ClaudeMailbox/plugin

claude-mailbox plugin

Lets Claude Code pull unread messages from a local claude-mailbox daemon before every prompt and inject them into the conversation context. Each Claude session gets a unique mailbox identity auto-derived from its session id, so two sessions in the same project never collide.

Setup (three prompts, all inside Claude Code)

/plugin marketplace add https://git.kuns.dev/releases/ClaudeMailbox.git
/plugin install claude-mailbox@claude-mailbox
/claude-mailbox:mailbox-doctor

The doctor walks the rest:

1. installs the claude-mailbox binary via npm install -g @kuns/claude-mailbox if missing (asks first)
2. registers the daemon for autostart and starts it if needed
3. health-probes http://127.0.0.1:37849/health
4. optionally lets you set a base prefix (e.g., backend) — without one, mailbox names are anonymous (claude-XXXXXXXX)
5. runs a self → self smoke test

Restart Claude Code only if step 4 wrote a new prefix. After that, every prompt auto-pulls unread messages.

Mailbox identity (the important bit)

Each Claude Code session gets its own mailbox name, derived from the session's UUID:

Configuration	Resulting mailbox name
No CLAUDE_MAILBOX_NAME set	claude-a8b3c1d2 (first 8 hex chars of session_id)
CLAUDE_MAILBOX_NAME=backend in .claude/settings.json env	backend-a8b3c1d2

So if you open two Claude Code sessions in the same project, they'll be e.g. backend-a8b3c1d2 and backend-d4e5f6a7 — distinct, addressable, no manual setup.

The SessionStart hook announces the current session's mailbox name in the conversation context on startup. Peers discover each other via claude-mailbox list or the mcp__mailbox__list_mailboxes MCP tool.

What the hooks do

Hook	Command	Effect
SessionStart	claude-mailbox session-announce	Registers the session with the daemon, then prints (a) this session's mailbox name, (b) the exact from / name args to pass to MCP tools, and (c) a list of other mailboxes active in the last hour — so Claude knows who's around without needing to call list_mailboxes first.
UserPromptSubmit	claude-mailbox check --hook	Pulls unread messages for the session's mailbox and injects them as context. Silent on empty inbox; emits a one-line setup hint when the daemon is unreachable.
SubagentStop	claude-mailbox check --hook	Same as UserPromptSubmit, but fires when a subagent finishes (Task tool). Lets the parent see peer messages that arrived during a long-running subagent run, instead of waiting until the next user prompt.

Cost: one local HTTP round-trip per prompt and per subagent stop + Node coldstart (~100ms on Windows).

MCP tools

The plugin ships a .mcp.json that spawns a stdio MCP wrapper (claude-mailbox mcp-stdio) so the daemon URL is configurable per machine via the CLAUDE_MAILBOX_URL env var (Claude Code doesn't yet support env substitution in HTTP MCP URLs — see issue #46889). The wrapper proxies tool calls to the daemon's REST API.

Each MCP tool takes the caller's mailbox name as an explicit argument (from the SessionStart announcement):

Tool	Required args	Purpose
mcp__mailbox__send	from, to, body	Send a message to another mailbox.
mcp__mailbox__check_inbox	name	Pull all undelivered messages for your mailbox (marks delivered).
mcp__mailbox__peek_inbox	name	Non-consuming count of pending messages.
mcp__mailbox__list_mailboxes	name	Discover known mailboxes and pendingForYou counts.

The SessionStart announcement spells out the exact args to pass, so Claude picks them up automatically.

Slash commands

Command	What it does
/claude-mailbox:mailbox-doctor	Diagnose + auto-fix the full setup.
/claude-mailbox:mailbox-status	Read-only health check. No changes.
/claude-mailbox:mailbox-update	Update the daemon to the latest npm version and restart it.

Coordinating two Claude Code sessions

1. Open two Claude Code sessions in the same (or different) project.
2. Each session's SessionStart hook registers itself with the daemon and prints both its own mailbox name and the list of currently active peers into context.
3. In session A you can simply say: "I started a second session, coordinate with it." Because the peer's mailbox name is already in context, Claude can call mcp__mailbox__send(from="<my-name>", to="<peer-name>", body="...") straight away — no manual list_mailboxes step needed.
4. Session B's UserPromptSubmit hook pulls the message on its next prompt and injects it as context.

You can also send from any shell:

claude-mailbox list
claude-mailbox send --from probe --to backend-a8b3c1d2 --body "hi"

Uninstall

/plugin uninstall claude-mailbox@claude-mailbox
npm uninstall -g @kuns/claude-mailbox
claude-mailbox uninstall-autostart    # if you registered it