Sourced from h2's changelog.

4.3.0 (2025-08-23)

API Changes (Backward Incompatible)

• Reject header names and values containing illegal characters, based on RFC 9113, section 8.2.1. The main Python API is compatible, but some previously valid requests/response headers might now be blocked. Use the validate_inbound_headers config option if needed. Thanks to Sebastiano Sartor (sebsrt) for the report.

API Changes (Backward Compatible)

• h2 events now have tighter type bounds, e.g. stream_id is guaranteed to not be None for most events now. This simplifies downstream type checking.
• Various typing-related improvements.

Bugfixes

• Fix error value when opening a new stream on too many open streams.

• 1aae569 v4.3.0
• 9e4bbed merge surrounding whitespace and uppercase validators into illegal character ...
• 035e989 be stricter about which characters to accept for headers
• 883ed37 reject header names and values containing unpermitted characters \r, \n, ...
• 0583911 lint: fix TC006
• bbd3d90 fix(packaging): bump twine to pass meta check wildcard bugs
• ea3140f cleanup
• 9ce83ff exclude RDT from sdist
• 492d3db Update .readthedocs.yaml
• 243461d Create RTD config
• Additional commits viewable in compare view

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