Backport: docs: fix openai provider section of migration guide (#9592)

This is an automated backport of #9590 to the release-v5.0 branch.

Co-authored-by: Nico Albanese <49612682+nicoalbanese@users.noreply.github.com>

Author: vercel-ai-sdk[bot]

content/docs/08-migration-guides/26-migration-guide-5-0.mdx

@@ -3049,33 +3049,105 @@ return createUIMessageStreamResponse({ stream });
 
 ### OpenAI
 
-#### Structured Outputs
+#### Default Provider Instance Uses Responses API
 
-The `structuredOutputs` parameter has been replaced with the `strictJsonSchema` provider option. It is now disabled by default.
+In AI SDK 5, the default OpenAI provider instance uses the Responses API, while AI SDK 4 used the Chat Completions API. The Chat Completions API remains fully supported and you can use it with `openai.chat(...)`.
 
 ```tsx filename="AI SDK 4.0"
 import { openai } from '@ai-sdk/openai';
 
+const defaultModel = openai('gpt-4.1-mini'); // Chat Completions API
+```
+
+```tsx filename="AI SDK 5.0"
+import { openai } from '@ai-sdk/openai';
+
+const defaultModel = openai('gpt-4.1-mini'); // Responses API
+
+// Specify a specific API when needed:
+const chatCompletionsModel = openai.chat('gpt-4.1-mini');
+const responsesModel = openai.responses('gpt-4.1-mini');
+```
+
+<Note>
+  The Responses and Chat Completions APIs have different behavior and defaults.
+  If you depend on the Chat Completions API, switch your model instance to
+  `openai.chat(...)` and audit your configuration.
+</Note>
+
+#### Strict Schemas (`strictSchemas`) with Responses API
+
+In AI SDK 4.0, you could set the `strictSchemas` option on Responses models (which defaulted to `true`). This option has been renamed to `strictJsonSchema` in AI SDK 5.0 and now defaults to `false`.
+
+```tsx filename="AI SDK 4.0"
+import { z } from 'zod';
+import { generateObject } from 'ai';
+import { openai, type OpenAIResponsesProviderOptions } from '@ai-sdk/openai';
+
 const result = await generateObject({
-  model: openai('gpt-4.1-2024-08-06', { structuredOutputs: true }),
-  schema: z.object({ name: z.string() }),
+  model: openai.responses('gpt-4.1'),
+  schema: z.object({
+    // ...
+  }),
+  providerOptions: {
+    openai: {
+      strictSchemas: true, // default behaviour in AI SDK 4
+    } satisfies OpenAIResponsesProviderOptions,
+  },
 });
 ```
 
 ```tsx filename="AI SDK 5.0"
+import { z } from 'zod';
+import { generateObject } from 'ai';
 import { openai, type OpenAIResponsesProviderOptions } from '@ai-sdk/openai';
 
 const result = await generateObject({
-  model: openai('gpt-4.1-2024-08-06'),
-  schema: z.object({ name: z.string() }),
+  model: openai('gpt-4.1-2024'), // uses Responses API
+  schema: z.object({
+    // ...
+  }),
   providerOptions: {
     openai: {
-      strictJsonSchema: true, // renamed and opt-in via provider options
+      strictJsonSchema: true, // defaults to false, opt back in to the AI SDK 4 strict behaviour
     } satisfies OpenAIResponsesProviderOptions,
   },
 });
 ```
 
+If you call `openai.chat(...)` to use the Chat Completions API directly, you can type it with `OpenAIChatLanguageModelOptions`. AI SDK 5 adds the same `strictJsonSchema` option there as well.
+
+#### Structured Outputs
+
+The `structuredOutputs` option is now configured using provider options rather than as a setting on the model instance.
+
+```tsx filename="AI SDK 4.0"
+import { z } from 'zod';
+import { generateObject } from 'ai';
+import { openai } from '@ai-sdk/openai';
+
+const result = await generateObject({
+  model: openai('gpt-4.1', { structuredOutputs: true }), // use Chat Completions API
+  schema: z.object({ name: z.string() }),
+});
+```
+
+```tsx filename="AI SDK 5.0 (Chat Completions API)"
+import { z } from 'zod';
+import { generateObject } from 'ai';
+import { openai, type OpenAIChatLanguageModelOptions } from '@ai-sdk/openai';
+
+const result = await generateObject({
+  model: openai.chat('gpt-4.1'), // use Chat Completions API
+  schema: z.object({ name: z.string() }),
+  providerOptions: {
+    openai: {
+      structuredOutputs: true,
+    } satisfies OpenAIChatLanguageModelOptions,
+  },
+});
+```
+
 #### Compatibility Option Removal
 
 The `compatibility` option has been removed; strict compatibility mode is now the default.