Installation

Cap’n Web is a single npm package with no dependencies.

npm i capnweb

There is no build step, no schema compiler, and no code generation. Import it and go:

import { RpcTarget, RpcStub, newWebSocketRpcSession } from 'capnweb';

Runtime support

Cap’n Web works in all major browsers, Cloudflare Workers, Node.js, Bun, Deno, and other modern JavaScript runtimes. The package ships runtime-specific entry points and your bundler or runtime will pick the right one automatically:

Beyond fetch or WebSocket, Cap’n Web’s serializer inspects a handful of WHATWG globals to decide how to encode a value, and expects them to exist: ReadableStream, WritableStream, Blob, URL, Headers, Request and Response. All five runtimes above provide them.

Other JavaScript environments may need help. React Native is the usual example: it has fetch, WebSocket, Blob and URL, but no ReadableStream or WritableStream, so you will need a WHATWG streams polyfill loaded before capnweb. React Native is not covered by CI, so treat it as untested rather than unsupported; a CI contribution would be very welcome.

If a runtime has no suitable network API at all, you can still use Cap’n Web by supplying a custom transport over any bidirectional message stream.

TypeScript setup

Cap’n Web is written in TypeScript and ships its own types; you do not need a @types package.

Stubs integrate with JavaScript’s explicit resource management, so many examples in these docs use using declarations. To compile using, your tsconfig.json needs a recent target and the matching libs:

{
  "compilerOptions": {
    "target": "esnext",
    "lib": ["esnext", "dom"],
    "module": "nodenext",
    "moduleResolution": "nodenext",
    "strict": true
  }
}

using became widely available in JavaScript engines in mid-2025, and has been supported via transpilers and polyfills for a few years before that. If you cannot use it, every disposable value also has an explicit [Symbol.dispose]() method you can call yourself. See Disposal.

Optional: build-time validation

Cap’n Web does not perform runtime type checking by default. The companion package capnweb-validate generates runtime validators from your TypeScript types at build time.

npm i -D capnweb-validate