fix(discord): auto-join threads under allowed_channels to enable bot-to-bot @mention delivery

[chaodu-agent]

Summary

Bot-to-bot @mention in threads is broken because Discord Gateway does not deliver MESSAGE_CREATE to bots that are not thread members. This makes the trusted_bot_ids bypass added in #967 ineffective — the application-layer logic never fires.

Related: #974 (bug report), #967 (application-layer fix), #950 (involvement gate docs)

Root Cause

Discord treats thread event delivery differently for bots vs humans:

• Human @mentions a bot in a thread → Discord auto-adds the bot to thread member list → bot receives MESSAGE_CREATE
• Bot @mentions another bot in a thread → Discord does NOT auto-add → target bot never receives the event

This is confirmed by Discord official docs (Threads - Thread Membership):

Each thread tracks explicit membership.

And validated by live testing: human mention works, bot mention in the same thread does not.

Proposed Fix

Implement a THREAD_CREATE / THREAD_UPDATE Gateway event handler that auto-joins threads whose parent channel is in allowed_channels (or allow_all_channels = true).

When the bot receives a THREAD_CREATE event for a new thread under a monitored channel, call:

PUT /channels/{thread_id}/thread-members/@me

This ensures the bot is always a thread member for relevant threads, so:

1. It receives all MESSAGE_CREATE events in those threads
2. The existing feat(discord): trusted bot @mention bypasses involvement gate #967 trusted_bot_ids bypass logic can actually fire
3. Bot-to-bot @mention collaboration works as documented

Alternatives Considered

1. Reactive approach: When Bot A sends a message mentioning Bot B, Bot A calls PUT /channels/{thread_id}/thread-members/{bot_b_id} to add Bot B. Downside: requires Bot A to have MANAGE_THREADS permission and know Bot B's ID.

2. Proactive auto-join (proposed): Each bot joins threads under its own allowed_channels on THREAD_CREATE. Simpler, no cross-bot coordination needed, each bot is self-sufficient.

Impact

Without this fix, the multi-agent collaboration feature (trusted_bot_ids + allow_bot_messages=mentions) cannot work for bot-initiated conversations in threads — only human-initiated ones work.

Environment

• Version: 0.8.5+
• Platform: Discord
• Config: allow_bot_messages = "mentions", trusted_bot_ids set

[github-actions]

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[ChunHao-dev]

Issue at a Glance

   Bot A (thread)          Discord Gateway          Bot B
      │                         │                      │
      ├─ @mention Bot B ───────►│                      │
      │                         │  (Bot B not in       │
      │                         │   thread member list)│
      │                         │                      │
      │                         ├──── ✗ no delivery ──►│
      │                         │                      │
      │  ┌─────────────────────────────────────────┐   │
      │  │ FIX: on THREAD_CREATE, if parent chan   │   │
      │  │ is in allowed_channels → auto-join      │   │
      │  │ PUT /channels/{thread_id}/              │   │
      │  │      thread-members/@me                 │   │
      │  └─────────────────────────────────────────┘   │
      │                         │                      │
      ├─ @mention Bot B ───────►├──── ✓ delivered ────►│

Hi @chaodu-agent,

This is a gateway-level limitation where Discord does not auto-add bots to thread membership when mentioned by another bot (only human mentions trigger auto-add). As a result, the trusted_bot_ids bypass logic from #967 never fires for bot-initiated threads because MESSAGE_CREATE is never delivered to the target bot.

Related:

• trusted_bot_ids admission override ineffective in new threads (missing auto-join) #974 — trusted_bot_ids admission override ineffective in new threads (OPEN, same root cause)
• feat(discord): trusted bot @mention bypasses involvement gate #967 by @chaodu-agent — MERGED (application-layer trusted bot bypass)
• docs: document involvement gate design #950 by @chaodu-agent — MERGED (involvement gate docs)

The proposed proactive auto-join approach (handle THREAD_CREATE → join threads under allowed_channels) is self-contained and requires no cross-bot coordination. A PR to implement this is welcome — feel free to open one if you are interested.