diff --git a/tests/integration/test_api_server.ts b/tests/integration/test_api_server.ts
index 8b6337d40..55159bfd2 100644
--- a/tests/integration/test_api_server.ts
+++ b/tests/integration/test_api_server.ts
@@ -5,6 +5,7 @@
  */
 
 import {spawn} from 'node:child_process';
+import * as fs from 'node:fs';
 import * as path from 'node:path';
 import {AdkApiClient} from '../../dev/src/server/adk_api_client.js';
 import {BaseTestServer} from './test_case_utils.js';
@@ -24,6 +25,30 @@ export interface TestApiServerParams {
 
 const DEFAULT_TIMEOUT = 60000;
 
+/** The built ADK CLI entrypoint that the test server spawns. */
+const CLI_ENTRYPOINT_PATH = path.resolve(
+  __dirname,
+  '../../dev/dist/esm/cli_entrypoint.js',
+);
+
+/**
+ * Throws if the ADK CLI has not been built.
+ *
+ * The integration tests spawn the compiled CLI. On an unbuilt checkout the
+ * child exits 1 with `ERR_MODULE_NOT_FOUND` and the harness reports only
+ * `CLI exited prematurely with code 1`. This check verifies the entrypoint
+ * alone, not that every workspace is built.
+ */
+export function assertAdkCliBuilt(cliPath: string): void {
+  if (!fs.existsSync(cliPath)) {
+    throw new Error(
+      `ADK CLI entrypoint not found at ${cliPath}. ` +
+        'The integration tests spawn the built CLI; run `npm run build` from ' +
+        'the repository root first.',
+    );
+  }
+}
+
 /**
  * ADK API server for testing via the CLI. This is useful for integration tests
  * that require an ADK API server to be running.
@@ -37,6 +62,8 @@ export class AdkTsApiServer extends BaseTestServer {
   }
 
   async start(): Promise<AdkApiClient> {
+    assertAdkCliBuilt(CLI_ENTRYPOINT_PATH);
+
     await this.startProcess({
       spawnProcess: () => {
         return spawn('node', this.getAdkCliArgs(this.params), {
@@ -56,12 +83,8 @@ export class AdkTsApiServer extends BaseTestServer {
   }
 
   private getAdkCliArgs(params: TestApiServerParams): string[] {
-    const cliPath = path.resolve(
-      __dirname,
-      '../../dev/dist/esm/cli_entrypoint.js',
-    );
     const args = [
-      cliPath,
+      CLI_ENTRYPOINT_PATH,
       params.serveDebugUI ? 'web' : 'api_server',
       params.agentsDir,
       '--port',
diff --git a/tests/integration/test_api_server_test.ts b/tests/integration/test_api_server_test.ts
new file mode 100644
index 000000000..c5fc62830
--- /dev/null
+++ b/tests/integration/test_api_server_test.ts
@@ -0,0 +1,59 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import * as childProcess from 'node:child_process';
+import * as fs from 'node:fs';
+import * as path from 'node:path';
+import {describe, expect, it, vi} from 'vitest';
+import {AdkTsApiServer, assertAdkCliBuilt} from './test_api_server.js';
+
+// Reports the CLI entrypoint as missing so start() takes the unbuilt-checkout
+// path on a built tree. Every other path keeps the real answer.
+vi.mock('node:fs', async (importActual) => {
+  const actual = await importActual<typeof import('node:fs')>();
+  return {
+    ...actual,
+    existsSync: (target: fs.PathLike) =>
+      String(target).endsWith('cli_entrypoint.js')
+        ? false
+        : actual.existsSync(target),
+  };
+});
+
+vi.mock('node:child_process', async (importActual) => {
+  const actual = await importActual<typeof import('node:child_process')>();
+  return {...actual, spawn: vi.fn(actual.spawn)};
+});
+
+const START_BUDGET_MS = 5000;
+
+describe('assertAdkCliBuilt', () => {
+  it('names the missing entrypoint and the build command', () => {
+    const missing = path.resolve(__dirname, 'not_built/cli_entrypoint.js');
+
+    expect(() => assertAdkCliBuilt(missing)).toThrowError(missing);
+    expect(() => assertAdkCliBuilt(missing)).toThrowError('npm run build');
+  });
+
+  it('accepts an entrypoint that exists', () => {
+    const present = path.resolve(__dirname, 'test_api_server.ts');
+
+    expect(() => assertAdkCliBuilt(present)).not.toThrow();
+  });
+});
+
+describe('AdkTsApiServer.start', () => {
+  it(
+    'rejects before it spawns the CLI',
+    async () => {
+      const server = new AdkTsApiServer({agentsDir: __dirname});
+
+      await expect(server.start()).rejects.toThrowError('npm run build');
+      expect(vi.mocked(childProcess.spawn)).not.toHaveBeenCalled();
+    },
+    START_BUDGET_MS,
+  );
+});