Parametric CAD in the browser with a bouncer for AI agents

What it does

PartMode is a browser-based mechanical CAD application running on OpenCascade WebAssembly, replicad, and three.js. It handles constrained sketches, editable feature history, solid B-rep evaluation, assemblies, drawings, and standard exchange formats without requiring a desktop installation. Both people and typed AI agents work against a single document schema and exact kernel; the shaded scene is just a view of the B-rep result, not the source of truth.

The interesting bit

Agent access is treated as a permissioned security boundary rather than an opaque side channel. Anonymous human CAD lives in browser storage, while agents authenticate via revocable keys and can only edit through browser-approved sessions with visible human consent or separate, explicit headless grants—each path with its own storage and authority rules.

Key highlights

• Exact B-rep geometry from OpenCascade WASM; three.js provides the shaded view, not the truth
• Local-first: anonymous modeling in browser storage with no required account
• Typed agent support via MCP tools, with revision-safe commits and detached previews
• Full assembly support including mates, in-context component editing, exploded views, and mass properties
• STEP import/export, plus STL, AMF, and 3MF mesh exports

Caveats

• Headless committed records survive until account deletion; there is no individual project-delete control yet
• Explicitly not certification software: no engineering simulation, CAM, certified GD&T/PMI workbench, or native DWG authoring
• Imported STEP does not reconstruct a vendor’s native feature history, mates, or PMI

Verdict

A solid option for developers and designers who want local-first, browser-native parametric CAD with a principled agent integration story. Look elsewhere if you need certified manufacturing analysis or full-featured vendor-native CAD reconstruction.