Try a different approach to importing "cloudflare:workers".

__tests__/test-server-workerd.js

@@ -10,7 +10,7 @@
 // build step for it. Instead, we're getting by configuring the worker in vitest.config.ts by
 // just specifying the raw JS modules.
 
-import { newWorkersRpcResponse } from "../dist/index.js";
+import { newWorkersRpcResponse } from "../dist/index-workers.js";
 import { RpcTarget, DurableObject } from "cloudflare:workers";
 
 // TODO(cleanup): At present we clone the implementation of Counter and TestTarget because

__tests__/workerd.test.ts

@@ -5,7 +5,7 @@
 /// <reference types="@cloudflare/workers-types" />
 import { expect, it, describe } from "vitest";
 import { RpcStub as NativeRpcStub, RpcTarget as NativeRpcTarget, env, DurableObject } from "cloudflare:workers";
-import { newHttpBatchRpcSession, newWebSocketRpcSession, RpcStub, RpcTarget } from "../src/index.js";
+import { newHttpBatchRpcSession, newWebSocketRpcSession, RpcStub, RpcTarget } from "../src/index-workers.js";
 import { Counter, TestTarget } from "./test-util.js";
 
 class JsCounter extends RpcTarget {

package.json

@@ -11,7 +11,10 @@
   ],
   "exports": {
     ".": {
-      "import": "./dist/index.js",
+      "import": {
+        "workerd": "./dist/index-workers.js",
+        "default": "./dist/index.js"
+      },
       "types": "./dist/index.d.ts"
     }
   },

src/core.ts

@@ -3,6 +3,7 @@
 //     https://opensource.org/license/mit
 
 import type { RpcTargetBranded, __RPC_TARGET_BRAND } from "./types.js";
+import { WORKERS_MODULE_SYMBOL } from "./symbols.js"
 
 // Polyfill Symbol.dispose for browsers that don't support it yet
 if (!Symbol.dispose) {
@@ -12,11 +13,7 @@ if (!Symbol.asyncDispose) {
   (Symbol as any).asyncDispose = Symbol.for('asyncDispose');
 }
 
-let workersModuleName = navigator.userAgent === "Cloudflare-Workers" ? "cloudflare:workers" : null;
-let workersModule: any;
-if (workersModuleName) {
-  workersModule = await import(/* @vite-ignore */workersModuleName);
-}
+let workersModule: any = (globalThis as any)[WORKERS_MODULE_SYMBOL];
 
 export interface RpcTarget {
   [__RPC_TARGET_BRAND]: never;

src/index-workers.ts

@@ -0,0 +1,8 @@
+// Copyright (c) 2025 Cloudflare, Inc.
+// Licensed under the MIT license found in the LICENSE.txt file or at:
+//     https://opensource.org/license/mit
+
+// When building for Cloudflare Workers, this file is the top-level module, instead of index.ts.
+// This ensures that inject-workers-module.js gets imported before the rest of the library.
+import "./inject-workers-module.js";
+export * from "./index.js";

src/inject-workers-module.ts

@@ -0,0 +1,14 @@
+// Copyright (c) 2025 Cloudflare, Inc.
+// Licensed under the MIT license found in the LICENSE.txt file or at:
+//     https://opensource.org/license/mit
+
+import { WORKERS_MODULE_SYMBOL } from "./symbols.js";
+
+// Import cloudflare:workers and stick it in the global scope where in can be used conditionally.
+// As long as inject-workers-module.ts is imported before the rest of the library, this allows the
+// library to set up automatic interoperability with Cloudflare Workers' built-in RPC.
+//
+// Meanwhile, we define our `exports` in package.json such that when building on Workers, this
+// module is in fact imported first.
+import * as cfw from "cloudflare:workers";
+(globalThis as any)[WORKERS_MODULE_SYMBOL] = cfw;

src/symbols.ts

@@ -0,0 +1,5 @@
+// Copyright (c) 2025 Cloudflare, Inc.
+// Licensed under the MIT license found in the LICENSE.txt file or at:
+//     https://opensource.org/license/mit
+
+export let WORKERS_MODULE_SYMBOL = Symbol("workers-module");

tsup.config.ts

@@ -5,8 +5,9 @@
 import { defineConfig } from 'tsup'
 
 export default defineConfig({
-  entry: ['src/index.ts'],
+  entry: ['src/index.ts', 'src/index-workers.ts'],
   format: ['esm'],
+  external: ['cloudflare:workers'],
   dts: true,
   sourcemap: true,
   clean: true,

vitest.config.ts

@@ -49,7 +49,7 @@ export default defineConfig({
                       },
                       {
                         type: "ESModule",
-                        path: "./dist/index.js",
+                        path: "./dist/index-workers.js",
                       },
                     ],
                     durableObjects: {