Slack emits `message_changed` for a link unfurl carrying a DIFFERENT event ts
than the original message. That ts legitimately misses the `_dedup` check, so
`_processed_message_ts` is the only guard against it becoming a second user
turn -- but it was only populated at the END of `_handle_slack_message`, after
thread context, permalink resolution and file downloads had all awaited.
An unfurl landing inside that window found the guard empty and was promoted to
a duplicate turn: a spurious "Interrupting current task" banner plus the same
answer posted twice.
Production capture (adminbot, 2026-08-22 02:23:30-31Z, channel C0BF1EYUA9H):
02:23:30.718 message ts=1787365409.908499 dedup_hit=False
02:23:30.737 app_mention ts=1787365409.908499 dedup_hit=True
02:23:31.675 message ts=1787365411.012100 dedup_hit=False <- leaked
subtype=message_changed
The original copy was still resolving two Slack permalinks when the unfurl
arrived 957ms later.
Claim the message ts once every filter has passed and the event is certain to
be delivered, before the slow enrichment awaits. Claiming any earlier (right
after the dedup check) also claims messages the handler then discards, which
breaks summoning the bot by editing "@bot" into a previously ignored message
(tests/gateway/test_slack.py::TestMessageRouting::
test_message_edit_with_new_mention_processed).
Eviction logic is extracted to `_remember_processed_message_ts` so both call
sites share one bounded implementation.
Richard Hojun Jang
708f84c477