ThunderClaude

ThunderClaude is a Thunderbird MailExtension for rewriting the current compose-window email with LLM presets or a custom instruction. It rewrites only the text you select, or the whole draft after a confirmation step, and leaves every inline image untouched by default.

Current Status

The latest release is v0.1.2, available from the GitHub Releases page. Listing on addons.thunderbird.net is in progress; until it is approved, install the packaged XPI as described below.

Features

• Presets and custom instructions: make formal or casual, shorten, expand, fix grammar, translate, or draft a reply, plus your own free-text instruction.
• Selection-aware scope: by default ThunderClaude rewrites only the text you have selected, so quoted replies and forwarded threads keep their original styling. With nothing selected it asks for confirmation before rewriting the whole draft.
• Inline images preserved by default: every inline image (cid: attachment, remote, or data: image) stays exactly in place. An optional toggle lets the model move images when you want it to, and it can never silently drop one.
• Bring your own provider: Anthropic, OpenAI, Gemini, MiniMax, DeepSeek, OpenRouter, any OpenAI-compatible endpoint, or local models via Ollama or LM Studio.
• Local-first key storage: your API key is stored encrypted at rest and is sent only to the provider you configured. No telemetry.

Screenshots

The compose-window popup. Choose a preset (Formal, Casual, Shorten, Expand, Grammar, Translate, or Reply draft) or type a custom instruction that overrides it. Inline images stay exactly where they are unless you tick "Let the AI move inline images", and your provider and model are one click away. The header shows whether the draft is HTML or plain text and lets you flip between light and dark.

The options page. Pick any supported provider from the sidebar, set its default model and base URL, and save an API key that is encrypted at rest (AES-GCM) with a key kept inside your Thunderbird profile. "Test connection" checks the key before you depend on it, and a blank key field leaves an existing key in place.

Requirements

• Thunderbird 128 or newer.
• At least one LLM provider:
  • an API key for Anthropic, OpenAI, Gemini, MiniMax, DeepSeek, OpenRouter, or any OpenAI-compatible endpoint, or
  • a local model served by Ollama or LM Studio (no key required).
• Node.js 22 or newer, only if you build from source.

Development

Install dependencies:

pnpm install

Load the extension in a temporary Thunderbird profile:

pnpm dev

By default this launches thunderbird. Set THUNDERBIRD_BINARY when the binary has a different name or path:

THUNDERBIRD_BINARY=/path/to/thunderbird pnpm dev

Run the required gates before committing:

pnpm test
pnpm lint

Prepare the unpacked extension files:

pnpm build

This writes dist/unpacked/ for inspection and dist/thunderclaude-0.1.2.xpi for local installation testing.

Install

Development Install

Use pnpm dev for active development. It starts web-ext with Thunderbird and loads the repository as a temporary extension.

On first run:

1. Open a Thunderbird compose window.
2. Click the ThunderClaude compose-action button.
3. Complete onboarding by choosing a provider, testing it, and saving the default model.
4. Reopen the compose-action popup and choose a rewrite preset.

Packaged Install

Build the XPI:

pnpm build

Then install it:

1. Open Thunderbird Add-ons Manager.
2. Use the gear menu to choose "Install Add-on From File".
3. Select dist/thunderclaude-0.1.2.xpi.

Thunderbird's official add-on install guide documents the Add-ons Manager file install flow: https://support.mozilla.org/kb/installing-addon-thunderbird

Privacy

ThunderClaude has no servers and collects no analytics or telemetry. Your draft and instruction are sent only to the LLM provider you configure, only when you trigger a rewrite; your API key is stored locally and encrypted at rest. See PRIVACY.md for the full policy.

Contributing

Contributions are welcome. Please open an issue or pull request, keep changes focused, and run pnpm test and pnpm lint before every commit.

License

Copyright (C) 2026 Kenzo Wijnants

ThunderClaude is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License v3.0 as published by the Free Software Foundation. See LICENSE for the full text.

This means you are free to use, study, fork, and modify ThunderClaude. If you distribute a modified version, you must release your changes under the same GPL-3.0 license with source, so nobody can take this code, close it up, and resell it as a proprietary product.

ThunderClaude is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.