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Onshape MCP Server

Project Conventions

  • PRs target develop, never main. main is only for releases.
  • Tests: pytest (all tests must pass). Coverage: pytest --cov.
  • Style: ruff check and ruff format --check.

Assembly Workflow (5-Step Methodology)

When building assemblies, follow this order:

  1. Ground a reference part — First instance is grounded by default. All positions are relative to this part.
  2. Get face IDs — Use get_body_details on the Part Studio. Face IDs are reusable across all instances of the same part.
  3. Create mates in batches — Do 3-5 at a time, verify between batches with get_assembly_features (all OK?) and get_assembly_positions (positions correct?).
  4. Verify positions relative to the reference part — Always compute instance_pos - reference_pos since the reference may not be at origin.
  5. Test motion mates — Create slider/revolute mates without limits first. Verify direction and animation before adding limits.

See knowledge_base/assembly_workflow_guide.md for the full guide and examples/cabinet_assembly.md for a worked example.

Critical Assembly Gotchas

  • Instance order = direction: For slider/revolute/cylindrical mates, the first instance moves relative to the second. Swap the order to reverse the direction.
  • Limits can break the solver: API-set limits trigger a full re-solve that may flip parts. Create mates without limits first, add limits in the Onshape UI.
  • Fixed instances can't be moved: transform_instance and set_instance_position fail on grounded parts (API 400 error).
  • Absolute positions need reference offset: If the reference part is at (100, 200, 300), a part at relative (5, 10, 0) needs absolute (105, 210, 300).
  • MC offsets are in local coords: Flipping the Z-axis changes all axes (right-hand rule), which changes how X/Y offsets map to world space.

Key File Paths

  • onshape_mcp/server.py — MCP server with all tool definitions
  • onshape_mcp/builders/mate.py — MateConnectorBuilder, MateBuilder
  • onshape_mcp/api/assemblies.py — Assembly API operations
  • knowledge_base/assembly_workflow_guide.md — Full assembly methodology
  • knowledge_base/cad/cad_best_practices.md — CAD design principles
  • examples/cabinet_assembly.md — Complete cabinet assembly walkthrough