b123d-edit

Edit Existing build123d Geometry (b123d-edit)

Use this skill when the task is to change an existing build123d model or script: "make this bracket 5 mm taller", "move the holes inward", "add a counterbore", "remove this rib", "make it fit this mating part", or "repair the code so the intended edit remains parametric".

The goal is not to sculpt the final B-rep. The goal is to make an explicit, reviewable code edit that preserves design intent, then prove the changed model is valid and the intended delta happened.

Step 0 - Establish the Baseline

Before editing, capture what exists.

1. Read the source script or recover the session code with script().

2. Run the current model in the MCP session with execute().

3. Register the edited target with show(part, "baseline") or a meaningful name.

4. Capture numeric evidence:

   save_snapshot("before_edit")
   print(measure(part))
   

5. Run the MCP validate() tool on the registered baseline. For tasks whose deliverable is an exported solid, or where downstream CAD consumers will read a STEP/STL/BREP file, also export the unchanged baseline to a safe throwaway path such as _baseline_gate.step and read the export gate result. The throwaway baseline gate must never use the final deliverable path, such as output.step. For script-only work, a successful rebuild plus validate() is the baseline proof unless the task later requires a file export.

6. If the baseline fails the relevant proof (validate() for script-only work; validate() plus the throwaway export gate for exported-solid work), stop and switch to build123d://skill/repair. Until a repaired baseline passes that same proof, the only geometry-changing operations allowed are minimal baseline fixups. Do not move, cut, extend, remove, or replace the requested target feature while the baseline is still invalid. Do not combine a feature edit with geometry repair unless the requested edit is the repair.

7. After the baseline passes the relevant proof, save a snapshot such as save_snapshot("valid_baseline"). Only then begin the requested feature or parameter edit.