I built Aretu CAD, a parametric CAD program for 3D printing (Windows, C++), mostly with Claude Code. The video shows the end result from the user's side: Claude, as the in-app assistant, designing a wall-mounted drill holster from one brief. It plans, asks before changing my sizes, and leaves an editable Feature Tree and an STL. The build is sped up. Scale About 250,000 lines of C++. 749 of the project's 1,013 commits were made with Claude Code (Opus). That covers the geometry and modelling features, the UI, the AI integration, the installer, the licensing service and the website. I set the direction, write or approve the specs, and review the results. What made it work Specs as contracts. About 190 Markdown documents cover requirements, architecture and design, with an index. AGENTS.md tells Claude to read the owning contract before touching code. Most bad changes I've seen came from an ambiguous spec, not bad code. A ticket gate I control. I select text in a document and raise a ticket. Its state (Document, Implement, Review) is the permission for that piece of work. A separate review pass checks it, and only I can resolve a ticket. Hard rules in AGENTS.md. Platform code only in adapter folders, one portable core, files kept under about 2,000 lines. Every user-facing command must have a matching AI operation. That last rule pays twice. Because everything a user can do is also an operation Claude can call, the in-app assistant can build real parametric models. Claude Code can also use the running app to test it. What it does Sketches, extrudes, revolves, fillets, a Feature Tree you can edit, materials and STL export, plus an AI assistant that works through the same operations as the menus. Free to try The early beta is free on Windows (x64 and ARM64): https://aretucad.com/download.html. Modelling works without AI. For the assistant you connect your own Anthropic API key, or another provider.   submitted by   /u/Far_Marsupial_8739   to   r/ClaudeCode [link]   [comments]