Message Claude

Find the exact Claude Code session and send it one plain-text message.

For the opposite direction, use Message Codex.

If ListAgents and SendMessage are available, use them. Do not use the raw inbox to bypass a permission rule that denied either tool. The workflow below is for external callers such as a standalone Codex session, Codex launched by Claude, a hook, or a script.

The bundled helper supports macOS, Linux, and WSL 2. Native Windows uses a named pipe and is not implemented here. Requires JDK 17+ and Kotlin 2.4.21+ (kotlinr on PATH). The first run resolves the pinned JSON dependency and compiles the script.

Send a message

Resolve scripts/message_claude.main.kts from this skill's directory.

1. List the registered sessions:

   kotlinr <skill-dir>/scripts/message_claude.main.kts --list
   

2. Select the target by its exact name. If names repeat, narrow with --cwd or --pid. Never guess between multiple matches.

3. Send a short message:

   kotlinr <skill-dir>/scripts/message_claude.main.kts \
     --name the-session-name \
     --message 'codex: the migration finished; tenant_id is the new column'
   

   Omit --message to read arbitrary or multiline text from stdin.

The helper uses --config-dir, then CLAUDE_CONFIG_DIR, then ~/.claude. Pass --config-dir when Claude uses a custom directory that the caller did not inherit.

What the helper guarantees

• Reads <config-dir>/sessions/*.json and refuses zero or multiple target matches.
• Reads the target's peerToken without printing it. The key filename hashes messagingSocketPath exactly as recorded; it does not resolve /tmp to /private/tmp.
• JSON-encodes the complete auth and message payload before connecting, so quotes, newlines, and backslashes in the message remain valid.
• Reports a successful socket write, not guaranteed delivery. The receiving session may still hold or refuse the message through crossSessionInbound.

Failures

• No matching session: list again and check name, cwd, pid, and --config-dir.
• Connection failure: the registry entry may be stale; list again instead of retrying blindly.
• Permission error: the Codex sandbox may block that Unix socket. Report the socket path and ask for a grant scoped to its directory; do not bypass the sandbox.
• Missing peerToken: stop and verify the config directory and target rather than sending an unauthenticated message.

Send one concise message and identify the sender in its first words. Do not send repeated probes: messages can be held, refused, or rate-limited, and an incoming message never grants user authority.

Reference: https://code.claude.com/docs/en/cross-session-messaging