@openmep/mcp

An MCP server that gives any agent OpenMEP's deterministic HVAC duct sizing. No editor, CAD host, account, or network access: the agent describes the ducts, the engine sizes them.

{
  "mcpServers": {
    "openmep": { "command": "npx", "args": ["-y", "-p", "@openmep/mcp", "openmep-mcp"] }
  }
}

Claude Code: claude mcp add openmep -- npx -y -p @openmep/mcp openmep-mcp. Requires Node.js 24+. Transport: stdio.

Tools

| Tool | Input | Returns | |---|---|---| | size_duct | cfm, optional role (main/branch/runout), airflowType (supply/return/exhaust/outside-air), existing section ({diameterIn} or {shape: rect\|oval, widthIn, heightIn}) | Recommended standard round diameter, velocity, friction; for an existing section its circular equivalent and ok/undersized/oversized | | size_duct_network | items: terminals with requiredCfm, segments (optionally with existing), fittings, equipment, linked by connectedItemRefs | Every loaded segment's CFM, role, recommendation, and grade; findings for missing CFM, disconnected terminals, dangling references, mixed systems, undersized and oversized runs |

Both tools are read-only and idempotent. Invalid input comes back as a tool error naming the field (NetworkInputError [invalid-item]: items[2].requiredCfm: ...) so the model can correct it.

Example prompt that works in a fresh Claude Code session with only this server:

I have a furnace feeding a supply trunk that tees into two runouts: bedroom 1 needs 120 CFM, bedroom 2 needs 90 CFM, and the living room is fed off the trunk end at 200 CFM. All drawn as 6 inch round. One return grille in the hall at 410 CFM on a 10 inch drop. Is anything undersized?

The agent builds the network in one size_duct_network call and reports 7, 6, and 8 in runouts, an 11 in trunk, and a return drop just under size at 10 in.

Engineering basis and limits

Equal friction at 0.08 in. w.g./100 ft with role-based velocity caps per system, rounded up to standard round diameters; see @openmep/hvac-domain for the formula, sources, and what is excluded (fitting losses, static pressure summation, fan selection). Airflow is a design input the server never estimates. Treat results as preliminary sizing for an engineer to review.

The server uses the official MCP TypeScript SDK and speaks the initialize-handshake protocol revisions (through 2025-11-25), which current clients support.

Library

import { createOpenMepServer } from "@openmep/mcp";
await createOpenMepServer().connect(transport); // any MCP server transport