# Backend capabilities

## build123d through CAD Skill

Use as the default STEP-first backend.

Best fit:

- Brackets, connecting rods, cranks, levers, shafts, bushings, flanges, plates, housings, covers, enclosures, fixtures, and machined features.
- Parametric Python sources with holes, pockets, slots, bosses, ribs, shells, fillets, chamfers, lofts, sweeps, and revolves.
- Source-level assemblies, named datums, joints, imported components, measurements, alignment checks, and existing STEP inspection.
- STEP output compiled from executable DesignIR 2.0.

Weak signals: a specialized mechanical family already implemented by SimpleCADAPI or a browser-only execution requirement.

## SimpleCADAPI

Use for its standard mechanical factories and replayable modeling graph.

Best fit:

- Spur, helical, and herringbone gears; internal ring gears; racks; cycloidal discs; ball bearings.
- Planetary and cycloidal reducers, joint actuators, and mechanism-heavy assemblies.
- Parameter expressions, operation-graph JSON, replay, semantic tags, and topology queries.
- Reusable mechanical product families whose formulas should become deterministic library functions.

Before invocation, inspect the installed package and Skill versions together. Do not mix the current single-`Solid` boolean API with older `*rsolidlist` documentation. If the installed runtime and docs disagree, stop that route and use build123d or repair the installation.

## Router refinement and repair

Use text-to-cad's own requirement expansion and visual repair when requirements
are vague, reference images carry important intent, or visual complexity makes
one-pass generation unreliable:

1. Refine the request into a measurable brief.
2. Confirm important fit- or safety-critical unknowns.
3. Generate and execute backend source.
4. Validate deterministic geometry facts.
5. Inspect a multi-view render against the brief.
6. Repair visual mismatches, capped at three automatic visual rounds.

CAD Viewer reviews the generated or reconstructed STEP.