Thunderbird

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.

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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. While enabled, meetings with attendees other than the calendar user are read-only through MCP, and adding attendees is blocked: calendar servers can email invitations, updates, or cancellations without review. Automatic Forward/Reply filter actions have a separate opt-in, disabled by default.

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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.

The bridge starts when executed directly or loaded by a desktop client's Node bootstrap. Tests or library consumers that only need its exports must set THUNDERBIRD_MCP_NO_AUTOSTART=1 before requiring mcp-bridge.cjs.

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What you can do

Mail

Tool	Description
listAccounts	List all email accounts and their identities
listFolders	Browse folder tree with message counts and isFavorite in object or table format. Filter by account, subtree, or favoritesOnly: true; favorites nested under ordinary folders are included.
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 or copy between folders, or trash -- supports bulk via messageIds. copyTo preserves the source and can add a Gmail label.
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)

searchMessages scans cooperatively with a best-effort 20-second budget. Header searches count and sort collected matches before pagination, including those beyond 10,000; only the returned page loads full result fields. Counts and totals are best-effort when folders change during a long search, even if no truncation is reported. Unscoped searches still request an IMAP refresh for each visited folder, but results use the locally available cache. Opening a native database and taking its key snapshot are synchronous and cannot be interrupted by this budget.

Message results remain a plain array when offset is omitted or null, including incomplete searches and searchBody: true. To receive completeness information, provide offset: 0 (or another offset) for a paginated object. Only object responses can include truncated: true and a message explaining why; countOnly retains its existing object response and can also include these fields. Partial counts and totalMatches cover only observed matches, and date ordering covers only that subset. hasMore describes additional pages within the collected results and becomes false at their end, even if truncated is true; an empty returned page also sets it false. To improve coverage, narrow the folder, date range, or query (or set includeSubfolders: false).

For searchBody: true, object responses are conservatively marked truncated: true: Gloda returns candidates ranked by relevance and exposes no reliable completeness indicator. It applies its limit before removing stale index rows, so even a short result can omit other matches. Plain-array responses carry no completeness information. Narrowing the text query improves coverage; use header search when header/preview matching is sufficient.

updateMessage takes tag keys, not display labels: for example, addTags: ["my=20project"] uses a key generated for a label with a space. Keys must be non-empty printable ASCII without spaces, parentheses, brackets, braces, %, *, double quotes, backslashes, <, >, or ;. This follows Thunderbird's keyword handling, including its extra restrictions beyond RFC 3501 atoms. = and legacy modified UTF-7 keys containing & are accepted. Invalid entries in either addTags or removeTags fail the entire call with an error naming those entries before any messages are changed.

Use copyTo: "<destination folder URI>" to copy without removing the original. On Gmail, copying from Sent Mail to a project folder adds that label while preserving Sent Mail and existing labels. copyTo, moveTo, and trash: true are mutually exclusive. Both source and destination must be accessible. Copies and moves within or across accounts use the same Thunderbird native copy service call. On IMAP, completion is asynchronous and later failures are not reported back; success means only that the operation was submitted, so verify the destination. Tags applied together with a copy or move may not transfer on IMAP; verify the destination tags as well.

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.

Structured MIME extraction keeps the first body's representation outside multipart/alternative and joins later fragments of that same type in message order, including after inline attachments. A footer of another type cannot replace the main body. Alternatives select the requested representation; attached text files are excluded using Thunderbird's attachment metadata. Encryption checks also classify the joined content before exposing it.

includeInlineImages: true is a separate opt-in for supported inline CID images as MCP image blocks; it does not add image URLs to the Markdown body.

Compose

Tool	Description
sendMail	Compose a new email -- opens a review window; direct sending requires explicitly disabling the skipReview safety block
saveDraft	Save a new or replacement draft without sending or opening a window; supports threading headers and reports an accessible Drafts folder
replyToMessage	Reply with quoted original and proper threading -- skipReview is subject to the same safety block; saveAsDraft saves the threaded reply to Drafts without sending
forwardMessage	Forward with all original attachments preserved -- skipReview is subject to the same safety block

sendMail, replyToMessage, and forwardMessage 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.

Direct sends report success only after SMTP succeeds, and only then mark originals replied or forwarded. If Thunderbird's 120-second send timeout expires, the outcome is unknown: check Sent and the Outbox before retrying. The bridge waits 150 seconds for direct sends and draft saves, and 30 seconds for other calls. A timeout or lost connection after submitting a mail operation also reports an unknown outcome; check Sent/Outbox for sends or Drafts for saves before retrying.

replyToMessage accepts saveAsDraft: true to build a native reply with quoted text, the identity's signature and threading headers, save it, and close the compose window without sending. This requires the saveDraft tool to be enabled and cannot be combined with skipReview. Encrypted originals require the Allow MCP clients to read encrypted messages opt-in. Before saving, the current compose identity's configured destination must be accessible under account restrictions and carry the Drafts flag. A timeout returns saveOutcome: "uncertain": the outstanding save may still complete, so check Drafts before retrying. Failures and timeouts restore the window's prior close and save-dialog behavior.

saveDraft supports these optional parameters:

• inReplyTo: one bracketed Message-ID such as <original@example.com>. references: up to 100 such IDs, oldest first, separated by single ASCII spaces. Each ID is limited to 998 characters and the full References value to 16,384. Missing brackets, extra tokens, whitespace inside IDs, and control characters (including CR/LF) are rejected, never repaired. References defaults to inReplyTo when omitted, and may also be supplied independently. The caller supplies the subject and quoted text.
• replaceMessageId and replaceFolderPath: replace an existing draft with the supplied content. The folder must be accessible under account restrictions, carry Thunderbird's Drafts flag, and match the selected from identity's configured drafts folder. Other folders and missing messages are rejected before saving. Supply all fields and attachments you want retained; content is not merged from the old draft.
• includeSignature: append the identity's signature, defaulting to true for a new draft and false for a replacement so re-saving a fetched draft does not duplicate its signature. Set it explicitly when needed. sendMail with skipReview: true accepts the same option, defaulting to true; review windows use Thunderbird's own signature preferences. Signature files are limited to 1 MiB; unreadable or oversized signature files are omitted.

On success, saveDraft returns folderPath when Thunderbird exposes the destination and that folder is accessible under account restrictions. Otherwise the save still succeeds without disclosing the folder URI. It does not return the saved message's ID; look up the draft in the returned folder when available.

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.

Date conditions accept only YYYY-MM-DD as a local calendar day; date-times are rejected. listFilters reports a lone native ALL term as matchAll: true with an empty terms array. Compound rules retain their terms and Boolean operators, including ALL terms; an empty rule is not reported as matching all messages.

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.

New or replaced Move/Copy actions store the resolved folder's canonical URI, including when the supplied URI uses another accepted spelling.

applyFilters submits only enabled, parseable rules with the Manual type flag and skips rules with inaccessible or Outbox Move/Copy destinations, rules with address book conditions while address books are restricted, and 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 disabled, read-only, event, and task support flags
createEvent	Create an event through a review dialog, optionally with RRULE recurrence. Direct creation and non-empty attendees require disabling Block skipReview. Accepts status: tentative | confirmed | cancelled.
listEvents	Query events by date range with bounded recurrence expansion. Returns a plain array capped at maxResults, including status, recurrence, organizer, attendees, and your participation status. Series that cannot be expanded return a master marked recurrenceNotExpanded: true.
updateEvent	Modify an event or series; recurrenceId selects one occurrence. Meetings with other attendees are read-only while Block skipReview is on, even when attendees is omitted.
deleteEvent	Delete an event or series; recurrenceId excludes one occurrence. Meetings with other attendees cannot be deleted while Block skipReview is on.
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

recurrence accepts a single RRULE, such as FREQ=WEEKLY;BYDAY=MO,TU or RRULE:FREQ=DAILY;COUNT=10. The optional prefix is case-insensitive. Control characters (including CR/LF), malformed rules, SECONDLY/MINUTELY, and HOURLY on all-day events are rejected; Thunderbird's recurrence parser validates the rule before saving. On update, recurrence: "" or null clears the rule. Replacing a rule discards existing EXDATEs and modified occurrences.

Thunderbird creates the initial Home calendar disabled. Check listCalendars[].disabled and enable the calendar in Thunderbird's calendar properties before using it. Event and task tools report an error for an explicitly selected disabled calendar; listings and default direct creation use enabled calendars, and report an error if all calendars are disabled. MCP leaves the enabled/disabled setting unchanged.

Use the recurrenceId returned by listEvents to update or delete one occurrence. Omitted or null IDs select the series; a non-null ID cannot be combined with recurrence on update. All-day occurrence IDs preserve their calendar date across timezones. Existing Date-compatible inputs remain accepted for event start/end dates; ISO 8601 is recommended.

Attendees use { "email": "alice@example.com", "name": "Alice", "role": "optional" }; new attendees default to required, and a mailto: email prefix is accepted. Email addresses must be single mailboxes without control characters or URI headers; names cannot contain control characters either. On update, omitted/null attendees preserve the list, while [] removes everyone. Retained attendees match by case-insensitive email and keep their participation status and provider metadata. Only supplied name/role fields change; omitted/null fields preserve existing values, and an empty name clears it. New attendees start with NEEDS-ACTION.

Attendee management requires the calendar identity to match an existing organizer. Existing organizers are preserved; a missing organizer is initialized from the calendar identity, which may also set a missing calendar organizerId property. Creation with non-empty attendees requires disabling Block skipReview, even when skipReview is false: Exchange/Owl or CalDAV may email invitations containing the event title and description without review.

listEvents returns a plain array, capped at maxResults (default 100, maximum 500). RRULE generation is limited to maxResults + 1 candidates per series (hard ceiling 501) and 5,000 per request before expansion can allocate an unbounded occurrence array. Generation limits can leave fewer than maxResults results even when more occurrences exist, especially when exclusions remove generated candidates. Use narrower date ranges to reduce this effect.

If a series cannot be expanded safely with a bounded count, expansion fails, or the shared generation budget is exhausted before that series, its master is included once with recurrenceNotExpanded: true, subject to the same output cap. This includes EXRULE series: incomplete exclusion expansion could expose excluded dates, so their rules are not expanded. A flagged master carries the series' original dates, which may be outside the requested range; it does not represent a matching occurrence. Ordinary EXDATEs and modified occurrences remain supported during bounded expansion.

Each event includes organizer: { id, commonName }, up to 100 attendees, attendeeCount with the full count, and myParticipationStatus (empty when unavailable). Do not use a truncated attendee list as an update replacement, since that would remove the omitted attendees.

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. It also blocks adding attendees and makes meetings with attendees other than the calendar user read-only through MCP: updateEvent and deleteEvent reject every write, including time/description/recurrence edits, attendee removal, and occurrence deletion, even when attendees is omitted. Occurrence writes check both the series and selected occurrence; series writes also check stored exceptions. Events without attendees and events whose only attendee is the identified calendar user remain editable. If the calendar identity cannot be established, existing attendees cannot be treated as self-only. Disabling the setting permits these writes, which may email updates or cancellations without review. Event/task creation review dialogs remain available without attendees.

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.

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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: <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/<id>/thunderbird-mcp/connection.json and ~/.var/app/<id>/cache/tmp/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. Flatpak discovery accepts only these application IDs: org.mozilla.Thunderbird, org.mozilla.thunderbird, org.mozilla.thunderbird_esr, net.thunderbird.Thunderbird, and eu.betterbird.Betterbird. Set THUNDERBIRD_MCP_CONNECTION_FILE to force a single explicit path.

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.

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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 <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.
• 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. Clear-signed mail (multipart/signed with an OpenPGP or S/MIME detached signature) is not encrypted and stays readable. When Thunderbird hides such a message's signed parts as encrypted, the extension reads it from the stored message instead and never asks Thunderbird to decrypt; the message is withheld if anything inside the signature is encrypted, and a direct forward of it is refused when it has attachments. While it is off, searchMessages and getRecentMessages return the outer subject without a preview for messages Thunderbird has recorded as OpenPGP-encrypted, searches (including searchBody) do not match them, and displayMessage refuses encrypted messages. This relies on Thunderbird's recorded OpenPGP status, so a message decrypted without that status being recorded may still show a cached decrypted subject. 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.

Release channel and Experiment API add-ons

This add-on uses Thunderbird's Experiment APIs. The add-on team has announced plans to disable them on the Release channel; Thunderbird has since reported a postponement, so do not assume every Release version blocks them.

If Thunderbird disables the add-on for this reason, use Thunderbird ESR (Extended Support Release), which this add-on supports. It has been verified working on 140.16.0esr and 153.3.1esr. Check Tools > Add-ons and Themes to confirm the add-on is enabled, then open its Options page for the server status and any startup error.

---

Development

See CONTRIBUTING.md for build, isolated test, and pull request instructions.

# 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.

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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)
• 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

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License

This project uses the MIT license. The bundled extension/httpd.sys.mjs is derived from Mozilla's HTTP server and remains under MPL-2.0; its license notice is retained in the file.