fix: slack mention-gating thread participation

[bek91]

Summary

• Record Slack thread participation after successful visible threaded sends in the shared Slack send path.
• Remove narrower duplicate recording from the Slack action runtime so message-tool, outbound, and media delivery paths share the same behavior.
• Add focused coverage for threaded text sends, media uploads, and outbound replyToId-to-threadTs routing.

Root Cause

Slack inbound mention gating already supports implicit thread mentions through the bot thread participation cache, but not every visible outbound Slack path seeded that cache. In particular, sends routed through the shared message/outbound delivery path could visibly reply in a thread without recording participation, so later unmentioned thread replies could be dropped as no-mention despite documented bot participation behavior.

Fixes #77648.

Validation

• pnpm exec vitest run extensions/slack/src/send.blocks.test.ts extensions/slack/src/send.upload.test.ts extensions/slack/src/action-runtime.test.ts extensions/slack/src/outbound-delivery.test.ts extensions/slack/src/monitor/message-handler/prepare.test.ts --testTimeout=120000
• pnpm check:test-types
• pnpm build
• codex review --base origin/main

AI-assisted: yes.

[clawsweeper]

Codex review: needs maintainer review before merge.

Summary
The PR moves Slack thread-participation recording into sendMessageSlack after successful threaded sends, removes narrower action-runtime recording, and adds focused Slack send/outbound/media coverage plus a changelog entry.

Reproducibility: yes. Source inspection gives a high-confidence path: current main can post through sendMessageSlack(..., { threadTs }) without seeding the cache that inbound mention gating uses for bot-participated threads.

Next step before merge
No repair lane: the PR is already a narrow implementation with no discrete review finding for automation to fix, and the protected label requires maintainer handling.

Security
Cleared: The diff only touches Slack channel source/tests and the changelog, with no dependency, CI, permission, secret, or supply-chain changes.

Review details

Best possible solution:

Land this PR or an equivalent Slack-owned fix after maintainer review and targeted Slack checks so every successful visible threaded send path seeds participation state consistently.

Do we have a high-confidence way to reproduce the issue?

Yes. Source inspection gives a high-confidence path: current main can post through sendMessageSlack(..., { threadTs }) without seeding the cache that inbound mention gating uses for bot-participated threads.

Is this the best way to solve the issue?

Yes. Centralizing the cache write after the shared Slack send returns a resolved channel is narrower and more maintainable than duplicating participation recording in each caller.

What I checked:

• Documented Slack thread behavior: Slack docs say thread.requireExplicitMention defaults to false and bot-participated thread replies bypass requireMention gating. Public docs: docs/channels/slack.md. (docs/channels/slack.md:935, b546aa91e19b)
• Inbound gate already consumes participation state: Current main builds the bot_thread_participant implicit mention kind from hasSlackThreadParticipationWithPersistence(...). (extensions/slack/src/monitor/message-handler/prepare.ts:367, b546aa91e19b)
• Current shared send path lacks recording: Current main returns the queued sendMessageSlack result without recording successful threaded sends in the participation cache. (extensions/slack/src/send.ts:538, b546aa91e19b)
• Proposed fix is centralized: The PR records normalized opts.threadTs after runQueuedSlackSend(...) returns a result with channelId, covering shared text, blocks, outbound, and upload callers. (extensions/slack/src/send.ts:548, 686279cdee77)
• Action-runtime duplicate removed safely: The PR removes the action-runtime-only cache write so the action path relies on the same shared Slack send behavior as the other visible send paths. (extensions/slack/src/action-runtime.ts:271, 686279cdee77)
• Coverage added: The PR adds tests for threaded sendMessageSlack, invalid/unthreaded sends, upload participation, and outbound replyToId to Slack threadTs routing. (extensions/slack/src/send.blocks.test.ts:71, 686279cdee77)

Likely related people:

• steipete: Recent commits touch the central Slack send, outbound adapter, and thread-anchor behavior that this PR changes. (role: recent maintainer; confidence: high; commits: 374529d61255, e40d7abda9ff, 2dfa2663ecff; files: extensions/slack/src/send.ts, extensions/slack/src/outbound-adapter.ts, extensions/slack/src/action-runtime.ts)
• amknight: Commit d0ec3d1 added best-effort persistent Slack thread participation, which is the cache this PR seeds from more send paths. (role: introduced adjacent behavior; confidence: medium; commits: d0ec3d1f09b0; files: extensions/slack/src/sent-thread-cache.ts, extensions/slack/src/monitor/message-handler/prepare.ts)
• bek91: Before this PR, bek91 authored merged Slack action-thread target normalization work in the same action/threading area. (role: adjacent owner; confidence: medium; commits: d6c2280aab91; files: extensions/slack/src/action-runtime.ts)

Remaining risk / open question:

• This read-only review did not execute the PR validation commands; CI and maintainer review still need to gate the exact head.

Codex review notes: model gpt-5.5, reasoning high; reviewed against b546aa91e19b.

[steipete]

Landed via admin rebase onto main despite unrelated red CI in diagnostics-otel exhaustive-switch lint.

• Gate: pnpm test extensions/slack/src/send.blocks.test.ts extensions/slack/src/send.upload.test.ts extensions/slack/src/action-runtime.test.ts extensions/slack/src/outbound-delivery.test.ts extensions/slack/src/monitor/message-handler/prepare.test.ts --testTimeout=120000 (5 files, 130 passed); pnpm test extensions/slack/src/thread-ts.test.ts extensions/slack/src/sent-thread-cache.test.ts extensions/slack/src/outbound-adapter.test.ts extensions/slack/src/channel.test.ts --testTimeout=120000 (4 files, 54 passed); pnpm exec oxfmt --check --threads=1 CHANGELOG.md extensions/slack/src/action-runtime.test.ts extensions/slack/src/action-runtime.ts extensions/slack/src/outbound-delivery.test.ts extensions/slack/src/send.blocks.test.ts extensions/slack/src/send.ts extensions/slack/src/send.upload.test.ts
• Source SHA: 686279c
• Landed SHA: cf3ce08

Thanks @bek91!