diff --git a/internal/util/gemini_schema.go b/internal/util/gemini_schema.go
index 2daf0a79..33df61f9 100644
--- a/internal/util/gemini_schema.go
+++ b/internal/util/gemini_schema.go
@@ -344,7 +344,7 @@ func cleanupRequiredFields(jsonStr string) string {
 }
 
 // addEmptySchemaPlaceholder adds a placeholder "reason" property to empty object schemas.
-// Claude VALIDATED mode requires at least one property in tool schemas.
+// Claude VALIDATED mode requires at least one required property in tool schemas.
 func addEmptySchemaPlaceholder(jsonStr string) string {
 	// Find all "type" fields
 	paths := findPaths(jsonStr, "type")
@@ -364,6 +364,9 @@ func addEmptySchemaPlaceholder(jsonStr string) string {
 		// Check if properties exists and is empty or missing
 		propsPath := joinPath(parentPath, "properties")
 		propsVal := gjson.Get(jsonStr, propsPath)
+		reqPath := joinPath(parentPath, "required")
+		reqVal := gjson.Get(jsonStr, reqPath)
+		hasRequiredProperties := reqVal.IsArray() && len(reqVal.Array()) > 0
 
 		needsPlaceholder := false
 		if !propsVal.Exists() {
@@ -381,8 +384,17 @@ func addEmptySchemaPlaceholder(jsonStr string) string {
 			jsonStr, _ = sjson.Set(jsonStr, reasonPath+".description", "Brief explanation of why you are calling this tool")
 
 			// Add to required array
-			reqPath := joinPath(parentPath, "required")
 			jsonStr, _ = sjson.Set(jsonStr, reqPath, []string{"reason"})
+			continue
+		}
+
+		// If schema has properties but none are required, add a minimal placeholder.
+		if propsVal.IsObject() && !hasRequiredProperties {
+			placeholderPath := joinPath(propsPath, "_")
+			if !gjson.Get(jsonStr, placeholderPath).Exists() {
+				jsonStr, _ = sjson.Set(jsonStr, placeholderPath+".type", "boolean")
+			}
+			jsonStr, _ = sjson.Set(jsonStr, reqPath, []string{"_"})
 		}
 	}