TL;DR: as of version 6.136.9, Hypothesis supports running the same test simultaneously from multiple threads.

Hypothesis has historically had the following thread-safety policy:

• Running tests in multiple processes: fully supported.
• Running separate tests in multiple threads: not officially supported, but mostly worked.
• Running the same test in multiple threads: not supported, and didn't work.

No longer! In a series of releases spanning from v6.135.17 to v6.136.9, Hypothesis has gained official support for all three of these cases. You can read about the details of what we now guarantee here. The now-historic tracking issue is here.

Why now?

While we of course would always have loved for Hypothesis to be thread-safe, thread-safety has historically not been a priority, because running Hypothesis tests under multiple threads is not something we see often.

That changed recently. Python—as both a language, and a community—is gearing up to remove the global interpreter lock (GIL), in a build called free threading. Python packages, especially those that interact with the C API, will need to test that their code still works under the free threaded build. A great way to do this is to run each test in the suite in two or more threads simultaneously.

Where does Hypothesis fit into this? When I was at PyCon 2025 in May earlier this year, I talked with Nathan Goldbaum from Quansight, who is one of the people working on community free threading compatibility. Nathan mentioned that because Hypothesis is not thread-safe, Hypothesis tests in community packages have to be skipped when testing free threaded compatibility, which removes a substantial battery of coverage.

As a result, Quansight contracted me to work on making Hypothesis thread-safe. I enjoy contributing to Hypothesis in my free time even without a monetary incentive, so this was a pleasure to do, and Nathan and Quansight were great to work with. (Seriously: it's thanks to them funding my time that Hypothesis is now thread-safe!)

A note on compatibility

Free threading may have been the impetus for making Hypothesis thread-safe, but the thread-safety of Hypothesis is not tied to it. Even in the unlikely event that free threading is rolled back tomorrow by the Steering Council, Hypothesis will continue to remain thread-safe.

Now, go forth and run Hypothesis in parallel!