"Automate the CAD model" can mean two very different things. One is generating raw geometry from scratch — coordinates, curves, meshes — with no relationship to anything a human designer set up. The other is driving the same named parameters and feature tree a human designer already uses, and letting the CAD application's own rebuild do the rest. Only the second one is parametric, and the difference isn't academic — it's what makes an automated edit reviewable at all.
What a parametric model actually is
In a parametric CAD model, geometry is defined by named values and relationships rather than fixed coordinates: a hole positioned by a dimension relative to an edge, a boss whose height is an equation referencing another feature's parameter, a pattern whose count is itself a named value. The application records the sequence of operations that built the part — the feature tree — and can recompute everything downstream of any value that changes. That recompute is the rebuild: change one governing dimension, and every feature that depends on it, directly or through a chain of other features, updates to match.
Why this makes an AI edit contained
A tool call that sets a named dimension and triggers a rebuild can only change what that dimension and its downstream dependents are allowed to change — it isn't drawing anything, it's setting a value the model already knows how to propagate. That's a fundamentally narrower blast radius than free-form geometry generation, and it comes with the same audit trail a human edit gets: the feature tree shows exactly what changed and in what order, and the undo stack works exactly the way it would for a manual edit, because it is a manual edit's own mechanism — just triggered by a tool call instead of a mouse.
The three-stage pattern
In practice, a parametric build follows the same three stages whether a person or an AI assistant is doing it, and it's the pattern both recordings on /demos show end to end on a parametric flange:
1. Named parameters first. The governing dimensions — bore diameter, flange thickness, bolt-circle count — are defined before any geometry exists, so every later step references a name instead of a fixed number.
2. Sketch and feature construction from those parameters. Geometry is built referencing the named values, not typed in as constants — an extrude driven by the thickness parameter, a hole pattern driven by the bolt-circle count.
3. A rebuild that propagates a later change. Widen the bore or thicken the flange after the fact, and the rebuild carries that single parameter edit through every feature that depends on it — the same demonstration both recorded builds show, from two different AI assistants, on the same part.
Reads vs. writes, concretely
The real tool surfaces on the shipped desktop connectors keep this distinction explicit. Reading a feature tree, named dimensions, equations, or mass properties never mutates the document — it's pure inspection. Setting a named dimension and rebuilding, sketching and extruding, or placing an assembly component are writes: scoped, attributable changes that go through the model's own parametric relationships. See the tool references on SolidWorks and Inventor for the current read/write surface on each.