Evergreen Windows tasks can fail before tests start when the fixed host Python installation is unavailable. `prelude_python.sh` falls back to Bazel `@py_host` through `prelude_python_windows.sh`, which copies the complete runtime to `.tmp/evergreen-python` so later Bazel output-base changes do not invalidate venvs. A usable host interpreter remains preferred. A completion marker is written only after the runtime copy and interpreter check succeed, so interrupted copies are repaired on the next bootstrap attempt.

Downloaded Windows venvs are refreshed by `venv_adjust.sh` with the receiving host’s selected interpreter using `venv --upgrade --without-pip`. This rewrites the build host’s absolute runtime reference and refreshes the launchers while preserving installed dependencies.