fast-engineer-mcp

A local MCP server that gives AI assistants fast.engineer's verified electrical engineering calculations: cable sizing, fault current, maximum demand, unit conversion and more, each result with a provenance envelope (method, inputs, assumptions, warnings).

Nothing is calculated on your machine. The package starts a small server over stdio, lists the tools the hosted service offers, and forwards every call over HTTPS to https://fast.engineer/mcp. Accounts, quotas and the calculation engines live on the service.

Install

python -m pip install fast-engineer-mcp

or run it without installing, with uv:

uvx fast-engineer-mcp

Python 3.11 or newer.

Free key (optional)

The service works anonymously with a small per-IP quota. A free key raises it. Enter your email at https://fast.engineer/mcp/, then store the key that is emailed to you:

fast-engineer-mcp login

The key is written to a file only your user can read (~/.config/fast-engineer/credentials.json on Linux, ~/Library/Application Support/fast-engineer/ on macOS, %APPDATA%\fast-engineer\ on Windows). FAST_ENGINEER_API_KEY in the environment, or --key, takes precedence over the file. fast-engineer-mcp logout removes it.

Check the connection and see the current tool catalogue:

fast-engineer-mcp check

Connect a client

Claude Desktop — claude_desktop_config.json:

{
  "mcpServers": {
    "fast-engineer": {
      "command": "fast-engineer-mcp"
    }
  }
}

(Use "command": "uvx", "args": ["fast-engineer-mcp"] to run it with uv.)

Claude Code:

claude mcp add fast-engineer -- fast-engineer-mcp

Cursor — ~/.cursor/mcp.json, VS Code — .vscode/mcp.json, and any other client that launches stdio servers: the command is fast-engineer-mcp with no arguments. Pass the key through the environment instead of login if the client manages secrets for you:

{
  "mcpServers": {
    "fast-engineer": {
      "command": "fast-engineer-mcp",
      "env": { "FAST_ENGINEER_API_KEY": "YOUR_KEY" }
    }
  }
}

Clients that can connect to remote MCP servers directly do not need this package: point them at https://fast.engineer/mcp (see the connection guide). This package is for clients that only launch local servers, or that cannot send an Authorization header.

Using it

Ask the assistant an engineering question, for example:

Size a copper cable for a 40 A three-phase load over 100 m at 400 V with a 5 % voltage-drop limit.

The assistant calls the matching tool, and the result comes back with the calculation method, the inputs used, the assumptions made and any warnings. Quote them. Tools are listed with full input and output schemas, so the assistant knows which parameters it must ask you for.

Limits are per hour and per day. When a quota is used up the tool returns a message saying so, how long to wait, and how to get a key. Listing tools never uses quota.

Command reference

fast-engineer-mcp                 serve over stdio (what clients run)
fast-engineer-mcp login [--key K] store a key (prompts when --key is omitted)
fast-engineer-mcp logout          forget the stored key
fast-engineer-mcp check           connect and print the server and its tools
fast-engineer-mcp --version

Serve and check accept --url (another endpoint, also FAST_ENGINEER_MCP_URL), --key, --anonymous (ignore any configured key) and --verbose (debug logging on stderr).

How it works

AI client ──stdio──▶ fast-engineer-mcp ──HTTPS──▶ https://fast.engineer/mcp ──▶ calculation engines
                      (this package)               (auth, quotas, audit)

• tools/list, tools/call, prompts/list and prompts/get are forwarded as-is. The catalogue is whatever the service releases; nothing is hard-coded here, so an installed client never offers a withdrawn tool.
• The key travels as Authorization: Bearer …. It is never logged.
• HTTP-level replies from the service (429 quota, 503 paused, 401/403 credentials) become readable tool errors instead of a bare failure.
• If the service is unreachable the server still starts; each call reports the problem, and the connection is retried on the next call.

Development

git clone https://github.com/fast-engineer/fast-engineer-mcp
cd fast-engineer-mcp
python -m venv .venv && . .venv/bin/activate
python -m pip install -e ".[dev]"
pytest

The tests run a fake endpoint in-process (and, for the end-to-end test, on a local port with uvicorn); they never contact fast.engineer.

Releases: bump __version__ in src/fast_engineer_mcp/__init__.py, add a changelog entry, tag vX.Y.Z and publish a GitHub release. The publish.yml workflow tests, builds and uploads to PyPI through trusted publishing.

Licence

MIT, see LICENSE. The licence covers this client only; the calculation engines and the fast.engineer service are governed by the service's own terms of use.