Backport: fix(openai): allow 'revised_prompt' to be returned as null (#9900)

This is an automated backport of #9891 to the release-v5.0 branch.

Co-authored-by: Aayush Kapoor <83492835+aayush-kapoor@users.noreply.github.com>

Author: vercel-ai-sdk[bot]

.changeset/shy-kids-explain.md

@@ -0,0 +1,5 @@
+---
+'@ai-sdk/openai': patch
+---
+
+fix(openai): revised_prompt sometimes returns null, causing errors

packages/openai/src/image/openai-image-api.ts

@@ -1,15 +1,15 @@
 import { lazyValidator, zodSchema } from '@ai-sdk/provider-utils';
 import { z } from 'zod/v4';
 
-// minimal version of the schema, focussed on what is needed for the implementation
+// minimal version of the schema, focused on what is needed for the implementation
 // this approach limits breakages when the API changes and increases efficiency
 export const openaiImageResponseSchema = lazyValidator(() =>
   zodSchema(
     z.object({
       data: z.array(
         z.object({
           b64_json: z.string(),
-          revised_prompt: z.string().optional(),
+          revised_prompt: z.string().nullish(),
         }),
       ),
     }),

packages/openai/src/image/openai-image-model.test.ts

@@ -238,6 +238,38 @@ describe('doGenerate', () => {
     expect(requestBody).not.toHaveProperty('response_format');
   });
 
+  it('should handle null revised_prompt responses', async () => {
+    server.urls['https://api.openai.com/v1/images/generations'].response = {
+      type: 'json-value',
+      body: {
+        created: 1733837122,
+        data: [
+          {
+            revised_prompt: null,
+            b64_json: 'base64-image-1',
+          },
+        ],
+      },
+    };
+
+    const result = await provider.image('gpt-image-1').doGenerate({
+      prompt,
+      n: 1,
+      size: '1024x1024',
+      aspectRatio: undefined,
+      seed: undefined,
+      providerOptions: {},
+    });
+
+    expect(result.images).toStrictEqual(['base64-image-1']);
+    expect(result.warnings).toStrictEqual([]);
+    expect(result.providerMetadata).toStrictEqual({
+      openai: {
+        images: [null],
+      },
+    });
+  });
+
   it('should include response_format for dall-e-3', async () => {
     prepareJsonResponse();