# Choose a Kernel
URL: /runtime/guides/choosing-a-kernel



Pick kernels by source language or import format, and install only the packages your host needs.

| Kernel package        | Input                                     | Best fit                                     |
| --------------------- | ----------------------------------------- | -------------------------------------------- |
| `@taucad/replicad`    | TypeScript/JavaScript using Replicad      | High-level parametric BRep                   |
| `@taucad/opencascade` | TypeScript/JavaScript using OCCT bindings | Direct BRep and topology control             |
| `@taucad/manifold`    | TypeScript/JavaScript using Manifold      | Mesh-first robust booleans                   |
| `@taucad/jscad`       | TypeScript/JavaScript using JSCAD         | CSG modeling                                 |
| `@taucad/openrscad`   | OpenSCAD                                  | `.scad` models in browser or Node            |
| `@taucad/zoo`         | KCL                                       | Configured remote Zoo execution              |
| `@taucad/gltf`        | glTF/GLB                                  | glTF interchange with optional Draco         |
| `@taucad/brep`        | STEP/STP/IGES/IGS/BREP                    | BRep-family file import                      |
| `@taucad/rhino`       | 3DM                                       | Rhino file import                            |
| `@taucad/assimp`      | Mesh and scene formats                    | OBJ, FBX, STL, PLY, DAE, and related imports |
| `@taucad/build123d`   | Python using Build123d                    | Native desktop BRep, topology, and STEP      |
| `@taucad/tscircuit`   | TSX using tscircuit                       | Electronics: 3D board, schematic, PCB, BOM   |

There is no catch-all Tau kernel. Every first-party import extension has one explicit owner.

## Compose toolkits [#compose-toolkits]

Pass one factory per package to `defineRuntime`. Source-code kernels need a bundler beside them; file-import kernels such as `gltf()`, `brep()`, `rhino()`, and `assimp()` do not:

```typescript
import { defineRuntime } from '@taucad/runtime/worker';
import { esbuild } from '@taucad/esbuild';
import { manifold } from '@taucad/manifold';
import { replicad } from '@taucad/replicad';

export const modelingRuntime = defineRuntime({ plugins: [esbuild(), replicad(), manifold()] });
```

## Configure a packaged capability [#configure-a-packaged-capability]

Presets select capability sets — `assimp({ preset: 'import' })` installs only the import kernel, leaving the export transcoder out. Role-nested options configure the factories the preset selects:

```typescript
import { defineRuntime } from '@taucad/runtime/worker';
import { zoo } from '@taucad/zoo';

export const runtime = defineRuntime({
  plugins: [zoo({ kernels: { default: { baseUrl: 'wss://example.com/v1/kernels/zoo' } } })],
});
```

Use a direct role factory instead for app-local capabilities, isolated tests, or when a whole role must interleave outside plugin expansion.

## Choose Build123d only in a native host [#choose-build123d-only-in-a-native-host]

Build123d needs a packaged CPython payload and a project trust grant, so only a native host such as Tau Desktop can advertise it. Its `.py` files skip the JavaScript bundler entirely -- see [Build123d](/runtime/reference/build123d) for the authoring contract.

## Choose tscircuit for electronics [#choose-tscircuit-for-electronics]

A `.tsx` or `.jsx` file exports a board component. Rendering is offline: local autorouter, no parts engine; URL footprints and models are never fetched; each attempt or `kicad:`/`jlcpcb:` reference becomes a warning. See [tscircuit](/runtime/reference/tscircuit) for the authoring contract and install steps.

## Related [#related]

* [Kernel Selection](/runtime/concepts/kernel-selection)
* [Kernel Plugins](/runtime/api/kernels)
* [Plugin System](/runtime/concepts/plugin-system)