ClaudeMailbox

A standalone MCP mail server that lets parallel Claude sessions coordinate with each other. Any Claude session (plain terminal, ClaudeDo worktree, anything that consumes .mcp.json) can send messages to a peer session's inbox, check for pending messages, and discover other active mailboxes.

Not a substitute for run_in_background: true — that handles single-session responsiveness. This handles session-to-session coordination.

Architecture

One long-running daemon binds HTTP on loopback, hosts the MCP server at /mcp and a small REST API at /v1/*, and persists state in a single SQLite file. Sessions declare themselves via an X-Mailbox header in their .mcp.json.

 session-backend                 session-frontend           external sender
 (X-Mailbox: backend)            (X-Mailbox: frontend)      (CLI / UI / hook)
        |                                |                          |
        |  HTTP                          |                          |
        +--------------+-----------------+--------------------------+
                       v
         claude-mailbox serve  (ASP.NET Core + Kestrel)
           /mcp      MCP tools
           /v1/*     REST for non-MCP senders
           /health
                       v
         ~/.claude-mailbox/mailbox.db  (SQLite WAL)

Install

dotnet publish -c Release -r win-x64 --self-contained -p:PublishSingleFile=true

Put the resulting claude-mailbox.exe on your PATH.

Daemon lifecycle

The daemon is a normal console process. Pick whichever level of automation you want:

1. Manual. Open a terminal, run claude-mailbox serve, leave it open. Stops when you close the window.
2. Startup shortcut. Drop a shortcut to claude-mailbox serve in shell:startup — starts on login.
3. Windows Service. sc.exe create ClaudeMailbox binPath= "C:\path\to\claude-mailbox.exe serve" start= auto — same pattern ClaudeDo uses.

Defaults: port 47822, bind 127.0.0.1, database at %USERPROFILE%\.claude-mailbox\mailbox.db. All overridable:

claude-mailbox serve [--port 47822] [--bind 127.0.0.1] [--db-path <path>]

Use from a Claude session

Drop this into your project's .mcp.json (one per session, different X-Mailbox values):

{
  "mcpServers": {
    "mailbox": {
      "type": "http",
      "url": "http://127.0.0.1:47822/mcp",
      "headers": {
        "X-Mailbox": "backend"
      }
    }
  }
}

Four MCP tools are exposed:

Tool	Purpose
mcp__mailbox__send(to, body)	Send a message to another mailbox
mcp__mailbox__check_inbox()	Pull all pending messages for this mailbox (marks delivered)
mcp__mailbox__peek_inbox()	Non-consuming check — returns { pending, oldestAt }
mcp__mailbox__list_mailboxes()	Discover known mailboxes and who has mail for you

Suggested CLAUDE.md snippet for poll discipline

When coordinating with a peer session, call mcp__mailbox__peek_inbox
after each subagent completes. If pending > 0, call mcp__mailbox__check_inbox
and treat the messages as input with priority over the current plan.

CLI client mode

Any external process (scripts, UIs, hooks) can talk to a running daemon without needing MCP:

claude-mailbox send  --to <mailbox> --from <mailbox> --body <text> [--url http://127.0.0.1:47822]
claude-mailbox peek  --name <mailbox>                              [--url ...]
claude-mailbox check --name <mailbox>                              [--url ...]
claude-mailbox list                                                [--url ...]

The CLI subcommands are thin HTTP clients against the /v1/* endpoints.

REST surface

Method	Path	Requires X-Mailbox	Purpose
GET	/health	no	{ status, version, dbPath }
POST	/v1/send	yes (sender)	{ to, body }
GET	/v1/peek?name=<mailbox>	no	read-only status
POST	/v1/check-inbox?name=<mailbox>	yes (must match name)	consume inbox
GET	/v1/list	no	list all mailboxes

Development

dotnet build
dotnet test tests/ClaudeMailbox.Tests/ClaudeMailbox.Tests.csproj
dotnet run --project src/ClaudeMailbox -- serve

Test suite covers end-to-end coordination, concurrent check_inbox race safety, and schema idempotency.

Scope

• Loopback bind only (v1). Cross-machine coordination is a future extension — swap the middleware for token auth and change the bind address.
• No auth on loopback. Local filesystem permissions are the trust boundary.
• No message expiry or cleanup. Delivered messages stay as a timeline/audit log.