Summary

• Fix stale lastThreadId fallback causing bot to reply in wrong Telegram DM topic
• Add tests for thread ID persistence behavior

Problem/Context

When a user interacts with the bot inside a DM topic/thread, the session persists lastThreadId. If the user later sends a plain DM message (no topic), ctx.MessageThreadId is undefined and the || fallback in initSessionState picks up the stale persisted thread ID — causing the bot to reply into the old topic instead of the main conversation.

This is particularly confusing for Telegram DMs with topics enabled, where the bot creates a phantom topic visible only as "#" in the topic list.

Solution

Only fall back to baseEntry.lastThreadId for thread sessions (where isThread is true). Non-thread sessions now correctly leave threadId unset, preventing cross-session thread ID leakage.

- const lastThreadIdRaw = ctx.MessageThreadId || baseEntry?.lastThreadId;
+ const lastThreadIdRaw = ctx.MessageThreadId || (isThread ? baseEntry?.lastThreadId : undefined);

Test Plan

• Added test: non-thread session does not inherit stale threadId from a previous thread interaction
• Added test: thread session correctly preserves threadId across messages
• All 37 existing session tests pass

Sign-Off

• Models used: Claude Opus 4.6
• Submitter effort summary: Discovered while debugging bot replying in invisible Telegram DM topic. Root-caused to stale lastThreadId fallback in session state initialization. Verified fix with new tests.
• Agent notes: The isThread flag was already computed earlier in initSessionState — reusing it for the guard keeps the change minimal.

—Calculon, Actor Extraordinaire (feat. Opus 4.6)

Greptile Summary

Adds defensive guard to prevent non-thread sessions from inheriting stale lastThreadId from previous thread interactions. The fix correctly scopes the baseEntry?.lastThreadId fallback to only apply when isThread is true, preventing thread ID leakage between thread and non-thread sessions. Includes comprehensive tests covering both the fix scenario (non-thread not inheriting stale threadId) and the preservation scenario (thread sessions correctly maintaining threadId).

Confidence Score: 5/5

• This PR is safe to merge with minimal risk
• The change is a focused, defensive fix with clear logic: only allowing threadId fallback when actually in a thread session. The implementation is minimal (single line change + comment), the fix prevents a real bug (phantom thread replies), comprehensive tests verify both the fix and preservation behavior, and all 37 existing session tests pass.
• No files require special attention

_{Last reviewed commit: 4ba2da4}