kiro: only set additionalModelRequestFields for supported models

The field is rejected by models that don't support thinking/reasoning
configuration. Check model name before setting the field; 'auto' and
unknown models skip it to avoid ValidationException errors.

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 {
@@ -349,9 +362,13 @@ func buildKiroRequest(model string, messages []Message, tools []Tool, profileARN
 	}
 
 	// Set additionalModelRequestFields for reasoning/thinking support.
+	// 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
 }