fix: support thinking with structured output in AI SDK runner

AI SDK `generateObject()` seems to not work well with the Anthropic
extended thinking mode (which seems to be rarely used by people..?).
This commit configures the Anthropic provider differently when needed to
support extended thinking w/ structured responses.

Author: devversion

package.json

@@ -54,6 +54,7 @@
     "@ai-sdk/anthropic": "^2.0.45",
     "@ai-sdk/google": "^2.0.39",
     "@ai-sdk/openai": "^2.0.71",
+    "@ai-sdk/provider": "^2.0.0",
     "@anthropic-ai/sdk": "^0.68.0",
     "@axe-core/puppeteer": "^4.10.2",
     "@genkit-ai/compat-oai": "1.23.0",

pnpm-lock.yaml

@@ -0,0 +1,36 @@
+import type {LanguageModelV2Middleware} from '@ai-sdk/provider';
+
+/**
+ * Middleware for Anthropic AI SDK models that is necessary for enabling
+ * thinking mode + structured responses.
+ *
+ * This is necessary because Anthropic would be used with enforced tool usage
+ * by default with `generateObject()`. This is a workaround that makes the tool
+ * optional: https://github.com/vercel/ai/issues/9351.
+ */
+export const anthropicThinkingWithStructuredResponseMiddleware: LanguageModelV2Middleware = {
+  transformParams: ({params}) => {
+    if (params.responseFormat?.type === 'json' && params.responseFormat.schema) {
+      params.tools = [
+        {
+          type: 'function',
+          description: 'Respond with a JSON object for the structured output/answer.',
+          inputSchema: params.responseFormat.schema,
+          name: 'json',
+        },
+      ];
+      params.toolChoice = {type: 'auto'};
+
+      params.prompt.push({
+        role: 'user',
+        content: [
+          {
+            type: 'text',
+            text: 'Use the `json` tool to provide the structured output/answer. No other text is needed.',
+          },
+        ],
+      });
+    }
+    return Promise.resolve(params);
+  },
+};

runner/codegen/ai-sdk-claude-thinking-patch.ts

@@ -16,13 +16,15 @@ import {
   ModelMessage,
   SystemModelMessage,
   TextPart,
+  wrapLanguageModel,
 } from 'ai';
 import {google, GoogleGenerativeAIProviderOptions} from '@ai-sdk/google';
 import {anthropic, AnthropicProviderOptions} from '@ai-sdk/anthropic';
 import {openai, OpenAIResponsesProviderOptions} from '@ai-sdk/openai';
 import z from 'zod';
 import {callWithTimeout} from '../utils/timeout.js';
 import {combineAbortSignals} from '../utils/abort-signal.js';
+import {anthropicThinkingWithStructuredResponseMiddleware} from './ai-sdk-claude-thinking-patch.js';
 
 const SUPPORTED_MODELS = [
   'claude-opus-4.1-no-thinking',
@@ -159,26 +161,19 @@ export class AiSDKRunner implements LlmRunner {
     const modelName = request.model as (typeof SUPPORTED_MODELS)[number];
     switch (modelName) {
       case 'claude-opus-4.1-no-thinking':
-      case 'claude-opus-4.1-with-thinking-16k': {
-        const thinkingEnabled = modelName.includes('-with-thinking');
-        return {
-          model: anthropic('claude-opus-4-1'),
-          providerOptions: {
-            anthropic: {
-              sendReasoning: thinkingEnabled,
-              thinking: {
-                type: thinkingEnabled ? 'enabled' : 'disabled',
-                budgetTokens: thinkingEnabled ? claude16kThinkingTokenBudget : undefined,
-              },
-            } satisfies AnthropicProviderOptions,
-          },
-        };
-      }
+      case 'claude-opus-4.1-with-thinking-16k':
       case 'claude-sonnet-4.5-no-thinking':
       case 'claude-sonnet-4.5-with-thinking-16k': {
         const thinkingEnabled = modelName.includes('-with-thinking');
+        const isOpus4_1Model = modelName.includes('opus-4.1');
+        const model = anthropic(isOpus4_1Model ? 'claude-opus-4-1' : 'claude-sonnet-4-5');
         return {
-          model: anthropic('claude-sonnet-4-5'),
+          model: thinkingEnabled
+            ? wrapLanguageModel({
+                model,
+                middleware: anthropicThinkingWithStructuredResponseMiddleware,
+              })
+            : model,
           providerOptions: {
             anthropic: {
               sendReasoning: thinkingEnabled,