Possible errors and fixes
Common errors you may hit in the QuantCraft IDE and how to resolve them. This page will grow as more issues are documented. For package installs in general, see Dependencies.
Error index
Package install: "Access is denied" / [WinError 5] (Windows)
What you'll see
Installing a package from IDE Packages fails with something like:
Package install failed (polars): ERROR: Could not install packages due to an
OSError: [WinError 5] Access is denied:
'...\py-core\.venv\lib\site-packages\_polars_runtime\_polars_runtime.pyd'
Check the permissions.The filename is usually a compiled extension — a .pyd (Windows) or .so/.dll
file — such as _polars_runtime.pyd.
Why it happens
Some packages (polars, numpy, pyarrow, and other native libraries) ship a
compiled extension that gets loaded into memory when the package is imported.
The IDE's background workers import your installed packages to power autocomplete
and analysis, which loads that file. On Windows a file that is currently loaded
cannot be overwritten, so when pip tries to replace it during an install or
upgrade, Windows blocks it with Access is denied (WinError 5).
This is why you may see it when installing the latest version (pip has to overwrite the loaded file) but not when pinning a version that is already installed (pip sees nothing to change and skips the file entirely).
How to fix it
- Quit QuantCraft completely — this ends the background Python workers that are holding the file open.
- Reopen QuantCraft.
- Install the package again, ideally before opening or running a script that imports it.
Tips:
- If the package is already installed and working, you often don't need to reinstall at all.
- Pinning an exact version that is already installed (e.g.
polars==1.41.2) avoids the overwrite and will succeed without a restart.