OneCAD/docs/mcp.md

SolidCraft and AI agents (MCP)

SolidCraft speaks the Model Context Protocol over stdio (JSON-RPC 2.0, one message per line; protocol revisions 2025-06-18, 2025-03-26 and 2024-11-05). An agent models exactly like a person does: every tool runs engine commands, the same ones the toolbar, palette, scripts and control channel use, and never opens a dialog.

Start it

# Headless: an in-process design session, no window (CPU renders for screenshots).
solvecraft-cli mcp

# Headless, starting from a design file or a command script.
solvecraft-cli mcp --in part.solvecraft

# Bridged to the running app, so you can watch the agent model:
solvecraft --control 7878 &
solvecraft-cli mcp --connect 127.0.0.1:7878

stdout carries only protocol messages; errors go to stderr. Bridged screenshots are written to a temporary file by the app and read back by the server, so both must run on the same machine.

Client configuration

Claude Code (project .mcp.json, or claude mcp add solvecraft -- solvecraft-cli mcp):

{
  "mcpServers": {
    "solvecraft": { "command": "solvecraft-cli", "args": ["mcp"] }
  }
}

Claude Desktop (claude_desktop_config.json), bridged to a running app:

{
  "mcpServers": {
    "solvecraft": { "command": "/path/to/solvecraft-cli", "args": ["mcp", "--connect", "127.0.0.1:7878"] }
  }
}

Tools

Tool	Arguments	What it does
list_commands	filter?	The command catalog: id, label, tab/panel, shortcut, parameter docs, enabled now. filter matches id, label, tab and panel.
execute	command, params?	Run any command with JSON parameters (list_commands shows them). Failures leave the design unchanged.
batch	commands: [{command, params}], rollback? (default true)	Run steps in order, stop at the first error and undo the completed steps. The reply names the failing step (failed.index, failed.command, failed.error) and every result before it.
inspect_design	measure? (default true), sketches? (default true)	Parameters, timeline (type, suppressed, rolled back, error, warning, timing), bodies (volume, area, centre of mass, bbox, face/edge/vertex counts, visible), sketches (curves, points, constraints, profiles, DOF, solve status, visible), selection, undo depth. Hide helper sketches, bodies or construction planes with execute browser.visibility ({items, visible} or {folder, visible}).
measure	body?	Volume (mm³), area (mm²), centre of mass, bbox and topology counts per body, with totals.
body_topology	body	Edges (index, midpoint, length, ends) and faces (index, area, centroid, normal). An edge midpoint is the [x,y,z] that picks that edge for fillet and chamfer.
set_parameter	name, value (number or expression), unit?, comment?	Create or change a user parameter and recompute; reports recomputed (features rebuilt), restored (features whose earlier result for the same inputs was reused, e.g. a value set back) and features that now fail.
screenshot	view? (iso, top, front, back, bottom, left, right, home, fit), path?, width?, height?, source? (window, model)	A PNG image content block. Headless: CPU render fitted to the model. Bridged: the app window (default) or a model render with the app's camera.
export	path, format? (step, 3mf, stl, stla, obj), bodies?	STEP, 3MF or mesh export.
undo, redo	—	Undo / redo one change.
new_design	name?	Start an empty design.
open	path	Open a .solvecraft design, a STEP file (.step/.stp) as a new design holding its bodies (an Import base feature), or a 3MF/STL file as mesh bodies.
save	path?	Save the design (to its current file without path).

To change a feature after it is made, read its definition with execute timeline.inspect ({feature}: id or name) and change fields with timeline.edit ({feature, set}; e.g. {"extent": {"distance": "30"}}). The fields per feature type are listed in the book's Solid features chapter.

Resources: solvecraft://design (inspect JSON with measurements) and solvecraft://commands (the catalog).

Errors: an unknown tool or arguments that don't match the tool's schema are JSON-RPC -32602 errors; an engine failure (bad command parameters, a feature that fails) is a tool result with isError: true and the engine's message, which the model can read and correct.

batch rollback goes back to an engine checkpoint (edit.checkpoint) taken before the first step: the design, the active sketch and the undo history are restored exactly as they were, however long the history is. Batches that call file.new or doc.open get the design back, not its file path. (An older app without checkpoints falls back to undoing as many steps as the batch added to the undo history.)

Demo: an enclosure built over MCP

examples/demo/enclosure_mcp.py is a small MCP client. It drives a running app (solvecraft --control PORT) through solvecraft-cli mcp --connect and builds an electronics enclosure one execute call at a time (enclosure_steps.json): a rounded box, shelled open at the top, a USB port cut through a side wall, four screw bosses, and then a change of the width parameter that the timeline rebuilds. examples/demo/record.sh records the run with ffmpeg, with each step's caption burnt in.

examples/demo/enclosure_ui_mcp.py (v2) builds the same part the way a person would: the modelling goes through the app's UI over its control channel. The pointer glides to toolbar buttons and faces and clicks them, the real dialogs open, the sketches go on the model's faces, and the camera follows. The parameters, the sketch dimensions bound to them and two extrudes go over MCP. With --story v3 (the default) one standoff is patterned 2 × 2 in the Rectangular Pattern dialog with spacings from the parameters, and width and wall are then changed in the Parameters dialog. record_ui.sh records it and draws the pointer and the captions.

A typical session

→ {"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2025-06-18","capabilities":{},"clientInfo":{"name":"me","version":"1"}}}
→ {"jsonrpc":"2.0","method":"notifications/initialized"}
→ {"jsonrpc":"2.0","id":2,"method":"tools/call","params":{"name":"batch","arguments":{"commands":[
     {"command":"sketch.create","params":{"plane":"XY"}},
     {"command":"sketch.rectangle.two_point","params":{"p0":[0,0],"p1":[40,30]}},
     {"command":"sketch.finish"},
     {"command":"solid.extrude","params":{"distance":20}},
     {"command":"solid.fillet","params":{"edges":[[0,0,10]],"radius":3}},
     {"command":"sketch.create","params":{"plane":"XY"}},
     {"command":"sketch.circle.center","params":{"center":[20,15],"radius":5}},
     {"command":"solid.extrude","params":{"distance":20,"operation":"cut"}}]}}}
← {"jsonrpc":"2.0","id":2,"result":{"content":[{"type":"text","text":"{\"ok\": true, \"completed\": 8, \"results\": [...]}"}]}}
→ {"jsonrpc":"2.0","id":3,"method":"tools/call","params":{"name":"measure","arguments":{}}}
← … "total": {"volume_mm3": 22390.6, …}
→ {"jsonrpc":"2.0","id":4,"method":"tools/call","params":{"name":"screenshot","arguments":{"view":"iso"}}}
← {"jsonrpc":"2.0","id":4,"result":{"content":[{"type":"image","mimeType":"image/png","data":"iVBORw0…"},{"type":"text","text":"{\"source\":\"model\",…}"}]}}
→ {"jsonrpc":"2.0","id":5,"method":"tools/call","params":{"name":"export","arguments":{"path":"/tmp/part.step"}}}

The tests in crates/mcp/src/tests.rs run sessions like this (box → fillet → hole, parameters, batch rollback, hostile arguments) and check the results only through MCP.