diff --git a/src/common/utils/string.cpp b/src/common/utils/string.cpp
index b3d5119..d91a210 100644
--- a/src/common/utils/string.cpp
+++ b/src/common/utils/string.cpp
@@ -30,22 +30,24 @@ std::vector<std::string> split(const std::string& s, const char delim) {
   return elems;
 }
 
-std::string to_lower(std::string text) {
-  std::transform(text.begin(), text.end(), text.begin(),
-                 [](unsigned char input) {
-                   return static_cast<char>(std::tolower(input));
-                 });
+std::string to_lower(const std::string& text) {
+  std::string result;
+  std::ranges::transform(text, std::back_inserter(result),
+                         [](unsigned char input) {
+                           return static_cast<char>(std::tolower(input));
+                         });
 
-  return text;
+  return result;
 }
 
-std::string to_upper(std::string text) {
-  std::transform(text.begin(), text.end(), text.begin(),
-                 [](unsigned char input) {
-                   return static_cast<char>(std::toupper(input));
-                 });
+std::string to_upper(const std::string& text) {
+  std::string result;
+  std::ranges::transform(text, std::back_inserter(result),
+                         [](unsigned char input) {
+                           return static_cast<char>(std::toupper(input));
+                         });
 
-  return text;
+  return result;
 }
 
 bool starts_with(const std::string& text, const std::string& substring) {
diff --git a/src/common/utils/string.hpp b/src/common/utils/string.hpp
index eb78028..b05f91a 100644
--- a/src/common/utils/string.hpp
+++ b/src/common/utils/string.hpp
@@ -76,8 +76,9 @@ const char* va(const char* fmt, ...);
 
 std::vector<std::string> split(const std::string& s, char delim);
 
-std::string to_lower(std::string text);
-std::string to_upper(std::string text);
+std::string to_lower(const std::string& text);
+std::string to_upper(const std::string& text);
+
 bool starts_with(const std::string& text, const std::string& substring);
 bool ends_with(const std::string& text, const std::string& substring);