diff --git a/packages/ai/src/steps/analyst-step.ts b/packages/ai/src/steps/analyst-step.ts
index 3dcf3a91f..54a241abc 100644
--- a/packages/ai/src/steps/analyst-step.ts
+++ b/packages/ai/src/steps/analyst-step.ts
@@ -60,6 +60,7 @@ const outputSchema = z.object({
 
 const DEFAULT_CACHE_OPTIONS = {
   anthropic: { cacheControl: { type: 'ephemeral', ttl: '1h' } },
+  openai: { parallelToolCalls: false, reasoningEffort: 'minimal' },
 };
 
 /**
diff --git a/packages/ai/src/steps/think-and-prep-step.ts b/packages/ai/src/steps/think-and-prep-step.ts
index b4a384517..f916181ec 100644
--- a/packages/ai/src/steps/think-and-prep-step.ts
+++ b/packages/ai/src/steps/think-and-prep-step.ts
@@ -82,6 +82,7 @@ const outputSchema = ThinkAndPrepOutputSchema;
 
 const DEFAULT_CACHE_OPTIONS = {
   anthropic: { cacheControl: { type: 'ephemeral', ttl: '1h' } },
+  openai: { parallelToolCalls: false, reasoningEffort: 'minimal' },
 };
 
 // Helper function to create the result object
diff --git a/packages/ai/src/utils/models/sonnet-4.ts b/packages/ai/src/utils/models/sonnet-4.ts
index 03bb8ecd3..46ade010b 100644
--- a/packages/ai/src/utils/models/sonnet-4.ts
+++ b/packages/ai/src/utils/models/sonnet-4.ts
@@ -1,6 +1,7 @@
 import type { LanguageModelV1 } from '@ai-sdk/provider';
 import { createFallback } from './ai-fallback';
 import { anthropicModel } from './providers/anthropic';
+import { openaiModel } from './providers/openai';
 import { vertexModel } from './providers/vertex';
 
 // Lazy initialization to allow mocking in tests
@@ -24,13 +25,13 @@ function initializeSonnet4() {
     }
   }
 
-  // Only include Vertex if credentials are available
-  if (process.env.VERTEX_CLIENT_EMAIL && process.env.VERTEX_PRIVATE_KEY) {
+  if (process.env.OPENAI_API_KEY) {
     try {
-      models.push(vertexModel('claude-sonnet-4@20250514'));
-      console.info('Sonnet4: Vertex AI model added to fallback chain');
+      models.push(openaiModel('gpt-5'));
+
+      console.info('Sonnet4: OpenAI model added to fallback chain');
     } catch (error) {
-      console.warn('Sonnet4: Failed to initialize Vertex AI model:', error);
+      console.warn('Sonnet4: Failed to initialize OpenAI model:', error);
     }
   }