diff --git a/flake/dev/fmt.nix b/flake/dev/fmt.nix
index e3733497..7c2610d1 100644
--- a/flake/dev/fmt.nix
+++ b/flake/dev/fmt.nix
@@ -57,6 +57,7 @@
             "**.scm"
             "**.svg"
             "**/man/*.5"
+            "docs/mdbook/highlight.js"
             # Those files are generated by pytest-regression, which then `diff`s them.
             # Formatting them will make the tests fail.
             "docs/gfm-alerts-to-admonitions/tests/**/*.yml"