No module named pytest

The error

PS C:\work\app> python -m pytest
C:\Users\you\AppData\Local\Programs\Python\Python313\python.exe: No module named pytest

The path at the start says which Python ran, and that's the clue.

Why it happens

python -m pytest runs the pytest installed in that Python. Every Python install and every virtual environment has its own packages, and a project's pytest usually lives in its environment, such as .venv. A shell that hasn't activated it, like the one a coding agent starts, runs the global Python instead.

On Windows there can be several of each. On the machine we checked this on:

PS> where.exe python pytest
C:\Users\you\AppData\Local\Programs\Python\Python313\python.exe
C:\Users\you\AppData\Local\Microsoft\WindowsApps\python.exe
C:\Users\you\AppData\Roaming\Python\Python314\Scripts\pytest.exe

python is 3.13, the second one is the Microsoft Store's stand-in, and the pytest command belongs to a third, Python 3.14, so running pytest instead tests with a different Python again.

The fix

Run the environment's own Python. It needs no activating:

.venv\Scripts\python -m pytest

In PowerShell, a quoted path needs the call operator:

& '.venv\Scripts\python.exe' -m pytest

Or install pytest into the Python you ran:

python -m pip install pytest

To see which Python a command runs:

python -c "import sys; print(sys.executable)"