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

Compose 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. Automatic Forward/Reply filter actions have a separate opt-in, disabled by default.

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

| Tool | Description | |------|-------------| | listAccounts | List all email accounts and their identities | | listFolders | Browse folder tree with message counts -- filter by account or subtree | | searchMessages | Search by subject, sender, recipient, body preview, date range, or tags. Multi-word queries are AND-of-tokens (every word must appear somewhere). Prefix with from:, subject:, to:, or cc: to restrict to one field. Set searchBody: true for full-text body search via Thunderbird's Gloda index. Supports includeSubfolders, countOnly, and offset-based pagination. Results include threadId and preview snippet. By default, dedupByMessageId collapses the same RFC Message-ID found in multiple folders/labels into one row and reports the other folder paths in dupLocations; set dedupByMessageId: false to return every location. | | getMessage | Read full email content -- bodyFormat: markdown (default), text, or html. Set rawSource: true for the complete RFC 2822 source (all headers + MIME parts). 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. | | getRecentMessages | Get recent messages with date, unread, and tag filtering. Supports pagination. Results include threadId and preview. | | 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 | | renameFolder | Rename an existing mail folder | | deleteFolder | Delete a folder (moves to Trash, or permanently deletes if already in Trash) | | 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) |

Message body formats (getMessage and getMessages):

• markdown (default) converts HTML to visible structure, allowing only http:, https:, and mailto: link destinations. Other HTML links become their visible text. HTML images become alt text (or nothing); their source URLs are omitted. Text nodes and alt text escape backslashes, backticks, brackets, angle brackets, and ! before [ to prevent literal links, images, or raw HTML. A literal ! immediately before a generated link is also escaped, including across element boundaries. Other punctuation (including underscores, asterisks, hashes, pipes, and tildes) is preserved; HTML code spans and blocks use code formatting.
• For plain-text MIME bodies, markdown escapes Markdown image openers (![) while preserving the rest of the text, apart from the existing invisible-character removal. This also applies to coerced text and raw-MIME recovery. Other Markdown, links, and literal HTML in plain-text bodies are not sanitized; clients must disable raw HTML and remote-content loading when rendering untrusted message text.
• text extracts visible text. Both text formats remove hidden HTML content and invisible control characters.
• For text and markdown, HTML input over 2 MiB (UTF-8) is truncated at a Unicode character boundary before being parsed with the same HTML privacy rules. The output ends with [Message body truncated at 2 MiB]. Content after the cut is omitted; retained content keeps the normal formatting and link rules. Oversized compose fragments use the same capped parsing path. Encryption checks inspect the full source: oversized HTML containing -----BEGIN PGP MESSAGE----- anywhere is withheld unless encrypted-message access is enabled, even when the marker is only quoted in prose.
• html returns the original HTML unchanged. It is untrusted and may contain hidden content, scripts, unsafe links, and remote images; clients must sanitize it before rendering. rawSource: true also remains unchanged.

Compose

| Tool | Description | |------|-------------| | sendMail | Compose a new email -- opens a review window; direct sending requires explicitly disabling the skipReview safety block | | replyToMessage | Reply with quoted original and proper threading -- skipReview is subject to the same safety block | | forwardMessage | Forward with all original attachments preserved -- skipReview is subject to the same safety block |

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

| Tool | Description | |------|-------------| | listFilters | List all filter rules with human-readable conditions and actions | | 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 | Start eligible enabled Manual filters on a folder; report submitted rules and skipped rules with reasons |

Your AI can create sorting rules, adjust priorities, and run them on existing mail. Changes persist after validation; all updates, including name or enabled-state edits, validate a complete candidate before replacing the existing rule. Unparseable rules cannot be updated, but can still be deleted. Filter names, conditions, and action text reject control characters (U+0000–U+001F and U+007F) and backslashes. Custom add-on actions cannot be created, preserved by an update, or submitted for execution.

Allow automatic Forward/Reply filter actions is off by default in the extension settings. While off, createFilter and updateFilter reject any resulting rule containing Forward or Reply, including disabled rules or edits that retain an existing sending action. An update that only sets enabled: false can still disable a sending rule, and deletion remains available. Enabling this setting permits automatic sends without a compose review window, independently of Block skipReview. Turning it off does not disable saved filters or stop Thunderbird's own automatic filtering.

The sending-action setting governs rules MCP creates, modifies, or runs manually. Reordering or deleting a rule containing StopExecution can change which of your existing rules Thunderbird runs automatically, including Forward/Reply rules.

Move/Copy destinations must remain accessible under the current account restrictions, including actions retained by an update. An update that only sets enabled: false, or deletion, remains available for rules with inaccessible destinations.

applyFilters submits only enabled, parseable rules with the Manual type flag and skips rules with inaccessible Move/Copy destinations, as well as Forward/Reply rules while the setting is off. It returns submittedFilters, submitted rule names, and skipped rule names with reasons. An eligible rule containing a Custom action rejects the call before any rule is submitted. A successful submission means processing has started, not completed; if no rules are eligible, nothing is submitted.

Contacts

| Tool | Description | |------|-------------| | searchContacts | Search contacts across all address books by email or name and return full contact details. Supports maxResults. | | 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, 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. Returns status on each event. | | 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 | | updateTask | Update a task's title, due date, description, priority, completion status, or percent complete |

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 > Thunderbird MCP > 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.

The separate "Filter Send Actions" section controls Allow automatic Forward/Reply filter actions, which defaults to off. Only the user can change this preference in the settings page; MCP clients cannot enable 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.

Automatic updates: From v0.7.3 on, the add-on auto-updates through Thunderbird's add-on update check. Thunderbird downloads updates in the background and applies them on the next restart; because this add-on uses an experiment API, updates are not live hot-swapped. v0.7.3 is the last build you need to install by hand because older builds have no update_url and cannot auto-discover it. Thunderbird ships with xpinstall.signatures.required=false, so unsigned auto-updates work out of the box; a profile hardened to require signatures blocks both manual and automatic installs. If updates do not arrive, check the Add-ons gear menu and make sure Update Add-ons Automatically is enabled.

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"]
    }
  }
}

Connecting HTTP-capable clients directly

The extension serves MCP over HTTP itself (POST /: initialize, tools/list, tools/call, notifications), so clients that support HTTP transport can skip the bridge:

claude mcp add --transport http thunderbird http://127.0.0.1:8765/ --header "Authorization: Bearer <token>"

• The port defaults to 8765 (8766-8774 if taken); the token is in connection.json, or set a stable token in the settings page so it survives restarts.
• Responses are plain JSON; there is no SSE stream and no Mcp-Session-Id, both optional in the Streamable HTTP spec.
• Path attachments are then read by the extension itself, under the same attachment policy as the bridge, and inside the Snap/Flatpak sandbox where applicable (the bridge is what makes host paths work for sandboxed installs).
• Keep this on localhost. For a client on another machine or container, prefer an SSH tunnel or a TLS proxy bound to loopback over "Listen on all interfaces": that mode is plain HTTP, so the token and mail content cross the network unencrypted.

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: /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//environ, plus the official snap fallback under ~/Downloads/thunderbird.tmp
5. Linux Flatpak / Betterbird Flatpak: $XDG_RUNTIME_DIR/app//thunderbird-mcp/connection.json and ~/.var/app//cache/tmp/thunderbird-mcp/connection.json

Every connection file, including an explicit pin, must be a regular file of at most 4 KiB with a valid port and token. On Linux and macOS it must also belong to the bridge's user, have no group/other permissions (normally 0600), and must not be a symlink. Rejected files are reported, never repaired or deleted. An invalid explicit pin does not fall back to discovery.

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.

Outgoing attachments

For sendMail, saveDraft, replyToMessage, and forwardMessage, pass attachments as a JSON array, not a JSON-encoded string. The stdio bridge reads file-path attachments on the host and sends inline base64 to Thunderbird. The existing bridge limit is 18 MiB per path attachment, now also applied to saveDraft. The extension's direct-HTTP path limit remains 50 MiB; inline base64 remains limited to 25 MiB of encoded data. Both transports enforce at most 20 attachments and 50 MiB of decoded attachments per operation.

UNC/network and device paths, dotfiles and dot-directories, application-data directories (Windows AppData and the macOS home Library), and credential/key filenames are refused. Windows alternate data streams and path components ending in dots or spaces are also refused. Files exported by getMessage(saveAttachments: true) can still be attached again, and ordinary local document paths continue to work. If any attachment is refused, missing, invalid, or over a limit, the entire operation fails before sending, saving a draft, or opening a review window. The error identifies refused entries; temporary files created during the failed conversion are removed. Callers that previously relied on partial attachment success must handle this error explicitly.

Security

• Auth tokens: The HTTP server requires a session-scoped bearer token. Generated on startup, written to /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.
• Stable tokens: Optional stable tokens persist across restarts. The stored token is cleared on uninstall only while the add-on is enabled (best effort). If you disable the add-on before uninstalling, regenerate the token before using it again.
• 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. A corrupt stored setting is shown as blocked, and saving it unchanged does not lift the restriction.
• Calendars and address books under account restrictions: While any account restriction is active, calendar and address-book tools (listing, lookups by ID, and writes) are blocked unless you enable "Allow all calendars" or "Allow all address books" in the settings page. Both are off by default and grant access to all calendars or all address books respectively. Without an account restriction, calendars and address books behave as before.
• Tool access control: Disable specific tools via the settings page. Disabled tools are hidden from tools/list and blocked at dispatch.
• Encrypted-message access: "Allow MCP clients to read encrypted messages" is off by default. It gates full-message bodies, attachments, inline images, raw-source reads, and direct reply/forward; Thunderbird may still decrypt S/MIME locally. Opaque-signed S/MIME messages are also withheld while this setting is off. This setting does not redact cached headers/previews in searchMessages, getRecentMessages, or displayMessage, or exclude encrypted mail from searchBody matches. Cached subjects may already contain decrypted protected headers. Thunderbird has no reliable encryption signal on every message header: OpenPGP's enigmail property is cached display status, while S/MIME's URI registry only tracks currently displayed decrypted messages. In particular, OpenPGP can update a protected subject during background processing without the display callback that records that property; the S/MIME registry is scoped to the current display.
• 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 tomaszkasperczyk.name registration.

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 THUNDERBIRD_MCP_CONNECTION_FILE explicitly if your environment uses a non-standard temp/runtime path | | 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

# 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"}'

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. Regular users should install the latest release once and let auto-update handle later releases.

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 40 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
• Thunderbird itself still runs pre-existing filters with cross-account move/copy targets automatically; the MCP filter tools refuse to update such rules (except disabling or deleting them) and applyFilters skips them
• 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.