fix (ai/ui): forward streaming errors in useChat

.changeset/tender-poets-itch.md

@@ -0,0 +1,6 @@
+---
+'@ai-sdk/ui-utils': patch
+'@ai-sdk/react': patch
+---
+
+fix (ai/ui): forward streaming errors in useChat

packages/react/src/use-chat.ui.test.tsx

@@ -121,7 +121,7 @@ describe('stream data stream', () => {
   );
 
   it(
-    'should show error response',
+    'should show error response when there is a server error',
     withTestServer(
       { type: 'error', url: '/api/chat', status: 404, content: 'Not found' },
       async () => {
@@ -135,6 +135,25 @@ describe('stream data stream', () => {
     ),
   );
 
+  it(
+    'should show error response when there is a streaming error',
+    withTestServer(
+      {
+        type: 'stream-values',
+        url: '/api/chat',
+        content: ['3:"custom error message"\n'],
+      },
+      async () => {
+        await userEvent.click(screen.getByTestId('do-append'));
+
+        await screen.findByTestId('error');
+        expect(screen.getByTestId('error')).toHaveTextContent(
+          'Error: custom error message',
+        );
+      },
+    ),
+  );
+
   describe('loading state', () => {
     it(
       'should show loading state',

packages/ui-utils/src/parse-complex-response.ts

@@ -89,6 +89,10 @@ export async function parseComplexResponse({
   for await (const { type, value } of readDataStream(reader, {
     isAborted: () => abortControllerRef?.current === null,
   })) {
+    if (type === 'error') {
+      throw new Error(value);
+    }
+
     if (type === 'text') {
       if (prefixMap['text']) {
         prefixMap['text'] = {