Enable the new module registry by default from 2026-11-17

src/workerd/api/tests/wasm-source-url-test.wd-test

@@ -4,14 +4,16 @@ using Workerd = import "/workerd/workerd.capnp";
 # must recompile it rather than keep this worker's URL.
 const unitTests :Workerd.Config = (
   services = [
-    # With the flag, the module's frames name it by its module name.
+    # With the flag and the legacy registry, the module's frames name it by its module name.
     ( name = "wasm-source-url-test",
       worker = (
         modules = [
           (name = "worker", esModule = embed "wasm-source-url-test.js"),
           (name = "trap.wasm", wasm = embed "wasm-source-url.wasm"),
         ],
-        compatibilityFlags = ["nodejs_compat", "experimental", "wasm_source_url_support"],
+        compatibilityFlags = [
+          "nodejs_compat", "experimental", "wasm_source_url_support", "legacy_module_registry",
+        ],
         bindings = [
           (name = "expectedFrame", text = "at trap.wasm:wasm-function[0]:0x"),
           (name = "childFlag", text = "no_wasm_source_url_support"),
@@ -45,7 +47,9 @@ const unitTests :Workerd.Config = (
           (name = "worker", esModule = embed "wasm-source-url-test.js"),
           (name = "trap.wasm", wasm = embed "wasm-source-url.wasm"),
         ],
-        compatibilityFlags = ["nodejs_compat", "experimental", "no_wasm_source_url_support"],
+        compatibilityFlags = [
+          "nodejs_compat", "experimental", "no_wasm_source_url_support", "legacy_module_registry",
+        ],
         bindings = [
           (name = "expectedFrame", text = "at wasm://wasm/"),
           (name = "childFlag", text = "wasm_source_url_support"),

src/workerd/io/compatibility-date-test.c++

@@ -589,6 +589,33 @@ KJ_TEST("encode to full flag list") {
   }
 }
 
+KJ_TEST("new module registry defaults on at 2026-11-17") {
+  auto compileFlags = [](kj::StringPtr date, kj::ArrayPtr<const kj::StringPtr> flags) {
+    capnp::MallocMessageBuilder message;
+    auto flagList = message.initRoot<capnp::List<capnp::Text>>(flags.size());
+    for (auto i: kj::indices(flags)) {
+      flagList.set(i, flags[i]);
+    }
+    auto output = message.getOrphanage().newOrphan<CompatibilityFlags>();
+    SimpleWorkerErrorReporter errorReporter;
+    compileCompatibilityFlags(date, flagList.asReader(), output.get(), errorReporter,
+        /*allowExperimentalFeatures=*/false, CompatibilityDateValidation::FUTURE_FOR_TEST, nullptr);
+    KJ_ASSERT(errorReporter.errors.empty());
+    return capnp::clone(output.getReader());
+  };
+
+  auto before = compileFlags("2026-11-16", {});
+  KJ_EXPECT(!isNewModuleRegistryEnabled(*before));
+  auto enabled = compileFlags("2026-11-17", {});
+  KJ_EXPECT(isNewModuleRegistryEnabled(*enabled));
+  auto optedIn = compileFlags("2026-11-16", {"new_module_registry"_kj});
+  KJ_EXPECT(isNewModuleRegistryEnabled(*optedIn));
+  auto optedOut = compileFlags("2026-11-17", {"legacy_module_registry"_kj});
+  KJ_EXPECT(!isNewModuleRegistryEnabled(*optedOut));
+  auto python = compileFlags("2026-11-17", {"python_workers"_kj});
+  KJ_EXPECT(!isNewModuleRegistryEnabled(*python));
+}
+
 KJ_TEST("compatibility dates must be Tuesday, Wednesday, or Thursday") {
   // List of specific flags that are allowed to use non-conformant dates
   // (already deployed and can't be changed for compatibility reasons)
@@ -766,7 +793,7 @@ KJ_TEST("isNewModuleRegistryEnabled ignores the flag for Python workers") {
   KJ_EXPECT(check(true, false));
   // Python workers always use the original module registry: the
   // new_module_registry flag is ignored for them, whether set explicitly or
-  // (once the flag has a default-on date) implied by the compatibility date.
+  // implied by the compatibility date.
   KJ_EXPECT(!check(true, true));
   KJ_EXPECT(!check(false, true));
 }

src/workerd/io/compatibility-date.capnp

@@ -1,3 +1,3 @@
 # Copyright (c) 2017-2022 Cloudflare, Inc.
 # Licensed under the Apache 2.0 license found in the LICENSE file or at:
 #     https://opensource.org/licenses/Apache-2.0
@@ -513,7 +513,8 @@
 
   newModuleRegistry @52 :Bool
       $compatEnableFlag("new_module_registry")
-      $compatDisableFlag("legacy_module_registry");
+      $compatDisableFlag("legacy_module_registry")
+      $compatEnableDate("2026-11-17");
   # Enables the new module registry implementation, which handles module
   # specifiers as URLs, implements import.meta (url/main/resolve), supports
   # import attributes, and allows a registry to be shared across isolate

src/workerd/io/features.h

@@ -67,10 +67,10 @@ inline void configureJsgIsolate(jsg::Lock& js, CompatibilityFlags::Reader flags)
 // Returns whether the new module registry implementation should be used for a
 // worker with the given flags. Python workers always use the original module
 // registry: the new_module_registry flag is ignored for them, whether it was
-// listed explicitly or (once the flag gains an enable date) implied by the
-// worker's compatibility date. Every decision about which registry to use MUST
-// go through this function rather than reading getNewModuleRegistry() directly;
-// otherwise a Python worker could end up split across the two registries.
+// listed explicitly or implied by the worker's compatibility date. Every decision
+// about which registry to use MUST go through this function rather than reading
+// getNewModuleRegistry() directly; otherwise a Python worker could end up split
+// across the two registries.
 //
 // Note that Cloudflare.compatibilityFlags.new_module_registry (as observed by
 // user code) still reflects the configured flag value, not this function's

src/workerd/server/factory/worker-factory.c++

@@ -1567,13 +1567,14 @@ kj::Promise<kj::Own<CompiledWorker>> factory_new_worker(const WorkerFactory& con
   // The file system roots use their defaults; the config does not expose mount points.
   auto workerFs = newWorkerFileSystem(kj::heap<FsMap>(), getBundleDirectory(workerSource));
 
+  KJ_REQUIRE(experimental || moduleFallback == kj::none,
+      "The module fallback service is an experimental feature. "
+      "You must run workerd with `--experimental` to use the module fallback service.");
+
   // Python workers never use the new module registry, whatever their flags say.
   bool usingNewModuleRegistry = isNewModuleRegistryEnabled(featureFlags);
   kj::Maybe<kj::Arc<jsg::modules::ModuleRegistry>> newModuleRegistry;
   if (usingNewModuleRegistry) {
-    KJ_REQUIRE(experimental,
-        "The new ModuleRegistry implementation is an experimental feature. "
-        "You must run workerd with `--experimental` to use this feature.");
     // Module URLs follow the virtual file system: a module at "/foo/bar/baz.js" in the bundle
     // is "file:///foo/bar/baz.js".
     const jsg::Url& bundleBase = workerFs->getBundleRoot();
@@ -1670,9 +1671,6 @@ kj::Promise<kj::Own<CompiledWorker>> factory_new_worker(const WorkerFactory& con
 
   if (!usingNewModuleRegistry) {
     KJ_IF_SOME(fallback, moduleFallback) {
-      KJ_REQUIRE(experimental,
-          "The module fallback service is an experimental feature. "
-          "You must run workerd with `--experimental` to use the module fallback service.");
       auto& apiIsolate = isolate->getApi();
       apiIsolate.setModuleFallbackCallback(
           [client = kj::heap<fallback::FallbackServiceClient>(kj::str(fallback)),

src/workerd/server/server-test.c++

@@ -412,6 +412,9 @@ class Server: public kj::PtrTarget {
   void setDefaultCompatibilityDate(kj::String date) {
     options->defaultCompatibilityDate = kj::mv(date);
   }
+  void setTestCompatibilityDateOverride(kj::String date) {
+    options->testCompatibilityDateOverride = kj::mv(date);
+  }
   void enableDebugPort(kj::String addr) {
     debugPort = kj::str("loopback:", addr);
   }
@@ -1033,6 +1036,27 @@ KJ_TEST("Server: serve Service Worker using the new module registry") {
   conn.httpGet200("/service-worker", "NMR: http://foo/service-worker");
 }
 
+KJ_TEST("Server: module registry enabled by date starts without experimental mode") {
+  TestServer test(singleWorker(R"((
+    modules = [
+      ( name = "worker",
+        esModule =
+          `import message from "./message.js";
+          `export default {
+          `  fetch() { return new Response(message + ":" + import.meta.main); }
+          `}
+      ),
+      ( name = "message.js", esModule = "export default 'date-default';" )
+    ]
+  ))"_kj));
+
+  test.server.setTestCompatibilityDateOverride(kj::str("2026-11-17"));
+  test.start();
+
+  auto conn = test.connect("test-addr");
+  conn.httpGet200("/", "date-default:true");
+}
+
 // Note: Python workers ignore the new_module_registry flag entirely — they
 // always use the original module registry. That decision is centralized in
 // isNewModuleRegistryEnabled() (io/features.h) and unit-tested in

src/workerd/server/tests/module-fallback/module-fallback-test.mjs

@@ -88,12 +88,12 @@ function sendJson(response, value) {
   response.end(JSON.stringify(value));
 }
 
-async function runWorkerd(configPath) {
+async function runWorkerd(configPath, extraArgs = ['--experimental']) {
   return await new Promise((resolve, reject) => {
     let output = '';
     const child = spawn(
       env.WORKERD_BINARY,
-      ['test', configPath, '--experimental'],
+      ['test', configPath, ...extraArgs],
       { stdio: ['ignore', 'pipe', 'pipe'] }
     );
     const timeout = setTimeout(() => {
@@ -114,6 +114,48 @@ async function runWorkerd(configPath) {
   });
 }
 
+test('module fallback requires experimental mode with either registry', async (t) => {
+  const directory = await mkdtemp(
+    join(env.TEST_TMPDIR, 'module-fallback-gate-')
+  );
+  const cases = [
+    ['date-default', '2026-11-17', []],
+    ['explicit-opt-in', '2026-11-16', ['new_module_registry']],
+    ['legacy-opt-out', '2026-11-17', ['legacy_module_registry']],
+    ['legacy-default', '2026-11-16', []],
+  ];
+
+  for (const [name, date, flags] of cases) {
+    await t.test(name, async () => {
+      const configPath = join(directory, `${name}.capnp`);
+      await writeFile(
+        configPath,
+        `using Workerd = import "/workerd/workerd.capnp";
+const config :Workerd.Config = (
+  services = [(
+    name = "worker",
+    worker = (
+      compatibilityFlags = ${JSON.stringify(flags)},
+      moduleFallback = "127.0.0.1:1",
+      modules = [(
+        name = "worker.js",
+        esModule = "export const check = { test() {} };"
+      )]
+    )
+  )]
+);`
+      );
+
+      const result = await runWorkerd(configPath, [`--compat-date=${date}`]);
+      assert.notEqual(result.code, 0, result.output);
+      assert.match(
+        result.output,
+        /The module fallback service is an experimental feature/
+      );
+    });
+  }
+});
+
 test('module fallback serves V1 and V2 module types', async () => {
   const requests = [];
   const handlerErrors = [];