Baryon MCP

A local, permission-limited bridge between Proton Mail and the AI you choose

Baryon connects an MCP-compatible AI assistant to Proton Mail through a local Proton Mail Bridge.

It lets your assistant search and read email, understand conversations, work with attachments, and prepare drafts for you to review. Baryon cannot send email, move or delete ordinary messages, or mark messages as read.

Baryon has no hosted service. You decide which MCP client and AI model receive your email content.

[!IMPORTANT] Baryon and Proton Mail Bridge run locally, but your AI may not.

Email content requested through Baryon is returned to your MCP client and can be passed to the model it uses. With a local or self-hosted model, processing can remain on infrastructure you control, depending on your client configuration. With a hosted service, selected email content is sent to that provider.

For example, using Baryon with hosted Claude or Codex models sends requested email content to Anthropic or OpenAI infrastructure respectively. Review your client's and provider's privacy and data-retention terms before connecting it to your mailbox.

What you can do

• Find messages by text, sender, recipient, subject, date, or unread state.
• Read an email or an entire conversation.
• Inspect, retrieve, and save attachments.
• Prepare new messages as Proton Mail drafts.
• Review, edit, and send those drafts yourself in Proton Mail.

For example, you could ask your assistant:

Find the renewal notices I received this month and summarise the price changes.

Read my conversation with Alice and prepare a polite follow-up draft.

Find the PDF invoice attached to the latest message from Acme and save it locally.

Summarise this support thread and draft a reply covering the unresolved questions.

Useful access, deliberate limits

Replacing an existing draft saves its replacement before removing the previous draft copy. Baryon has no tool for deleting ordinary received or sent messages.

How it works

Proton Mail
    │
    ▼
Proton Mail Bridge       local
    │
    ▼
Baryon                   local
    │
    ▼
Your MCP client
    │
    ├── Local model      can stay on infrastructure you control
    │
    └── Hosted model     content is sent to its provider

Baryon talks only to Proton Mail Bridge on your computer. It refuses to send Bridge credentials to a non-loopback address and verifies Bridge using its exported TLS certificate by default.

Before you start

You will need:

• Proton Mail Bridge installed, signed in, and running.
• An MCP client that can launch local MCP servers over stdio.
• The IMAP username and generated password shown in Bridge's mailbox settings.
• Bridge's exported TLS certificate for a verified connection.

Use the password generated by Bridge, not your normal Proton account password. Export cert.pem from Settings → Advanced settings → Export TLS certificates in Proton Mail Bridge.

Installation

1. Install the binary

macOS:

brew install --cask combor/tap/baryon-mcp

Windows (after the generated manifest is accepted into the WinGet Community Repository):

winget install --exact --id Combor.BaryonMCP

Arch Linux — baryon-mcp-bin from the AUR:

yay -S baryon-mcp-bin

Debian-based Linux distributions (amd64 and arm64):

sudo apt install ./baryon-mcp_*.deb

RPM-based Linux distributions (amd64 and arm64):

sudo rpm --install ./baryon-mcp_*.rpm

Anywhere else, download the archive for your platform from the latest release and extract baryon-mcp onto your PATH. Releases provide macOS, Linux, FreeBSD, and OpenBSD builds for amd64 and arm64, Windows amd64 builds, and DEB and RPM packages for Linux amd64 and arm64, alongside a SHA256SUMS file. Hash your download and compare it with the matching line in that file:

sha256sum baryon-mcp_*.tar.gz            # Linux archive
sha256sum baryon-mcp_*.deb               # Linux DEB
sha256sum baryon-mcp_*.rpm               # Linux RPM
shasum -a 256 baryon-mcp_*.tar.gz        # macOS
sha256 baryon-mcp_*.tar.gz               # FreeBSD and OpenBSD

Get-FileHash baryon-mcp_*.zip -Algorithm SHA256   # Windows

2. Run the setup

baryon-mcp setup

Setup pins Bridge's TLS certificate, stores your Bridge credentials, and registers baryon with any installed Claude Code and Codex CLIs, which then launch the binary directly — no wrapper script and no credentials in a client configuration file. The Bridge password goes into the system keyring (Secret Service on Linux with gnome-keyring, KWallet or KeePassXC running; Login Keychain on macOS; Credential Manager on Windows), and into a mode-600 file under ~/.config/baryon-mcp when no keyring answers.

On Linux, if no exported certificate is found, setup offers to pin the certificate the running Bridge is serving — after verifying through procfs that the process owning the port is a Bridge executable running as you — and shows its fingerprint before asking. Bridge installed headlessly (protonmail-bridge-core) has no certificate-export UI, so this is the expected path there. baryon-mcp setup --help lists the options, including --reset-credentials, --tls-cert and --client.

Configuring a client by hand

baryon-mcp setup knows how to configure Claude Code and Codex. For any other MCP client, point it at the baryon-mcp binary yourself. The surrounding configuration format varies between clients, but the server entry looks like this:

{
  "mcpServers": {
    "baryon": {
      "command": "/absolute/path/to/baryon-mcp",
      "env": {
        "PROTON_BRIDGE_USERNAME": "you@proton.me",
        "PROTON_BRIDGE_PASSWORD": "bridge-generated-password",
        "PROTON_BRIDGE_TLS_CERT": "/absolute/path/to/cert.pem"
      }
    }
  }
}

This basic configuration may store Bridge credentials as plaintext. Prefer baryon-mcp setup, which keeps them out of client configuration entirely, or your client's secure secret storage. Any variable set here takes precedence over the stored credentials, so the two can be mixed — a client entry can override the endpoint while the password stays in the keyring.

Proton Pass CLI

If you keep the Bridge password in Proton Pass, pass-cli run can resolve it at launch, so the client configuration holds a reference instead of the password:

claude mcp add --transport stdio --scope user baryon \
  -e PROTON_BRIDGE_USERNAME=you@proton.me \
  -e "PROTON_BRIDGE_PASSWORD=pass://Personal/Proton Bridge/password" \
  -- pass-cli run --no-masking -- /usr/bin/baryon-mcp

pass-cli run passes the MCP stdio stream through unchanged and preserves the exit code. --no-masking matters: by default pass-cli rewrites any occurrence of the resolved secret in the child's output, which would corrupt a tool result whose mailbox content happens to contain the Bridge password. An expired Proton Pass session or an unresolvable reference fails on stderr before the server starts, which surfaces in the client's MCP log.

Hosted AI clients

[!CAUTION] The installation methods below connect Baryon to hosted AI products. Email content retrieved through Baryon is sent to the relevant provider when the client gives it to the model.

Claude Desktop

Download the .mcpb bundle for your platform from the latest release, open it, and enter the Bridge settings when prompted.

Claude Code and Codex

Install the binary and run baryon-mcp setup, as above. It registers both CLIs it finds on PATH; --client claude or --client codex configures only one, and an existing baryon entry is left alone unless --force-client-config is given.

Docker on Linux

ghcr.io/combor/baryon-mcp is published for linux/amd64 and linux/arm64. It exists so MCP directories can start the server and read its tool schemas, and so Linux users can run Baryon in a container. On macOS and Windows use the native binaries: Docker Desktop does not share the host loopback interface that Bridge listens on.

Baryon speaks stdio, so the client launches the container rather than connecting to a port:

{
  "mcpServers": {
    "baryon": {
      "command": "docker",
      "args": [
        "run", "--rm", "-i",
        "--network=host",
        "--user", "1000:1000",
        "--env-file", "/absolute/path/to/baryon.env",
        "-v", "/absolute/path/to/cert.pem:/run/secrets/bridge-cert.pem:ro",
        "-v", "/absolute/path/to/attachments:/attachments",
        "-e", "PROTON_BRIDGE_TLS_CERT=/run/secrets/bridge-cert.pem",
        "-e", "BARYON_ATTACHMENT_ROOTS=/attachments",
        "ghcr.io/combor/baryon-mcp:latest"
      ]
    }
  }
}

• --network=host is required. Bridge listens on the host's loopback interface, and Baryon refuses to send credentials to any other address.
• Keep PROTON_BRIDGE_USERNAME and PROTON_BRIDGE_PASSWORD in the env file and supply them at run time. It holds the Bridge password in plain text, so create it outside any repository and chmod 600 it. Never bake credentials into an image or commit them in a Compose file.
• Replace 1000:1000 with your own id -u and id -g. Bind mounts keep host ownership, so without --user the container runs as UID 65532 and cannot read a mode-0600 certificate or write into your attachment directory. save_attachment creates files mode 0600 owned by whichever UID the container ran as, so running as yourself keeps them readable.
• Keep BARYON_ATTACHMENT_ROOTS pointed at the mounted directory, as above. Without it the server creates its attachment directory under the container user's home, which a --user override usually cannot write to, and startup fails rather than falling back to unrestricted access.
• Anyone who can reach the Docker daemon can read a running container's environment. On a desktop, prefer a native binary configured with baryon-mcp setup, which keeps credentials in the system keyring or in mode-600 files.
• With both credentials absent the image still starts and serves tool schemas, so a directory can inspect it; every tool call then reports the missing configuration. Supplying only one credential fails at startup.

Security boundaries

• Baryon refuses to send Bridge credentials to a non-loopback host.
• Bridge's TLS certificate is pinned by default. Without a certificate, Baryon refuses to start unless insecure mode is explicitly enabled.
• Read tools select mailboxes read-only and do not mark messages as read.
• save_draft and save_reply_draft are the only tools that change the mailbox, and only inside Drafts. There are no send, move, general delete, or flag-changing tools.
• save_attachment is the only tool that writes to local disk. It never overwrites an existing file or creates a missing parent directory.
• Local attachment reads and writes are confined to a managed folder by default, or to the directories chosen with BARYON_ATTACHMENT_ROOTS.
• Reading can be limited to an allowlist of folders with BARYON_ALLOWED_FOLDERS. A folder outside it is refused before the mailbox is opened, and never appears in list_folders.
• Replies can only be sent from the addresses in BARYON_SENDER_IDENTITIES, and never carry the original message's Bcc recipients.
• Message content is labelled content_trust: "untrusted_email" wherever it is returned, and fenced with an unguessable delimiter for clients that read only text blocks.
• MCP clients can access the message content and attachments you request. Connect only clients and models you trust.

Advanced reference

secret-tool lookup service baryon-mcp-proton-bridge username you@proton.me

Development

Building from source requires Go 1.26.5 or later.

make build      # build ./baryon-mcp
make test       # formatting, shell script checks, vet, and race-enabled tests
make snapshot   # local GoReleaser build and MCPB packaging into dist/

make snapshot also requires GoReleaser, jq, and npx.

License

BSD 3-Clause