Constant MCP_MAX_SSE_FRAME_BYTES 

pub const MCP_MAX_SSE_FRAME_BYTES: usize = _; // 16_777_216usize

Hardening cap (same review finding as MCP_MAX_RESPONSE_BYTES): the maximum size a single un-terminated SSE frame may grow to inside SseLineAccumulator before it’s treated as malformed/hostile and the connection is torn down, rather than the accumulator buffer growing without bound while waiting forever for a blank-line terminator that never arrives.