# Build123d
URL: /runtime/reference/build123d



Build123d is Tau's native Python BRep kernel. Tau Desktop packages the pinned CPython, Build123d, and OCP payload and asks you to trust the physical project directory before Python starts; the web runtime neither advertises the kernel nor ships the payload.

## Authoring contract [#authoring-contract]

Each entry file defines one frozen `Params` dataclass and a synchronous `main(params)` function. `main` returns a `build123d.Shape` or a non-empty list or tuple of shapes. Root labels must be unique because Tau uses them for topology identity, selection, and export.

```python
from dataclasses import dataclass
from typing import Literal

from build123d import Box, Color, Shape


@dataclass(frozen=True)
class Params:
    width: float = 80.0
    depth: float = 40.0
    height: float = 12.0
    finish: Literal["blue", "orange"] = "blue"


__tau__ = {
    "parameters": {
        "width": {
            "minimum": 1.0,
            "maximum": 500.0,
            "description": "Body width in millimeters",
        }
    },
    "dependencies": ["profile.json"],
}


def main(params: Params) -> Shape:
    body = Box(params.width, params.depth, params.height)
    body.label = "Body"
    body.color = Color(params.finish)
    return body
```

Supported parameter annotations are `bool`, `int`, `float`, `str`, and scalar `Literal` values. Every field needs a literal default. `__tau__.parameters` may refine the generated JSON Schema with numeric bounds, string bounds/patterns, descriptions, and enums. Put non-Python files read by the model in `__tau__.dependencies` so edits invalidate the render.

Local imports must be static and project-relative. Tau parses the entry and its local import closure without executing it, mirrors the rooted project into a private directory, then calls `main(Params(...))` in a warm native process. Editing an imported `.py` file or declared data dependency triggers the same autonomous rerender loop as other kernels.

## Geometry, topology, and export [#geometry-topology-and-export]

The kernel retains the Build123d/OCP shapes from the successful build. Preview meshing emits a GLB carrying the canonical `TAU_cad_topology` payload, and STEP export consumes the same retained shapes without running the source again.

Tau maps Build123d's Z-up millimeter coordinates to glTF Y-up meters. Preview tessellation is a runtime render option, not a model parameter; STEP remains an exact BRep export.

## Trust and resources [#trust-and-resources]

Python executes with the desktop user's operating-system permissions. Utility-process isolation protects the UI from a crash or hang but is not a malicious-code sandbox: trust only code you know, and revoke native execution from the visible project control. A moved or replaced directory does not inherit an old grant.

Production hosts pass exact packaged interpreter, worker, and support-file hashes plus a trust marker to `build123d()`. The kernel never searches `PATH`, runs `pip`, downloads wheels, or falls back to system Python.

## Related [#related]

* [Choose a Kernel](/runtime/guides/choosing-a-kernel)
* [Kernel Plugins](/runtime/api/kernels)
* [Worker Model](/runtime/concepts/worker-model)
* [Set Up the Filesystem](/runtime/guides/filesystem-setup)