diff --git a/build_tools/style.fish b/build_tools/style.fish
index cff0c8ea8..31b0d1b64 100755
--- a/build_tools/style.fish
+++ b/build_tools/style.fish
@@ -74,13 +74,20 @@ if set -q c_files[1]
     else if type -q clang-format
         echo === Running "$red"clang-format"$normal"
         for file in $c_files
-            cp $file $file.new # preserves mode bits
-            clang-format $file >$file.new
-            if cmp --quiet $file $file.new
-                rm $file.new
-            else
-                echo $file was NOT correctly formatted
-                mv $file.new $file
+            if clang-format --dry-run -Werror $file
+                # file was clean, remove it from the list
+                set -e c_files[(contains -i $file $c_files)]
+            end
+        end
+        if set -q c_files[1]
+            printf "Reformat those %d files?\n" (count $c_files)
+            read -P 'y/N? ' -n1 -l ans
+            if string match -qi "y" -- $ans
+                clang-format -i --verbose $c_files
+            else if string match -qi "n" -- $ans
+                echo Skipping
+            else # like they ctrl-C'd or something.
+                exit 1
             end
         end
     else