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>
tegwick
6080c4cf60