Feature: Cross-agent message delivery in shared-bot multi-agent setups

Feature Request: Cross-agent message delivery in shared-bot multi-agent setups

Problem

When running multiple agents in a single spacebot instance, all agents share one Discord bot user. This means the Discord message handler filters all messages from that bot user ID — including messages sent by one agent to another agent's channel via send_message_to_another_channel.

The result: Agent A posts to Agent B's channel → Agent B's handler drops it as "self-message" → cross-agent communication through Discord channels is impossible.

This is a fundamental limitation for multi-agent workflows where agents coordinate via shared Discord channels.

Related

• #435 (GNAP: git-native task coordination layer) — discusses multi-agent coordination patterns
• The existing send_message_to_another_channel tool already provides the sending side; this addresses the receiving side

Proposed Solution

We implemented a self-contained approach using Discord embed footers as a signal:

Sending side (send_message_to_another_channel)

When the target is a Discord channel, wrap the outbound message in a RichMessage with a Card containing:

footer: "cross-agent:{source_agent_display_name}"

Receiving side (Handler::message() in discord.rs)

Before the self-message filter:

1. Check if the message has embeds with a footer starting with "cross-agent:"
2. If found, set is_cross_agent = true
3. Modify the self-message filter: && !is_cross_agent
4. Extract source agent name and add to metadata as cross_agent_source

Why embed footers?

• No shared state between sender and receiver — the footer is self-contained in the message
• No trait signature changes — broadcast still returns Result<()>
• No infinite loops — only send_message_to_another_channel adds the footer; regular agent replies don't, so they're still filtered
• Observable — footer text is visible to Discord users (small embed text), which is actually useful for transparency

Example flow

1. Overseer posts to Liaison's #observatory channel via send_message_to_another_channel
2. Message arrives with embed footer "cross-agent:Overseer"
3. Liaison's handler sees the footer, allows the message through, adds cross_agent_source: "Overseer" to metadata
4. Liaison processes and replies normally — its own reply lacks the footer, so it gets filtered (no loop)

Impact

• Unblocks multi-agent coordination through Discord channels
• Zero impact on single-agent setups (no footer = existing behavior)
• No database migrations or config changes
• Footer visibility provides transparency to human observers

Implementation Notes

We've been running this on v0.3.3 in a multi-agent BIM experiment setup. The approach is deliberately minimal — no new tables, no shared state, no protocol changes. Happy to submit a PR, though I'd appreciate feedback on the embed-footer approach vs. alternatives (e.g., a dedicated agent-to-agent message bus bypassing Discord entirely).

Review send_message_to_another_channel and Handler::message() in discord.rs, then trace the existing self-message filtering and Discord embed handling. Done means cross-agent messages reach the target agent with the source metadata, while ordinary agent replies remain filtered to avoid loops.