diff --git a/handlers/handleCustomResource.js b/handlers/handleCustomResource.js
index f32837f..58ab3ba 100644
--- a/handlers/handleCustomResource.js
+++ b/handlers/handleCustomResource.js
@@ -19,5 +19,23 @@ export default async function handleCustomResource(event, context) {
   logDebug('invocation', invocation);
   logDebug('handleCustomResource', { requestType: event.RequestType, requestId: context.awsRequestId });
   console.log('Custom Resource request:', event.RequestType);
-  return {};
+
+  if (!event.ResponseURL) {
+    throw new Error('Missing ResponseURL');
+  }
+
+  const responseBody = JSON.stringify({
+    Status: 'SUCCESS',
+    PhysicalResourceId: event.PhysicalResourceId || context.awsRequestId,
+    StackId: event.StackId,
+    RequestId: event.RequestId,
+    LogicalResourceId: event.LogicalResourceId
+  });
+
+  await fetch(event.ResponseURL, {
+    method: 'PUT',
+    body: responseBody
+  });
+
+  return;
 }
diff --git a/tests/handleCustomResource.test.js b/tests/handleCustomResource.test.js
new file mode 100644
index 0000000..1561e50
--- /dev/null
+++ b/tests/handleCustomResource.test.js
@@ -0,0 +1,33 @@
+import { jest } from '@jest/globals';
+import handleCustomResource from '../handlers/handleCustomResource.js';
+
+describe('handleCustomResource', () => {
+  test('PUTs response to ResponseURL', async () => {
+    const fetchMock = jest.spyOn(global, 'fetch').mockResolvedValue({ ok: true });
+    const event = {
+      RequestType: 'Create',
+      ResponseURL: 'https://example.com',
+      StackId: 's',
+      RequestId: 'r',
+      LogicalResourceId: 'l'
+    };
+    const context = { awsRequestId: 'id1' };
+    await handleCustomResource(event, context);
+    expect(fetchMock).toHaveBeenCalledWith('https://example.com', expect.objectContaining({ method: 'PUT' }));
+    const body = fetchMock.mock.calls[0][1].body;
+    expect(JSON.parse(body)).toMatchObject({
+      Status: 'SUCCESS',
+      PhysicalResourceId: 'id1',
+      StackId: 's',
+      RequestId: 'r',
+      LogicalResourceId: 'l'
+    });
+    fetchMock.mockRestore();
+  });
+
+  test('throws when ResponseURL missing', async () => {
+    const event = { RequestType: 'Create' };
+    const context = { awsRequestId: 'id1' };
+    await expect(handleCustomResource(event, context)).rejects.toThrow('Missing ResponseURL');
+  });
+});
diff --git a/tests/handlers.test.js b/tests/handlers.test.js
index 665810b..ddfcdee 100644
--- a/tests/handlers.test.js
+++ b/tests/handlers.test.js
@@ -163,10 +163,13 @@ describe('handler dispatch', () => {
   });
 
   test('handles Custom Resource event', async () => {
-    const event = { RequestType: 'Create', ResponseURL: 'https://example.com' };
+    const fetchMock = jest.spyOn(global, 'fetch').mockResolvedValue({ ok: true });
+    const event = { RequestType: 'Create', ResponseURL: 'https://example.com', StackId: 's', RequestId: 'r', LogicalResourceId: 'l' };
     const context = { awsRequestId: '1' };
     const result = await handler(event, context);
-    expect(result).toEqual({});
+    expect(result).toBeUndefined();
+    expect(fetchMock).toHaveBeenCalledWith('https://example.com', expect.objectContaining({ method: 'PUT' }));
+    fetchMock.mockRestore();
   });
 
   test('handles Cognito event', async () => {