diff --git a/pyproject.toml b/pyproject.toml
index 44b92485..59b98c5c 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -65,7 +65,10 @@ docs = [
     "mkdocs-material==9.7.7",
     "mkdocstrings==0.18.1",
     "mkdocs_autorefs==1.0.1",
-    "mkdocs-macros-plugin==1.3.7"
+    "mkdocs-macros-plugin==1.3.7",
+    # 12.2 made md a required arg of Highlight.__init__, which
+    # mkdocstrings 0.18.1 does not pass. Remove after upgrading mkdocstrings.
+    "pymdown-extensions<12.2"
 ]
 dev = [
     "planet[test, docs, lint, validate_models]",