The experimental Just-in-Time (JIT) compiler has been part of CPython’s main branch since Python 3.13. PEP 744 described part of its initial design and explicitly deferred a number of questions about the JIT’s long-term status. Since then, the JIT has been re-architected and matured considerably. In Python 3.15, it delivers a measurable, reproducible speedup over the interpreter (about 4-12% geometric mean performance improvement across measured Tier 1 platforms (see Appendix), emits frames that native debuggers can unwind through, and reduces the memory footprint of generated code relative to 3.14. Along the way, we have learned a good deal about what works for a JIT in CPython.