Thunderbird MCP

Give your AI assistant full access to Thunderbird -- search mail, compose messages, manage filters, and organize your inbox. All through the Model Context Protocol.

Inspired by bb1/thunderbird-mcp. Rewritten from scratch with a bundled HTTP server, proper MIME decoding, and UTF-8 handling throughout.

Why?

Thunderbird has no official API for AI tools. Your AI assistant can't read your email, can't help you draft replies, can't organize your inbox. This extension fixes that -- it exposes 36 tools over MCP so any compatible AI (Claude, GPT, local models) can work with your mail the way you'd expect.

Compose tools open a review window before sending by default. Set skipReview to send directly when you've already approved the content upstream. Nothing gets sent without your approval.

How it works

                    stdio              HTTP (localhost:8765-8774)
  MCP Client  <----------->  Bridge  <--------------------->  Thunderbird
  (Claude, etc.)           mcp-bridge.cjs                    Extension + HTTP Server

The Thunderbird extension embeds a local HTTP server with session-scoped auth tokens. The Node.js bridge translates between MCP's stdio protocol and HTTP, discovering the port and token automatically via a connection file. The bridge handles MCP lifecycle methods (initialize, ping) locally, so clients can connect even before Thunderbird is fully loaded.

What you can do

Mail

Compose

All compose tools open a window for you to review and edit before sending by default. Set skipReview: true to send directly when you've already approved the content. Attachments can be file paths or inline base64 objects.

Compose tools validate the from identity strictly -- if the specified sender doesn't match any configured Thunderbird identity, the tool returns an error instead of silently substituting another account.

Filters

Full control over Thunderbird's message filters. Changes persist immediately. Your AI can create sorting rules, adjust priorities, and run them on existing mail.

Contacts

Calendar

Access Control

Account and tool access are configured via the extension settings page (Tools > Add-ons > Thunderbird MCP > Options). Access control is not MCP-exposed -- only the user can change it.

The same settings page has a "Send Safety" section: toggle Block skipReview to reject sendMail / replyToMessage / forwardMessage calls that pass skipReview: true. The review-window path still works, so clients stay usable -- they just can't send silently.

Setup

1. Install the extension

git clone https://github.com/TKasperczyk/thunderbird-mcp.git

Install dist/thunderbird-mcp.xpi in Thunderbird (Tools > Add-ons > Install from File), then restart. A pre-built XPI is included in the repo -- no build step needed.

2. Configure your MCP client

Add to your MCP client config (e.g. ~/.claude.json for Claude Code):

{
  "mcpServers": {
    "thunderbird-mail": {
      "command": "node",
      "args": ["/absolute/path/to/thunderbird-mcp/mcp-bridge.cjs"]
    }
  }
}

Sandbox-aware connection discovery

The bridge re-discovers connection.json on every cache miss. It tries these locations in order:

1. THUNDERBIRD_MCP_CONNECTION_FILE, if set
2. Native temp dir: <os.tmpdir()>/thunderbird-mcp/connection.json
3. macOS fallback: /var/folders/*/*/T/thunderbird-mcp/connection.json owned by the current user
4. Linux Snap: Thunderbird's live TMPDIR from /proc/<pid>/environ, plus the official snap fallback under ~/Downloads/thunderbird.tmp
5. Linux Flatpak / Betterbird Flatpak: $XDG_RUNTIME_DIR/app/*/thunderbird-mcp/connection.json

This covers native installs, the official Thunderbird snap, Thunderbird Flatpak, Thunderbird Beta Flatpak, and Betterbird Flatpak without changing the extension side. If multiple sandbox candidates exist at once, the bridge tries the newest file first. Set THUNDERBIRD_MCP_CONNECTION_FILE to force a single explicit path.

Example override:

{
  "mcpServers": {
    "thunderbird-mail": {
      "command": "node",
      "args": ["/absolute/path/to/thunderbird-mcp/mcp-bridge.cjs"],
      "env": {
        "THUNDERBIRD_MCP_CONNECTION_FILE": "/absolute/path/to/connection.json"
      }
    }
  }
}

That's it. Your AI can now access Thunderbird.

Security

• Auth tokens: The HTTP server requires a session-scoped bearer token. Generated on startup, written to <TmpD>/thunderbird-mcp/connection.json with 0600 permissions. The bridge re-discovers that file automatically across native installs, Snap, Flatpak, Betterbird Flatpak, and macOS temp directories.
• Dynamic port: Tries ports 8765-8774, records the actual port in the connection file. No hardcoded port dependency.
• Account access control: Restrict which email accounts are visible to MCP clients via the settings page. Changes take effect immediately.
• Tool access control: Disable specific tools via the settings page. Disabled tools are hidden from tools/list and blocked at dispatch.
• Localhost only: By default, the server binds to localhost only. The "Listen on all interfaces" option in settings binds to all IPv4 interfaces for WSL, Docker, or remote access. This exposes the MCP server to every device on your local network. Only enable on trusted networks. Auth token is always required.

Troubleshooting

Development

# Build the extension
./scripts/build.sh

# Test via the bridge (handles auth automatically)
echo '{"jsonrpc":"2.0","id":1,"method":"tools/list"}' | node mcp-bridge.cjs

# Test the HTTP API directly.
# On Snap / Flatpak / Betterbird Flatpak / macOS, point CONN_FILE at the
# real file or export THUNDERBIRD_MCP_CONNECTION_FILE first.
CONN_FILE="${THUNDERBIRD_MCP_CONNECTION_FILE:-/tmp/thunderbird-mcp/connection.json}"
TOKEN=$(jq -r .token "$CONN_FILE")
PORT=$(jq -r .port "$CONN_FILE")
curl -X POST http://127.0.0.1:$PORT \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer $TOKEN" \
  -d '{"jsonrpc":"2.0","id":1,"method":"tools/list"}'

After changing extension code: remove from Thunderbird, restart, reinstall the XPI, restart again. Thunderbird caches aggressively.

Project structure

thunderbird-mcp/
├── mcp-bridge.cjs              # stdio <-> HTTP bridge (auth, port discovery)
├── extension/
│   ├── manifest.json
│   ├── background.js           # Extension entry point
│   ├── httpd.sys.mjs           # Embedded HTTP server (Mozilla)
│   ├── options.html            # Settings page UI
│   ├── options.js              # Settings page logic
│   ├── icons/                  # Extension icons
│   └── mcp_server/
│       ├── api.js              # All 36 MCP tools + auth + access control
│       └── schema.json
├── test/                       # Test suite (node:test, zero dependencies)
└── scripts/
    ├── build.sh
    └── install.sh

Known issues

• IMAP folder databases can be stale until you click on them in Thunderbird
• HTML-only emails are converted to plain text (original formatting is lost)
• Recurring calendar event CRUD operates on the series, not individual occurrences
• IMAP folder operations (rename, delete, move) are async -- verify with listFolders after
• Combining tags with move/trash on IMAP may not preserve tags on the moved copy -- use separate calls
• Pre-existing Thunderbird filters with cross-account move/copy targets are not restricted by account access control
• searchBody on IMAP without offline sync only searches headers (Gloda limitation)
• rawSource requires offline message copy for IMAP -- online-only messages will error

License

MIT. The bundled httpd.sys.mjs is from Mozilla and licensed under MPL-2.0.