diff --git a/packages/codex-native-api/src/codex_app_client.ts b/packages/codex-native-api/src/codex_app_client.ts
index 216daae..a543453 100644
--- a/packages/codex-native-api/src/codex_app_client.ts
+++ b/packages/codex-native-api/src/codex_app_client.ts
@@ -342,6 +342,8 @@ export class CodexAppClient extends EventEmitter {
 
   socket: WebSocket | null;
 
+  stdioLineBuffer: string;
+
   pending: Map<string, PendingRequest>;
 
   pendingApprovals: Map<string, PendingApproval>;
@@ -398,6 +400,7 @@ export class CodexAppClient extends EventEmitter {
 
     this.child = null;
     this.socket = null;
+    this.stdioLineBuffer = '';
     this.pending = new Map();
     this.pendingApprovals = new Map();
     this.approvedExecutions = new Map();
@@ -1019,21 +1022,22 @@ export class CodexAppClient extends EventEmitter {
     }
     this.childStartError = null;
     this.childStderrTail = [];
-    this.port = await reservePort();
+    this.stdioLineBuffer = '';
+    this.port = null;
     const featureArgs = this.enabledFeatures.flatMap((feature) => ['--enable', feature]);
     const launchSpec = createCodexAppServerLaunchSpec({
       command: this.codexCliBin,
-      args: [...this.codexCliArgs, 'app-server', ...featureArgs, '--listen', `ws://127.0.0.1:${this.port}`],
+      args: [...this.codexCliArgs, 'app-server', ...featureArgs, '--listen', 'stdio://'],
       platform: this.platform,
     });
     try {
       this.child = launchSpec.args
         ? this.spawnImpl(launchSpec.command, launchSpec.args, {
-          stdio: ['ignore', 'pipe', 'pipe'],
+          stdio: ['pipe', 'pipe', 'pipe'],
           ...launchSpec.options,
         })
         : this.spawnImpl(launchSpec.command, {
-          stdio: ['ignore', 'pipe', 'pipe'],
+          stdio: ['pipe', 'pipe', 'pipe'],
           ...launchSpec.options,
         });
     } catch (error) {
@@ -1053,6 +1057,7 @@ export class CodexAppClient extends EventEmitter {
       autolaunch: this.autolaunch,
       launchCommand: this.launchCommand,
     });
+    this.child.stdout?.on('data', (chunk) => this.handleStdioData(chunk));
     this.child.stderr?.on('data', (chunk) => {
       const text = String(chunk).trim();
       if (text) {
@@ -1071,10 +1076,36 @@ export class CodexAppClient extends EventEmitter {
       this.connected = false;
       this.socket = null;
     });
-    await this.connectWebSocket();
+    this.connected = true;
+    await new Promise<void>((resolve) => setImmediate(resolve));
+    if (this.childStartError) {
+      throw this.childStartError;
+    }
+    if (this.child && this.child.exitCode !== null) {
+      throw createCodexAppServerExitedError({
+        command: this.codexCliBin,
+        exitCode: this.child.exitCode,
+        stderrTail: this.childStderrTail,
+      });
+    }
     await this.initialize();
   }
 
+  handleStdioData(chunk: unknown): void {
+    this.stdioLineBuffer += Buffer.isBuffer(chunk) ? chunk.toString('utf8') : String(chunk ?? '');
+    for (;;) {
+      const newlineIndex = this.stdioLineBuffer.indexOf('\n');
+      if (newlineIndex < 0) {
+        break;
+      }
+      const line = this.stdioLineBuffer.slice(0, newlineIndex).trim();
+      this.stdioLineBuffer = this.stdioLineBuffer.slice(newlineIndex + 1);
+      if (line) {
+        this.handleMessage(line);
+      }
+    }
+  }
+
   async connectWebSocket(): Promise<void> {
     const url = `ws://127.0.0.1:${this.port}`;
     const started = Date.now();
@@ -1140,7 +1171,7 @@ export class CodexAppClient extends EventEmitter {
   }
 
   async request(method: string, params: any, { timeoutMs = 30_000 }: { timeoutMs?: number } = {}): Promise<any> {
-    if (!this.socket || !this.connected) {
+    if (!this.child || !this.connected) {
       await this.start();
     }
     const id = String(++this.requestId);
@@ -1191,10 +1222,10 @@ export class CodexAppClient extends EventEmitter {
   }
 
   send(payload: any): void {
-    if (!this.socket || this.socket.readyState !== WebSocket.OPEN) {
-      throw new Error('Codex app-server socket is not open');
+    if (!this.child?.stdin?.writable) {
+      throw new Error('Codex app-server stdio is not open');
     }
-    this.socket.send(JSON.stringify(payload));
+    this.child.stdin.write(`${JSON.stringify(payload)}\n`);
   }
 
   handleMessage(raw: string): void {
diff --git a/src/providers/codex/app_client.ts b/src/providers/codex/app_client.ts
index 78d11b5..5349f84 100644
--- a/src/providers/codex/app_client.ts
+++ b/src/providers/codex/app_client.ts
@@ -1066,7 +1066,7 @@ export class CodexAppClient extends EventEmitter {
     const featureArgs = this.enabledFeatures.flatMap((feature) => ['--enable', feature]);
     const appServerArgs = transportKind === 'websocket'
       ? [...this.codexCliArgs, 'app-server', ...featureArgs, '--listen', `ws://127.0.0.1:${this.port}`]
-      : [...this.codexCliArgs, 'app-server', ...featureArgs];
+      : [...this.codexCliArgs, 'app-server', ...featureArgs, '--listen', 'stdio://'];
     const launchSpec = createCodexAppServerLaunchSpec({
       command: this.codexCliBin,
       args: appServerArgs,
@@ -1125,6 +1125,17 @@ export class CodexAppClient extends EventEmitter {
     if (transportKind === 'stdio') {
       this.transportKind = 'stdio';
       this.connected = true;
+      await new Promise<void>((resolve) => setImmediate(resolve));
+      if (this.childStartError) {
+        throw this.childStartError;
+      }
+      if (this.child && this.child.exitCode !== null) {
+        throw createCodexAppServerExitedError({
+          command: this.codexCliBin,
+          exitCode: this.child.exitCode,
+          stderrTail: this.childStderrTail,
+        });
+      }
     } else {
       await this.connectWebSocket();
     }
@@ -1132,10 +1143,10 @@ export class CodexAppClient extends EventEmitter {
   }
 
   resolveAppServerTransportKind(): 'websocket' | 'stdio' {
-    if (this.appServerTransport === 'stdio') {
-      return 'stdio';
+    if (this.appServerTransport === 'websocket') {
+      return 'websocket';
     }
-    return 'websocket';
+    return 'stdio';
   }
 
   handleStdioData(chunk: unknown): void {
diff --git a/test/providers/codex/app_client.test.ts b/test/providers/codex/app_client.test.ts
index 605bfeb..2d8452d 100644
--- a/test/providers/codex/app_client.test.ts
+++ b/test/providers/codex/app_client.test.ts
@@ -618,7 +618,7 @@ test('CodexAppClient startServer inherits the default Codex feature config when
     exitCode: number | null;
   };
   child.stderr = new EventEmitter();
-  child.exitCode = 0;
+  child.exitCode = null;
 
   const client = new CodexAppClient({
     codexCliBin: 'codex',
@@ -638,8 +638,7 @@ test('CodexAppClient startServer inherits the default Codex feature config when
   assert.equal(calls.length, 1);
   assert.equal(calls[0]?.command, 'codex');
   assert.equal(calls[0]?.args?.[0], 'app-server');
-  assert.deepEqual(calls[0]?.args?.slice(1, 2), ['--listen']);
-  assert.match(String(calls[0]?.args?.[2]), /^ws:\/\/127\.0\.0\.1:\d+$/);
+  assert.deepEqual(calls[0]?.args?.slice(1, 3), ['--listen', 'stdio://']);
 });
 
 test('CodexAppClient startServer prepends configured Codex CLI args', async () => {
@@ -649,7 +648,7 @@ test('CodexAppClient startServer prepends configured Codex CLI args', async () =
     exitCode: number | null;
   };
   child.stderr = new EventEmitter();
-  child.exitCode = 0;
+  child.exitCode = null;
 
   const client = new CodexAppClient({
     codexCliBin: 'codex',
@@ -677,7 +676,7 @@ test('CodexAppClient startServer wraps Windows cmd launchers through cmd.exe', a
     exitCode: number | null;
   };
   child.stderr = new EventEmitter();
-  child.exitCode = 0;
+  child.exitCode = null;
 
   const client = new CodexAppClient({
     codexCliBin: 'C:\\Program Files\\Codex\\codex.cmd',
@@ -703,7 +702,7 @@ test('CodexAppClient startServer wraps Windows cmd launchers through cmd.exe', a
   assert.equal(calls[0]?.args, null);
   assert.equal(calls[0]?.options?.shell, true);
   assert.equal(calls[0]?.options?.windowsHide, true);
-  assert.match(String(calls[0]?.command), /^"C:\\Program Files\\Codex\\codex\.cmd" app-server --listen ws:\/\/127\.0\.0\.1:\d+$/);
+  assert.match(String(calls[0]?.command), /^"C:\\Program Files\\Codex\\codex\.cmd" app-server --listen stdio:\/\/$/);
 });
 
 test('CodexAppClient startServer surfaces a helpful Windows Codex ENOENT error', async () => {