MAJOR REWRITE: The Python and C implementations of the encoder and decoder were replaced with a single, Rust-based implementation in the interest of maintainability.

Here are some of the highlights:

• Improved memory safety (100% safe-mode Rust)
• Complete elimination of reference leaks
• Support for free-threading and subinterpreters
• Substantially improved performance
• Improved decoder error handling where any non-base exception gets wrapped in a CBORDecodeError
• Iterative, rather than recursive decoding, meaning the container nesting depth is limited only by the available memory, rather than the C stack size

BACKWARD INCOMPATIBLE Changed the signature of the tag_hook decoder callables to accept (CBORTag, immutable as arguments instead of CBORDecoder, CBORTag)

BACKWARD INCOMPATIBLE Changed the signature of the object_hook decoder callables to accept (Mapping[Any, Any], bool) instead of (CBORDecoder, dict[Any, Any])

BACKWARD INCOMPATIBLE Removed the break_marker singleton as no longer necessary

BACKWARD INCOMPATIBLE Removed the CBORDecodeValueError exception, instead chaining ValueError or TypeError to a CBORDecodeError

BACKWARD INCOMPATIBLE Changed the decoding of semantic tag 261 to yield an IPv4Interface or IPv6Interface if the address contains host bits

BACKWARD INCOMPATIBLE Removed the individual decoding functions from the API as they were mistakenly called directly by users. Please open an issue if you need them back.

BACKWARD INCOMPATIBLE Changed the encoding of IP addresses to use the semantic tags 52 and 54 instead of the deprecated 260 and 261 (#232)

BACKWARD INCOMPATIBLE Dropped the deprecated cbor2.decoder and cbor2.encoder modules – everything in the API is now importable directly from cbor2

BACKWARD INCOMPATIBLE The cbor2.FrozenDict class has now been renamed frozendict and is not available on Python 3.15 where the built-in frozendict class must be used instead

Added the semantic_decoders decoder option to add or override decoders for specific semantic tags

Added the immutable decoder flag to always use immutable containers where possible when decoding a CBOR stream

Added the allow_indefinite decoder option to optionally disallow indefinite-length strings and containers

Dropped support for Python 3.9

Fixed the decoder not rejecting invalid two-byte simple value sequences (0xF800 - 0xF81F)