what makes a slow test look like a hung one, and a slow test
+that looks hung gets deleted.
+
 ---
 
 ## 3. Known broken / unfinished

goodwe_controller/CHANGELOG.md

@@ -67,6 +67,121 @@ A refusal is legible as a refusal: a banner that says in words that this is a
 configuration problem and nothing is broken, the numbered problems, no traceback,
 and exit status 78 (`EX_CONFIG`).
 
+**SAFETY-12.** The control loop now evaluates on a clock as well as on a change.
+`run_control()` ran `cycle()` only when the meter value differed from the
+previous poll, so a meter that repeated a value ran **no control cycles at
+all** - while the heartbeat went on re-emitting the last command and the
+firmware went on putting it on the wire every 2 s. Layer 1 stayed fed, every
+indicator read healthy, and a command computed from an old reading kept being
+executed through whatever the load did next. Measured on the rig during TEL-05:
+70 s at a literally constant 350.0 W with 14 arrivals and zero cycles.
+
+**It was worse than a latency limit: it was a safety claim that could not
+fire.** `stale_input_s` and the `inputs missing -> 0 W` claim both live *inside*
+`cycle()`. Pre-fix, if the meter value happened to be constant, `cycle()` was
+never called again - so that claim had no way to run. Constant meter, SoC entity
+dying at t=4 s, same binary:
+
+```
+pre-fix    writes: [(0.0, 220.0), (0.0, 220.0), (10.1, 220.0)]
+           final target: 220.0 W   reason: 'loop -> 220 W'
+post-fix   writes: [(0.0, 220.0), (0.0, 220.0), (3.0, 430.0), (9.1, 0.0), (10.1, 0.0)]
+           final target: 0.0 W     reason: 'safety -> 0 W'
+```
+
+The pre-fix controller held 220 W through the entire input outage with its
+status line still reading `loop -> 220 W`, and layer 1 does not cover that case
+on a `meter_source: off` install: no P1 age sensor is published, so the firmware
+has nothing to veto on, while the heartbeat keeps its watchdog fed. So the old
+behaviour was not "holds a stale command safely" - it was a failsafe that a
+repeating meter switched off. `test_loop.py` asserts both halves.
+
+`cycle_floor_s` (default **15 s**) is a deadline on the gap between cycles, not
+a second scheduler: every cycle resets it, whichever trigger fired. On a healthy
+meter it never elapses, so the cycle rate, the write sequence and therefore the
+per-cycle gain the tuning was measured against are unchanged - asserted in
+`test_loop.py` by driving the real loop twice on one binary (a huge
+`cycle_floor_s` reproduces the old behaviour exactly) and comparing the
+commanded values.
+
+The reset is not the only thing holding that up, and the other argument is the
+sturdier one: the combined cycle rate is `max(arrival rate, 1/floor)`, never the
+sum. At 1/15 s the floor is strictly slower than the ~1/5 s arrival rate, so
+even a hypothetical floor racing the arrivals could not exceed the nominal
+healthy rate. The re-tune is unreachable by construction. ⚠️ That holds only
+while `cycle_floor_s` stays above the meter's update interval, and the schema
+minimum is 5 s - which is exactly the reference meter's interval. Nothing
+enforces the relationship; `validate.py` is the obvious home for it and does not
+have it yet.
+
+Observed over six real minutes on the same rig, one binary, meter held at a
+literally constant 350.0 W: **before, 1 cycle** (at startup) and then nothing,
+the command stuck at 220 W against a 350 W import for the rest of the window.
+**After, 24 cycles**, worst gap 15.14 s, and the ordinary PI ramp doing its job -
+220, 430, 650, 860, 1080, 1300, 1510, 1730, 1940, 2000 W, one step per cycle,
+rail at t=136 s and flat after. Three minutes with the meter parked at the
+target instead: **1 cycle before, 12 after, and 19 writes either way - the same
+19**. Three minutes on a normally-changing meter: **36 cycles and 49 writes on
+both sides, the same values in the same order.** The heartbeat write count never
+moves; the only writes the fix adds are the ten the loop was supposed to be
+making all along.
+
+15 s is bracketed, not round.
+
+**Below it, ~11 s, and it arises more routinely than a dropped poll.**
+`HomeWizardLocalSource` polls at 5.0 s against a meter that updates every 4.97 s
+(NOTES.md:640). Two near-equal periods beat, so roughly once every ~14 minutes
+the poller steps over an update entirely - a ~10 s gap between changed values on
+a perfectly healthy meter, plus up to 1 s for the 1 s tick to notice. A failed
+HTTP poll (retried on the next tick, no backoff) adds another 5 s. The floor
+sits above the beat-skip so it does not fire on that alone; what it takes to
+reach 15 s is a beat-skip *and* a failed poll coinciding.
+
+**Above it, 30 s.** `meter_max_age_s` is 30 s, past which there is no reading to
+evaluate, and the firmware's ingest veto is also 30 s - so the loop is
+guaranteed to have re-decided at least twice inside any window in which layer 1
+might otherwise be the only thing deciding. ⚠️ The default is therefore not
+independently justified at 15: if the firmware's veto ever moves, this moves
+with it. Noted beside the constant in `main.py`.
+
+It is also exactly `stale_input_s`, this system's existing answer to "how long
+may we hold without re-deciding".
+
+A floor cycle runs the **full** law on the same inputs, integrator included. In
+P1 mode the freshness question is already answered upstream and answered harder
+- past `meter_max_age_s` the reading is None, which is "inputs missing", which
+is a `P_SAFETY` 0 W claim - so a floor cycle there can only ever run on a reading
+this system already calls fresh. ⚠️ That is transport-conditional: with
+`meter_source: off`, `read_inputs()` takes the grid figure straight from
+`hass.number()` with **no age gate at all**, so a Home Assistant entity that has
+gone quiet reads as current on that path, with or without this change. Running
+the full law is still right there - the alternative is a degraded second law
+reasoning about an age nobody measured - but the floor does not make that path
+fresh. The integrator advances per *cycle*, not per second,
+so bounding the gap bounds the accumulation; `integrator_max_w` bounds it after
+that. A gentler second law for floor cycles would be a second set of dynamics
+nobody can test on this hardware.
+
+⚠️ **This is not a freeze detector**, and TEL-05 established one cannot be built
+here: at the converged -10 W target a 1 Wh register needs ~6 minutes to move
+while the power figure legitimately repeats, so any threshold on `unchanged_s`
+false-trips at the exact operating point we run at. Nothing in this change
+decides whether a repeated reading is *true*. It stops "the number is the same"
+from meaning "do not think". The consequence, stated plainly: against a meter
+genuinely frozen at a wrong value the loop will now correct toward that wrong
+value rather than hold a stale command, bounded by `max_w`, the arbiter and the
+device rating. That was already what happened to a lying meter whose value
+merely jittered; bit-exact repetition was never a safety property.
+
+`DOCS.md`'s "Saturation is counted in cycles, not seconds" section documented
+the old behaviour as an accepted deviation. It is **qualified, not deleted**:
+the deviation survives (the window is still counted in cycles, and 3 x 15 s is
+still not the specification's 10 s) but it is now bounded at 45 s instead of
+open-ended, and the passage's second justification - "nothing accumulates while
+it is stalled" - is called out as having covered integrator windup and not
+staleness, which was the actual hazard. The same wrong argument in
+`control.py`'s saturation comment is corrected too.
+
 **TEL-04.** `meter_source: ha_signed` reads a single *signed* power entity, which
 is the shape a HomeWizard P1 publishes. Both transports 0.3.0 shipped require two
 unsigned registers and refuse negatives, so neither fits the meter installed here.

goodwe_controller/DOCS.md

@@ -272,9 +272,10 @@ behind it would trip the firmware watchdog on a system that is working fine.
 | `target_grid_w` | -10 | What the meter should rest at. Negative = a slight export |
 | `step_w` | 10 | Quantisation |
 | `saturation_w` | 500 | Divergence that counts as "the inverter is at a limit" |
-| `saturation_cycles` | 3 | How many consecutive cycles before freezing. A cycle is one *changed* meter reading, not a fixed period - see the note below. **Do not set to 1** |
+| `saturation_cycles` | 3 | How many consecutive cycles before freezing. A cycle is one meter update or `cycle_floor_s`, whichever comes first - see the note below. **Do not set to 1** |
 | `integrator_max_w` | 0 | Bound on the loop's accumulator, and 0 means "same as `max_w`". Caps how much stale error can be waiting to unwind when the sign flips. **Do not raise it above `max_w`** - the output clamp already bounds what is commanded, so the only thing extra headroom buys is more cycles of wrong-direction power after every saturation event. Lowering it below `max_w` is the useful direction |
 | `heartbeat_s` | 10 | Refresh interval; must stay well under the firmware watchdog |
+| `cycle_floor_s` | 15 | The longest the loop may go without evaluating. A deadline on the gap between cycles, reset by every cycle — on a healthy meter it never elapses and changes nothing. Raise it only if your meter is slower than ~10 s; it must stay under `meter_max_age_s`, past which there is no reading to evaluate anyway |
 | `stale_input_s` | 15 | How long inputs may be missing before commanding 0 W. In P1 mode this clock starts only *after* `meter_max_age_s` has already expired — the two stack, see "How long a dead meter takes to reach 0 W" |
 | `auto_start` | false | Start controlling on boot (only after commissioning) |
 
@@ -284,22 +285,43 @@ The specification states the saturation window as **"> 10 s"**. This add-on coun
 **cycles** instead, and that is a deliberate, accepted deviation rather than an
 oversight - the acceptance criterion is not met as literally written.
 
-A cycle here is one *changed* meter reading: the controller only runs the loop when the
+A cycle here is one meter update **or `cycle_floor_s` elapsing, whichever comes
-meter value differs from the previous poll. At the reference P1's ~5 s update rate the
+first**. At the reference P1's ~5 s update rate the default of 3 cycles is usually
-default of 3 cycles is usually around 15 s, but there is **no guaranteed wall-clock
+around 15 s, and the worst case is bounded at `3 x cycle_floor_s` = 45 s. It is still
-window** - a meter that repeats the same value stalls the counter for as long as it
+not the 10 s the specification asks for, so the deviation stands - but it is now a
-repeats.
+bounded number rather than an open one.
 
-Two reasons that is acceptable:
+⚠️ **This passage used to say something different, and the second half of it was
+wrong.** Until SAFETY-12 the loop ran only when the meter value *changed*, so a meter
+that repeated a value stalled the counter for as long as it repeated, with no upper
+bound at all. That was defended here on the grounds that "a stalled counter is a
+detection-latency limit and not a runaway risk - the condition that stalls it stops the
+whole loop, so nothing accumulates while it is stalled". That argument covers
+**integrator windup and nothing else**. The hazard it misses is **staleness**: while the
+loop was stopped the heartbeat kept re-emitting the last command and the firmware kept
+putting it on the wire every 2 s, so a command computed from an old reading went on
+being executed through a real load change, with every indicator reading healthy.
+Measured on the rig during TEL-05: 70 s at a literally constant 350.0 W, 14 arrivals,
+zero control cycles.
 
-- the control law is a pure function with no clock, which is what makes it testable
+The half that survives is the first one: the control law is still a pure function with
-  without hardware, and a seconds-based window would have to live in the controller;
+no clock, and the wall-clock window still lives in `Controller`, which has one - see
-- a stalled counter is a detection-latency limit and not a runaway risk. The condition
+`run_control()`. What changed is that the controller now enforces a floor on the gap
-  that stalls it - an unchanging meter - stops the whole loop, so nothing accumulates
+between cycles, so `saturation_cycles` finally means a bounded number of seconds.
-  while it is stalled.
 
-If a guaranteed window matters on your site, raise `saturation_cycles` for a fast meter,
+If a tighter window matters on your site, lower `cycle_floor_s` (not below your meter's
-and treat the figure as "N meter updates" rather than "N seconds".
+update interval, or the floor starts firing on a healthy meter) or raise
+`saturation_cycles` for a fast meter.
+
+**This is not a freeze detector, and it deliberately cannot be one.** Nothing here
+decides whether a repeated reading is true — see `unchanged_s` in the meter section for
+why no threshold on that is safe at the converged operating point. The floor only stops
+"the number is the same" from meaning "do not think". The consequence is worth stating
+plainly: against a meter that is genuinely frozen at a wrong value, the loop will now
+correct toward that wrong value instead of holding a stale command. It is bounded by
+`max_w`, the arbiter and the device rating, and it is the same thing that already
+happened to a lying meter whose value merely jittered — bit-exact repetition was never a
+safety property.
 
 #### Why `target_grid_w` is not zero
 

goodwe_controller/app/control.py

@@ -95,19 +95,23 @@ def compute(
     # lets slew be larger than saturation_w.
     #
     # The spec states this window twice and differently: "> 10 s" (§11.2) and
-    # "3 samples" (§10.3). This counts CYCLES, and a cycle is not a unit of
+    # "3 samples" (§10.3). This counts CYCLES, and a cycle is still not a unit
-    # time: run_control() calls cycle() only when the meter value CHANGES
+    # of time - but it is now a BOUNDED one. run_control() runs a cycle on a
-    # (`if self.grid != last_grid`), so three cycles is three distinct meter
+    # changed meter value OR on `cycle_floor_s` elapsing, whichever comes first
-    # readings and nothing more. At the reference P1's ~5 s update rate that is
+    # (SAFETY-12), so three cycles is at most 3 x cycle_floor_s = 45 s at the
-    # usually ~15 s, but there is no upper bound on it - a meter that repeats a
+    # defaults, and usually ~15 s at the reference P1's ~5 s update rate.
-    # value stalls the counter.
     #
-    # That is a detection-latency limit, not a windup hazard: the same
+    # ⚠️ THIS COMMENT USED TO CLAIM there was no upper bound and that it did not
-    # condition that stalls the counter stalls the whole loop, so nothing
+    # matter, "because the same condition that stalls the counter stalls the
-    # accumulates in the meantime either. If a wall-clock window is ever
+    # whole loop, so nothing accumulates in the meantime". The first clause was
-    # required, it belongs in Controller (which has a clock) and not here.
+    # true and the second was the bug: a stalled loop does not stop commanding,
-    # ponytail: this function is worth keeping clockless; the ceiling is that
+    # it goes on emitting its last command through the heartbeat. Windup was
-    # saturation_cycles cannot express a guaranteed number of seconds.
+    # never the hazard; staleness was.
+    #
+    # The wall-clock window lives in Controller, which has a clock, exactly as
+    # this comment always said it should. ponytail: this function stays
+    # clockless; the remaining ceiling is that saturation_cycles expresses a
+    # bound in seconds rather than an exact number of them.
     saturated_now = abs(prev_w - actual_w) > tuning.saturation_w
     sat_count = min(sat_count + 1, 10) if saturated_now else 0
     frozen = sat_count >= tuning.saturation_cycles

goodwe_controller/app/main.py

@@ -342,11 +342,85 @@ class Controller:
         last_grid = object()
         heartbeat = float(self.o.get("heartbeat_s", 10))
         last_beat = 0.0
+        # ⚠️ SAFETY-12. The floor is a DEADLINE ON THE GAP BETWEEN CYCLES, not a
+        # second scheduler running alongside the arrival trigger. Every cycle
+        # resets it, whichever trigger fired, so on a healthy changing meter it
+        # never elapses and not one existing cycle moves - and therefore no
+        # change to the per-cycle gain the tuning was measured against.
+        #
+        # That is true twice over, and the second reason is the sturdier one:
+        # the combined cycle rate is max(arrival rate, 1/floor), never the sum.
+        # At the defaults 1/15 s is strictly slower than the ~1/5 s arrival
+        # rate, so even a floor that raced the arrivals could not push the loop
+        # past its nominal healthy rate. The re-tune is unreachable by
+        # construction, not merely by the reset. ⚠️ That argument holds only
+        # while floor > the meter's update interval - which is why the schema
+        # minimum of 5 s deserves the second look validate.py has not given it
+        # yet: 5 s IS the reference meter's interval, i.e. the one configurable
+        # value that would double the cycle rate.
+        #
+        # Why it has to exist, and it is not only about latency: `self.grid !=
+        # last_grid` alone means a meter that repeats a value runs NO control
+        # cycles at all. The heartbeat below keeps re-emitting the last command
+        # and the firmware keeps putting it on the wire, so every indicator
+        # reads healthy - and, worse, the `inputs missing -> 0 W` claim lives
+        # INSIDE cycle(), so on a repeating meter THAT SAFETY CLAIM CANNOT FIRE.
+        # Demonstrated in test_loop.py: constant meter, SoC entity dies at t=4 s,
+        # and the pre-fix controller holds 220 W through the whole outage with
+        # the status line still reading "loop -> 220 W". Layer 1 does not cover
+        # it on a `meter_source: off` install either - no P1 age sensor is
+        # published, so the firmware has nothing to veto on. Measured on the rig
+        # during TEL-05: 70 s at a literally constant 350.0 W, 14 arrivals, zero
+        # cycles.
+        #
+        # This is NOT a freeze detector, and deliberately so - see
+        # HomeWizardLocalSource.unchanged_s for why one cannot be built here (at
+        # the converged -10 W target a 1 Wh register needs ~6 minutes to move,
+        # so any freeze threshold false-trips at the exact operating point we
+        # run at). Nothing here decides whether a repeated reading is true. It
+        # only stops "the number is the same" from meaning "do not think".
+        #
+        # ⚠️ THE 15 s DEFAULT HAS A CEILING IT DOES NOT OWN. It is under
+        # validate.FIRMWARE_INGEST_VETO_S (30 s) and under meter_max_age_s on
+        # purpose, so the loop re-decides at least twice inside any window in
+        # which layer 1 might otherwise be the only thing deciding. If the
+        # firmware's veto ever moves, this default moves with it - it is not
+        # independently justified at 15.
+        floor = float(self.o.get("cycle_floor_s", 15))
+        last_cycle = None
         while not self.stopping:
             await self.read_inputs()
-            # A cycle is one meter update, exactly as on the reference install.
+            now_mono = asyncio.get_running_loop().time()
-            if self.grid != last_grid:
+            # A cycle is one meter update, exactly as on the reference install -
+            # or the floor elapsing, whichever comes first.
+            overdue = last_cycle is None or (now_mono - last_cycle) >= floor
+            if self.grid != last_grid or overdue:
                 last_grid = self.grid
+                last_cycle = now_mono
+                # ⚠️ The FULL cycle, not a degraded one. A floor cycle runs the
+                # same law on the same inputs as an arrival cycle, because in P1
+                # mode the freshness question is already answered upstream and
+                # answered harder: past `meter_max_age_s` (30 s) p1.net_w is
+                # None, which is "inputs missing", which becomes a P_SAFETY 0 W
+                # claim. So a floor cycle cannot run the PI on a 60 s old
+                # figure, because at 60 s there is no figure.
+                #
+                # ⚠️ THAT GUARANTEE IS TRANSPORT-CONDITIONAL, and saying so
+                # matters more than the guarantee: with `meter_source: off`,
+                # read_inputs() takes self.grid straight from hass.number() with
+                # NO age gate at all, so a Home Assistant entity that has gone
+                # quiet reads as a current value on that path with or without
+                # this change. Running the full law is still the right choice
+                # there - the alternative is a degraded second law reasoning
+                # about an age nobody measured - but the floor does not make
+                # that path fresh, and nothing here should be read as claiming
+                # it does. And the integrator
+                # advances per CYCLE, not per second (control.py), so bounding
+                # the gap bounds the accumulation: at most one extra
+                # gain x error per floor interval, under the same
+                # integrator_max_w as every other cycle. A second, gentler
+                # control law for floor cycles would be a second set of
+                # dynamics nobody can test on this hardware.
                 await self.cycle()
             # ⚠️ The heartbeat is not an optimisation. The ESP32 treats silence
             # longer than ~30 s as "the controller is gone" and zeroes the

goodwe_controller/config.yaml

@@ -90,6 +90,7 @@ options:
   integrator_max_w: 0
   heartbeat_s: 10
   stale_input_s: 15
+  cycle_floor_s: 15
   auto_start: false
 
   # --- maintenance -----------------------------------------------------------
@@ -151,6 +152,7 @@ schema:
   integrator_max_w: int(0,15000)
   heartbeat_s: int(2,25)
   stale_input_s: int(5,120)
+  cycle_floor_s: int(5,60)
   auto_start: bool
 
   maintenance_enabled: bool

goodwe_controller/test_loop.py

@@ -0,0 +1,213 @@
+"""Runnable check for the control loop's cadence.  `python3 test_loop.py`
+
+SAFETY-12: the loop must evaluate on a clock as well as on a change. Before the
+fix, `run_control()` ran `cycle()` only when the meter value differed from the
+previous poll, so a meter that repeats a value ran NO control cycles while the
+heartbeat kept re-emitting the last command - measured on the rig during TEL-05
+as 70 s at a literally constant 350.0 W with 14 arrivals and zero cycles.
+
+⚠️ THIS TEST TAKES ~95 SECONDS OF WALL CLOCK (measured 93 s), on purpose, and
+that figure is the only thing standing between it and a future developer who
+decides a slow test is a broken one. The thing under test is a wall-clock floor
+inside a loop that sleeps 1 s; mocking the clock would leave the assertion
+testing the mock. It drives the real `run_control()` and counts what actually
+happened. Eight runs of the real loop is what the eight scenarios cost.
+
+⚠️ If you shorten the windows, re-measure and update this number and the one in
+HANDOVER.md. An understated runtime reads as a hang.
+
+`cycle_floor_s` is read from options with no lower bound in code, so a huge
+value reproduces the pre-fix behaviour exactly. Every claim below is therefore
+an A/B on one binary, not a comparison against a remembered old one.
+"""
+
+import asyncio
+import sys
+import time
+
+from app.main import Controller
+
+fails = []
+NEVER = 1e9          # cycle_floor_s so large the floor can never elapse: pre-fix
+
+
+def check(name, cond):
+    print(f"  {'ok  ' if cond else 'FAIL'} {name}")
+    if not cond:
+        fails.append(name)
+
+
+class FakeHass:
+    """A scripted meter and a plant that tracks the command instantly."""
+
+    def __init__(self, meter, soc_dies_at=None):
+        self.meter = meter
+        self.soc_dies_at = soc_dies_at
+        self.batt = 0.0
+        self.writes = []
+        self.t0 = time.monotonic()
+
+    async def limits(self, entity):
+        return (-5000.0, 5000.0, 1.0)
+
+    async def number(self, entity, invert=False):
+        now = time.monotonic() - self.t0
+        if entity == "sensor.grid":
+            return self.meter(now)
+        if entity == "sensor.batt":
+            return self.batt
+        if entity == "sensor.soc":
+            if self.soc_dies_at is not None and now >= self.soc_dies_at:
+                return None
+            return 50.0
+        return None
+
+    async def set_number(self, entity, value):
+        self.writes.append(float(value))
+        self.batt = float(value)
+        return True
+
+
+class _Store:
+    data = {}
+
+    def set(self, *a):
+        pass
+
+
+class _Pub:
+    enabled = False
+
+    def publish(self, *a, **k):
+        pass
+
+    def close(self):
+        pass
+
+
+BASE = {"setpoint_entity": "number.sp", "meter_entity": "sensor.grid",
+        "soc_entity": "sensor.soc", "batt_entity": "sensor.batt",
+        "meter_source": "off", "max_w": 2000, "gain": 0.6, "slew_w": 1000,
+        "deadband_w": 15, "target_grid_w": -10, "step_w": 10,
+        "heartbeat_s": 10, "stale_input_s": 15}
+
+
+def drive(meter, floor, seconds, soc_dies_at=None, **over):
+    """Run the real control loop for `seconds`; return (cycle times, writes)."""
+    async def go():
+        hass = FakeHass(meter, soc_dies_at)
+        c = Controller(dict(BASE, cycle_floor_s=floor, **over), hass, _Store(),
+                       _Pub(), stop=asyncio.Event())
+        c.auto = True
+        stamps = []
+        real = c.cycle
+
+        async def counted():
+            stamps.append(time.monotonic() - hass.t0)
+            await real()
+
+        c.cycle = counted
+        task = asyncio.create_task(c.run_control())
+        await asyncio.sleep(seconds)
+        c.stopping = True
+        await asyncio.wait_for(task, timeout=10)
+        return stamps, hass.writes, c.target, c.reason
+    return asyncio.run(go())
+
+
+def worst_gap(stamps, seconds):
+    """Longest stretch with no cycle, including the tail after the last one."""
+    edges = [0.0] + list(stamps) + [seconds]
+    return max(b - a for a, b in zip(edges, edges[1:]))
+
+
+# --------------------------------------------------------------------------- #
+print("a meter that repeats the same value")
+FLAT = lambda t: 350.0                                          # noqa: E731
+FLOOR, SECONDS = 3.0, 12.0
+
+pre_stamps, pre_writes, _, _ = drive(FLAT, NEVER, SECONDS)
+# The defect itself, demonstrated rather than described: one cycle at startup
+# and then nothing for the rest of the window, while writes keep happening.
+check("pre-fix: a constant meter runs exactly one cycle, at startup",
+      len(pre_stamps) == 1)
+check("...while the heartbeat keeps writing anyway, so nothing looks wrong",
+      len(pre_writes) >= 2)
+
+post_stamps, post_writes, _, _ = drive(FLAT, FLOOR, SECONDS)
+check("post-fix: the same constant meter runs cycles throughout",
+      len(post_stamps) >= int(SECONDS / FLOOR) - 1)
+# The actual acceptance criterion: a BOUND on the gap, not a count.
+gap = worst_gap(post_stamps, SECONDS)
+check(f"...with no gap longer than the floor ({gap:.1f}s <= {FLOOR + 1.5:.1f}s)",
+      gap <= FLOOR + 1.5)
+check("...and the loop actually corrected, instead of holding a stale command",
+      len(post_writes) > len(pre_writes))
+
+# --------------------------------------------------------------------------- #
+print("a meter that changes normally - nothing may move")
+# ⚠️ THE REGRESSION GUARD, and the more important half. The floor is a deadline
+# on the gap between cycles, reset by every cycle whichever trigger fired - not
+# a second scheduler. On a healthy meter it must never elapse, so the cycle
+# count, the write count and the commanded values must be IDENTICAL either side
+# of the fix. Anything else is a new write cadence, which SAFETY-12 forbids:
+# the deadband, step_w and the 10 s force-write decide what reaches the wire,
+# and this change is only allowed to alter when the loop thinks.
+STEPS = [0.0, 300.0, 850.0, -400.0, 120.0, -900.0, 60.0]
+LIVE = lambda t: STEPS[int(t // 2) % len(STEPS)]                # noqa: E731
+
+a_stamps, a_writes, _, _ = drive(LIVE, NEVER, 12.0)
+b_stamps, b_writes, _, _ = drive(LIVE, FLOOR, 12.0)
+check("a changing meter runs the same number of cycles as before the fix",
+      len(a_stamps) == len(b_stamps))
+check("...commanding exactly the same sequence of values", a_writes == b_writes)
+
+# --------------------------------------------------------------------------- #
+print("a meter parked at the target - the converged operating point")
+# The case TEL-05 warned about: at the converged target the power figure
+# legitimately repeats for minutes. The loop must still run, and must still
+# write nothing but the heartbeat, because the deadband - not the arrival
+# trigger - is what holds the command still.
+AT_TARGET = lambda t: -10.0                                     # noqa: E731
+c_stamps, c_writes, _, _ = drive(AT_TARGET, NEVER, 12.0)
+d_stamps, d_writes, _, _ = drive(AT_TARGET, FLOOR, 12.0)
+check("the loop evaluates at the converged point too", len(d_stamps) > len(c_stamps))
+check("...and the deadband still holds the command, so writes are unchanged",
+      d_writes == c_writes)
+
+# --------------------------------------------------------------------------- #
+print("an input dies while the meter repeats - the safety claim must still fire")
+# ⚠️ THE STRONGEST PROPERTY IN SAFETY-12, and the one that makes it a safety fix
+# rather than a latency fix. The `inputs missing -> 0 W` claim lives INSIDE
+# cycle() (main.py, the stale_input_s branch and the P_SAFETY claim below it).
+# Pre-fix, on a meter whose value happened to repeat, cycle() was never called
+# again - so that claim could not fire AT ALL. The controller held its last
+# command through an input outage of unbounded length with the status line still
+# reading "loop -> N W".
+#
+# Layer 1 does not cover this either on a `meter_source: off` install: no P1 age
+# sensor is published (see Controller.publish), so the firmware has nothing to
+# veto on, and the heartbeat keeps its watchdog fed the whole time.
+#
+# stale_input_s is shortened to 3 s here so the outage resolves inside a test
+# rather than inside a quarter of a minute. The mechanism is the default one.
+c2_stamps, c2_writes, pre_target, pre_reason = drive(
+    FLAT, NEVER, 10.0, soc_dies_at=4.0, stale_input_s=3)
+check("pre-fix: a dead input during a constant meter never reaches the safety claim",
+      pre_target != 0.0 and "safety" not in pre_reason)
+check(f"...so it holds its last command instead ({pre_target:g} W, {pre_reason!r})",
+      pre_target == 220.0)
+
+d2_stamps, d2_writes, post_target, post_reason = drive(
+    FLAT, FLOOR, 10.0, soc_dies_at=4.0, stale_input_s=3)
+check("post-fix: the same outage commands 0 W", post_target == 0.0)
+check(f"...through the safety claim, not by accident ({post_reason!r})",
+      "safety" in post_reason)
+check("...and the 0 W actually reached the device", 0.0 in d2_writes)
+
+# --------------------------------------------------------------------------- #
+print()
+if fails:
+    print(f"{len(fails)} FAILED: " + "; ".join(fails))
+    sys.exit(1)
+print("all checks passed")