FastBuild123

A browser UI for build123d models that is generated directly from the code and has two way caching (both of the UI inputs and the code itself) to enable live, responsive updates of the parameters.

Quick start

Try a bundled example with uv:

uvx fastbuild123 demo enclosure

Install in your project:

uv add fastbuild123      # or: pip install fastbuild123

Save this as my_model.py:

from typing import Annotated
from build123d import Box, Part
from fastbuild import App, Range

app = App()

@app.part()
def block(
    width: Annotated[float, Range(20, 200, step=1, unit="mm")] = 80,
    depth: float = 50,
    height: float = 30,
) -> Part:
    return Box(width, depth, height)

Open it in the browser:

fastbuild view my_model.py

The viewer preserves your camera when rebuilding. If a build fails, it shows the error and keeps the last working model.

Parameters

Every parameter needs a type annotation and a default value.

Use Group("Label") and Desc("Help text") from fastbuild inside Annotated to group controls and add tooltips. Multiple @app.part() functions get a part selector.

Export files for printing/milling/etc

fastbuild export my_model.py block --param width=120 -o block.step

Values are validated against each parameter's Range; output format follows the extension (.step/.stp/.stl).

AI

AI coding agents

Install the bundled skill with API usage and caching guidance:

fastbuild agent-docs --install

Writes to .claude/skills/fastbuild123/ and .agents/skills/fastbuild123/. Re-run after upgrading. Omit --install to print the docs.

Requirements

Python 3.10–3.13 on macOS, Linux (glibc 2.31+, x86-64 or aarch64), or 64-bit Windows. The install is roughly 500 MB, mostly OpenCASCADE. Alpine/musl is not supported.

License

MIT — see LICENSE. Bundles three-cad-viewer (MIT), whose license is included at fastbuild/static/vendor/LICENSE-three-cad-viewer.