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 32 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. 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. Attachments can be file paths or inline base64 objects.

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.

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"]
    }
  }
}

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 reads this automatically.

• 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: No remote access. The bridge fails closed -- refuses to forward requests without a valid token.

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 (requires auth token from connection file)
TOKEN=$(cat /tmp/thunderbird-mcp/connection.json | jq -r .token)
PORT=$(cat /tmp/thunderbird-mcp/connection.json | jq -r .port)
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 32 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

• Email bodies with control characters are sanitized to avoid breaking JSON

• 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

License

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