Test proxying an RPC stub all the way to Durable Objects.

In particular, the DO keeps a dup() of the stub and uses it later, after the original subscription call has returned. This requires the `rpc_params_dup_stubs` compat flag, which is new in yesterday's workerd release.

Since wrangler is not updated yet, this requires a package version override for now. Once wrangler has been updated, I'll remove the override and then merge this.

DO NOT MERGE until the override can be removed.

Author: kentonv

__tests__/test-server-workerd.js

@@ -35,6 +35,15 @@ export class TestDo extends DurableObject {
   getValue() {
     return this.value;
   }
+
+  subscribe(callback) {
+    this.subscriber = callback.dup();
+  }
+
+  async notify(value) {
+    await this.subscriber(value);
+    this.subscriber[Symbol.dispose]();
+  }
 }
 
 export class TestTarget extends RpcTarget {

__tests__/workerd.test.ts

@@ -230,6 +230,9 @@ interface Env {
 interface TestDo extends DurableObject {
   setValue(val: any): void;
   getValue(): any;
+
+  subscribe(callback: (s: string) => void): void;
+  notify(value: string): void;
 }
 
 interface WorkerdTestTarget extends TestTarget {
@@ -268,6 +271,18 @@ describe("workerd RPC server", () => {
       expect(await foo.getValue()).toBe(123);
       expect(await bar.getValue()).toBe("abc");
     }
+
+    {
+      let baz = cap.getDurableObject("baz");
+
+      let receivedValue: any;
+
+      await baz.subscribe((value: any) => {receivedValue = value});
+
+      await baz.notify("hello");
+
+      expect(receivedValue).toBe("hello");
+    }
   })
 
   it("can accept HTTP batch RPC connections", async () => {

package-lock.json

@@ -46,6 +46,7 @@
     "tsx": "^4.21.0",
     "typescript": "^5.9.3",
     "vitest": "^3.2.4",
+    "workerd": "1.20251223.0",
     "ws": "^8.18.3"
   },
   "repository": {
@@ -55,5 +56,8 @@
   "bugs": {
     "url": "https://github.com/cloudflare/capnweb/issues"
   },
-  "homepage": "https://github.com/cloudflare/capnweb#readme"
+  "homepage": "https://github.com/cloudflare/capnweb#readme",
+  "overrides": {
+    "workerd": "1.20251223.0"
+  }
 }

package.json

@@ -30,7 +30,7 @@ export default defineConfig({
             workers: {
               miniflare: {
                 compatibilityDate: '2025-07-01',
-                compatibilityFlags: ["expose_global_message_channel"],
+                compatibilityFlags: ["expose_global_message_channel", "rpc_params_dup_stubs"],
 
                 // Define a backend worker to test server-side functionality. The tests will
                 // talk to it over a service binding. (Only the workerd client tests will talk
@@ -42,6 +42,7 @@ export default defineConfig({
                   {
                     name: "test-server-workerd",
                     compatibilityDate: '2025-07-01',
+                    compatibilityFlags: ["rpc_params_dup_stubs"],
                     modules: [
                       {
                         type: "ESModule",