Problem

The Slack adapter unconditionally drops all bot messages at line 631 of gateway/platforms/slack.py:

# Ignore bot messages (including our own)
if event.get("bot_id") or event.get("subtype") == "bot_message":
    return

There is no configuration option to override this behavior. This means bot-to-bot communication is impossible on Slack — even when another bot explicitly @mentions the Hermes bot, the message is silently dropped.

Discord already solved this

The Discord adapter (gateway/platforms/discord.py, lines 539-551) has a DISCORD_ALLOW_BOTS env var with three modes:

• "none" — ignore all other bots (default)
• "mentions" — accept bot messages only when they @mention us
• "all" — accept all bot messages

if getattr(message.author, "bot", False):
    allow_bots = os.getenv("DISCORD_ALLOW_BOTS", "none").lower().strip()
    if allow_bots == "none":
        return
    elif allow_bots == "mentions":
        if not self._client.user or self._client.user not in message.mentions:
            return
    # "all" falls through to handle_message

Requested change

Add an equivalent SLACK_ALLOW_BOTS env var to the Slack adapter with the same three modes (none / mentions / all), defaulting to "none" for backward compatibility.

The implementation would replace the current hard filter with:

if event.get("bot_id") or event.get("subtype") == "bot_message":
    allow_bots = os.getenv("SLACK_ALLOW_BOTS", "none").lower().strip()
    if allow_bots == "none":
        return
    elif allow_bots == "mentions":
        bot_user_id = getattr(self, "_bot_user_id", "")
        if bot_user_id and f"<@{bot_user_id}>" not in text:
            return
    # "all" falls through
    # Still ignore our own messages
    if event.get("bot_id") == getattr(self, "_bot_id", None):
        return

Note: The self-message guard should remain to prevent echo loops regardless of mode.

Use case

Multi-agent setups where different AI agents (running on different platforms/frameworks) coexist in the same Slack workspace and need to communicate via @mentions. Without this, Slack is the only platform where bot-to-bot handoffs are impossible.

Additional context

The SLACK_ALLOWED_USERS allowlist may also need consideration — bot user IDs should either be exempt from this check when SLACK_ALLOW_BOTS is set, or documentated as needing to be added to the allowlist separately.