Commonpost MCP for Thunderbird

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

A continuation of thunderbird-mcp by Tomasz Kasperczyk (MIT), started from its 0.7.5 release with its full history kept. It is an independent project by the Commonpost community, not the original and not endorsed by it; it can be installed beside the original (see Migrating). If the original author wants any of this work back, we hand it over: our changes are MIT-licensed and offered upstream too. Thunderbird is a trademark of the Mozilla Foundation; this project is not affiliated with Mozilla or MZLA.

The original was inspired by bb1/thunderbird-mcp. What is in this release is listed in the changelog.

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 40 tools over MCP so any compatible AI (Claude, GPT, local models) can work with your mail the way you'd expect.

Mail sends and event/task creation require review by default because Block skipReview starts enabled. skipReview: true is honored only after you explicitly disable that safety setting. By default, nothing is sent or created without your review.

Related MCP server: Thunderbird MCP Server

How it works

                    stdio              HTTP (localhost:8780-8789)
  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. The Block skipReview preference is on by default, so skipReview: true is rejected until you explicitly disable the preference; only then can it send directly. 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.

Filter rules that forward or reply send mail without the review window that the compose tools keep. The Block filter forward/reply setting (Options > Send Safety) is on by default: while it is on, such rules cannot be created or changed through MCP, and a filter list that holds one cannot be run through applyFilters. Deleting the rule stays possible.

Contacts

Calendar

Access Control

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

The same settings page has a "Send Safety" section. Block skipReview is enabled by default and rejects skipReview: true for sendMail, replyToMessage, forwardMessage, createEvent, and createTask; their review window or dialog still opens normally. skipReview is honored only after you explicitly disable this preference.

Setup

1. Install the extension

Download commonpost-mcp-v<version>.xpi from the latest release, then in Thunderbird: Tools > Add-ons and Themes > gear menu > Install Add-on From File, and restart. Each release also carries a provenance attestation (*.sigstore.json) that ties the XPI to the commit and the workflow that built it; you can check it with gh attestation verify <file>.xpi --repo commonpost/thunderbird-mcp. The XPI can be rebuilt byte for byte from the tagged source with node scripts/build-xpi-reproducible.cjs.

The MCP bridge, mcp-bridge.cjs, is attached to each release next to the XPI (with its own provenance attestation). You can also git clone --branch v<version> https://github.com/commonpost/thunderbird-mcp.git and use the file from the clone; use the bridge of the same version as the extension.

Requires Thunderbird 156 or later, and Node.js 22 or later for the bridge (mcp-bridge.cjs): these are the versions the project is tested on.

Automatic updates: the add-on checks https://commonpost.github.io/thunderbird-mcp/updates.json through Thunderbird's add-on update check; the file lists the hash of each release. Thunderbird downloads updates in the background and applies them on the next restart; because this add-on uses an Experiment API, updates are not hot-swapped. If updates do not arrive, check the Add-ons gear menu and make sure Update Add-ons Automatically is enabled. Thunderbird's default xpinstall.signatures.required=false lets unsigned add-ons install; a profile hardened to require signatures blocks both manual and automatic installs. Because the auto-update channel is a code-delivery channel, you can turn it off (per add-on, in its details page) and update by hand.

2. Configure your MCP client

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

{
  "mcpServers": {
    "commonpost-mail": {
      "command": "node",
      "args": ["/absolute/path/to/commonpost-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. COMMONPOST_MCP_CONNECTION_FILE, if set

2. Native temp dir: <os.tmpdir()>/commonpost-mcp/connection.json

3. macOS fallback: /var/folders/*/*/T/commonpost-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/*/commonpost-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 COMMONPOST_MCP_CONNECTION_FILE to force a single explicit path.

Example override:

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

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

Migrating from thunderbird-mcp

Both add-ons can be installed at the same time: they use different ids, preferences, ports (8780-8789 here, 8765-8774 there) and temporary directories. Nothing is shared or migrated automatically:

• settings (extensions.thunderbird-mcp.*) are not copied; set them again in this add-on's options page. Until you do, every account and tool is visible to MCP clients: your account and tool restrictions are not carried over;

• your MCP client must point its bridge at this repository's mcp-bridge.cjs (or the commonpost-mcp binary) and, if you set it, use COMMONPOST_MCP_CONNECTION_FILE (the old THUNDERBIRD_MCP_CONNECTION_FILE is ignored on purpose, so that one bridge never reads the other add-on's file);

• if you keep both, the options page shows a notice, and each MCP client must be configured for the one you want. To avoid two servers touching the same mailbox, disable the one you do not use.

Security

• Auth tokens: The HTTP server requires a session-scoped bearer token. Generated on startup, written to <TmpD>/commonpost-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 8780-8789, 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.

• Auto-update integrity: Auto-update is a code-delivery channel whose integrity depends on continued control of the GitHub repository, the GitHub Actions token, and the commonpost GitHub organization.

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 COMMONPOST_MCP_CONNECTION_FILE first.
CONN_FILE="${COMMONPOST_MCP_CONNECTION_FILE:-/tmp/commonpost-mcp/connection.json}"
TOKEN=$(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"}'

Dev-only extension reload: After changing extension source locally, remove the add-on from Thunderbird, restart, reinstall the XPI, and restart again. Thunderbird caches aggressively.

Run the tests with npm ci --ignore-scripts && npm test; see CONTRIBUTING.md. Everything that needs Thunderbird's full privileges is listed in docs/experiment-inventory.md.

Project structure

commonpost-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 40 MCP tools + auth + access control
│       └── schema.json
├── test/                       # Test suite (node:test; fast-check for the property tests)
├── docs/                       # Experiment inventory, filter API notes
└── scripts/
    ├── build-xpi-reproducible.cjs   # release build (byte-for-byte reproducible)
    ├── build.sh                # quick development build
    └── 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 and credits

MIT (see LICENSE): copyright Tomasz Kasperczyk (original project) and the Commonpost contributors. extension/httpd.sys.mjs is Mozilla's embedded HTTP server under the MPL-2.0 (see THIRD-PARTY.md). Fixes and ideas taken from upstream pull requests keep their authors: Tony (TKasperczyk/thunderbird-mcp#209), safrano9999 (TKasperczyk/thunderbird-mcp#214), Daniel Glaser (the78mole; TKasperczyk/thunderbird-mcp#195, since superseded by TKasperczyk/thunderbird-mcp#222), Neel Radhakrishnan (TKasperczyk/thunderbird-mcp#175), JordanRO2 (TKasperczyk/thunderbird-mcp#126, and the idea and preference name of the filter send guard, TKasperczyk/thunderbird-mcp#127), KinJLy and coulof (forks).