diff --git a/plugins/mock/src/lib/app.ts b/plugins/mock/src/lib/app.ts
index 8c3ff02f9a..6710ab4aeb 100644
--- a/plugins/mock/src/lib/app.ts
+++ b/plugins/mock/src/lib/app.ts
@@ -99,13 +99,15 @@ class MockApplicationWorker extends Base {
     this[APP_INIT] = true;
     debug('this[APP_INIT] = true');
     this.#bindEvent();
+    // Koa normally installs this in callback(), but startup can emit errors before then.
+    if (!app.listenerCount('error')) {
+      app.on('error', (app as any).onerror.bind(app));
+    }
+    // Wait for middleware loading before app.callback() composes the request handler.
+    await app.ready();
     debug('http server instantiate');
     createServer(app);
-    await app.ready();
-    // emit `server` after ready: egg core registers its `once('server', ...)`
-    // listener inside `Application.load()` (during `app.ready()` above), and
-    // `onServer` reads loaded config, so the event must be emitted now rather
-    // than at createServer() time. Mirrors @eggjs/cluster's post-ready emit.
+    // Application.load() registers the onServer listener during app.ready().
     if (app.server) {
       app.emit('server', app.server);
     }
diff --git a/plugins/mock/src/lib/mock_http_server.ts b/plugins/mock/src/lib/mock_http_server.ts
index efb0d1fd14..126d15ee03 100644
--- a/plugins/mock/src/lib/mock_http_server.ts
+++ b/plugins/mock/src/lib/mock_http_server.ts
@@ -12,11 +12,7 @@ export function createServer(app: any): Server {
       app.server = server;
     }
   }
-  // NOTE: don't emit the `server` event here. egg core registers its
-  // `once('server', ...)` listener inside `Application.load()` (during
-  // `app.ready()`), so emitting at server-creation time (before ready) would be
-  // missed and `onServer` (clientError logging / graceful / timeout / websocket)
-  // never runs. The caller emits `server` after `app.ready()` instead — just
-  // like @eggjs/cluster, which emits it after the app is ready.
+  // Mock startup emits `server` once after app.ready(). HTTP requests also use
+  // this helper to get the cached server and must not emit the event again.
   return server;
 }
diff --git a/plugins/mock/src/lib/parallel/app.ts b/plugins/mock/src/lib/parallel/app.ts
index 969369e77f..e09d1794fa 100644
--- a/plugins/mock/src/lib/parallel/app.ts
+++ b/plugins/mock/src/lib/parallel/app.ts
@@ -53,11 +53,15 @@ export class MockParallelApplication extends Base {
     this.__APP_INIT__ = true;
     debug('this[APP_INIT] = true');
     this.#bindEvents();
+    // Koa normally installs this in callback(), but startup can emit errors before then.
+    if (!app.listenerCount('error')) {
+      app.on('error', (app as any).onerror.bind(app));
+    }
+    // Wait for middleware loading before app.callback() composes the request handler.
+    await app.ready();
     debug('http server instantiate');
     createServer(app);
-    await app.ready();
-    // emit `server` after ready so egg core's onServer listener (registered in
-    // Application.load()) is wired up; createServer no longer emits it.
+    // Application.load() registers the onServer listener during app.ready().
     if (app.server) {
       app.emit('server', app.server);
     }
diff --git a/plugins/mock/test/app.test.ts b/plugins/mock/test/app.test.ts
index ca1c2462b5..3d948f69f1 100644
--- a/plugins/mock/test/app.test.ts
+++ b/plugins/mock/test/app.test.ts
@@ -1,5 +1,6 @@
 import { strict as assert } from 'node:assert';
 
+import { request } from '@eggjs/supertest';
 import { describe, it, beforeAll, afterAll, afterEach } from 'vitest';
 
 import mm, { type MockApplication } from '../src/index.ts';
@@ -81,12 +82,8 @@ describe('test/app.test.ts', { concurrent: false }, () => {
   });
 
   it('should run onServer after ready (server event not lost)', async () => {
-    // @eggjs/mock emits the `server` event before `app.ready()`, while egg core
-    // registers its `once("server", ...)` listener inside Application.load()
-    // (during app.ready()). egg core re-emits `server` after the listener is
-    // registered so onServer still runs and wires up the `clientError` handler
-    // (plus graceful shutdown / server timeout / websocket). Regression guard:
-    // assert the clientError listener is wired after ready.
+    // Application.load() registers the server listener during app.ready().
+    // Mock startup must emit server afterward so onServer attaches clientError.
     const baseDir = getFixtures('server');
     const app = mm.app({
       baseDir,
@@ -104,7 +101,23 @@ describe('test/app.test.ts', { concurrent: false }, () => {
     }
   });
 
-  it('should emit server after ready in parallel app', async () => {
+  it('should compose HTTP middleware after app is ready', async () => {
+    const app = mm.app({
+      baseDir: getFixtures('app'),
+      framework: getFixtures('snapshot-framework'),
+      cache: false,
+    });
+    try {
+      await app.ready();
+      assert(app.server, 'app.server not exists');
+      await request(app.server).get('/').expect(200, 'foo');
+      await app.httpRequest().get('/').expect(200, 'foo');
+    } finally {
+      await app.close();
+    }
+  });
+
+  it('should create and emit server after ready in parallel app', async () => {
     // cover the parallel app worker's post-ready `server` emit (lib/parallel/app.ts)
     const baseDir = getFixtures('server');
     let emittedServer: unknown;
@@ -127,6 +140,7 @@ describe('test/app.test.ts', { concurrent: false }, () => {
       assert(app.server, 'app.server not exists');
       assert.equal(emittedServer, app.server);
       assert.equal(emittedAfterReady, true);
+      await request(app.server).get('/').expect(200, 'ok');
     } finally {
       await app.close();
     }
@@ -159,6 +173,31 @@ describe('test/app.test.ts', { concurrent: false }, () => {
     await app.close();
   });
 
+  it.each([false, true])('should handle parallel startup errors (custom listener: %s)', async (customListener) => {
+    const error = new Error('parallel startup failed');
+    const errors: Error[] = [];
+    const app = createParallelApp({
+      baseDir: getFixtures('server'),
+      framework: getFixtures('parallel-framework'),
+      cache: false,
+      clean: false,
+      beforeInit: async (parallelApp) => {
+        parallelApp.options.clusterPort = 1;
+        parallelApp.options.readyError = error;
+      },
+    });
+    if (customListener) {
+      app.on('error', (err: Error) => errors.push(err));
+    }
+    try {
+      await assert.rejects(app.ready(), (err) => err === error);
+      assert.deepEqual((app as any).errors, customListener ? [] : [error]);
+      assert.deepEqual(errors, customListener ? [error] : []);
+    } finally {
+      await app.close();
+    }
+  });
+
   // Node.js v20: SyntaxError: Unexpected identifier 'SingleModeApplication'
   it.skipIf(process.version.startsWith('v20.'))('should FrameworkErrorformater work during app boot', async () => {
     // let logMsg = '';
diff --git a/plugins/mock/test/fixtures/parallel-framework/index.js b/plugins/mock/test/fixtures/parallel-framework/index.js
index 098c5dd5f8..45f8b491df 100644
--- a/plugins/mock/test/fixtures/parallel-framework/index.js
+++ b/plugins/mock/test/fixtures/parallel-framework/index.js
@@ -6,6 +6,7 @@ class Application extends EventEmitter {
     this.options = options;
     this.context = {};
     this.config = {};
+    this.errors = [];
     this.messenger = {
       messages: [],
       onMessage: (msg) => this.messenger.messages.push(msg),
@@ -13,13 +14,25 @@ class Application extends EventEmitter {
   }
 
   callback() {
-    return (_req, res) => res.end('ok');
+    const ready = this.readyAt;
+    return (_req, res) => {
+      res.statusCode = ready ? 200 : 503;
+      res.end(ready ? 'ok' : 'not ready');
+    };
   }
 
   async ready() {
+    if (this.options.readyError) {
+      this.emit('error', this.options.readyError);
+      throw this.options.readyError;
+    }
     this.readyAt = true;
   }
 
+  onerror(err) {
+    this.errors.push(err);
+  }
+
   async close() {}
 }
 
diff --git a/plugins/mock/test/fixtures/snapshot-framework/index.js b/plugins/mock/test/fixtures/snapshot-framework/index.js
new file mode 100644
index 0000000000..c49b0f2ced
--- /dev/null
+++ b/plugins/mock/test/fixtures/snapshot-framework/index.js
@@ -0,0 +1,17 @@
+const egg = require('egg');
+
+class Application extends egg.Application {
+  callback() {
+    // Model middleware composition that captures the current list, as koa-compose 4.2 does.
+    const middleware = this.middleware;
+    this.middleware = [...middleware];
+    try {
+      return super.callback();
+    } finally {
+      this.middleware = middleware;
+    }
+  }
+}
+
+exports.Application = Application;
+exports.Agent = egg.Agent;