Proton Safe MCP

Proton Safe MCP

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A client-agnostic FastMCP server for Proton Mail through the official Proton Mail Bridge. It can read and search mail and create drafts with attachments. It deliberately cannot send email — you review every draft in Proton Mail and press Send yourself.

The server runs locally over STDIO for any MCP-compatible client (Claude Desktop, Claude Code, or anything else that speaks MCP). Received PDF, TXT, and CSV attachments can be inspected through bounded text extraction without exposing their raw bytes. Outgoing attachments are uploaded as bounded base64 chunks, so the server never receives or reads a client filesystem path.

Read the full documentation for setup, configuration, tool inputs, security boundaries, and troubleshooting.

Why "safe"?

Email is attacker-controlled input. Any sender can put adversarial instructions in a message body, and an AI agent that reads mail and holds write-capable tools is one prompt injection away from doing something you did not ask for. This project limits the blast radius by construction:

• No send. There is no SMTP client and no send_message tool in the codebase — a test asserts it.
• No delete, no move, and no raw received-attachment download tool.
• Human in the loop. Draft creation requires explicit confirmation of the exact content in the conversation, and Proton Mail still requires manual review and sending.
• No filesystem access for clients. Attachment bytes are streamed in chunks with declared size and SHA-256 verification; paths are rejected.
• Loopback only. STDIO transport, no listening socket, and the Bridge host is hard-coded to 127.0.0.1.

These controls reduce risk but do not make email trusted. Never expose unrelated write-capable tools in the same unattended agent workflow.

Architecture

Security properties

• STDIO only: the server opens no listening network socket.
• Proton Bridge host is hard-coded to 127.0.0.1 (PROTON_BRIDGE_HOST is intentionally unsupported).
• No SMTP client, send tool, delete tool, move tool, or raw received-attachment download tool.
• Message and attachment reads use BODY.PEEK; attachment inspection returns bounded extracted text, metadata, and a SHA-256 digest, never raw bytes or files.
• Attachment input is chunked base64 with declared size and SHA-256 verification.
• Only PDF, DOCX, XLSX, PPTX, TXT, CSV, PNG, and JPEG uploads are accepted.
• Attachment tokens are random, short-lived, single-use, and reverified before draft creation.
• Draft creation requires an explicit client assertion that the user confirmed the exact recipients, subject, body, and attachments in the conversation.
• The Bridge-generated IMAP password is stored in the operating-system keyring.
• Draft bodies are stored as plain text plus a server-generated HTML alternative that HTML-escapes the confirmed body, so quoted markup can never be rendered. Nothing is ever appended to a body server-side, including a reply quote.
• Proton Bridge does not preserve reply threading in saved drafts. Reply targets are refused unless the user explicitly accepts a possibly separate draft. Accepted replies submit headers only. Recipients are never derived from the message being replied to, its Message-ID is reverified at the IMAP write, and both identifiers are validated so neither can continue into a header of its own.
• Recipient, subject, and folder inputs are validated against header/criteria injection.
• State files are private to one account. On Linux: 0700 directories, 0600 files and O_NOFOLLOW. On Windows: a protected DACL granting only the owning user and the system accounts, with reparse points refused in place of O_NOFOLLOW.

Requirements

• Linux with the official Proton Mail Bridge installed, signed in, and running (developed and tested on Ubuntu).
• A Proton plan that supports Bridge.
• Python 3.11–3.14 (tested in CI) for the command-line installation; bundled in the desktop installer.
• uv for the command-line installation.
• A working Secret Service keyring (gnome-keyring or compatible).

Windows 11 x64 support is implemented in this repository but not released: automated checks run on Windows Server, while end-to-end Windows 11 acceptance remains untested. See the Windows installer notes and its acceptance sheet.

Installation

Desktop assistant — Ubuntu 24.04, x86_64

Download the installer, open it with Ubuntu's package installer, then launch Proton Safe. The assistant guides you through Bridge connection and plugin activation in a compatible local ChatGPT desktop or Codex installation. Python and uv are included; no terminal setup is required.

You still need Proton Mail Bridge, a compatible Proton plan and a session keyring. See the desktop guide for prerequisites, screenshots and repair options. Checksums and build provenance are in the release.

Desktop assistant — Windows 11, x64

Not available yet. The installer, the Windows platform layer and the build pipeline exist, but no acceptance scenario has been run on Windows and there is no download. Two external dependencies remain open: a Windows AI client whose full path is qualified, and a signing identity usable from CI.

Command-line installation — other MCP clients

Install the reviewed release from PyPI with uv:

uv tool install proton-safe-mcp==2.3.0

For development from source instead:

git clone https://github.com/fbossiere/proton-safe-mcp.git
cd proton-safe-mcp
uv sync --extra dev

Set the Proton address and Bridge IMAP port in the MCP process environment. No credential is ever set here:

export PROTON_BRIDGE_USER="your-address@proton.me"
export PROTON_IMAP_PORT="1143"

To draft as another Proton address of the same account, add the optional sender allowlist in that same environment, as comma-separated bare addresses:

export PROTON_BRIDGE_ALIASES="billing@example.com,legal@example.com"

Store the Bridge-generated IMAP password (shown in the Bridge UI), not your Proton account password:

proton-safe-mcp setup

The value shown by Bridge is installation-specific and works only against the local Bridge.

Register with an MCP client

Configure a local STDIO server with these logical fields. Use command -v proton-safe-mcp to obtain the absolute command path when your client does not inherit your shell PATH:

{
  "name": "proton-safe",
  "transport": "stdio",
  "command": "/absolute/path/to/proton-safe-mcp",
  "args": ["serve"],
  "env": {
    "PROTON_BRIDGE_USER": "your-address@proton.me",
    "PROTON_IMAP_PORT": "1143"
  }
}

PROTON_BRIDGE_ALIASES goes in the same env block when a draft may use more than one From address. See Sender addresses.

Do not put PROTON_BRIDGE_PASSWORD in the client configuration. The server reads it from the OS keyring established by setup.

Copy-paste instructions are available for Claude Code, Cursor, and VS Code. AI coding agents can follow the safety-constrained llms-install.md guide.

Verify the complete local setup without printing credentials, addresses, or mailbox data:

proton-safe-mcp doctor

OpenAI plugin

The repository includes a private, local-first Proton Safe plugin for ChatGPT and Codex. It packages safety-focused mail review and draft workflows with the same restricted MCP server:

• ChatGPT desktop and Codex can launch the bundled STDIO configuration directly on the Bridge machine, with no tunnel or dedicated server;
• direct MCP registration remains available when the packaged workflow skills are not needed;
• ChatGPT web or a client on another machine can optionally reach the Bridge host through OpenAI Secure MCP Tunnel without an inbound port;
• a remote deployment still keeps proton-safe-mcp and Proton Mail Bridge together, with IMAP fixed to 127.0.0.1.

See the OpenAI plugin guide for local ChatGPT desktop/Codex installation, direct MCP registration, and the optional remote tunnel.

Plugin installed but no Proton tools? The bundled MCP configuration forwards PROTON_BRIDGE_USER and PROTON_IMAP_PORT from the environment that started Codex; it does not define their values. On Ubuntu, a correct ~/.config/environment.d/*.conf file can still require a user-manager reload, while GNOME and already-running terminals can keep their earlier environment. If a menu relaunch still has no tools, start ChatGPT once from a terminal that has loaded the file. That terminal launch is a diagnostic, not a requirement for every start; the troubleshooting guide also provides a persistent per-user menu launcher. Follow the privacy-safe Ubuntu recovery procedure or the FAQ before reinstalling anything.

Help test the onboarding. Linux and Proton Mail Bridge users can run the 10-minute external test and submit privacy-safe installation feedback.

Available MCP tools

Category	Tool	Notes
Read-only mail	mailbox_status	Bridge connectivity + INBOX counts
	list_folders
	list_sender_addresses	The fixed From allowlist a draft may use
	list_messages	Never marks messages as read
	search_messages	Injection-safe IMAP TEXT search
	read_message	Bounded plain text; no attachment bytes
	extract_attachment_text	Bounded PDF/TXT/CSV text; no raw bytes or files
	get_reply_context	Candidate reply recipients, Re: subject, and a bounded quote — all suggestions
Attachment staging	begin_attachment_upload	Declares filename, type, size, SHA-256
	upload_attachment_chunk	Ordered base64 chunks
	finish_attachment_upload	Verifies hash, returns single-use token
	discard_attachment
Drafts	create_confirmed_draft	Requires exact confirmation; reply targets also require acceptance of a possibly separate draft

There is deliberately no send_message tool.

Attachment and draft workflow

The MCP client only needs the ability to obtain the bytes of a file and call tools. There is no dependency on a particular client, service, or local directory.

1. Calculate the exact byte length and SHA-256 of the file.
2. Call begin_attachment_upload(filename, content_type, size_bytes, sha256_hex).
3. Base64-encode consecutive binary chunks no larger than the returned max_chunk_bytes.
4. Call upload_attachment_chunk(upload_id, chunk_index, data_base64) for indexes 0, 1, 2….
5. Call finish_attachment_upload(upload_id) and retain the returned attachment_token.
6. Present the exact recipients, subject, complete body, and attachment list to the user.
7. After explicit confirmation in the conversation, call create_confirmed_draft(..., user_confirmed=true, attachment_tokens=[token]).
8. Open Proton Mail, review the draft, and send it manually.

Step 8 is the human gate, and it is the only one that matters: the server has no SMTP implementation, so a draft it creates cannot leave your account until you press Send in Proton Mail. Uploaded attachments expire after 30 minutes if no draft consumes them.

Local administration

proton-safe-mcp doctor   # check the local setup without printing private data
proton-safe-mcp setup    # store the Bridge-generated IMAP password in the OS keyring
proton-safe-mcp serve    # run the MCP server over STDIO

Configuration reference

Variable	Default	Purpose
PROTON_BRIDGE_USER	required	Proton address configured in Bridge
PROTON_BRIDGE_ALIASES	empty	Comma-separated additional From addresses a draft may use
PROTON_IMAP_PORT	1143	Local Bridge IMAP port
PROTON_MCP_STATE_DIR	~/.local/state/proton-safe-mcp	Private attachment staging state
PROTON_MCP_MAX_ATTACHMENT_BYTES	20971520	Maximum per file and per draft, capped at 25 MiB
PROTON_MCP_MAX_RECEIVED_ATTACHMENT_BYTES	10485760	Received-file extraction maximum, capped at 25 MiB
PROTON_MCP_MAX_CHUNK_BYTES	393216	Decoded chunk maximum, capped at 1 MiB
PROTON_MCP_UPLOAD_TTL_SECONDS	1800	Attachment staging lifetime
PROTON_MCP_MAX_BODY_CHARS	100000	Maximum outgoing draft body length

PROTON_BRIDGE_HOST is intentionally unsupported.

For isolated containers without a Secret Service keyring, PROTON_BRIDGE_PASSWORD may contain the Bridge-generated IMAP password. Avoid this fallback in desktop MCP client configuration: environment values may be visible to the client process or its diagnostics.

Development

uv sync --extra dev
uv run pytest --cov      # tests with coverage
uv run ruff check .      # lint
uv run ruff format .     # format
uv run mypy              # strict type checking

Proton Bridge is not needed for development: the test suite fakes the IMAP layer. Tests cover path rejection, MIME restrictions, received-attachment size and format rejection, bounded PDF/text extraction, ordered chunks, size/hash verification, token consumption, header injection, draft confirmation, recipient and attachment bounds, and HTML-to-text sanitization.

See CONTRIBUTING.md for the design rules that reviews enforce.

Threat-model limitations

Read this before relying on the server in an autonomous workflow:

• A model necessarily sees any mail it reads and any attachment it creates or uploads.
• Extracted attachment text is attacker-controlled and may contain prompt injection. The server bounds the returned text but does not make it trustworthy or perform OCR or malware scanning.
• Uploaded attachment bytes are stored temporarily in files readable only by the Unix account. Use full-disk encryption.
• The server cannot inspect the surrounding conversation. user_confirmed: true is a client assertion, so the confirmation step depends on the client honoring the rule. It reduces accidents; it is not a boundary against a client under an attacker's influence.
• A draft this server creates is inert until you send it, but it can still be crafted to look like something you wrote. Read drafts in Proton Mail before sending, especially recipients.
• Secure MCP Tunnel keeps the server off the public internet, but mail content returned to an OpenAI product still crosses the local Bridge boundary and is processed by that product.
• Tool annotations and conversational-confirmation instructions are client guidance, not authorization controls. What the server enforces is input validation and the absence of any send, delete, or move capability.
• Proton Bridge's self-signed TLS certificate is not verified. This is acceptable here only because the target host is unchangeably 127.0.0.1.

Security

To report a vulnerability, use GitHub private vulnerability reporting — never a public issue. See SECURITY.md.

License

MIT © 2026 Francois Bossiere.

This project is not affiliated with or endorsed by Proton AG. "Proton Mail" and "Proton Mail Bridge" are trademarks of Proton AG.