diff --git a/integration-tests/acp-jsonrpc.test.ts b/integration-tests/acp-jsonrpc.test.ts
new file mode 100644
index 00000000000..6003dfee044
--- /dev/null
+++ b/integration-tests/acp-jsonrpc.test.ts
@@ -0,0 +1,81 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { expect, describe, it, beforeEach, afterEach } from 'vitest';
+import { TestRig } from './test-helper.js';
+
+describe('ACP JSON-RPC Protocol', () => {
+  let rig: TestRig;
+
+  beforeEach(async () => {
+    rig = new TestRig();
+    await rig.setup('acp-jsonrpc-test');
+  });
+
+  afterEach(async () => {
+    await rig.cleanup();
+  });
+
+  it('should return valid JSON-RPC 2.0 response for initialize method', async () => {
+    // Prepare the JSON-RPC initialize request
+    const initializeRequest = {
+      jsonrpc: '2.0',
+      id: 0,
+      method: 'initialize',
+      params: {
+        protocolVersion: 1,
+        clientCapabilities: {
+          fs: {
+            readTextFile: false,
+            writeTextFile: false,
+          },
+          terminal: false,
+        },
+      },
+    };
+
+    // Send the request via stdin with --experimental-acp flag
+    const result = await rig.run(
+      {
+        stdin: JSON.stringify(initializeRequest) + '\n',
+        yolo: false,
+      },
+      '--experimental-acp',
+    );
+
+    // Extract JSON lines from output (filter out debug/log output)
+    const jsonLines = result
+      .split('\n')
+      .map((line) => line.trim())
+      .filter((line) => line.startsWith('{') && line.includes('"jsonrpc"'));
+
+    // Should have at least one JSON-RPC response
+    expect(jsonLines.length).toBeGreaterThanOrEqual(1);
+
+    // Parse the first JSON-RPC response
+    const parsed = JSON.parse(jsonLines[0]);
+
+    // Verify it's a valid JSON-RPC 2.0 response
+    expect(parsed).toHaveProperty('jsonrpc');
+    expect(parsed.jsonrpc).toBe('2.0');
+
+    // Verify response structure
+    expect(parsed).toHaveProperty('id');
+    expect(parsed.id).toBe(0);
+
+    // Should have either 'result' or 'error', but not both
+    const hasResult = 'result' in parsed;
+    const hasError = 'error' in parsed;
+    expect(hasResult || hasError).toBe(true);
+    expect(hasResult && hasError).toBe(false);
+
+    // For a successful initialize, we expect a result
+    if (hasResult) {
+      expect(parsed.result).toBeDefined();
+      expect(typeof parsed.result).toBe('object');
+    }
+  });
+});
diff --git a/integration-tests/test-helper.ts b/integration-tests/test-helper.ts
index 94d3e697b8c..59289f4c6d6 100644
--- a/integration-tests/test-helper.ts
+++ b/integration-tests/test-helper.ts
@@ -503,9 +503,10 @@ export class TestRig {
           const isJsonOutput =
             commandArgs.includes('--output-format') &&
             commandArgs.includes('json');
+          const isAcpMode = commandArgs.includes('--experimental-acp');
 
-          // If we have stderr output and it's not a JSON test, include that also
-          if (stderr && !isJsonOutput) {
+          // If we have stderr output and it's not a JSON test or ACP mode (ACP uses JRPC), include that also
+          if (stderr && !isJsonOutput && !isAcpMode) {
             result += `\n\nStdErr:\n${stderr}`;
           }