diff --git a/pymol-open-source/build b/pymol-open-source/build
index c4ede00..c238132 100755
--- a/pymol-open-source/build
+++ b/pymol-open-source/build
@@ -41,9 +41,15 @@ pbuild::install() {
     # -C k=v reaches setup.py as `build_ext --k=v` (_custom_build/backend.py).
     # str2bool accepts ONLY true/false (yes/no -> "Boolean value expected").
     # no-vmd-plugins=false is deliberate: upstream dest is vmd_plugins, false disables them.
+    # upstream's documented install is `pip install --no-build-isolation .`; with
+    # default isolation the wheel builds in an env upstream never tests and comes
+    # out pure-python (no _cmd.so) WITHOUT any error. Build deps must therefore
+    # be present in the venv itself.
+    "$UV" pip install --python "$PREFIX/venv/bin/python" setuptools numpy || \
+        std::die 42 "build deps (setuptools, numpy) install failed"
     # --no-cache: never reuse a cached pymol wheel — a poisoned pure-python wheel
     # (built from a half-failed compile) otherwise gets reinstalled forever
-    "$UV" pip install --no-cache --python "$PREFIX/venv/bin/python" \
+    "$UV" pip install --no-cache --no-build-isolation --python "$PREFIX/venv/bin/python" \
         -C use-msgpackc=no -C no-libxml=true -C no-vmd-plugins=false \
         "$SRC_DIR" || \
         std::die 42 "pymol build failed - check system devel RPMs (glew, libpng, freetype, mesa-libGL)"