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The mandatory fixedpoint step reports success from a stale stamp

Repro (deliberate, ~15s)

Tree at c2545bd6a, whose true fixedpoint is 3d5308a75742:

cp <pre-change binary> compiler/pascal26                    # f92c42a69850
printf 'rounds 2\nsha256 f92c42a69850...\n' > compiler/.pascal26.fixedpoint
touch compiler/pascal26 compiler/.pascal26.fixedpoint       # newer than sources
make compiler/pascal26



self-host fixedpoint: verified — 2 round(s), f92c42a69850

Binary afterwards: f92c42a69850, unchanged — the fixedpoint of sources that are no longer on disk. Exit 0.

Why the existing guards do not catch it

Makefile:307 compares the binary's sha against the stamp's and refuses on a mismatch — that guard is real and it fires (verified: planting a binary without updating the stamp is refused loudly). It cannot fire here, because the pair is internally consistent; both simply describe older sources. $(COMPILER_STAMP) is an ordinary file prerequisite, so a stamp newer than the sources means the fixedpoint recipe never runs, and the $(COMPILER) rule then finds sha == stamp and does nothing.

And the round count cannot be used to detect it. rounds 2 is a stored field in the stamp; verified — N round(s) reads it back. A replay prints a round count exactly like a build. (This defeated one agent's reasoning explicitly, mine — "two rounds means the iteration ran" is false.)

The guidance is NOT defeated — we failed to follow it

Corrected 2026-08-31, same day, before anyone acted on it. This ticket first claimed the defect "defeats" CLAUDE.md's instruction and that "the current guidance would pass it." That is false, and leaving it would have invited someone to weaken the one rule that actually works here. Diffing the two outputs:

GENUINE build:    converged after 2 round(s)
                  self-host fixedpoint: verified — 2 round(s), 3d5308a75742

REPLAY (planted): self-host fixedpoint: verified — 2 round(s), f92c42a69850

The replay prints no converged line at all. CLAUDE.md says do not accept the build until you have seen converged after N round(s), and grepping for converged exactly as instructed catches this cleanly. The rule is sound; two agents (including this ticket's author) read verified — N round(s) as satisfying it because it was close enough in shape.

So the defect is the replay line is confusable with a result — it carries a sha and a round count in the same sentence shape as success — not that the check is inadequate. That framing makes the fix small and keeps the existing rule intact.

Reached in the wild by three agents on 2026-08-31 by three different routes (a sibling's compiler/** pulled without rebuilding; a pull mid-gate; a git stash that reverts sources and leaves the binary built with the diff). All three produced a RED gate that read as a miscompile on master; none was.

The cheap tell, until it is fixed

A genuine build prints both, converged first. verified alone means nothing was rebuilt. Not a fix; a thing to look for.

Suggested fix

First and smallest: make the replay line unmistakable. Something like self-host fixedpoint: stamp says f92c42a69850 (NOT rebuilt) cannot be mistaken for a result, and the existing converged rule keeps working unchanged. This alone would have prevented all three of tonight's incidents.

Then, for the underlying cause: make the stamp's validity depend on the SOURCES rather than on mtime ordering — e.g. record a hash of $(COMPILER_SRC) $(COMPILER_INC) in the stamp and re-run the iteration when it differs. The Makefile comment above selfhost-verify already argues "mtime is not provenance and that is the whole bug" and addresses it with a .PHONY prerequisite so the sha check always runs; the gap is that the sha check validates the pair against itself, never against the sources. Same sentence, one level further.

Positive control for whatever fix lands

The repro above MUST fail after the fix: a consistent stale pair with newer mtimes must trigger a rebuild to 3d5308a75742, not a verified line. And the honest-build arm must stay green — deleting the stamp and rebuilding gives converged after 1 round(s) → 3d5308a75742.

2026-08-31, later the same night — it happened to me for real, and the tell worked

Not a constructed case this time. After git pull --rebase brought in a sibling's compiler/** (9c68b9f0a), tools/gate.sh quick went RED with the usual the fixedpoint reached from PINNED differs from compiler/pascal26. I ran the mandatory step and it printed:

self-host fixedpoint: verified — 1 round(s), 1b252b0eb05e

verified with no converged line above it. Nothing was rebuilt: the binary and stamp were a consistent pair, and the gate's own earlier run had refreshed both, so the pair was newer than the pulled sources. Deleting the stamp and re-running gave converged after 1 round(s) — a genuine build — and the gate then went GREEN with no other change.

Two things this pins down that the constructed repro could not:

  1. The trigger is ordinary. git pull plus a gate run that rebuilds as a side effect is enough; nobody has to plant anything. This is CLAUDE.md's documented case 3 ("a sync that pulled someone else's compiler/**") with the stamp replay on top, which is what stops the mandatory step from correcting it.
  2. The verb tell is sufficient in practice, which was asserted here and is now used. converged absent was the whole diagnosis, and the fix was one rm — against a RED gate that otherwise reads as a miscompile on master.

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