# What changed in msgpack

> 12 changes in msgpack 1 from its official release notes, upgrade guides and advisories: 2 breaking, 4 security. Each one cited. Sources checked 2026-10-18. HTML: https://unstale.sh/pypi/msgpack/

To see which of these apply to a repository, run `npx @unstale/cli init` in it.

## msgpack 1

### Breaking changes

- Negative array and map header sizes are now rejected. (1.2.3) — [release notes](https://github.com/msgpack/msgpack-python/releases/tag/v1.2.3)
- Raises OverflowError when use_single_float cannot represent a value. (1.2.2) — [release notes](https://github.com/msgpack/msgpack-python/releases/tag/v1.2.2)

### Security

- The C extension validates the nanoseconds range when unpacking timestamps. (1.2.2) — [release notes](https://github.com/msgpack/msgpack-python/releases/tag/v1.2.2)
- Fixes a use-after-free in the ExtraData path when input is non-contiguous. (1.2.2) — [release notes](https://github.com/msgpack/msgpack-python/releases/tag/v1.2.2)
- Adds a reentrant guard to feed(), preventing unsafe reentrant calls. (1.2.2) — [release notes](https://github.com/msgpack/msgpack-python/releases/tag/v1.2.2)
- MessagePack for Python: Out-of-bounds read / crash on Unpacker reuse after a caught error (1.2.1) — [security advisory](https://github.com/advisories/GHSA-6V7P-G79W-8964)

### Behavior and default changes

- Packing now supports Python 3.15 frozentdict. (1.2.3) — [release notes](https://github.com/msgpack/msgpack-python/releases/tag/v1.2.3)

### Tooling

- Wheels are now also built for Pyodide using Emscripten. (1.2.3) — [release notes](https://github.com/msgpack/msgpack-python/releases/tag/v1.2.3)

### Fixes and other changes

- Fixes memory leaks when map key validation or container hooks fail. (1.2.3) — [release notes](https://github.com/msgpack/msgpack-python/releases/tag/v1.2.3)
- Fixes timestamp precision for far-future datetimes. (1.2.2) — [release notes](https://github.com/msgpack/msgpack-python/releases/tag/v1.2.2)
- The fallback Unpacker.skip() translates RecursionError to StackError. (1.2.2) — [release notes](https://github.com/msgpack/msgpack-python/releases/tag/v1.2.2)
- Improves error handling and reporting when unpacking. (1.2.2) — [release notes](https://github.com/msgpack/msgpack-python/releases/tag/v1.2.2)