TL;DR: gilguard is a tiny, zero-dependency package for free-threaded Python (3.13t, 3.14t, 3.15t). Import it first. If any later import quietly turns the GIL back on, you get an error naming the extension module, the package that shipped it, and the line of your code that pulled it in. python -m gilguard audits an entire environment in one go. It’s on PyPI now, and it’s the second piece of a larger free-threaded Python effort I’ve been working on.The quiet failure mode of free-threaded PythonFree-threaded Python is officially supported as of 3.14, and I’m all in. Threads that actually run Python code in parallel? Yes please. But the migration has a failure mode that I think is going to bite a lot of teams, and there isn’t much written about it.Here’s the scenario. You move a service to 3.14t. You give the hot path a thread pool. You deploy, pull up the dashboard, and the throughput graph does… nothing. It’s the exact same shape as last week, as if the whole migration had been a dream.An hour later you find the reason, one line in a startup log that scrolled past in half a second:RuntimeWarning: The global interpreter lock (GIL) has been enabled to load module 'somepackage._speedups', which has not declared that it can run safely without the GIL.That line is CPython being careful. Every C extension has to declare that it’s safe to run without the GIL (via the Py_mod_gil slot). When the interpreter loads one that hasn’t, it turns the GIL back on for the rest of the process and carries on. Your threads still run, they just take turns again. And here’s the kicker: once the GIL is on, nothing can turn it back off for the life of that process.The warning names a private module you’ve never heard of. It doesn’t tell you which package you installed shipped it, or which of your imports dragged it in. So now you’re bisecting imports.What is gilguard?gilguard turns that one line into an exception, raised at the import statement that caused it. Make it the first import in your program and you’re done. You don’t call anything or configure anything:import gilguard # must be firstimport fastapiimport pandas #