diff --git a/pymol-open-source/build b/pymol-open-source/build
index f8ec693..ac74239 100755
--- a/pymol-open-source/build
+++ b/pymol-open-source/build
@@ -44,4 +44,8 @@ pbuild::install() {
     "$UV" pip install --python "$PREFIX/venv/bin/python" PyQt5 || \
         std::die 42 "PyQt5 install failed"
     [[ -x "$PREFIX/venv/bin/pymol" ]] || std::die 42 "pymol launcher missing from venv"
+    # launcher can exist while the compiled ext is missing (pure-python wheel from a
+    # poisoned uv cache); import _cmd directly so a broken build dies here, not at runtime
+    "$PREFIX/venv/bin/python" -c "import pymol._cmd" || \
+        std::die 42 "pymol._cmd import failed - ext .so missing; try 'uv cache clean pymol-open-source' and rebuild"
 }