# SolveCraft and AI agents (MCP)

SolveCraft speaks the [Model Context Protocol](https://modelcontextprotocol.io) 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

```sh
# 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`):

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

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

```json
{
  "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. |
| `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), sketches (curves, points, constraints, profiles, DOF, solve status), selection, undo depth. |
| `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 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`). |

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 undoes as many steps as the batch added to the undo history; batches that
themselves call `edit.undo`, `file.new` or `doc.open` are not rolled back exactly.

## A typical session

```text
→ {"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.