rubicon Session Messaging
Overview
CCD sessions message each other via mcp__ccd_session_mgmt__send_message, addressed by
a sessionId in the form local_<uuid>. A session cannot look up its own sessionId
through list_sessions or get_session — both tools explicitly exclude the calling
session from their results. The title shown in the app UI is not a valid address either;
passing a title into send_message fails.
Self-identification
A session's own address is the env var CLAUDE_CODE_HOST_SESSION_ID — already in
local_<uuid> form, ready to hand to another session.
CLAUDE_CODE_SESSION_ID (no prefix) is a different ID — the bare transcript/CLI
session UUID — and does not work as a send_message target. Concatenating local_
onto it produces a plausible-looking but wrong address.
Verified 2026-08-23: calling get_session with the correct CLAUDE_CODE_HOST_SESSION_ID
value returns "Refusing to return the current session" — proof it's a real, recognized
ID that's simply self-excluded. Calling it with the wrong (CLAUDE_CODE_SESSION_ID-based)
guess returns a generic "... not found" — indistinguishable from a typo. Don't treat
"not found" as proof an ID's format is wrong; it's also what a correct-but-self-excluded
ID looks like from certain angles, and what a genuinely wrong ID looks like. If a "not
found" result matters, disambiguate it against a different session rather than concluding
from the message alone.
Quick reference
| Need | Do |
|---|---|
| Get a session's own address | In that session: echo $CLAUDE_CODE_HOST_SESSION_ID |
| Send to a known address | mcp__ccd_session_mgmt__send_message(session_id, message) |
| Find an address by title/PR/branch | mcp__ccd_session_mgmt__list_sessions — excludes the caller, but works for any other session, running or not |
Common mistakes
- Using the app UI title as
session_id. Titles aren't addresses;send_messagewill fail even though the title is real and visible. - Using
ListAgents/SendMessage's short slug for a session that might go idle. That roster only covers currently-running peers and the slug isn't guaranteed stable across restarts. Fine for live in-process subagents; wrong tool when the target might be idle or closed by the time the other session sends. - Assuming a session can self-report via
list_sessions. It's excluded from its own results by design — use the env var instead.
Related commands: /session-messaging:session-whoami, /session-messaging:session-send