diff --git a/cmd/olliesrv/internal/backend/kiro.go b/cmd/olliesrv/internal/backend/kiro.go
index 902d2b6..ea3fa7b 100644
--- a/cmd/olliesrv/internal/backend/kiro.go
+++ b/cmd/olliesrv/internal/backend/kiro.go
@@ -283,6 +283,19 @@ func kiroSanitizeModel(model string) string {
 	return model
 }
 
+// kiroModelSupportsThinking returns true if the model supports
+// additionalModelRequestFields for thinking/reasoning configuration.
+// Only explicit model names are supported; "auto" routes through Kiro's
+// model selection which may choose models that don't support this field.
+func kiroModelSupportsThinking(model string) bool {
+	switch model {
+	case "claude-opus-4-5", "claude-opus-4", "claude-sonnet-4-5", "claude-sonnet-4":
+		return true
+	default:
+		return false
+	}
+}
+
 func buildKiroRequest(model string, messages []Message, tools []Tool, profileARN string, params GenerationParams) (*kiroGenerateRequest, error) {
 	model = kiroSanitizeModel(model)
 	if len(messages) == 0 {
@@ -349,8 +362,12 @@ func buildKiroRequest(model string, messages []Message, tools []Tool, profileARN
 	}
 
 	// Set additionalModelRequestFields for reasoning/thinking support.
-	if fields := kiroModelRequestFieldsFromParams(params); fields != nil {
-		req.AdditionalModelRequestFields = fields
+	// Only set for models known to support it (claude-opus-4, claude-sonnet-4).
+	// Other models (including "auto") may route to models that reject this field.
+	if kiroModelSupportsThinking(model) {
+		if fields := kiroModelRequestFieldsFromParams(params); fields != nil {
+			req.AdditionalModelRequestFields = fields
+		}
 	}
 
 	return req, nil