fusion360-mcp-bridge: Instructions file for Claude Code

Fusion 360 MCP — Scripting Knowledge

This file is read automatically by Claude at the start of every session. It encodes hard-won knowledge about the Fusion 360 Python API so that fusion_execute scripts work first time rather than through trial and error.

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Architecture

Claude  ←→  server.py (MCP, stdio)  ←→  HTTP :7654  ←→  FusionMCPBridge add-in  ←→  Fusion API

Two tools:

• fusion_execute(script) — run arbitrary Python inside Fusion's process
• fusion_screenshot(...) — capture the active viewport as a base64 PNG

All geometry work goes through fusion_execute. Write scripts that print() their results back.

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Units

Fusion stores all geometry in centimetres (cm). When calling API methods that accept a ValueInput, pass strings like "3cm", "10mm", "45 deg" — Fusion converts them. Raw floats are always in cm.

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Script structure

import adsk.core, adsk.fusion, math

def run(_context):
    comp = design.activeComponent
    # ... work here ...
    print("done")

The context provides adsk, app, ui, and design (may be None if no design is open). Defining run(_context) is optional but conventional. Always print() results — return values are ignored.

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Design mode

Check before scripting anything parametric:

import adsk.fusion
def run(_context):
    mode = design.designType
    if mode == adsk.fusion.DesignTypes.ParametricDesignType:
        print("parametric")
    else:
        print("direct")

• Parametric mode: features are recorded in the timeline. TemporaryBRepManager bodies need a BaseFeature wrapper (see below).
• Direct mode: operations happen immediately, no timeline. Some parametric APIs are unavailable.

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Revolve / profile rules

The single most common error: profile crossing the revolve axis.

Fusion's modelling kernel rejects any revolve where the sketch profile has geometry on both sides of the axis — even if the crossing point is just a line endpoint at the origin.