Problem

Slack delivers both message and app_mention events for the same @mention in channels, with no guaranteed delivery order. OpenClaw's dedup cache uses the same channelId:ts key for both event types — whichever arrives first marks the message as seen, and the second is silently dropped.

When message arrives first in a channel with requireMention: true:

1. message event is dropped by the requireMention check (no mention flag) — but the dedup key is still set
2. app_mention event arrives with wasMentioned: true — silently deduped

Result

• ~50% of @mentions in channels are silently ignored
• Thread context never established (threadTsResolver.resolve never runs)
• invalid_thread_ts errors on reply → fallback to main channel

Reproduction

This is timing-dependent. In a multi-agent Slack setup where agents @mention each other in channels, approximately half of mentions are dropped. Slack provides no ordering guarantees between message and app_mention (confirmed by Slack team).

Proposed Fix

Exempt app_mention events from the dedup check in message-handler.ts. This is safe because:

• The downstream debouncer already handles dedup at the flush level via buildKey
• prepareSlackMessage correctly merges wasMentioned across debounced entries

Related

• [Bug]: app_mention events silently dropped by dedup cache when message event arrives first #20151 — original report (closed, but PR fix(slack): allow app_mention events to bypass dedup cache #20152 was never merged)
• a1ee60549 — fixes a different issue: duplicate DM processing. That fix skips app_mention for im/mpim channels. This issue is about channel mentions being dropped by the dedup cache.