diff --git a/dev/opentitan.nix b/dev/opentitan.nix
index b1fe29a..8801786 100644
--- a/dev/opentitan.nix
+++ b/dev/opentitan.nix
@@ -70,6 +70,7 @@ in
           # For documentation
           hugo
           doxygen
+          libxslt # util/mdbook/difgen.py, requires `xsltproc` to be in the path.
         ]
         # Binaries generated by the EDA tools do no have RPATH set so they also need runtime deps.
         ++ edaExtraDeps