remote-fusion-mcp

A remote MCP server for Fusion 360, hosted on an Azure VM. It exposes 36 Fusion tools over Streamable HTTP at one stable HTTPS domain. Hermes calls it from a WSL sandbox; a connector on the Windows laptop dials out to it and drives the local Autodesk Fusion MCP. The user watches geometry change in their own Fusion window.

Fusion's API only runs in-process on the desktop, so the modeling itself can never happen in the cloud. What the remote server provides is the endpoint, identity, tenant isolation, tool allowlisting, rate limiting, replay protection, and audit.

Azure VM                    apps/server
  /mcp        Streamable HTTP MCP endpoint (stateless, JWT)
  /connector  WebSocket hub for desktop connectors
     ^                    ^
     | HTTPS              | outbound WSS
     |                    |
WSL sandbox          Windows host           <- same laptop
NemoHermes           connector + Fusion 360
                       apps/connector

Both arrows point up. Nothing dials into the laptop, which is why this works behind NAT and through an OpenShell egress allowlist.

Layout

See docs/architecture.md for the full module map, the boundary rules and the conventions. Short version:

• packages/contracts — the only thing both sides share: tool definitions, wire schemas, auth claims, error codes. The broker never imports Fusion code; the connector never imports MCP transport code.
• apps/server — one Node service: MCP endpoint, connector hub, auth, registry, guard (rate limit + replay), audit.
• apps/connector — desktop client: outbound WS, auto-reconnect, and a pluggable Fusion backend (mock for dev, mcp to wrap the real local server).
• tests/smoke.mjs — end to end test, no Fusion required.

Quick start (no Fusion needed)

npm install
cp .env.example .env        # dev secrets are fine for local

# terminal 1
npm run server

# terminal 2: mint a connector token, then run the connector
CONNECTOR_TOKEN=$(node tests/mint-token.mjs connector t1 userA devA) \
BROKER_WS_URL=ws://localhost:8080/connector FUSION_BACKEND=mock \
npm run connector

Then drive it as Hermes would, using a Hermes token:

node tests/mint-token.mjs hermes t1 userA   # prints a JWT for the Authorization header

Point any MCP client at http://localhost:8080/mcp with Authorization: Bearer <that token>.

Run the tests

npm test          # coverage + smoke

coverage checks the surface is fully wired: every tool has a mock handler and a local name mapping, no tool exposes arbitrary execution, all are annotated.

smoke runs the whole path end to end with no Fusion: registration, tools/list, document, rectangle, parameter update, extrude, fillet, pattern, mass properties, export, undo, invalid arguments, cross-user isolation, unavailable session, and scope-based denial.

Skills

Five workflow templates served as MCP prompts from the VM: create-box, parametric-part, mounting-plate, inspect-design, safe-edit. They live on the server, so editing one changes it for every connected user with no desktop redeploy. They only reference declared tools, and npm run coverage fails the build if that ever stops being true.

Tool surface

36 tools across document, inspection, sketch, parameters, features, bodies, measurement, export, view, and timeline. All lengths in centimeters, all angles in degrees. The map in packages/contracts/src/index.ts is the whole surface — nothing outside it exists, which is how default-deny is enforced structurally. There is deliberately no run_python or generic api_call tool; those are remote code execution on a user's desktop.

Wiring the real Fusion

On the Windows machine, with Fusion open and its MCP add-in started:

FUSION_MCP_URL=http://127.0.0.1:8765/mcp npm run introspect

That prints the local server's real tool names. Update LOCAL_TOOL_NAMES in apps/connector/src/fusion/tool-map.ts to match — the shipped names are placeholders and are not to be trusted. Then set FUSION_BACKEND=mcp.

The local schema is the source of truth for argument shapes. If it disagrees with ours, translate in backend.ts rather than bending the remote surface.

Deploy to the Azure VM

See docs/azure-vm.md for the full walkthrough. Short version:

sudo ./infrastructure/azure/setup-vm.sh fusion-mcp.example.com

Installs Node and Caddy (automatic TLS), creates an unprivileged service user, generates fresh secrets, and runs the server under systemd. Open 443 and 80 in the NSG; leave 8080 closed — the server binds locally and Caddy fronts it.

docker compose up runs the server plus a demo mock connector for local dev.

One VM, one server process is the intended shape, and the in-memory session registry is correct for it. Only multi-replica needs Redis; SessionRegistry in apps/server/src/registry.ts is the seam if that day comes.

Security model

• North (Hermes to broker): OIDC + short-lived JWT. Dev uses HS256; production swaps to RS256 with JWKS.
• South (connector to broker): device token on the outbound WS. Production swaps to mTLS or a device bound credential. The connector never opens an inbound port.
• Default deny: only the closed tool surface exists. A token scope can further restrict it per user. No arbitrary code execution tool exists.
• Every call is rate limited, deduped within a short window, and audited with a correlation id.

custom-fusion-mcp