Summary

Ports the merge-safe continuity fixes from #23090 onto current main:

• fork thread/topic sessions from parent even when a preseeded thread session key already exists
• persist a forkedFromParent flag so parent transcript forking only happens once
• always fetch configured thread history window in Slack thread sessions (not only first turn)
• always inject available thread history/starter context into reply prompt composition
• add regression tests for all three gates
• add Unreleased changelog entry

Why

Thread session continuity was gated behind first-turn checks in multiple paths, which caused thread context to disappear after the first interaction and prevented expected parent transcript branching in preseeded thread sessions.

Validation

• pnpm test -- src/auto-reply/reply/session.test.ts src/slack/monitor/message-handler/prepare.test.ts src/auto-reply/reply/get-reply-run.media-only.test.ts
• pnpm exec oxfmt --check src/slack/monitor/message-handler/prepare.ts src/auto-reply/reply/get-reply-run.ts src/auto-reply/reply/session.ts src/config/sessions/types.ts src/slack/monitor/message-handler/prepare.test.ts src/auto-reply/reply/get-reply-run.media-only.test.ts src/auto-reply/reply/session.test.ts CHANGELOG.md
• pnpm exec oxlint src/slack/monitor/message-handler/prepare.ts src/auto-reply/reply/get-reply-run.ts src/auto-reply/reply/session.ts src/config/sessions/types.ts src/slack/monitor/message-handler/prepare.test.ts src/auto-reply/reply/get-reply-run.media-only.test.ts src/auto-reply/reply/session.test.ts

Supersedes: #23090

Greptile Summary

Ports thread session continuity fixes from #23090. Removes first-turn-only gates that prevented thread context from persisting beyond the initial interaction.

Key changes:

• Fork thread sessions from parent even when preseeded thread session key exists (session.ts:340-362)
• Add forkedFromParent flag to prevent repeated forking (types.ts:39, session.ts:339)
• Always fetch thread history, not just on first turn (prepare.ts:524)
• Always inject thread history/starter context into prompts (get-reply-run.ts:263-267)

Test coverage:

• Regression test for preseeded thread session forking (session.test.ts)
• Regression test for continued thread history loading (prepare.test.ts)
• Regression test for thread context in follow-up turns (get-reply-run.media-only.test.ts)

Confidence Score: 5/5

• Safe to merge - well-tested fix for thread session continuity bug
• Changes are focused, well-tested with comprehensive regression tests covering all three gates, and fix a clear bug where thread context was lost after first turn. Logic is sound: forkedFromParent flag prevents duplicate forking, and removing first-turn gates enables consistent context.
• No files require special attention

_{Last reviewed commit: 35a05d1}