brepjs vs Replicad

brepjs and Replicad are siblings. Both are JavaScript CAD libraries that wrap the OpenCascade B-Rep kernel and run in the browser, and both produce exact solids you can export to STEP. Replicad came first and proved the approach works. brepjs is a TypeScript-first take on the same idea, built around type safety, structured errors, and a verification loop for AI-authored parts.

This is an honest comparison: where each library fits, and where Replicad is still the better choice.

In short

• Reach for brepjs if you want a TypeScript-first API where invalid geometry is caught at compile time, fallible operations return a Result instead of throwing, and — increasingly — if you're authoring parts with an AI agent that needs to check its own output.
• Reach for Replicad if you want a mature, proven library with an established community, a polished in-browser workbench, and the shortest path to your first model. It pioneered code-first CAD in the browser and is an excellent choice.

Both are exact B-Rep, both export STEP, both are open source. Neither is a toy.

Side by side

	brepjs	Replicad
Geometry	Exact B-Rep (OpenCascade)	Exact B-Rep (OpenCascade)
Language	TypeScript-first	TypeScript / JavaScript
Type safety	Branded types; invalid geometry fails to compile	Standard TypeScript types
Errors	Result<T, BrepError> with codes + suggestions	Throws exceptions
Runtime	Browser and Node	Browser
Kernel	OpenCascade WASM; pluggable (Rust brepkit in development)	OpenCascade WASM
Export	STEP, STL, IGES, glTF, DXF, 3MF, OBJ	STEP, STL, glTF
AI authoring	brepjs-cad — deterministic validity, measurement, and snapshot loop	—
Maturity	Newer, smaller surface	Mature and battle-tested
Community & docs	Growing	Larger, with a community manual
Time to first model	Comparable	A little shorter — fewer concepts up front

The bold rows are the honest summary: brepjs leads on type safety, structured errors, kernel choice, and AI authoring; Replicad leads on maturity, community, and getting started quickly.

Where brepjs is different

Invalid geometry is a compile error

brepjs distinguishes Edge, Wire, and Face at the type level, and stamps ClosedWire, OrientedFace, and ValidSolid onto shapes proven to satisfy those invariants. An operation like extrude requires an OrientedFace, so a wire you forgot to close won't type-check. Replicad treats most shapes as a single Shape type, so the same mistake reaches runtime.

Errors are values, not exceptions

Fallible operations — booleans, fillets, imports, exports — return Result<T, BrepError> rather than throwing. You handle the failure where it happens, with an error code and a suggestion attached.

// Replicad — throws
const part = makeBox(20, 20, 20).cut(makeCylinder(5, 25));

// brepjs — returns a Result you check
const result = cut(box(20, 20, 20), cylinder(5, 25));
if (isOk(result)) {
  /* use result.value */
} else {
  console.error(result.error.code, result.error.suggestion);
}

A chainable shape() wrapper is available if you prefer Replicad's throwing, fluent style — it just throws a typed BrepWrapperError instead.

Built for AI authoring

This is the clearest gap. brepjs-cad runs a part on a real kernel and returns a deterministic report — solid validity, measured dimensions, multi-view snapshots, and a STEP export — so an agent can check its output against intent instead of guessing. It ships as a CLI and a Claude Code skill. Replicad has no equivalent.

One API, swappable kernels

brepjs runs on OpenCascade WASM today and is built so the kernel is pluggable; a Rust kernel (brepkit) is in development as a drop-in alternative aimed at a smaller bundle. Replicad is OpenCascade-only.

When Replicad is the better choice

A fair comparison has to say this plainly:

• It's mature and proven. Replicad has been used in production longer, across more models, so you'll hit fewer rough edges.
• The community is larger. More examples, more answered questions, and a thorough community manual.
• It's quicker to start. Fewer concepts up front. If you just want a model on screen today, Replicad gets you there with less to learn.
• You're already invested and it's working. If your Replicad code isn't hitting silent failures or runtime topology errors, there's no urgent reason to switch.

brepjs's pitch is narrower and honest: the type system catches a class of bugs that otherwise surface at runtime, and the verify loop makes AI-authored parts checkable. If those matter to you, the trade is worth it. If they don't, Replicad is a great tool.

Already using Replicad?

The two libraries are close enough that switching is mostly a search-and-replace plus a couple of pattern shifts. The Coming from Replicad guide has a function-by-function map and the two patterns to internalize (Result instead of try/catch, and type guards on imported shapes).

How it compares to the rest of the field

If you're choosing a JavaScript CAD library more broadly, here's where brepjs sits:

• vs OpenSCAD — OpenSCAD uses its own language and a mesh-based engine, so it has no true fillets and no STEP export. brepjs is exact B-Rep, written in TypeScript, with STEP out of the box.
• vs JSCAD — JSCAD is JavaScript but polygon/mesh-based, so it can't produce exact solids or true STEP. brepjs is a B-Rep kernel.
• vs CadQuery / build123d — excellent, but Python and server-side. brepjs runs in the browser, in TypeScript.
• vs opencascade.js — a raw OpenCascade WASM binding. Replicad builds on its own custom version (replicad-opencascadejs); brepjs builds on its own (occt-wasm) and wraps it in a high-level, typed API, so you don't work with the kernel directly.

FAQ

Is brepjs production-ready? It's newer than Replicad, with a smaller API surface and fewer examples, but it's well tested and grew out of a tool people use daily (the Gridfinity Layout Tool). For exact, parametric, manufacturable parts it's solid; for organic sculpting or dense lattices, it isn't the right tool — and neither is Replicad.

Can I migrate my Replicad models? Yes — see Coming from Replicad. Most code is a search-and-replace away.

Does it work in Node, not just the browser? Yes. The same API runs in both; the verify CLI runs on Node.

What's the brepkit kernel? A Rust-based B-Rep kernel in development, intended as a drop-in alternative to OpenCascade WASM behind the same API.

Is it free? Yes. brepjs is open source under Apache-2.0, and Replicad under MIT — both are free to use.

Try it

The fastest way to judge a CAD library is to build something in it.

• Open the Playground — write TypeScript, see the solid, export STEP. No install.
• Get started — install and initialize in your own project.
• npm i brepjs occt-wasm — or read Why brepjs first.