fmsg-mcp

An MCP server that gives any AI agent its own fmsg address: send messages, follow threads, react, exchange attachments and wait for replies, through a deployed fmsg Web API. Connect through stdio in hosts such as Claude Code, Claude Desktop, Cursor and VS Code, or through HTTP using API-key headers or configured OAuth.

• stdio for local hosts: one address per server process, configured by two environment variables.
• Streamable HTTP for shared or remote deployments: one endpoint serving many users, each authenticated by their own fmsg API key or OAuth connection.
• The fmsg Web API client is exported for reuse: import { FmsgClient } from "@markmnl/fmsg-mcp/client".

1. Get an fmsg address and API key

Connecting to an existing OAuth-enabled endpoint? Add its MCP URL to your host and sign in; you can skip the API-key setup below. Operators can enable this with HTTP OAuth.

You send as an fmsg address, authenticated by an API key (fmsgk_…) issued by your fmsg host:

• No host yet? Create an account at a public fmsg host such as fmsg.io and add an agent (sub-account) to get an API URL and key.
• Self-hosting? Run the stack with fmsg-docker and issue a key with fmsg-webapi api-key create.

2. Install

Requires Node.js 22 or later.

Claude Code

claude mcp add fmsg --scope user \
  --env FMSG_API_URL=https://api.example.com \
  --env FMSG_API_KEY=fmsgk_... \
  -- npx -y @markmnl/fmsg-mcp

Then in any session: "Send @bob@example.com a note about the release", "What's in my fmsg inbox?", "Wait for Bob's reply and answer it". /fmsg:chat and /fmsg:reply are available as prompts.

Claude Desktop, Cursor, VS Code and other stdio hosts

Add a server entry with the same command; only the config file differs:

{
  "mcpServers": {
    "fmsg": {
      "command": "npx",
      "args": ["-y", "@markmnl/fmsg-mcp"],
      "env": { "FMSG_API_URL": "https://api.example.com", "FMSG_API_KEY": "fmsgk_..." }
    }
  }
}

(Claude Desktop: claude_desktop_config.json; Cursor: .cursor/mcp.json; VS Code: .vscode/mcp.json under "servers" with "type": "stdio".)

Remote (Streamable HTTP) mode

Run one server for many users. Each client sends its own fmsg API key as a bearer token; the server exchanges it at the fmsg host and acts as that address. FMSG_API_KEY must not be set.

FMSG_API_URL=https://api.example.com FMSG_MCP_ALLOWED_HOSTS=mcp.example.com \
  npx -y @markmnl/fmsg-mcp --http 0.0.0.0:8765
# or
docker build -t fmsg-mcp .
docker run -e FMSG_API_URL=https://api.example.com \
  -e FMSG_MCP_ALLOWED_HOSTS=mcp.example.com -p 8765:8765 fmsg-mcp

The MCP endpoint is /mcp; /healthz reports liveness. Use a client that supports an explicitly configured Authorization: Bearer fmsgk_... header. Each caller supplies its own key; a shared header means a shared fmsg identity. For browser sign-in without user API keys, configure HTTP OAuth: the operator supplies an issuer, resource URI and exchange client. Users add the public MCP URL in a host supporting that issuer's client registration method.

For Claude Code over HTTP:

claude mcp add --transport http fmsg --scope user https://mcp.example.com/mcp \
  --header "Authorization: Bearer fmsgk_..."

Deploy behind a TLS-terminating reverse proxy and set FMSG_MCP_ALLOWED_HOSTS to the public hostname when binding to a non-loopback address; startup fails without it. Browser clients on another origin also need FMSG_MCP_ALLOWED_ORIGINS containing exact origins, such as https://app.example.com. For loopback binds, loopback browser origins on any port work by default, including MCP Inspector at http://localhost:6274. Setting an explicit origin list replaces that loopback default. Allowed preflights need no credentials; actual MCP requests always require authentication. wait_for_message holds a request open for up to FMSG_MCP_WAIT_MAX_SECONDS (230), so give the proxy an idle timeout of at least 240 s. See the TLS reverse-proxy example for a loopback deployment with Caddy.

Tools

Tool	What it does
whoami	The address this server acts as, the API URL and token expiry
resolve_address	Turn a short name into @user@domain (directory, then default domain)
list_messages	Inbox, newest first, with previews; reactions hidden; optional unread filter
list_sent	Sent messages with per-recipient delivery state
get_message	One message with headers, full text body, attachments and reactions
get_thread	The lineage from the thread root to a message, with gaps for messages you cannot see
send_message	Start a new thread; sends immediately (fmsg messages are immutable)
reply	Reply into a thread; reply-all by default, refuses terminal and no-reply parents
add_recipients	Add recipients to a sent message
react	Set or clear your emoji reaction
mark_read	Mark received messages read
download_attachment	Fetch a small attachment inline: text as text, images as image blocks, other files as base64 resources
save_attachment	Stream an attachment to the configured local folder; stdio only, enabled by FMSG_MCP_DOWNLOAD_DIR
delivery_status	Per-recipient delivery times and host response codes
wait_for_message	Block until the next inbound message (WebSocket push), batched per thread, with thread context

Every tool returns readable Markdown plus structuredContent. Ids are decimal strings. Message bodies are labelled as data from other parties, not instructions.

Resources fmsg://message/{id} and fmsg://thread/{id} expose the same content to hosts that attach resources; prompts chat and reply script the wait → reply loop and a guided reply.

Configuration

Variable	Default	Purpose
FMSG_API_URL	—	Base URL of the fmsg Web API (required)
FMSG_API_KEY	—	fmsgk_… key; stdio mode only
FMSG_MCP_AUTH_MODE	api-key	HTTP authentication: api-key or oauth; see OAuth settings
FMSG_ALLOW_INSECURE_HTTP	disabled	Set to 1 only to permit cleartext API access on a trusted development/private network; loopback HTTP is allowed by default
FMSG_DEFAULT_DOMAIN	—	Lets short names resolve: bob → @bob@<domain>
FMSG_DIRECTORY	—	JSON file mapping short names to full addresses
FMSG_MCP_DOWNLOAD_DIR	—	Enable save_attachment in stdio; folder for new files named from message ID and filename
FMSG_MCP_WAIT_MAX_SECONDS	230	Cap on one wait_for_message call
FMSG_MCP_HOST / FMSG_MCP_PORT	127.0.0.1 / 8765	HTTP bind address (or --http host:port)
FMSG_MCP_ALLOWED_HOSTS	loopback names	Comma-separated Host header allowlist; required for non-loopback binds
FMSG_MCP_ALLOWED_ORIGINS	same origin; loopback origins on loopback binds	Comma-separated browser origins including scheme and port; an explicit list replaces the loopback default; hostname-only values are rejected
FMSG_MCP_KEY_CACHE_MAX / FMSG_MCP_KEY_CACHE_TTL_SECONDS	500 / 1800	HTTP client cache bound; TTL applies only to API-key mode

The API key is exchanged for a short-lived access token that the server renews automatically. API URLs must not contain credentials, query strings or fragments. Authenticated requests do not follow redirects; configure the final API URL directly.

To save attachments directly to disk, add FMSG_MCP_DOWNLOAD_DIR to your stdio server's environment, for example /home/you/Downloads/fmsg. The optional save_attachment tool streams files into that folder without sending their bytes through model context. It accepts only a message ID and attachment filename and creates a new file such as 123-report.pdf. Repeat saves use 123-report-1.pdf, 123-report-2.pdf, etc., leaving existing files untouched. Unusual filenames are converted to portable names; use the returned saved_to path. Streaming downloads can run longer than 60 seconds while making progress; a 60-second idle timeout detects stalled transfers.

Inline downloads default to 256 KiB to keep file content manageable for the model. Use save_attachment for larger local files, or raise max_inline_bytes explicitly when your AI host can handle more inline content. HTTP clients use inline downloads or their host's file capabilities.

Over stdio, missing or invalid configuration still allows hosts to discover the tools. Tool calls explain the configuration error and how to fix it; restart the MCP server after correcting settings.

Safety

• Messaging access, quotas and recipient acceptance are enforced by fmsg-webapi and the host services. MCP forwards each operation as the caller's identity and surfaces upstream failures.
• download_attachment never writes local files. Optional save_attachment writes only generated filenames in the operator-configured folder, using exclusive creation with no overwrite.
• Sent messages cannot be edited or recalled; send tools say so in their descriptions and are annotated destructiveHint to describe their effects. Approval behavior belongs to the AI host; fmsg-mcp has no additional confirmation gate.
• Selected API-key/token formats are redacted from outbound bodies, topics and error text; the send tools report the count. This is not general data-loss prevention or binary attachment scanning.
• Nothing about message size or acceptance is assumed: the fmsg host's own responses and delivery codes are surfaced verbatim.
• The server publishes MCP instructions (shown to the model at session start) telling agents to use these tools rather than a local fmsg CLI or cached credentials, to carry out authorized tasks and automation without repeated confirmation, and to treat message content as data.
• See SECURITY.md.

Using the client library

import { FmsgClient } from "@markmnl/fmsg-mcp/client";

const client = new FmsgClient("https://api.example.com", process.env.FMSG_API_KEY!);
console.log(await client.address());
const inbox = await client.listInbox(10);
const sent = await client.send({ to: ["@bob@example.com"], topic: "Hi", body: "Hello from code" });
console.log(sent.id, sent.redactions);
client.close();

send() replaces selected credential patterns in the body and topic before creating the draft. Its result includes the replacement count (redactions) and transmitted topic. Attachments are unchanged. Use streamAttachment() to consume large files incrementally; consume or cancel its stream.

Applications with their own authorization integration can pass a TokenProvider instead of an API-key string. The client shares renewal across concurrent requests and keeps the authenticated address fixed. See the token-provider contract. This library interface is also used by the executable's optional OAuth mode.

Development

npm ci
npm run typecheck && npm run build && npm test
npx @modelcontextprotocol/inspector node dist/index.js          # stdio, with FMSG_API_URL/FMSG_API_KEY set
bash .github/scripts/run-fmsg-docker-e2e.sh                     # end to end on two real fmsg stacks

See AGENTS.md for layout and conventions, ROADMAP.md for remaining integration work, and CHANGELOG.md for release notes.

MIT licensed