Build-time runtime validation for Cap'n Web and Workers RPC services.

capnweb-validate keeps TypeScript method signatures as the source of truth. Add @validateRpc() to the service class; a bundler plugin or CLI rewrites the decorator and injects validators generated from the resolved TypeScript types.

If a validation decorator is left untransformed, it throws with a configuration error instead of silently running without validation.

npm install capnweb capnweb-validate

Workers RPC users can install capnweb-validate without installing capnweb. The root package has no runtime dependency on capnweb; Cap'n Web-specific helpers live under capnweb-validate/capnweb and internal transform outputs.

import { newWorkersRpcResponse, RpcTarget } from "capnweb";
import { validateRpc } from "capnweb-validate";

type User = { id: string; name: string };

@validateRpc()
export class Api extends RpcTarget {
  async authenticate(sessionToken: string): Promise<User> {
    // ...
  }
}

export default {
  async fetch(request: Request, env: Env) {
    return newWorkersRpcResponse(request, new Api());
  },
};

@validateRpc() validates calls on class instances, so it works with Cap'n Web, Workers WorkerEntrypoint, and Workers DurableObject services.

With no explicit type argument, the RPC surface is the class's public string-named methods and RPC-readable getters/properties, matching Cap'n Web dispatch. implements SomeInterface can sharpen matching signatures, but it does not hide extra public class methods. Keep local-only helpers private or symbol-named.

An explicit @validateRpc<SomeInterface>() makes SomeInterface the RPC surface. Public class methods outside that interface are rejected over RPC.

A decorator emits one validator at the class declaration. If the class itself is generic, the transform cannot specialize that validator for each later new expression.

Use an explicit RPC surface when type arguments are known at the decorator site:

@validateRpc<Gatekeeper<GmailSession, number, undefined>>()
class GmailGatekeeper
  extends RpcTarget
  implements Gatekeeper<GmailSession, number, undefined> {
  // ...
}

A generic implementation class needs no annotation. An unconstrained type parameter defaults to any with a warning; a constrained parameter validates against its constraint:

@validateRpc()
class ArrayCursor<T> extends RpcTarget implements Cursor<T> {
  // `T` defaults to `any` (warned). `<T extends Session>` would validate
  // those positions against `Session`.
}

Pass @validateRpc<Cursor<string>>() to validate the Cursor<string> surface, or @validateRpc<Cursor<any>>() to silence the warning while keeping Cursor positions permissive.

Client-side stub validation is explicit. Wrap a Cap'n Web client stub with validateStub<T>() when the caller wants return values and pipelined calls checked against a concrete surface:

import { newHttpBatchRpcSession } from "capnweb";
import { validateStub } from "capnweb-validate";

import type { Api } from "./worker";

export const api = validateStub<Api>(newHttpBatchRpcSession<Api>("/rpc"));

The same helper can wrap Workers RPC service stubs:

import { validateStub } from "capnweb-validate";

import type { Api } from "./worker";

export default {
  async fetch(_request: Request, env: { SERVICE: unknown }) {
    const api = validateStub<Api>(env.SERVICE as object);
    return Response.json(await api.status());
  },
};

validateStub<T>() validates resolved return values on the caller side. It does not validate outgoing arguments; the receiver validates those on arrival. Normal Cap'n Web client constructors are not rewritten automatically.

Use the adapter that matches your bundler:

import capnwebValidate from "capnweb-validate/vite";     // or
import capnwebValidate from "capnweb-validate/rollup";    // or
import capnwebValidate from "capnweb-validate/webpack";   // or
import capnwebValidate from "capnweb-validate/rspack";    // or
import capnwebValidate from "capnweb-validate/esbuild";   // or
import capnwebValidate from "capnweb-validate/farm";

export default {
  plugins: [capnwebValidate()],
};

The plugin transforms matching modules in memory; user source files are not modified on disk.

Wrangler does not expose a bundler plugin hook. For Wrangler, CI, or any flow that needs transformed files on disk, run:

capnweb-validate build --out .capnweb-validate

Options:

• --out <dir> writes the transformed source tree. Required.
• --tsconfig <path> defaults to ./tsconfig.json.
• --cwd <dir> defaults to process.cwd().

Point the downstream build tool at the generated entry under --out.

Use @skipRpcValidation() when one RPC method should not get generated argument or return validators:

import { RpcTarget } from "capnweb";
import { skipRpcValidation, validateRpc } from "capnweb-validate";

@validateRpc()
class Api extends RpcTarget {
  @skipRpcValidation()
  unsafe(payload: unknown): unknown {
    return payload;
  }
}

The method still goes through capnweb normally. This only disables capnweb-validate validation for that method.

Validation failures throw TypeError, so validation errors keep their standard error type when they cross RPC boundaries. The message includes the failing path, expected type, and actual type:

try {
  await api.authenticate(123 as never);
} catch (err) {
  if (err instanceof TypeError) {
    console.log(err.message);
  }
}

Where errors surface depends on which boundary failed:

The supported set matches capnweb's published wire format. Every type capnweb guarantees can travel over RPC also has a precise build-time validator:

Pass-by-value:

• Primitives: string, number, boolean, null, undefined, bigint, any, unknown, plus string / number / boolean literal types. any and unknown use a permissive validator.
• Containers: arrays, tuples (validated by exact length and per-position element type), Map<K, V> / ReadonlyMap<K, V>, Set<T> / ReadonlySet<T>, plain object shapes, unions, Record<K, T> and index signatures (string and numeric keys both validate their value type, since object keys cross the wire as strings), and Promise<T> return values (unwrapped one level). Optional properties (foo?: T) widen to T | undefined, matching how the wire deserializes a missing key.
• Built-ins from the RPC-compatible runtime set: Date, ArrayBuffer, DataView, RegExp, Uint8Array, other typed arrays, Error and its standard subclasses (EvalError, RangeError, ReferenceError, SyntaxError, TypeError, URIError, AggregateError), Blob, ReadableStream, WritableStream, Headers, Request, Response.

Some host objects are checked by exact prototype to match the transport behavior; Error, ArrayBuffer, RegExp, DataView, and typed arrays use instanceof. File is rejected at build time because it is a common Blob subclass that does not match the supported Blob validator.

Pass-by-reference:

• Plain functions (validated as typeof === "function").
• RpcStub<T> and RpcPromise<T> by symbol name, and any user-defined class that extends RpcTarget (the resolver walks getBaseTypes).
• Workers RPC Fetcher<T> values and branded RpcTarget / WorkerEntrypoint capabilities. These are validated as pass-through stubs; lifecycle methods such as fetch, queue, and tail remain pass-through.

Rejected types: these cannot be validated as supported RPC values, and the transform refuses to compile a service that uses them so the user finds out at build time, not at the first RPC call:

If a method signature contains a leaf the resolver cannot lower, such as a generic type parameter with no inference source, an unsupported recursive corner, or a rejected built-in nested inside an object, the transform fails at the call site with a class-qualified list of every offending field. You fix them in one pass rather than rebuilding once per field.

Recursive object and union shapes are emitted with lazy back-references. The resolver also has a guardrail for pathological non-recursive nesting. If the lowered type graph exceeds the internal resolution depth limit, it fails with type exceeds maximum resolution depth (64).

An overloaded method exposes several call signatures; validating against one would reject valid calls to the others. Overloaded methods are passed through unvalidated with a warning. Collapse the overloads into a single signature with union parameters to validate the method, or @skipRpcValidation() to silence the warning.

MIT.