MCP server (feelc mcp)

feelc ships as a Model Context Protocol server so any MCP-capable agent (Claude Code/Desktop, Cursor, …) can author rules and have the deterministic engine verify, run and explain them — the LLM writes, feelc decides (ADR 0008, ADR 0026). The model never sits on the execution path; every tool is pure, deterministic Go that reuses the same code as feelc run/verify/….

Run it

feelc mcp        # speaks JSON-RPC 2.0 over stdin/stdout (newline-delimited); no flags

Configure a client

One command — feelc mcp install writes/merges the config for you, idempotently (it never clobbers other servers or sibling keys, and points at this binary's absolute path):

feelc mcp install                          # → ./.mcp.json (Claude Code project scope)
feelc mcp install --target claude-desktop  # → the per-OS claude_desktop_config.json
feelc mcp install --print                  # just print the JSON snippet, write nothing

Or add it by hand — Claude Desktop / Code (claude_desktop_config.json or .mcp.json):

{
  "mcpServers": {
    "feelc": { "command": "feelc", "args": ["mcp"] }
  }
}

Any client that launches a stdio MCP server works the same way — point it at the feelc binary with the mcp argument. Installing the Claude Code plugin (/plugin install feelc@feelc) wires this server up automatically.

Tools

Every tool takes the .rules DSL source as rules; the engine is the oracle.

Tool Arguments Returns
feelc_verify rules {hash, findings, blockers} — blockers==0 ⇒ buildable
feelc_run rules, decision, input {decision, output} (exact decimals)
feelc_explain rules, decision, input justification trace (winning rule + cells)
feelc_required rules, decision {decision, inputs} — only ask the user for these
feelc_check rules, claims[] per-claim verdict (supported/contradicted/error) + blockers
feelc_graph rules {mermaid, dot, findings}
feelc_model rules {name, inputs, decisions} — the model surface

A tool that fails (e.g. a compile error) returns an MCP tool error carrying the structured diagnostic ({file, line, col, code, message, suggestion}), so the agent can read it and repair — the red→green authoring loop.

Quick check

printf '%s\n%s\n' \
  '{"jsonrpc":"2.0","id":1,"method":"initialize","params":{}}' \
  '{"jsonrpc":"2.0","id":2,"method":"tools/list"}' \
  | feelc mcp

This prints the server handshake and the seven tool descriptors.