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Should the one-target rule name the build host, not only the architecture?

This ticket proposes TEXT and does not apply it. CLAUDE.md is the owner's file. Three sessions declined to edit it on a peer's ask on 2026-09-05 and were right each time; this seat was the asking peer in two of those.

What the rule says now

"NOTHING OBSERVABLY DIFFERS" IS A CLAIM ABOUT ONE TARGET, AND IT IS HOW REAL BUGS GET RANKED AS REFACTORS. … The dev loop, gate.sh quick and the pin all run there, so a whole defect class — anything whose width, alignment or pointer size is native-only — is structurally invisible … Before ranking one down, ask which target the absence was measured on.

The measured instances behind it are NativeInt folding wrongly on i386/arm32/ riscv32, and a method-pointer record hard-sized 16 bytes. Both authors were honest and both measured on x86-64.

The instance the rule does not cover

2026-09-05. Five test-c-gtk* regressions, auto-filed from a run on seven, were closed on evidence: each ran its own Repro line at HEAD, full job rather than failing step, 1/1 GREEN. Retracted within the hour.

I measured on plexus, which has both /usr/include/gtk-2.0/gtk/gtk.h and /usr/include/gtk-3.0/gtk/gtk.h. A host with the headers passes whether or not a bug exists. "1/1 pass GREEN at HEAD" was true about plexus and silent about seven, where the failure happened. — frankB

The two moves that normally rescue you both ran and neither could work: full job rather than failing step, and HEAD rather than the filed sha. The discriminator was never in the tree at all.

Cost of not having the rule: 18 tickets, four batches — 3 on 08-21, then 5 each on 08-30, 09-01 and 09-05 — the same five tests, closed every time, every closure verified on a host that has the headers.

(The gtk instance's own final cause is still being settled — a dist-upgrade sits between the failing run at 17:54:19Z and a passing run at 19:15:54Z, and the tickets may name jobs that were never the failing source. None of that affects this ticket: whatever the cause turns out to be, the closure was measured on a host that could not have observed it.)

Proposed addition — the smallest version

Appended to that section, after "ask which target the absence was measured on":

And ask which HOST it was measured on. The same failure occurs one level out, where the variable is not the target architecture but the build host's installed packages, toolchain version or absent dev libraries — and there the usual rescues do not work. Measured 2026-09-05: five regressions filed from a host with no GTK dev headers were closed GREEN on a host that has them, four times across four batches. The author ran the full job rather than the failing step and measured at HEAD rather than the filed sha; neither could have caught it, because the discriminator was never in the tree. Reproduce a host-filed failure ON THE HOST THAT FILED IT, or say which host your green was measured on.

The general form, if a broader edit is preferred

frankB's framing, which subsumes both instances:

"The discriminator was never in the tree" is the general form. The architecture version is one instance and the build-host version is another, and neither has a gate row or a ticket field.

That is more useful and more dangerous — a rule stated that generally may not tell anyone what to do. The narrow version above names an action. Recommend the narrow one; the general form belongs in the rationale file.

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