Summary

• stop heartbeat-mode target resolution from inheriting session lastThreadId unless a thread/topic is explicit
• use originating channel context (not synthetic provider tags) for messaging-tool duplicate suppression and embedded run channel hints
• preserve queued overflow summary routing metadata (originatingChannel, originatingTo, originatingThreadId) during drain
• enforce strict followup origin isolation by not falling back to the active dispatcher when explicit origin route delivery fails
• add regression coverage across outbound target resolution, heartbeat context wiring, followup routing, payload dedupe, and queue drain routing

Tests

• pnpm check
• pnpm build
• pnpm test

Fixes #25864
Fixes #25835
Fixes #25730

Thanks @Gamedesigner, @jadeathena84-arch, and @markshields-tl for the reports and analysis.

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Greptile Summary

This PR hardens routing and session isolation across heartbeat, followup, and messaging tool contexts to prevent cross-channel message leakage. The changes ensure that heartbeat/cron deliveries don't inherit cached thread IDs unless explicitly requested, followup routing strictly enforces origin isolation without dispatcher fallback on failure, and messaging tool deduplication uses originating channel context instead of synthetic provider tags. Queue drain logic now preserves routing metadata (originatingChannel, originatingTo, originatingAccountId, originatingThreadId) during overflow summary generation.

Key improvements:

• Heartbeat mode stops inheriting lastThreadId from session state (src/infra/outbound/targets.ts:121)
• Followup runner removes dispatcher fallback when explicit origin routing fails (src/auto-reply/reply/followup-runner.ts:101-105)
• Messaging tool dedupe prefers OriginatingChannel over synthetic Provider tags (src/auto-reply/reply/agent-runner-payloads.ts:90, src/auto-reply/reply/followup-runner.ts:259)
• Queue drain preserves routing metadata for overflow summaries (src/auto-reply/reply/queue/drain.ts:119-122)
• Heartbeat runner wires delivery context into session context (src/infra/heartbeat-runner.ts:666-669)

Test coverage includes regression tests for all four main fixes plus the overflow summary routing preservation.

Confidence Score: 5/5

• This PR is safe to merge with no critical issues identified
• The changes are well-structured with clear intent, comprehensive test coverage for all four bug fixes, and consistent application of the isolation pattern across multiple files. The logic correctly handles edge cases (e.g., checking mode !== "heartbeat" before inheriting threadId, preserving routing metadata during queue drain). No security concerns, breaking changes, or logical errors detected.
• No files require special attention

_{Last reviewed commit: e1607a8}