diff --git a/tox.ini b/tox.ini
index a448b49..de0810d 100644
--- a/tox.ini
+++ b/tox.ini
@@ -20,6 +20,8 @@ extras =
 [testenv:coverage]
 basepython = python3.12
 commands =
+# Install in develop mode to ensure GHA coverage picks up the extension.
+    pip install -e .
     coverage run --source=hypatia {envbindir}/pytest
     coverage report -m --fail-under=100
 setenv =