What's broken?

The spec is ambiguous or self-contradictory

Where in the spec or docs?

https://modelcontextprotocol.io/community/seps/2243-http-standardization

What should happen?

The two statements about the =?base64?...?= sentinel need to match. The normative prose states that the markers are case-sensitive and should be lowercase only. So, we should change the "Base64 Decoding" conformance row for =?BASE64?SGVsbG8=?= from "Server MUST accept (case-insensitive prefix)" to "Server treats as literal value, not Base64." This aligns with the adjacent Missing prefix and Missing suffix rows in the same table, which already specify that it should be treated as literal and not Base64 when the sentinel doesn't match exactly.

What actually happens?

Two parts of the SEP directly contradict each other on whether the sentinel prefix is case-sensitive.

https://modelcontextprotocol.io/seps/2243-http-standardization#header-processing

https://modelcontextprotocol.io/seps/2243-http-standardization#base64-decoding

Anything else?

Surfaced during a Rust SDK SEP-2243 implementation review.