Claude Session Bus

A tiny Claude Code plugin that lets separate Claude sessions working the same project pass each other short messages: "holding the dev server on :1420", "editing dynamics.rs", "branch X pushed, safe to rebase".

• Send = an MCP tool (broadcast) the calling session invokes.
• Receive = a UserPromptSubmit hook that prepends new peer messages to the other session's context before its next turn.
• Transport = one append-only JSONL mailbox per project, under ${CLAUDE_PROJECT_DIR}/.claude/.session-bus/.

Broadcast-to-all, one-way, fire-and-forget. Advisory: it announces, it does not enforce. Project-agnostic - the same install only ever bridges sessions that share a project root.

Full design: docs/specs/2026-06-29-claude-session-bus-design.md.

Layout

claude-session-bus/
├── .claude-plugin/
│   ├── plugin.json         # plugin manifest: hooks + mcpServers
│   └── marketplace.json    # lets you add this dir as a local marketplace
├── server/index.mjs        # MCP stdio server: broadcast(), inbox()
├── scripts/drain.mjs       # UserPromptSubmit hook (the receive half)
├── scripts/selftest.mjs    # offline smoke test of the mailbox primitives
├── lib/bus.mjs             # shared: paths, identity, locking, message shape
├── hooks/hooks.json        # declares the UserPromptSubmit hook
└── package.json            # type:module, no runtime deps

Quick start

1. Clone the repo:

   git clone https://github.com/exalsch/claude-session-bus.git
   cd claude-session-bus
   

2. Smoke-test the primitives (no Claude needed):

   node scripts/selftest.mjs    # expect: selftest OK - { ... }
   

3. Install as a local plugin, then enable it:

   /plugin marketplace add /path/to/claude-session-bus    # the directory you just cloned
   /plugin install claude-session-bus@session-bus-dev
   

   Restart Claude Code so the MCP server + hook load. Verify the broadcast and inbox tools appear, and that the session-bus MCP server is connected.

4. Use it. From any session:

   • broadcast("holding the dev server on :1420", kind="lock")
   • inbox() to re-read peer messages on demand.

   The other session in the same project sees the message prepended to its context the next time you submit a prompt there.

End-to-end test

Open two terminals, both cd into the same project (e.g. ~/projects/your-app):

1. In session A: ask it to broadcast("test from A").
2. In session B: submit any prompt. B should report a [session-bus] line from A.
3. In session A: submit any prompt. A should not see its own message (origin filter).

If A does see its own message, the ppid correlation did not hold (see Risks in the spec) - the fallback is to render own posts tagged "(you)" instead of filtering them.

Config

• SESSION_BUS_BACKFILL_MIN (default 0) - minutes of backlog a session sees on its first prompt. 0 = forward-only (a new session only sees what arrives after it joins).
• Retention (MAX_LINES, MAX_AGE_MS) lives in lib/bus.mjs.

Status & next steps

v0.1. The headline risk - ppid correlation on Windows - was tested and did not hold: Claude Code spawns the UserPromptSubmit hook through a shell, so the hook's parent PID is a transient shell, never the directly-spawned MCP server's PID. Sessions now correlate on CLAUDE_CODE_SESSION_ID (an env var both halves inherit from claude.exe) instead of process.ppid, with a ppid fallback where that var is absent. Verified end-to-end across two live sessions and covered by scripts/test-origin.mjs (unit) and scripts/test-drain.mjs (integration); npm test runs the full suite.

Out of scope today: cross-project messaging. The mailbox lives inside each project (${CLAUDE_PROJECT_DIR}/.claude/.session-bus/), so only sessions sharing a project root bridge. A global bus would need a shared mailbox path plus a proj field per message.

This is dev tooling - it must never live inside a product repo.