diff --git a/.tools/envs/testenv-numpy.yml b/.tools/envs/testenv-numpy.yml
index ef75ece28..9c96854fa 100644
--- a/.tools/envs/testenv-numpy.yml
+++ b/.tools/envs/testenv-numpy.yml
@@ -34,4 +34,5 @@ dependencies:
       - types-openpyxl  # dev, tests
       - types-jinja2  # dev, tests
       - sqlalchemy-stubs  # dev, tests
+      - sphinxcontrib-mermaid  # dev, tests, docs
       - -e ../../