Fm MCP Server

fm-mcp

An MCP server that lets coding agents such as Claude Code and Codex hand simple text work to the on-device model for Apple Foundation Models, the fm that ships with macOS 27. It's free, private, and runs offline.

mcp-name: io.github.yrangana/fm-mcp

Site: yrangana.github.io/fm-mcp, with what we learned building it: the measured limits of the on-device model.

Why

Coding agents spend paid, remote tokens on simple jobs, like summarising a log or pulling fields out of a document. A small model already on your Mac can do much of that for nothing, and your text never leaves the machine. fm-mcp lets the agent hand off that work, and tells it plainly what the small model can't do.

Install

fm-mcp is a public beta (0.1.x): it works and is tested, but tool details may change between releases. CHANGELOG.md lists what changed.

You need a Mac with Apple Silicon (Intel Macs are not supported), macOS 27 with Apple Intelligence turned on, and the fm licence accepted once (fm license). That's the whole support policy: fm-mcp doctor checks each of these, and nothing else is supported.

No Homebrew yet? Install it from brew.sh first. Its installer asks to install Xcode's command line tools: accept, and let it finish.

brew install yrangana/tap/fm-mcp
fm-mcp install    # sets up Claude Code and Codex
fm-mcp doctor     # checks everything fm-mcp needs

Then start a new Claude Code or Codex session. fm-mcp starts the model server (fm serve) itself the first time a tool is called, and stops it when the session ends.

Other ways to get the binary:

curl --proto '=https' --tlsv1.2 -LsSf https://github.com/yrangana/fm-mcp/releases/latest/download/fm-mcp-installer.sh | sh
cargo install fm-mcp

What fm-mcp install changes

• Claude Code: adds the fm-mcp MCP server to ~/.claude.json (through claude mcp add when the claude command is there), and installs the fm-delegate skill in ~/.claude/skills/.
• Codex: adds [mcp_servers.fm-mcp] to ~/.codex/config.toml, and a marked section in ~/.codex/AGENTS.md (or in AGENTS.override.md, if you use one).

It backs up each file before changing it and prints every path it touched; running it again changes nothing. --claude or --codex picks one agent, --dry-run shows the changes without writing, and --no-guidance skips the skill and the AGENTS.md section. fm-mcp install --uninstall removes everything it added.

The skill and the AGENTS.md section tell the agent when to delegate. Without them the agent sees only the tool descriptions, and delegates less well.

Or: the Claude Code plugin

The plugin gives Claude Code the MCP server and the skill, without fm-mcp install. It still needs the binary on your PATH, so install that first (Homebrew above); otherwise Claude Code reports the server as failed to connect.

/plugin marketplace add yrangana/fm-mcp
/plugin install fm-mcp@fm-mcp

Use the plugin or fm-mcp install for Claude Code, not both: both together give Claude Code two fm-mcp servers. With the plugin, run fm-mcp install --codex to set up Codex only.

Other MCP clients

fm-mcp is a plain stdio MCP server, so other clients that run local servers on the same Mac can use it. fm-mcp install doesn't set them up, and they don't get the delegation guidance: the agent decides from the tool descriptions alone. Each client below was tried on a real Mac; others may work but are untested.

Claude Desktop (tried with 2.31226.1, 2026-10-11). In Settings › Developer › Edit Config, add this to mcpServers in claude_desktop_config.json, then quit and reopen Claude Desktop:

"fm-mcp": {
  "command": "/opt/homebrew/bin/fm-mcp",
  "args": []
}

Use the full path (which fm-mcp prints it). fm-mcp helps most with files on your Mac: give Claude the file's path and it passes path, so the text never goes through Claude. For a web page, Claude Desktop reads the page itself first, so delegating it saves nothing.

Devin (formerly Windsurf) (tried with Devin.app 3.8.20, 2026-10-11). Add this to mcpServers in ~/.config/devin/mcp_config.json:

"fm-mcp": {
  "type": "stdio",
  "command": "/opt/homebrew/bin/fm-mcp",
  "args": []
}

Tools

summarise, extract and classify take text, or a path to a text file (up to 1 MB) so the agent never has to read the file itself. docs/TOOLS.md lists every parameter, with example calls.

Limits

• Small model, small context: about 8K tokens, shared by the input and the answer. Logs full of IDs and numbers use far more tokens per word than prose.
• Not for code, maths, reasoning, facts the text doesn't contain, or anything where a wrong answer is costly and you can't check it.
• It makes mistakes. Summaries can drop or distort details. extract often returns null for a field the text doesn't contain, but not reliably: on short invoices a missing PO number took the supplier's name in 10 of 25 runs. When the model gets stuck on a field, fm-mcp keeps the finished fields, sets the stuck one to null and says so; that call takes about 15 s. Ask only for fields the text is likely to have. Nested schemas are less reliable than flat ones. Check what matters.
• One request at a time. The model is shared by every session on the Mac, so calls queue: a short extract takes 1–2 s, a long summarise a minute or more.
• Safety filter: the on-device model sometimes refuses harmless input. fm-mcp reports that clearly, and the agent does the task itself.

Privacy

Everything runs on your Mac. fm-mcp talks to fm serve over a local Unix socket and sends nothing over the network. Your text goes only to the on-device model. SECURITY.md has the details, and how to report a security problem privately.

Troubleshooting

Run fm-mcp doctor. It checks the Mac, macOS, fm, the licence, the model, a test request, and both agents' setup, and says how to fix anything that fails.

Uninstall

fm-mcp install --uninstall
brew uninstall fm-mcp

Development

git clone https://github.com/yrangana/fm-mcp.git
cd fm-mcp
cargo build --release
./target/release/fm-mcp install    # points your agents at this build

cargo test --features fake-fm      # needs no Apple Intelligence: uses a fake fm
cargo clippy --all-targets --all-features -- -D warnings
cargo fmt --check
scripts/real_fm_smoke.sh           # checks against the real fm on this Mac

AGENTS.md has the architecture, the tested facts about fm serve, and the project rules. docs/MANUAL_TEST.md is the checklist for each release.

Licence

MIT. fm-mcp is not affiliated with or endorsed by Apple. It calls the fm tool installed on your Mac and includes nothing from Apple.