The prefix budget measures a quantity that changes with what is installed on the machine: grep's description is 134 bytes longer without ripgrep #981

Symptom

TestTheLeanPrefixStaysUnderItsBudget reads a different number on every machine at the same commit. Measured at d84120b30's parent shape, one sha, three boxes:

The gate failed on CI while passing on both machines the lane could run it on, which is the worst shape a budget can have: the lane cutting bytes cannot see the number the gate will read.

Cause

internal/exec/bare/grepfindls.go. ripgrepPath probes exec.LookPath("rg") once per process and grepToolDescription picks the description from it — grepDescription where ripgrep is installed, grepFallbackDescription where it is not, and the fallback carries two extra sentences ("Walks the tree itself (ripgrep is not on this machine), so it does NOT read .gitignore — it skips .git, node_modules, vendor and files that look binary"). The GitHub runner has no rg; this laptop has one at /opt/homebrew/bin/rg.

That is 134 bytes of difference in one tool, and it lands in the tool block the budget weighs.

The behaviour itself is right and was deliberate (#: "A CAPABILITY THAT CANNOT WORK IS ABSENT, AND THIS ONE CAN ALWAYS WORK" — the tool keeps its name and only its engine and one sentence change). What is wrong is that a LEDGER measures it.

Replication — a stranger can run this

git checkout <any commit on dev>
go test -run TestTheLeanPrefixStaysUnderItsBudget -v ./internal/session   # with rg on PATH
env PATH=/usr/bin:/bin:$(dirname $(which go)) \
  go test -run TestTheLeanPrefixStaysUnderItsBudget -v ./internal/session # with rg hidden

On dev at 84016ad09 those two read 31,341 and 31,448: a 107-byte spread with nothing changed but PATH. The second figure is the one CI reads.

Acceptance

The budget test measures a figure that does not move with the machine, and says which shape it measured. Either:

• it weighs the belt with the probe pinned to a stated answer (both arms, or the worst arm), and the failure message names the arm; or
• the environment-dependent sentence leaves the description and becomes a fact the tool answers with, so the description is one string everywhere.

Either way, internal/session/prefixbudget_test.go fails identically on the laptop, the Spark and the GitHub runner at one sha, and a lane that measures 40 bytes of headroom has 40 bytes of headroom.

Found while cutting #923's prefix cost: that PR read 18 bytes under the budget on the laptop and 55 over on CI, and the difference was entirely this.

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