New Features:

• A pybind11::detail::type_caster_std_function_specializations feature was added, to support specializations for std::function's with return types that require custom to-Python conversion behavior (to primary use case is to catch and convert exceptions). #4597

Changes:

• Use PyMutex instead of std::mutex for internal locking in the free-threaded build. #5219
• Add a special type annotation for C++ empty tuple. #5214
• When compiling for WebAssembly, add the required exception flags (CMake 3.13+). #5298

Bug fixes:

• Make gil_safe_call_once_and_store thread-safe in free-threaded CPython. #5246
• A missing #include <algorithm> in pybind11/typing.h was added to fix build errors (in case user code does not already depend on that include). #5208
• Fix regression introduced in #5201 for GCC<10.3 in C++20 mode. #5205
• Remove extra = when assigning flto value in the case for Clang in CMake. #5207

Tests:

• Adding WASM testing to our CI (Pyodide / Emscripten via scikit-build-core). #4745
• clang-tidy (in GitHub Actions) was updated from clang 15 to clang 18. #5272

Full Changelog: https://github.com/pybind/pybind11/compare/v2.13.1...v2.13.2