Python's no-GIL build drops experimental label, penalty now 5-10%

The free-threaded build of CPython is now listed as officially supported in the documentation and is no longer labelled experimental, according to howtogeek.com. The build removes the global interpreter lock's restriction that only one thread can execute Python bytecode at a time within a process.

The switch does not change the default interpreter. The free-threaded version remains optional and separate, and users opt in by installing a distinct build, usually labelled 3.14t. Tools such as uv can keep both the standard and free-threaded builds installed side by side.

The single-threaded performance penalty of the free-threaded build has shrunk to roughly 5 to 10 percent versus the standard build, and the specialized adaptive interpreter now runs in free-threaded mode. The experimental JIT compiler in Python 3.14 is not yet available in the free-threaded build.

Python has carried the GIL since its early days. Free threading was proposed in PEP 703 and shipped as an experiment in Python 3.13 in October 2024; that first release ran single-threaded code noticeably slower and disabled some internal optimizations that were not yet thread-safe.

PEP 779, accepted in June 2025, set the criteria for promoting the free-threaded build. Python 3.14, released in October 2025, met those criteria. The core team describes the current status as Phase II of a gradual plan.