diff --git a/tools/parity_venv.sh b/tools/parity_venv.sh
index bf465ec..460c19a 100755
--- a/tools/parity_venv.sh
+++ b/tools/parity_venv.sh
@@ -10,8 +10,10 @@
 #   tools/parity_venv.sh            install from the Gitea registry
 #   tools/parity_venv.sh --local    install the local wheelhouse build
 #
-# Then run whichever slice of the app suite the current increment claims:
-#   cd ../app && UV_PROJECT_ENVIRONMENT=.venv-ocp-parity uv run pytest backend/tests/test_geom_memo.py
+# Then run whichever slice of the app suite the current increment claims — with
+# the venv's own interpreter, NOT `uv run`, which re-syncs the environment
+# against the manifest and so puts the stock wheel straight back:
+#   cd ../app && .venv-ocp-parity/bin/python -m pytest backend/tests/test_geom_memo.py
 set -euo pipefail
 
 HERE=$(cd "$(dirname "$0")" && pwd)
@@ -27,29 +29,46 @@ cd "$APP"
 export UV_PROJECT_ENVIRONMENT="$VENV_NAME"
 
 echo "--- syncing the app's dependencies into $VENV_NAME ---"
-uv sync --all-groups
+# 3.12 explicitly: the app's floor is >=3.12, so uv would otherwise pick the
+# newest interpreter on the box and the parity venv would stop resembling what
+# the image ships.
+uv sync --all-groups --python 3.12
+
+# `uv pip` does NOT honour UV_PROJECT_ENVIRONMENT — it discovers an environment
+# the way pip does, which here means the app's own .venv.  Without --python it
+# swaps the wrong environment entirely, and the sanity check below still passes
+# because `uv run` *does* honour the variable and reads the untouched parity
+# venv.  Pass the target explicitly to both commands.
+PY_BIN="$APP/$VENV_NAME/bin/python"
 
 # Removal must precede installation: both distributions install a top-level
 # OCP/, so installing over the stock wheel would leave a half-overwritten mix.
 echo "--- removing the stock wheel ---"
-uv pip uninstall cadquery-ocp-novtk || true
+uv pip uninstall --python "$PY_BIN" cadquery-ocp-novtk || true
 
 echo "--- installing n3xd-ocp ($SOURCE) ---"
 if [ "$SOURCE" = "local" ]; then
-    uv pip install "$OCP_REPO"/wheelhouse/*.whl
+    uv pip install --python "$PY_BIN" "$OCP_REPO"/wheelhouse/*.whl
 else
-    uv pip install --index-url "$INDEX" --prerelease=allow n3xd-ocp
+    uv pip install --python "$PY_BIN" --index-url "$INDEX" \
+        --prerelease=allow n3xd-ocp
 fi
 
+# Everything below runs the venv's interpreter directly.  `uv run` would re-sync
+# the environment against the app's manifest first, which still asks for
+# cadquery-ocp-novtk — reinstalling it on top of the swap.
 echo "--- sanity ---"
-uv run python -c "
+# Asserted, not printed: __occt_version__ exists only on our wheel, so this is
+# also the check that the swap landed in the environment we meant.
+"$PY_BIN" -c "
 import OCP
+assert hasattr(OCP, '__occt_version__'), 'this is not n3xd-ocp — the swap missed'
 print('OCP', OCP.__version__, '/ OCCT', OCP.__occt_version__)
 print('modules:', len(OCP._OCP.__all_modules__))
 "
 
 echo "--- coverage against the app's symbol inventory ---"
-uv run python "$HERE/inventory.py" --check || \
+"$PY_BIN" "$HERE/inventory.py" --check || \
     echo "(incomplete coverage is expected until the increments land)"
 
 # The binding's own suite, run from the swapped venv: this is what exercises the
@@ -57,4 +76,4 @@ uv run python "$HERE/inventory.py" --check || \
 # OCP/n3xd_ocp, so it works long before the app's own tests can collect (their
 # conftest imports n3xd.main, i.e. the whole OCP surface).
 echo "--- ocp suite under the swapped venv ---"
-uv run pytest "$OCP_REPO/tests" -q
+"$PY_BIN" -m pytest "$OCP_REPO/tests" -q