The model writes the model.

Code is the model. A SimpleCAD model is a marimo notebook — a plain .py file and the only source of truth. Each feature is one cell, each dimension a scad.var. An agent edits text; the BRep follows, cell by cell, with a fact card after every step.

Name the edge, not its index.

Select by meaning. Agents cannot click. QL selects topology by what it is — type, length, position — and asserts how many it must find. The faces a fillet makes are tagged, so the next feature asks for fillet.flange_edge, not face #14.

Errors are feedback.

Verifier first. When the kernel refuses, the SDK says why in numbers: the operation, what it measured, the room it had, and how to fix it. An agent reads it, changes one value, and moves on — no stack-trace archaeology.

Change the intent, not the geometry.

Parameters and guards. Every dimension is a parameter and every assumption is a guard. Change bolt_count and the notebook rebuilds; push the bolt circle past the rim and the guard refuses before the kernel ever sees it.

Solved, not placed.

Assemblies. Assemblies are constraints — revolute joints, gear meshes, bearing seats — solved strictly. Gears, bearings and fasteners come from scad.std as real parts. Sub-assemblies are notebooks too, composed with scad.use.

Simulation survives the next revision.

One source, every solver. One package exports AP242 STEP, editable FreeCAD and MJCF. Boundary faces keep their interface.* tags through Gmsh into CalculiX, so the load case finds its faces again after every change to the model.

One notebook, many parts.

Notebooks compose. scad.use runs a part notebook with its own id and parameters, so one link_bar.py becomes crank, coupler, rocker and ground. The assembly is just another notebook, and a per-cell cache makes every rerun incremental.

Design becomes design.

Describe what the part must do. Let the model write, verify and revise the CAD.