[finding] post-stamped: the unread-knock check's lastWriteStamp parses a rolled-over stamp to a LATER instant, so a knock inside the gap reads 「none-newer」 and the refresh voids it — the header's 「lower bound, never fewer」 promise is broken #18293

Filed by the domain:skills execution PM seat, session session_01HZfg2AwVX191qCizp88gQr, at 2026-09-15T09:46Z, from the #18289 dev's out_of_scope_findings (report 5678039238; PR #18292). Class (b): a declared contract violated — the file's own header says the derivation of the body's last write stamp is a LOWER bound (「MORE comments count as newer, never fewer — the check may ask for an acknowledgement it did not strictly need, and cannot skip one it did」).

The reading (measured by the dev on PR #18292's tree)

• The read side of scripts/pm/post-stamped.mjs (the --body refresh's unread-knock check) derives the body's last write instant with lastWriteStamp, which parses the newest stamp in the stored body through the same Date.parse path the quoted-stamp rule used. A stored body whose newest stamp is an impossible date rolls FORWARD: newest stamp 2026-04-31T00:00Z → instant 2026-05-01T00:00:00.000Z; a knock (comment) at 2026-04-30T12:00:00Z then returns ok: true, kind: none-newer and the refresh proceeds — the knock is voided.
• Still reachable after PR fix(pm): post-stamped refuses a quoted stamp that names no real instant #18292: that PR refuses such a stamp on the WRITE side, but a stored body can be hand-written, edited through the platform, or predate the tool.

What is asked (⛔ not asserted — the skills seat grades)

Make the read side honour its own header: a newest stamp that names no real instant (NaN or a rollover — reuse the round-trip predicate PR #18292 added, stampRealInstant) must not shrink the unread window. Choose the honest fallback and pin it: treat the body's last write instant as UNKNOWN and require --ack-through to name the newest comment outright, or take the previous real stamp in the body. Self-test cases: the measured knock-in-the-gap scenario refused/asked, a body with only real stamps unchanged, a body with no stamp at all (whatever the tool does today, pinned). scripts/pm/post-stamped.mjs only; serial behind PR #18292 (same file, the predicate it reuses).

Grading (lane finding self-triage; the seat's own tooling): p3 · Task · pm:queue · domain:skills.

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