bug(discord): bot-to-bot @mention in multibot-mentions thread silently dropped

[neonvision7724]

Description

When Bot A mentions Bot B inside a thread that Bot A does not own (thread was created by a human user or another bot), Bot B silently ignores the mention. However, if a human user copy-pastes the exact same message content (including the <@BOT_B_ID> mention), Bot B responds correctly.

Steps to Reproduce

1. Human user creates a thread by @mentioning Bot A (e.g. Anis) in a channel
2. Bot A completes its task and sends a message mentioning Bot B (e.g. Rapi): <@BOT_B_ID> please review this PR
3. Bot B does not respond — no reaction, no thread activity
4. Human user copies Bot A's exact message text and pastes it in the same thread
5. Bot B responds correctly

Expected Behavior

Bot B should process the mention from Bot A and respond, since:

• Bot B has allow_bot_messages = "mentions" configured
• Bot B has allow_user_messages = "multibot-mentions" configured
• Bot A explicitly @mentions Bot B by Discord user ID
• Both bots share the same allowed_role_ids and the mentioning bot has the required role

Actual Behavior

Bot B silently drops the message from Bot A. No error in logs, no reaction. The same message content from a human user triggers a response.

Configuration (Bot B — the one that should respond)

[discord]
bot_token = "${DISCORD_BOT_TOKEN}"
allow_bot_messages = "mentions"
allowed_role_ids = ["1504213593523224867"]
allow_user_messages = "multibot-mentions"

[reactions]
tool_display = "compact"

Environment

• OpenAB version: stable (Docker image ghcr.io/openabdev/openab:stable)
• Deployment: Zeabur template (independent containers per agent)
• Platform: Discord
• Thread type: Human-created thread (human @mentioned Bot A to start)
• Both bots have the same allowed_role_ids role assigned in Discord server settings

Additional Context

• This occurs in a multi-agent setup with 5 bots in the same server
• The mentioning bot (Bot A) is a Claude Code agent (claude-agent-acp)
• The target bot (Bot B) is an OpenCode agent (opencode acp)
• The thread was created when the human mentioned Bot A — so Bot A owns the thread
• Bot B has never participated in this thread before the mention

Possible Cause

The bot message filter pipeline may be checking thread ownership before processing allow_bot_messages = "mentions". If Bot B is not the thread owner and has never participated, the message might be dropped before reaching the bot-message-mentions check.

[CHC-Agent]

Issue at a Glance

Human @mentions Bot A in channel
        │
        ▼
Bot A creates thread (Bot A = thread owner)
        │
        ▼
Bot A sends "<@BOT_B_ID> please review" in thread
        │
        ▼
Discord checks: is Bot B a thread member?
        │
        ├─ NO → MESSAGE_CREATE not delivered to Bot B (silent drop)
        │
        └─ YES → Bot B receives event → allow_bot_messages="mentions" → responds ✓

Hi @neonvision7724,

This is an upstream Discord platform limitation, not an openab bug. Discord does not auto-join a bot to a thread when another bot @mentions it — only human-initiated mentions trigger the thread auto-join behavior. Since Bot B is not a thread member, Discord never delivers the MESSAGE_CREATE event to it in the first place, so openab never gets a chance to process the mention.

This is the same root cause as #916, which was confirmed by maintainer analysis.

Related:

• 本地雙 Process 架構下，Bot-to-Bot 訊息無法傳遞（副 Bot 收不到主 Bot 在 Thread 內的 @mention） #916 — same scenario (bot-to-bot mention in thread, target bot not a member), CLOSED as guidance
• fix(discord): multibot-mentions fails to detect multi-bot thread after restart in long threads #832 — multibot detection after restart in long threads (different issue), CLOSED

Workaround: Have a human @mention Bot B in the thread first. Once Bot B is a thread member (via human mention), subsequent bot-to-bot mentions will be delivered and processed correctly.

A potential enhancement would be for openab to proactively join threads where the bot is likely to be needed (e.g. via a THREAD_CREATE handler or periodic active-thread sync), but this would be a new feature rather than a bug fix.

[chaodu-agent]

This is expected behavior — Involvement Gate design

The behavior you observed is by design. OpenAB enforces an Involvement Gate that runs before any allow_bot_messages or allow_user_messages mode checks.

How it works

A bot must be involved (thread owner or has previously replied) before it will process any message in that thread. The key constraint:

Only a human @mention can pull a bot into a thread for the first time. A bot @mentioning another bot that is not yet involved will be silently dropped.

Why your scenario is dropped

In your case:

1. Human creates thread by @mentioning Bot A → Bot A is involved (thread owner)
2. Bot A @mentions Bot B → Bot B is NOT involved yet → message is dropped at the involvement gate, never reaches allow_bot_messages check
3. Human @mentions Bot B → passes the gate (human can break the gate) → Bot B replies and becomes involved
4. After that, Bot A @mentioning Bot B would work fine

Why this constraint exists

1. Resource control — each involved thread creates an agent session. Allowing bots to pull other bots in would let a single bot consume session slots without human oversight.
2. Human authority — humans decide which bots participate in which conversations.
3. Loop prevention — without this gate, Bot A → Bot B → Bot C chain reactions are possible.

Workaround

If you need multi-bot collaboration from the start:

• Pre-involve all bots — have the human @mention all relevant bots at thread creation (or use a shared role via allowed_role_ids with the human mentioning the role)
• The human can simply @mention Bot B once in the thread, after which Bot A→Bot B mentions will work

Documentation

Full details: docs/messaging.md — Involvement Gate and docs/discord.md — Involvement Gate

---

Closing as working as designed. The involvement gate is an intentional safety mechanism.