Fix gpt-5-chat-latest model not supported error

Exclude gpt-5-chat-latest variants from reasoning model detection to prevent
sending unsupported reasoning.summary parameter.

The gpt-5-chat-latest model variant does not support the reasoning.summary
parameter, causing 400 Bad Request errors when the Responses API is used.

Fixes: #2166

Signed-off-by: Md Yunus <admin@yunuscollege.eu.org>
Co-authored-by: Md Yunus <admin@yunuscollege.eu.org>

Author: yunus25jmi1

pkg/model/provider/openai/client.go

@@ -903,6 +903,10 @@ func isResponsesModel(model string) bool {
 
 func isOpenAIReasoningModel(model string) bool {
 	m := strings.ToLower(model)
+	// Exclude -chat-latest variants which don't support reasoning parameters
+	if strings.Contains(m, "-chat-latest") {
+		return false
+	}
 	return strings.HasPrefix(m, "o1") ||
 		strings.HasPrefix(m, "o3") ||
 		strings.HasPrefix(m, "o4") ||

pkg/model/provider/openai/thinking_budget_test.go

@@ -40,6 +40,11 @@ func TestIsOpenAIReasoningModel(t *testing.T) {
 		{"gpt-5-turbo", "gpt-5-turbo", true},
 		{"GPT-5 uppercase", "GPT-5", true},
 
+		// GPT-5 -chat-latest variants - should NOT support reasoning
+		{"gpt-5-chat-latest", "gpt-5-chat-latest", false},
+		{"gpt-5-chat-latest with suffix", "gpt-5-chat-latest-2025-10-01", false},
+		{"GPT-5-CHAT-LATEST uppercase", "GPT-5-CHAT-LATEST", false},
+
 		// GPT-4 series models - should NOT support reasoning
 		{"gpt-4", "gpt-4", false},
 		{"gpt-4o", "gpt-4o", false},