diff --git a/src/execute.ts b/src/execute.ts
index 150c4d7..5e84e1c 100644
--- a/src/execute.ts
+++ b/src/execute.ts
@@ -1,4 +1,3 @@
-import type { Worker } from 'node:worker_threads';
 import { MessageChannel } from 'node:worker_threads';
 import type { MessagePort, Transferable } from 'node:worker_threads';
 import { transferableAbortSignal } from 'node:util';
@@ -18,8 +17,15 @@ let nextCallId = 0;
 const pending = new Map<number, { resolve: (value: any) => void; reject: (reason: any) => void }>();
 const streamPortStack: MessagePort[][] = [];
 
-export function setupWorker(worker: Worker): void {
-  worker.on('message', (msg: { callId: number; value?: unknown; error?: Error }) => {
+/** Anything dispatch can talk over: a Worker from the main thread, or a
+ *  MessagePort between peers. Structurally satisfied by both. */
+export interface Endpoint {
+  postMessage(value: unknown, transferList?: readonly Transferable[]): void;
+  on(event: 'message', listener: (value: any) => void): unknown;
+}
+
+export function setupWorker(endpoint: Endpoint): void {
+  endpoint.on('message', (msg: { callId: number; value?: unknown; error?: Error }) => {
     const call = pending.get(msg.callId);
     if (!call) return;
     pending.delete(msg.callId);
@@ -85,7 +91,7 @@ function prepareArg(arg: unknown): unknown {
   return serializeArg(arg);
 }
 
-export function execute<T>(worker: Worker, id: string, args: unknown[]): Promise<T> {
+export function execute<T>(endpoint: Endpoint, id: string, args: unknown[]): Promise<T> {
   const url = id.slice(0, id.lastIndexOf('#'));
   freezeModule(url);
   const callId = nextCallId++;
@@ -96,7 +102,7 @@ export function execute<T>(worker: Worker, id: string, args: unknown[]): Promise
     const preparedArgs = extracted.args.map(prepareArg);
     const ports = streamPortStack.pop()!;
     const msg = { callId, id, args: preparedArgs };
-    worker.postMessage(msg, [...extracted.transfer, ...ports] as any[]);
+    endpoint.postMessage(msg, [...extracted.transfer, ...ports] as any[]);
   });
 }
 
@@ -106,7 +112,7 @@ export interface StreamDispatch<T> {
 }
 
 export function dispatchStream<T>(
-  worker: Worker,
+  endpoint: Endpoint,
   id: string,
   args: unknown[],
   opts?: ChannelOptions,
@@ -128,7 +134,7 @@ export function dispatchStream<T>(
   const preparedArgs = extracted.args.map(prepareArg);
   const ports = streamPortStack.pop()!;
   const msg = { id, args: preparedArgs, stream: serializeStreamHandle(handle) };
-  worker.postMessage(msg, [...extracted.transfer, ...ports, port2] as any[]);
+  endpoint.postMessage(msg, [...extracted.transfer, ...ports, port2] as any[]);
 
   let resolveDone: () => void;
   const donePromise = new Promise<void>((r) => {