diff --git a/tox.ini b/tox.ini
index bc7322f9..7d250352 100644
--- a/tox.ini
+++ b/tox.ini
@@ -4,6 +4,10 @@ envlist = py313, py314
 [testenv]
 allowlist_externals = uv
 skip_install = true
+# without this, uv ignores the tox env and syncs/runs the project's .venv,
+# so every env would test against the same interpreter
+setenv =
+    UV_PROJECT_ENVIRONMENT = {envdir}
 
 commands =
     uv sync --all-extras --dev