diff --git a/packages/void/src/index.ts b/packages/void/src/index.ts
index a4771a181..b5335a783 100644
--- a/packages/void/src/index.ts
+++ b/packages/void/src/index.ts
@@ -396,6 +396,25 @@ function createDefaultWorkerEnvironmentPlugin(): Plugin {
   };
 }
 
+function createWorkerEnvironmentSanitizerPlugin(environmentNames: ReadonlyArray<string>): Plugin {
+  const names = new Set(environmentNames);
+  return {
+    name: 'void:worker-environment-sanitizer',
+    enforce: 'pre',
+    configResolved(config) {
+      for (const name of names) {
+        const resolve = config.environments[name]?.resolve;
+        if (resolve && 'external' in resolve) {
+          // Test runners may seed Node externals into every Vite environment.
+          // Cloudflare Worker environments cannot externalize dependencies, but
+          // Cloudflare's validator expects the normalized array shape to remain.
+          resolve.external = [];
+        }
+      }
+    },
+  };
+}
+
 function createSandboxSdkResolvePlugin(): Plugin {
   let resolved: string | undefined;
 
@@ -1370,6 +1389,10 @@ export function voidPlugin(options?: {
 
     return [
       createCloudflareBuiltinsExternalPlugin(),
+      createWorkerEnvironmentSanitizerPlugin([
+        viteEnvironment.name,
+        ...(viteEnvironment.childEnvironments ?? []),
+      ]),
       createWranglerCompatPersistPlugin(root, config),
       restartPlugin(),
       bindingWatchPlugin,
@@ -1500,6 +1523,7 @@ export function voidPlugin(options?: {
   return [
     createCloudflareBuiltinsExternalPlugin(),
     createDefaultWorkerEnvironmentPlugin(),
+    createWorkerEnvironmentSanitizerPlugin([DEFAULT_WORKER_ENVIRONMENT_NAME]),
     createWranglerCompatPersistPlugin(root, config),
     restartPlugin(),
     bindingWatchPlugin,
diff --git a/packages/void/test/unit/plugin-vite-environment.test.ts b/packages/void/test/unit/plugin-vite-environment.test.ts
index 3c20eb107..ae2150333 100644
--- a/packages/void/test/unit/plugin-vite-environment.test.ts
+++ b/packages/void/test/unit/plugin-vite-environment.test.ts
@@ -79,4 +79,70 @@ describe('voidPlugin Vite environments', () => {
       },
     });
   });
+
+  it('clears resolve.external from the worker environment configured by test runners', async () => {
+    const root = createProjectRoot();
+    vi.spyOn(process, 'cwd').mockReturnValue(root);
+
+    const config = await resolveConfig(
+      {
+        root,
+        plugins: [voidPlugin()],
+        environments: {
+          void_worker: {
+            resolve: {
+              external: ['node:fs'],
+              conditions: ['workerd'],
+            },
+            build: {
+              emptyOutDir: false,
+            },
+          },
+        },
+      },
+      'serve',
+      'test',
+    );
+
+    expect(config.environments.void_worker.resolve.external).toEqual([]);
+    expect(config.environments.void_worker.resolve.conditions).toContain('workerd');
+    expect(config.environments.void_worker.build).toMatchObject({
+      emptyOutDir: false,
+      outDir: 'dist/ssr',
+    });
+  });
+
+  it('clears resolve.external from class A framework worker environments', async () => {
+    const root = createProjectRoot();
+    vi.spyOn(process, 'cwd').mockReturnValue(root);
+    writeFileSync(
+      join(root, 'package.json'),
+      JSON.stringify({
+        type: 'module',
+        dependencies: {
+          '@tanstack/react-start': '^1.0.0',
+        },
+      }),
+    );
+
+    const config = await resolveConfig(
+      {
+        root,
+        plugins: [voidPlugin()],
+        environments: {
+          ssr: {
+            resolve: {
+              external: ['node:fs'],
+              conditions: ['workerd'],
+            },
+          },
+        },
+      },
+      'serve',
+      'test',
+    );
+
+    expect(config.environments.ssr.resolve.external).toEqual([]);
+    expect(config.environments.ssr.resolve.conditions).toContain('workerd');
+  });
 });