Problem

A downstream repo hit:

Uncaught TypeError: capnweb-validate: Class1.field1 is not in the generated validator

and calls to subclass-only methods failed with the same misleading error.

Root cause: @validateRpc() returned a new Proxy(...) from the decorated constructor. When a decorated class extends another decorated class (Class2 extends Class1), super() returns the base class's validator proxy, so every subclass-only method (method2, method3, ...) is checked against the base validator and refused. The generated validator itself is correct; only the runtime wrapping was wrong.

The constructor-returns-Proxy approach had a second cost: instances were Proxies rather than real branded RpcTargets, a serialization hazard over native workerd RPC.

Fix

• __validateRpcClass now wraps the declared methods in place on the class's prototype instead of returning a Proxy from the constructor.
• Wrapping is idempotent (a WRAPPED_METHOD symbol marks wrapped functions), so decorated-extends-decorated and double-decoration compose: each class's methods are validated against its own validator, and inheritance behaves like ordinary JS.
• Instances stay real branded RpcTargets with intact instanceof identity and #-field privacy.
• Incoming argument stubs (callbacks declared with v.stubOf(...)) now pass through as native stubs instead of being wrapped in validation proxies. The wrapper proxy could not be forwarded over native workerd RPC (for example, handing a received callback to a Durable Object). The trade-off: return values from those callbacks are no longer validated by default. This follows the review direction on Add capnweb-validate RPC validators #169: incoming wrappers validate only arguments, and stub validation is explicit opt-in via validateStub<T>(stub) rather than automatic.
• Refusal errors now distinguish instance-property access from methods genuinely missing from the RPC interface, and suggest rebuilding if the validator may be stale.

Verification

• Full capnweb-validate suite: 179/179 pass. New regression file decorated-class-runtime.test.ts covers subclass-only methods, inherited methods, double-decoration, prototype/identity preservation, and refusal semantics.
• workerd suite: 146/146 pass, including a new integration test that receives a native RpcStub callback through a validated service and forwards it to a Durable Object, both as a top-level argument and nested inside an object. The test fails against the previous wrapping behavior.
• Return direction over native workerd RPC, verified with the same wrangler repro: a decorated method whose declared return type resolves to a stub-shaped validator (a class type) returns the instance across the boundary correctly, and declared methods on the returned stub still validate because the prototype methods are wrapped in place. Interface-typed returns emit object shapes and pass through unchanged. Over capnweb sessions the returned stub keeps the validation wrapper, preserving nested capability validation per the Add capnweb-validate RPC validators #169 direction (receiver validates, sender wraps nested capabilities).
• Minimal repro (wrangler dev over native Workers RPC + a pure capnweb session), before vs after:

Note: the downstream Class1.field1 console error is independently legitimate. protected fields are runtime-public properties and should be #-private in the downstream code; this PR makes the refusal message say so explicitly.