diff --git a/lib/ai-providers.ts b/lib/ai-providers.ts
index a20b92a..da93069 100644
--- a/lib/ai-providers.ts
+++ b/lib/ai-providers.ts
@@ -95,8 +95,8 @@ function parseIntSafe(
  * Supports various AI SDK providers with their unique configuration options
  *
  * Environment variables:
- * - OPENAI_REASONING_EFFORT: OpenAI reasoning effort level (minimal/low/medium/high) - for o1/o3/gpt-5
- * - OPENAI_REASONING_SUMMARY: OpenAI reasoning summary (none/brief/detailed) - auto-enabled for o1/o3/gpt-5
+ * - OPENAI_REASONING_EFFORT: OpenAI reasoning effort level (minimal/low/medium/high) - for o1/o3/o4/gpt-5
+ * - OPENAI_REASONING_SUMMARY: OpenAI reasoning summary (auto/detailed) - auto-enabled for o1/o3/o4/gpt-5
  * - ANTHROPIC_THINKING_BUDGET_TOKENS: Anthropic thinking budget in tokens (1024-64000)
  * - ANTHROPIC_THINKING_TYPE: Anthropic thinking type (enabled)
  * - GOOGLE_THINKING_BUDGET: Google Gemini 2.5 thinking budget in tokens (1024-100000)
@@ -118,18 +118,19 @@ function buildProviderOptions(
             const reasoningEffort = process.env.OPENAI_REASONING_EFFORT
             const reasoningSummary = process.env.OPENAI_REASONING_SUMMARY
 
-            // OpenAI reasoning models (o1, o3, gpt-5) need reasoningSummary to return thoughts
+            // OpenAI reasoning models (o1, o3, o4, gpt-5) need reasoningSummary to return thoughts
             if (
                 modelId &&
                 (modelId.includes("o1") ||
                     modelId.includes("o3") ||
+                    modelId.includes("o4") ||
                     modelId.includes("gpt-5"))
             ) {
                 options.openai = {
-                    // Auto-enable reasoning summary for reasoning models (default: detailed)
+                    // Auto-enable reasoning summary for reasoning models
+                    // Use 'auto' as default since not all models support 'detailed'
                     reasoningSummary:
-                        (reasoningSummary as "none" | "brief" | "detailed") ||
-                        "detailed",
+                        (reasoningSummary as "auto" | "detailed") || "auto",
                 }
 
                 // Optionally configure reasoning effort
@@ -152,8 +153,7 @@ function buildProviderOptions(
                 }
                 if (reasoningSummary) {
                     options.openai.reasoningSummary = reasoningSummary as
-                        | "none"
-                        | "brief"
+                        | "auto"
                         | "detailed"
                 }
             }
@@ -593,7 +593,9 @@ export function getAIModel(overrides?: ClientOverrides): ModelConfig {
                     apiKey,
                     ...(baseURL && { baseURL }),
                 })
-                model = customOpenAI.chat(modelId)
+                // Use Responses API (default) instead of .chat() to support reasoning
+                // for gpt-5, o1, o3, o4 models. Chat Completions API does not emit reasoning events.
+                model = customOpenAI(modelId)
             } else {
                 model = openai(modelId)
             }