# 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

```powershell
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):

```json
{
  "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.