λ lambdacad-mcp

Let AI draw in your CAD. An MCP server for any AutoLISP-capable CAD — no COM, no SDK, no plugins. Pure AutoLISP. The reference adapter drives BricsCAD on Linux: the first MCP server for a professional DWG CAD on Linux.

"Draw a DN80 flange: front view, hatched section, dimensions — then model it in 3D."

Real-time capture, unedited: front view with bolt circle, hatched section, radial/diameter dimensions, leader, title block — then the same flange modeled as a 3D solid with an AI-written parametric AutoLISP loop and one boolean subtraction. ~20 seconds, 12 tool calls.

Complete 2D drawing, drawn by Claude	3D solids with booleans	Clean render for AI vision

Why this exists

Every other MCP server for a professional DWG CAD (multiCAD-mcp, autocad-mcp, CAD-MCP, ...) talks to the CAD through Windows COM — none of them can run on Linux, and none of them does real 3D.

BricsCAD is the professional DWG CAD that does run natively on Linux. This server drives it through a mechanism that needs no API at all: an AutoLISP file bridge. The MCP server writes LISP expressions to a file, triggers a tiny MCP command inside the CAD, and reads the result back. That's the whole trick — and it unlocks everything the CAD can do, including 3D solid modeling.

Because the bridge is plain AutoLISP + standard DXF entmake + AutoCAD-compatible commands, it is portable by construction: AutoCAD, ARES Commander (also Linux-native!), ZWCAD, GstarCAD and the IntelliCAD family all speak the same language. Porting means swapping the autoload file name and the keystroke trigger — adapters welcome.

MCP client (Claude, Cursor, ...)            BricsCAD (Linux)
        │                                        │
        ▼                                        ▼
  server.py (FastMCP, 105 tools)          c:MCP AutoLISP command
        │        writes LISP  ──►  /tmp/mcp_bricscad.in
        │        triggers via xdotool ──► reads, evals line by line
        │        reads result ◄──  /tmp/mcp_bricscad.out

What it can do

• Full 2D drafting — lines/arcs/polylines/splines, real HATCH (with islands), dimensions incl. radius/diameter/angular, leaders, linetypes (chain/hidden), text styles, trim/extend/fillet/chamfer/offset/mirror/arrays, blocks
• Semantic shapes — slot, bolt_circle, rounded_rect, polygon, center_mark: one call, the server does the math
• 3D solid modeling — box/cylinder/sphere/cone/wedge/torus, extrude/revolve/sweep, union/subtract/intersect, slice, fillet/chamfer on solid edges, 3D move/rotate around any axis — and flatshot_view: auto-generate correct 2D drawing views from the 3D model
• Batch & atomic — draw_batch executes N operations in one round-trip, wrapped in one undo group (one batch = one Ctrl+Z)
• Talk about what the user selected — get_selection reads the GUI pickfirst set; select_by/select_window highlight what the AI found before acting; modify tools accept "selection"
• Verification without screenshots — bounding boxes, key points, exact lengths/areas, intersection checks (collision detection), solid volume/centroid
• Eyes when needed — render_view: BricsCAD's internal PNG render (clean, no UI, ~30 KB) for vision models
• Deliverables — save DWG, export DXF, PDF and STL (export_stl: 3D solids straight to the slicer, parked on the origin)
• Escape hatch — run_lisp evaluates arbitrary AutoLISP (multi-line, multi-expression): capable models write parametric code and love it

Full reference: docs/TOOLS.md · How it works: docs/ARCHITECTURE.md

Measured, not promised

Details and model-behavior notes (Sonnet writes parametric LISP, Haiku uses typed tools): docs/BENCHMARKS.md

Tests

BRICS_DISPLAY=:0 python tests/test_tools.py     # 49 checks, ~2 min
python tests/test_tools.py 3d edges export      # or just some groups
python tests/test_tools.py --list

An integration suite: it drives the CAD you already have running, then measures what came out — closed-form volumes, bounding boxes, the value stored inside a dimension, triangle counts read out of the STL. Never "no error".

That distinction is the whole point. The bridge answers OK whenever the CAD accepted the command, which is not the same as the CAD having done what was asked. Every bug found so far was of that shape: the classic CHAMFER silently switching to all edges, edge picking swallowing the whole part from a wide zoom, and — the one that reached a release — running object snap dragging programmatic coordinates onto nearby geometry, so five blocks placed in a row each landed on the previous. A volume, a bounding box and a measured dimension are exact signatures: all three surfaced as arithmetic that did not add up.

The suite draws in a scratch region (x ≥ 50000), erases it afterwards, and fails if the entity count does not come back to where it started. It still works in the active drawing, so point it at a scratch one.

Usage telemetry (local only)

With 105 tools, the interesting question is which ones earn their place — and which fail often enough to signal a docstring or design problem. The server appends one JSON line per call to ~/.lambdacad/usage.jsonl:

{"t": 1786526823.4, "tool": "draw_batch", "ms": 198, "err": 0}

Tool name, duration, error flag. Never the arguments — no coordinates, no paths, no drawing data — and nothing ever leaves your machine. Disable with LAMBDACAD_TELEMETRY=0.

python scripts/usage_report.py              # per-tool calls, error rate, p50
python scripts/usage_report.py --core 95    # smallest set covering 95% of calls
python scripts/usage_report.py --reset      # archive the current log

The --core output is a data-driven candidate for CORE_TOOLS (compact mode): after a few weeks of real use, the choice of which tools deserve a place in a schema-limited client stops being a guess. Early signal from the integration suite itself: run_lisp alone is ~half of all calls — on clients that load schemas on demand (Claude Code's tool search), the long tail of typed tools costs nothing until used, so the rich docstrings stay.

Quick start

Requirements: Linux, BricsCAD (any recent version — tested on V26), Python 3.10+, xdotool.

git clone https://github.com/Psalmustrack/lambdacad-mcp
cd lambdacad-mcp
./install.sh

The installer creates a venv, installs the AutoLISP bridge into BricsCAD's Support/on_doc_load.lsp (idempotent, marker-delimited), and prints ready-to-paste config for Claude Code / Claude Desktop.

Then:

1. Start BricsCAD and open a drawing (the bridge loads when a document opens).
2. Start a new AI session — the bricscad tools appear.
3. Ask: "draw the plan of a 4×3 m room with a door and dimensions".

Configuration (env vars)

Security note

run_lisp and run_command execute arbitrary code inside BricsCAD by design — that's what makes the server fully general. Only connect MCP clients you trust, and treat the bridge files as trusted input. The bridge itself never loads foreign files (read+eval only) and honors BricsCAD's SECURELOAD.

Troubleshooting

Related projects

Credit where due: multiCAD-mcp inspired the project; puran-water/autocad-mcp independently validated the LISP-bridge approach on Windows.

Roadmap

• Focus-free dispatch (targeted X synthetic events — no window activation)
• In-process socket bridge (no keystroke trigger, ~0 latency)
• VIEWBASE paper-space drawing views; sheet metal (SMUNFOLD) and BIM commands
• Compact-mode behavioral benchmark

License

Apache 2.0 — with an explicit patent grant and trademark protection for the lambdacad-mcp name.

Trademarks

AutoCAD® and AutoLISP® are registered trademarks of Autodesk, Inc. BricsCAD® is a registered trademark of Bricsys NV (Hexagon). ARES Commander®, ZWCAD® and all other CAD product names mentioned are trademarks of their respective owners, used here only to identify compatibility. This project is independent open-source software: it is not affiliated with, endorsed or sponsored by Autodesk, Bricsys, or any CAD vendor.