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CB-WP-0013-T01: SH-3 refuses a window that cannot carry a rate

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The metric six evidence files reported as 0.0% was never measured.
--shape-budget windows since the last commit, and it is read while
writing the evidence file — right after a commit, when the window holds
one or two responses. SH-1/SH-2 are location statistics and survive n=2.
SH-3 is a rate: at n=2 its only possible values are 0%, 50%, 100%.

cb-cost.py gains sh3_line(), which below a minimum sample prints
"insufficient sample" and no verdict. The floor is derived: if the true
rate were exactly the 20% target, P(zero batched in n) = 0.8^n, and at
n=14 that is 4.4% — so "0 batched in 14" rules out a target-meeting rate
at ~95%. Below that the tool has nothing to say and now says so.

The window was kept rather than split. SH-3 could have been given a
per-pass window, but the budget's purpose is the open remainder since
the last commit, and giving one of three metrics a different window
makes "the window" ambiguous in a tool three specs cite.

Four controls, three mutations, each red for its stated reason —
including the one the evidence files actually hit, where a refusal is
printed as a measured zero.

SessionShape.md §4 carries the correction with the real per-pass figures
(1.1%-6.3%), beside the eighth trusted-arithmetic instance. This is the
ninth, and the second in this same metric. It also shows what the frozen
0.0% hid: against the pinned 7.8%-8.6%, batching has got worse, and six
passes reported a breach that was moving underneath them.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
This commit is contained in:
tegwick 2026-08-02 07:23:04 +02:00
parent 7105027173
commit 6080c4cf60
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workplans/CB-WP-0013-instrument-corrections.md
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@ -69,7 +69,7 @@ figure, `syn` alone 66,916.
```task
id: CB-WP-0013-T01
status: todo
status: done
priority: high
```
@ -95,6 +95,32 @@ already records as the eighth trusted-arithmetic instance. **This is the
ninth, and it is in the same metric.** That coincidence is worth a
sentence: SH-3 is the number this project keeps getting wrong.
**Done 2026-08-02.** `cb-cost.py` gains `sh3_line()`: below a minimum
sample it prints `insufficient sample` and **no verdict**.
**The window was kept, not split.** SH-3 could instead have been given a
per-pass window while SH-1/SH-2 kept the since-last-commit one. Rejected:
the budget's stated purpose is the *open remainder* since the last commit
— the only thing a live budget can fire on — and giving one of three
metrics a different window makes "the window" ambiguous in a tool that
three specs cite. Refusing to report is honest and keeps one window.
**The floor is derived, not round.** If the true rate were exactly the 20%
target, the chance of observing zero batched turns in `n` responses is
`0.8^n`. At **n = 14** that is 4.4%, so "0 batched in 14" rules out a
target-meeting rate at ~95%. Below 14 the tool has nothing to say.
Four controls, three mutations, each red for its stated reason:
| mutation | result |
|---|---|
| the guard never refuses | `SH-3 refuses a window too small` red, *and* `a refusal is distinguishable from a genuine 0.0%` red |
| the guard always refuses | reporting at exactly the minimum, and a real 40% rate, both red |
| the refusal prints as `0.0%` | `refuses a window too small` red — the case the six evidence files actually hit |
The correction is recorded in `specs/SessionShape.md` §4, beside the
eighth instance, with the real per-pass figures.
## Task: argue SH-3's floor, or move it
```task