Python 3.13 Virtual Environment Path Incompatibility on Windows

Problem

After upgrading Windows Python from 3.10 to 3.13, Poetry creates its own virtualenv instead of using the activated venv in Evergreen CI. This causes Poetry to install dependencies in the wrong location, breaking the build.

Root Cause

Python 3.11.4+ introduced a change to the venv activation script that automatically converts Windows paths to Cygwin format when running under Cygwin/MSYS bash (which Evergreen uses on Windows).

What Changed:

Python 3.10: VIRTUAL_ENV=C:\data\mci\6409\venv (Windows path)
Python 3.13: VIRTUAL_ENV=/cygdrive/c/data/mci/6409/venv (Cygwin path)

Why This Breaks Poetry:

Poetry detects if it's running in a virtual environment by comparing the VIRTUAL_ENV environment variable to sys.prefix. However:

VIRTUAL_ENV is now a Cygwin path: /cygdrive/c/data/mci/6409/venv
sys.prefix is always a Windows path: C:\data\mci\6409\venv

These paths don't match, so Poetry thinks it's not in a venv and creates its own.

Python Upstream Issue

This behavior was intentionally added in Python 3.11.4 to fix venvs not working across different drives on Windows.

References:

• Issue: gh-103088: If the working directory and the venv are on different disks, venv doesn't work in bash on Windows
• Fix PR: #103325
• Commit: ebc81034
• Changelog: Python 3.11.4 Release Notes (search for gh-103088)

Affected Python Versions:

• Python 3.11.4+
• Python 3.12.0+
• Python 3.13.0+

Observed Behavior

From Evergreen logs:

[2026/01/12 12:52:52.238] DEBUG: VIRTUAL_ENV variable: /cygdrive/c/data/mci/6409/venv
[2026/01/12 12:52:52.238] DEBUG: sys.prefix: C:\data\mci\6409\venv
[2026/01/12 12:52:53.902] Creating virtualenv mdb-python-deps-gAD-pCWv-py3.13 in C:\data\mci\6409\poetry_dir\cache\virtualenvs

Poetry creates a new venv because the paths don't match.