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Changes being requested

Strip the single leading UTF-8 byte-order mark allowed by the SSE format before interpreting the first line.

Today SSEDecoder decodes each completed line independently. A valid stream beginning with a BOM therefore presents its first field as \ufeffdata instead of data, so the decoder silently drops the first event. The WHATWG event-stream interpretation algorithm explicitly strips one leading UTF-8 BOM.

The decoder now tracks the first line of each stream and removes only a leading BOM there. The regression covers synchronous and asynchronous decoding with the three BOM bytes both contiguous and split across input chunks.

Additional context & links

• Specification: https://html.spec.whatwg.org/multipage/server-sent-events.html#event-stream-interpretation
• Before: BOM + data:first + blank line + data:second yielded only second
• After: it yields first, then second

Validation:

• uv run --locked --all-extras pytest tests/test_sse_framing.py — 48 passed
• ./scripts/test-pydantic-v1 tests/test_sse_framing.py — 48 passed
• ./scripts/lint — Ruff, Pyright, Mypy, and import checks passed
• Castiron custom-code budget check — passed (6,626 / 10,000 lines)
• git diff --check origin/main...HEAD — passed