diff --git a/AK/String.h b/AK/String.h
index 6d1af1945e7..3582e54ce3b 100644
--- a/AK/String.h
+++ b/AK/String.h
@@ -244,7 +244,7 @@ public:
         return String((const char*)buffer.data(), buffer.size(), should_chomp);
     }
 
-    static String format(const char*, ...);
+    static String format(const char*, ...) __attribute__((format(printf, 1, 2)));
 
     static String vformatted(StringView fmtstr, TypeErasedFormatParams);
 
diff --git a/AK/StringBuilder.h b/AK/StringBuilder.h
index d313c9147de..e791dbf5321 100644
--- a/AK/StringBuilder.h
+++ b/AK/StringBuilder.h
@@ -46,7 +46,7 @@ public:
     void append(char);
     void append_code_point(u32);
     void append(const char*, size_t);
-    void appendf(const char*, ...);
+    void appendf(const char*, ...) __attribute__((format(printf, 2, 3)));
     void appendvf(const char*, va_list);
 
     void append_escaped_for_json(const StringView&);