Commonpost MCP for Thunderbird
by commonpost
README.md
# Commonpost MCP for Thunderbird
[](https://github.com/commonpost/thunderbird-mcp/actions/workflows/ci.yml)
[](https://www.bestpractices.dev/projects/14985)
[](#what-you-can-do)
[](#security)
[](https://www.thunderbird.net/)
[](LICENSE)
Give your AI assistant full access to Thunderbird -- search mail, compose messages, manage filters, and organize your inbox. All through the [Model Context Protocol](https://modelcontextprotocol.io/).
<p align="center">
<img src="docs/demo.gif" alt="Demo of the MCP server for Thunderbird" width="600">
</p>
> **A continuation of [thunderbird-mcp](https://github.com/TKasperczyk/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](#migrating-from-thunderbird-mcp)). 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](https://github.com/bb1/thunderbird-mcp). What is in this release is listed in the [changelog](CHANGELOG.md).
---
## Quick install (about 3 minutes)
1. Install the extension. Download `commonpost-mcp-v<version>.xpi` from the [latest release](https://github.com/commonpost/thunderbird-mcp/releases/latest). In Thunderbird: Tools > Add-ons and Themes > gear menu > Install Add-on From File. Restart Thunderbird.
2. Download `mcp-bridge.cjs` from the **same release** and put it in a folder you will keep, for example `~/commonpost-mcp/`. The bridge is a single file and needs nothing else: the release does not ship a `package.json`. (The bridge carries its own version; it reads no `package.json`.)
3. Check Node.js: `node --version` must show 22 or later.
4. Register the bridge with your MCP client.
- Claude Code, with the plugin (no file to download or to update by hand; see [Claude Code plugin](#claude-code-plugin)):
```
claude plugin marketplace add commonpost/thunderbird-mcp
claude plugin install commonpost-mcp@commonpost
```
- Claude Code, with the file (use the absolute path of the file):
```
claude mcp add commonpost-mail -- node /absolute/path/to/mcp-bridge.cjs
```
- Claude Code on Windows, for example:
```
claude mcp add commonpost-mail -- node C:\Users\<you>\commonpost-mcp\mcp-bridge.cjs
```
- Other clients: add this to the client's MCP config (Claude Desktop, VS Code, Cursor, Codex CLI and Gemini CLI have their own file and format: see [Other MCP clients](#other-mcp-clients)). In JSON, write Windows paths with `/` or double the backslashes (`"C:/Users/<you>/commonpost-mcp/mcp-bridge.cjs"`).
```json
{
"mcpServers": {
"commonpost-mail": {
"command": "node",
"args": ["/absolute/path/to/mcp-bridge.cjs"]
}
}
}
```
5. Test it. Keep Thunderbird open and ask your assistant: "list my Thunderbird accounts".
**The bridge is not updated automatically, and each MCP client has its own.** When one should be replaced, the extension says so in a tool result (your assistant passes it on) and in its options page (section Bridge), which lists the bridges that connected with "Up to date", "Newer version available" (it works; updating is optional) or "Update recommended". For a bridge to update or one the extension refuses, Thunderbird also shows a system notification, once per bridge and per Thunderbird session; the hidden preference `extensions.commonpost-mcp.bridgeUpdateAlert` set to `false` turns it off. Every [release page](https://github.com/commonpost/thunderbird-mcp/releases) starts with what the release brings, whether it changes the bridge, and the steps: download `mcp-bridge.cjs` and put it in place of your copy (its path is in the MCP configuration of your client; in Claude Code: `claude mcp get <server name>`), then reconnect your MCP client (`/mcp` in Claude Code); in Claude Desktop, install the `.mcpb` bundle again; with the [Claude Code plugin](#claude-code-plugin), update the plugin. A release that does not change the bridge ships the same bridge again. The extension's version is shown in Tools > Add-ons and Themes.
Right after installing, open the extension's options page (Tools > Add-ons and Themes > Commonpost MCP for Thunderbird > Options) and choose which accounts and tools your assistant may use. Until you do, every account and tool is visible to MCP clients.
Snap, Flatpak and macOS need no extra step in most cases; see [Sandbox-aware connection discovery](#sandbox-aware-connection-discovery). If something fails, see [Troubleshooting](#troubleshooting).
---
## 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. It stays within Cursor's limit of 40 tools per server.
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.**
---
## 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
| Tool | Description |
|------|-------------|
| `listAccounts` | List all email accounts and their identities |
| `listFolders` | Browse folder tree with message counts -- filter by account or subtree; `savedSearches` lists saved searches instead |
| `searchMessages` | Search headers across accounts. Words must all match (subject, from, to, cc, preview); operators `from:`, `to:`, `cc:`, `subject:`, `participant:` (from/to/cc/bcc; `@domain` matches addresses in that domain only, commas list alternatives: `participant:@acme.com,@acme-group.com`) and quoted phrases can be combined, e.g. `participant:@acme.com subject:"invoice 42"`. `groupBy: "sender" \| "thread"` returns one row per sender or conversation (`latestId` is the newest message that is not a draft); `threadOf: {messageId, folderPath}` returns a whole conversation across folders (Sent included), oldest first (it reads at most 10,000 headers, then sets `incomplete: true`). A `Re:` message sent without threading headers joins by subject the earlier message its sender took part in (`linkedBy: "subject"`); the same mail sent separately to several people and repeated notifications stay separate. Filters pick messages of the conversation without cutting its links. Trash and Junk are skipped unless `includeTrash` or an explicit `folderPath`. Always returns `{ messages, totalMatches, offset, limit, hasMore }` (default 20 rows, max 200); `format: "table"` returns `{ columns, rows }`, and `format: "legacy"` the output of 0.10 and earlier (deprecated), see CHANGELOG. Also `countOnly`, `includeSubfolders`, date range, unread/flagged/tag filters, `searchBody` (Gloda full text: terms under 3 characters are left out and listed in `warning`, English words match other forms, Russian and other languages only the exact word form) and `dedupByMessageId` (copies in several folders collapse into one row with `dupLocations`). |
| `getMessage` | Read full email content -- `bodyFormat`: `markdown` (default), `text`, or `html`. Bodies are capped (`maxBodyChars`, default 20000); a longer body sets `bodyTruncated` and `nextBodyOffset`, read the rest with `bodyOffset`. Set `rawSource: true` for the complete RFC 2822 source (same cap), decoded as UTF-8 when valid, else by its top-level, part or detected charset (`rawCharset`; a `warning` when parts declare different charsets, listed in `rawMixedCharsets`, or when invalid UTF-8 was replaced), with hidden characters removed as in a body; `rawEncoding: "base64"` returns the exact bytes. Optional attachment saving. Set `includeInlineImages: true` to append supported inline CID images as MCP image blocks (PNG, JPEG, GIF, or WebP; max 1 MiB base64 per image and 4 MiB total). Skipped images are reported in attachment metadata. |
| `getMessages` | Read full email content for up to the configured batch limit in one call (default 10, max 20). Uses the same `bodyFormat`, `rawSource`, and attachment options as `getMessage`; each item supplies `messageId` and `folderPath`. Bodies are capped per message (`maxBodyChars`, default 4000). |
| `getRecentMessages` | Recent messages newest first from all folders (Trash/Junk skipped) or one folder -- `daysBack` (default 7), unread/flagged filters, `includeTrash`, pagination. Same envelope, rows and `format` as `searchMessages`. |
| `displayMessage` | Open a message in Thunderbird's GUI -- `3pane` (default), `tab`, or `window` mode |
| `updateMessage` | Mark read/unread, flag/unflag, add/remove tags, move between folders, or trash -- supports bulk via `messageIds` |
| `deleteMessages` | Delete messages -- drafts are safely moved to Trash |
| `createFolder` | Create new subfolders to organize your mail, or a saved search (virtual folder: a live view, nothing moved) with `savedSearch` |
| `renameFolder` | Rename an existing mail folder; returns its new URI as `path` |
| `deleteFolder` | Delete a folder (moves to Trash, or permanently deletes if already in Trash); a saved search is removed as a view, no mail deleted |
| `moveFolder` | Move a folder to a new parent within the same account |
| `emptyTrash` | Permanently delete all messages in Trash (including subfolders) |
| `emptyJunk` | Permanently delete all messages in Junk/Spam (including subfolders) |
### Compose
| Tool | Description |
|------|-------------|
| `sendMail` | Compose a new email -- opens a review window; direct sending requires explicitly disabling the `skipReview` safety block, and adds the identity signature as the compose window would |
| `replyToMessage` | Reply with quoted original and proper threading, from the identity Thunderbird would pick. `mode`: `window` (default) opens a review window, `draft` saves a draft with the recipients Thunderbird's Reply / Reply All computes and the body its reply window builds (cite line, quote, signature, reply position), `send` (or `skipReview: true`) sends directly -- it needs explicit `to` and `from` and is subject to the same safety block. `latestInThread: true` (window or draft only) replies to the newest message of the conversation, Sent included, and returns it as `repliedTo`; `threadIncomplete` in the result means a large mailbox was read in part and a newer message may exist |
| `forwardMessage` | Forward with all original attachments preserved, from the identity Thunderbird would pick. `mode` as for `replyToMessage` (`draft` and `send` get the subject and body Thunderbird's forward window builds: header table, original body, signature); `to` is optional except with `send`, which needs explicit `to` and `from` |
| `saveDraft` | Save a new message as a draft, as Thunderbird's compose window saves it (with the identity signature), or edit an existing draft with `draftId` -- returns its `messageId` and `folderPath` |
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` (and `mode: "send"`) is rejected until you explicitly disable the preference; only then can it send directly. `mode: "draft"` only saves a draft, so the preference does not block it (even next to `skipReview: true`); it needs the `saveDraft` tool to be enabled in the settings, as a tool you disabled is not reachable through another one. Attachments can be file paths or inline base64 objects.
A direct send or saving a draft (`saveDraft`, `mode: "draft"`) can take a while: the bridge waits up to 180 s for Thunderbird (30 s for every other call). Your MCP client may also have its own time limit for a tool call; if it is shorter than 180 s, the client can give up while Thunderbird goes on sending or saving, so check the Sent folder and the Outbox (or the Drafts folder) before retrying to avoid sending the message, or saving the draft, twice.
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
| Tool | Description |
|------|-------------|
| `listFilters` | List all filter rules with human-readable conditions and actions; with `confirmation: true`, read the state of a pending or recent confirmation instead, optionally one by `confirmationId` (see below) |
| `createFilter` | Create filters with structured conditions (from, subject, date...) and actions (move, tag, flag...) |
| `updateFilter` | Modify a filter's name, enabled state, conditions, or actions |
| `deleteFilter` | Remove a filter by index |
| `reorderFilters` | Change filter execution priority |
| `applyFilters` | Run filters on a folder on demand -- let your AI organize your inbox |
Full control over Thunderbird's message filters. Changes persist immediately. Your AI can create sorting rules, adjust priorities, and run them on existing mail. `applyFilters` runs only the enabled rules marked "Manually Run" (tick it in the filter editor on the rules the assistant may launch) and lists the ones it skipped.
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. If you choose **Ask me each time** instead, a request of that kind returns `pending_user_confirmation` and Thunderbird shows a dialog with the rule, its conditions and its full destination; nothing is written unless you confirm it there, and closing the dialog or not answering within ten minutes writes nothing.
### Encrypted messages
OpenPGP and S/MIME messages are not decrypted for the assistant by default: `getMessage` returns their headers and a short notice instead of their content. `searchMessages` leaves out a message its query matched, `getRecentMessages` lists one with the wire-level subject, no preview and `encrypted: true`, and `threadOf` leaves them out and never links a conversation through their subject. The setting **Let the assistant read encrypted messages** (Options > Encrypted messages) turns this on; the decrypted content is then handed to the assistant, and so to the service that runs it.
### Contacts
| Tool | Description |
|------|-------------|
| `searchContacts` | Search contacts across all address books by email, name, or organization (e.g. `acme` or `@acme.com`); empty fields are omitted. Supports `maxResults` and `format: "table"`. |
| `getContact` | Read full contact details by UID |
| `createContact` | Create a contact with optional email/name, phones, postal addresses, organization, title, note, and birthday. Phone-only contacts are supported. |
| `updateContact` | Update contact fields; omitted fields stay unchanged, while empty phone/address arrays clear those collections |
| `deleteContact` | Delete a contact by UID |
### Calendar
| Tool | Description |
|------|-------------|
| `listCalendars` | List all calendars with read-only, disabled, event, and task support flags |
| `createEvent` | Create a calendar event -- opens a review dialog; direct creation via `skipReview` requires explicitly disabling the default safety block. Accepts `status: tentative \| confirmed \| cancelled` (VEVENT STATUS per iCal RFC 5545). |
| `listEvents` | Query events by date range with recurring event expansion; a date without a time is a whole day in local time. Returns `status` on each event. Supports `maxResults` and `format: "table"`. |
| `updateEvent` | Modify an event's title, dates, location, description, or `status` |
| `deleteEvent` | Delete a calendar event by ID |
| `createTask` | Open a pre-filled task dialog for review; direct creation via `skipReview` requires explicitly disabling the default safety block |
| `listTasks` | List tasks/to-dos from calendars -- filter by completion status, due date, or calendar. Supports `maxResults` and `format: "table"`. |
| `updateTask` | Update a task's title, due date, description, priority, completion status, or percent complete |
| `listCategories` | List the calendar category names defined in Thunderbird, to use exactly in events and tasks |
### Access Control
| Tool | Description |
|------|-------------|
| `getAccountAccess` | View which accounts the MCP server can access |
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` (and `mode: "send"` for `replyToMessage` and `forwardMessage`); 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](https://github.com/commonpost/thunderbird-mcp/releases/latest), 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 140 or later, and Node.js 22 or later for the bridge (`mcp-bridge.cjs`). Tested on Thunderbird 140.16.0esr, 153.3.1esr and 156.0.1.
**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. **The bridge is not updated automatically:** the add-on tells you (in a tool result and in its options page, section Bridge) when to replace your `mcp-bridge.cjs` or `.mcpb` with the one of a newer release; then reconnect your MCP client. Some protections live in the bridge.
### 2. Configure your MCP client
Add to your MCP client config (e.g. `~/.claude.json` for Claude Code):
```json
{
"mcpServers": {
"commonpost-mail": {
"command": "node",
"args": ["/absolute/path/to/commonpost-mcp/mcp-bridge.cjs"]
}
}
}
```
### Claude Code plugin
This repository is also a [Claude Code plugin marketplace](https://code.claude.com/docs/en/plugins): the plugin
`commonpost-mcp` declares the `commonpost-mail` MCP server and carries the bridge of the release, so there is no
`mcp-bridge.cjs` to download or to replace by hand.
```
claude plugin marketplace add commonpost/thunderbird-mcp
claude plugin install commonpost-mcp@commonpost
```
- The plugin (`plugins/claude-code/`) contains **only the bridge**, a copy of this repository's `mcp-bridge.cjs` that
the version check keeps identical; install the Thunderbird extension separately (step 1 above). Node.js 22 or later
must be on the `PATH`.
- Updates: Claude Code does not update a third-party marketplace on its own unless you turn it on (`/plugin` >
Marketplaces > Enable auto-update). Otherwise run `/plugin marketplace update commonpost` (or `claude plugin update
commonpost-mcp@commonpost`), then `/reload-plugins`. Each release raises the plugin's version, so an update follows
every release, whether or not the bridge changed; the bridge in the plugin is always the one of the release.
- Tool names carry the plugin and server names: `mcp__plugin_commonpost-mcp_commonpost-mail__searchMessages`, for
example. Permission rules written for a server added by file (`mcp__commonpost-mail__*`) do not match them. The
options page of the add-on (section Bridge) lists this bridge as a `mcp-bridge.cjs` file.
- The plugin is fetched from this repository with git (`main`, or a release tag you pin with `claude plugin marketplace
add commonpost/thunderbird-mcp#vX.Y.Z`, from v0.17.0 on); it is not signed. The bridge inside it is the
`mcp-bridge.cjs` of that commit, the same file every release attests.
### Claude Desktop (one-click bundle)
On macOS and Windows, Claude Desktop can install the bridge from a `.mcpb` bundle instead of a hand-edited config file. Download `commonpost-mcp-v<version>.mcpb` from the [latest release](https://github.com/commonpost/thunderbird-mcp/releases/latest) and open it with Claude Desktop, then confirm the installation. If the file does not open in Claude Desktop (on Windows it may not be associated with it), use Settings > Extensions > Advanced settings > Install Extension and choose the file.
- The bundle contains **only the bridge** (`mcp-bridge.cjs`). Install the Thunderbird extension separately (step 1 above). The bundle's version is the version of the bridge inside, which changes only when the bridge changes: a release can carry a bundle with a lower number than the add-on, and it is still the current bridge. Claude Desktop never updates a bundle installed from a file: install the new `.mcpb` when the add-on (in a tool result or in its options page, section Bridge) says so.
- The bundle asks for Node.js 22 or later, like the bridge. It has no settings: if the bridge cannot find the connection file, use the manual configuration in [Other MCP clients](#other-mcp-clients) with `COMMONPOST_MCP_CONNECTION_FILE`.
- The bundle is not signed. Each release carries a provenance attestation for it (`*.mcpb.sigstore.json`): check it with `gh attestation verify <file>.mcpb --repo commonpost/thunderbird-mcp`. It can be rebuilt byte for byte from the tagged source with `node scripts/build-mcpb-reproducible.cjs`.
### Other MCP clients
The bridge is a standard MCP stdio server and is not tied to any client: it works with any client that can launch a local server. The `mcpServers` layout above is common but not universal, so each client below has its own file and format. In every case use the absolute path of `mcp-bridge.cjs` and the server name `commonpost-mail`, then restart the client.
**Claude Desktop.** Edit `claude_desktop_config.json` (Settings > Developer > Edit Config opens it): `~/Library/Application Support/Claude/claude_desktop_config.json` on macOS, `%APPDATA%\Claude\claude_desktop_config.json` on Windows. In JSON, double the backslashes of Windows paths.
```json
{
"mcpServers": {
"commonpost-mail": {
"command": "node",
"args": ["C:\\Users\\<you>\\commonpost-mcp\\mcp-bridge.cjs"]
}
}
}
```
**Visual Studio Code.** Edit `.vscode/mcp.json` in a workspace, or run "MCP: Open User Configuration" for your user profile. The top-level key is `servers` (not `mcpServers`), and a stdio server needs `"type": "stdio"`.
```json
{
"servers": {
"commonpost-mail": {
"type": "stdio",
"command": "node",
"args": ["/absolute/path/to/mcp-bridge.cjs"]
}
}
}
```
**Cursor.** Edit `~/.cursor/mcp.json` (all projects) or `.cursor/mcp.json` (one project). The top-level key is `mcpServers`.
```json
{
"mcpServers": {
"commonpost-mail": {
"command": "node",
"args": ["/absolute/path/to/mcp-bridge.cjs"]
}
}
}
```
**OpenAI Codex CLI.** Edit `~/.codex/config.toml` (or `.codex/config.toml` in a trusted project). This is TOML, not JSON: one table per server.
```toml
[mcp_servers.commonpost-mail]
command = "node"
args = ["/absolute/path/to/mcp-bridge.cjs"]
```
**Gemini CLI.** Edit `~/.gemini/settings.json` (or `.gemini/settings.json` in a project). The top-level key is `mcpServers`. Its `env` block expands `$VAR`, `${VAR}` and, on Windows, `%VAR%` references, so a value containing `$` or `%` would be altered.
```json
{
"mcpServers": {
"commonpost-mail": {
"command": "node",
"args": ["/absolute/path/to/mcp-bridge.cjs"]
}
}
}
```
The bridge normally finds Thunderbird's connection file by itself, so no environment variable is needed. Only if it cannot, set `COMMONPOST_MCP_CONNECTION_FILE` to the file's real path in the server's environment: an `"env": { ... }` object next to `args` in the JSON formats, a `[mcp_servers.commonpost-mail.env]` table for Codex (see [Sandbox-aware connection discovery](#sandbox-aware-connection-discovery); on Windows the file must be under the bridge's own `%TEMP%`). A path set there replaces the automatic discovery, so never leave a placeholder in it. Client formats change; if one of these no longer matches, follow the client's own MCP documentation and keep the same command and arguments.
### Sandbox-aware connection discovery
The bridge re-discovers `connection.json` on every cache miss. It tries these locations in order:
1. `COMMONPOST_MCP_CONNECTION_FILE`, if set
2. Native temp dir: `<os.tmpdir()>/commonpost-mcp/connection.json`
3. macOS fallback: `/var/folders/*/*/T/commonpost-mcp/connection.json` owned by the current user
4. Linux Snap: only from a process that is really the confined Thunderbird snap (`/proc/<pid>/exe` resolves under `/snap/thunderbird/` and `SNAP_NAME=thunderbird` is in its environment), read from its live `TMPDIR`; the official snap's own `~/Downloads/thunderbird.tmp` fallback is tried only once such a process was actually seen
5. Linux Flatpak / Betterbird Flatpak: the real `/run/user/<uid>` (verified by `lstat`, never trusted from `$XDG_RUNTIME_DIR`), under a closed list of app ids (`org.mozilla.thunderbird`, `org.mozilla.thunderbird_esr`, `org.mozilla.Thunderbird`, `eu.betterbird.Betterbird`)
This covers native installs, the official Thunderbird snap, Thunderbird Flatpak (including its beta channel, which shares the same app id installed from a different remote), 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:
```json
{
"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. The options page also offers a stable token that stays the same across restarts, stored in a Thunderbird preference instead of the connection file; removing the add-on while it is still enabled clears that preference, but removing an add-on that was disabled first does not, so clear it yourself (uncheck "Use stable token" in the options page) before removing a disabled add-on.
- **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.
- **Bridge version**: the bridge announces its version on every request (`X-Commonpost-Bridge` header). The add-on warns about bridges older than it recommends and can refuse bridges below a security floor written in its code (shipped disarmed); nothing is checked or downloaded over the Internet. The header is self-declared: the token, not the header, protects the server. See [SECURITY.md](SECURITY.md).
### Attachments refused
A `sendMail`, `saveDraft`, `replyToMessage` or `forwardMessage` call whose attachment path matches any of the
following is refused, and the whole call fails (nothing is sent, saved, or opened) rather than silently dropping
just that attachment:
- a dotfile or dot-directory anywhere in the path (`.ssh`, `.aws`, `.config/gcloud`, `.git-credentials`, ...);
- a filename that holds credentials or secrets on its own, wherever it is found (`credentials.json`, `*.ppk`,
`*.jks`, `*.pem`, `*.p12`, `*.kdbx`, `wallet.dat`, `terraform.tfstate*`, `Local State`, browser cookie/login
stores, ...);
- system directories (`/etc`, `/proc`, `C:\Windows\System32`, ...) and each mail client's own profile
(Thunderbird's, other mail clients' saved credentials);
- the folders of every Thunderbird profile and the local folder of every mail account, **wherever they are** (another
drive, a portable install, a folder you chose): the add-on lists them in the connection file, and the bridge
refuses anything under them by path, by real path and by folder identity, so a link to them or a second mount of
the same folder is refused too. This needs the add-on and the bridge of 0.17.1 or later. Not covered: the account
folders of profiles other than the running one, a mail store at the root of a drive, a folder the bridge sees
under another path (a bridge under WSL or in a container), and copies made outside these folders;
- on macOS, anything under the signed-in user's `Library` folder (Mail, Messages, Keychains, Application Support,
...), including its APFS Data-volume real-path form;
- on Windows, AppData and its compatibility-junction aliases (`Local Settings`, `Application Data`, `Cookies`,
`Recent`, `SendTo`, ...) -- with one exception: the extension's own `<TEMP>\commonpost-mcp\` backup folder, so a
previously-saved attachment there can still be attached; `connection.json` itself is refused even there;
- a UNC or device path (`\\server\share\...`, `\\?\...`), an alternate data stream, or a Windows form (8.3 short
name, trailing dot or space) that resolves to a different name than the one given.
The bridge additionally resolves the attachment's **real path** (through symlinks and, on Windows, junctions and
reparse points) and checks that too, so a denied file reached under an innocuous-looking name is still refused.
The extension cannot do this on Windows (Gecko's `nsLocalFile::IsSymlink` has no implementation there) and instead
refuses every string file path attachment on Windows outright; attach files through the bridge there.
---
## If Thunderbird disables Experiment add-ons on the Release channel
This add-on uses a Thunderbird Experiment API (see [docs/experiment-inventory.md](docs/experiment-inventory.md)).
Thunderbird's Add-ons team has announced plans to stop running Experiment-API add-ons on the Release channel, and to
keep supporting them on the ESR (Extended Support Release) channel. As of September 2026 this had not happened yet.
If that happens on your Thunderbird and this add-on stops working: switch to Thunderbird ESR. Downloads for every
platform are on the [official Thunderbird site](https://www.thunderbird.net/en-US/thunderbird/all/) (look for the
"ESR" builds). This continuation has supported Thunderbird ESR since 0.8.2 (minimum 140.0; see the badge above and
[CHANGELOG.md](CHANGELOG.md)).
---
## Troubleshooting
| Problem | Fix |
|---------|-----|
| Extension not loading | Check Tools > Add-ons and Themes. Errors: Tools > Developer Tools > Error Console |
| Connection refused | Make sure Thunderbird is running and the extension is enabled |
| Bridge can't find `connection.json` | Set `COMMONPOST_MCP_CONNECTION_FILE` explicitly if your environment uses a non-standard temp/runtime path |
| On Windows, bridge finds the file but still refuses it | The file must be under the *bridge process's own* `%TEMP%`. Running the bridge under WSL or in a container gives it a different `%TEMP%` than the one Thunderbird (native Windows) used, even if the file is reachable through `\\wsl.localhost\...` or a mount -- this is a correct refusal, not a bug. Set `COMMONPOST_MCP_CONNECTION_FILE` to the file's real path instead |
| Bridge in a container or under WSL, which used to find the connection file through a mounted temp folder, is now refused (stale or foreign process) | The bridge no longer trusts a file it cannot tie to a running Thunderbird. Set `COMMONPOST_MCP_CONNECTION_FILE` to the file's real path |
| On Windows, an attachment under `%TEMP%` is refused | `%TEMP%` sits under `AppData\Local`, which the deny-list treats like the rest of AppData (it holds credentials and app data for other programs on Windows). Copy the file to another folder first, for example Documents |
| Missing recent emails | IMAP folders can be stale. Click the folder in Thunderbird to sync, or right-click > Properties > Repair Folder |
| Tool not found after update | Reconnect MCP (`/mcp` in Claude Code) to pick up new tools |
| `searchBody` returns no results | IMAP accounts need offline sync enabled for Gloda to index message bodies |
| `rawSource` fails on IMAP | Requires local/offline message copy. Enable offline sync or click the message first to cache it. |
---
## Development
```bash
# 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"}'
```
A program that calls the HTTP API directly, without the bridge, sends no `X-Commonpost-Bridge` header and is treated like a bridge 0.11 or older: tool results can carry a notice to update the bridge, and `replyToMessage` / `forwardMessage` with `mode: "send"` or `"draft"` are refused (an old bridge stops waiting after 30 s). A direct client that waits at least 180 s for those calls can declare itself with `-H "X-Commonpost-Bridge: 0.12.0; packaging=file"`. If a release arms the bridge security floor, a client without the header is refused entirely.
**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](CONTRIBUTING.md). Everything that needs Thunderbird's full privileges is listed in [docs/experiment-inventory.md](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](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](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).
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