Function parse_mcp_request 

fn parse_mcp_request(
    headers: &HeaderMap,
    body: &[u8],
) -> Result<McpRequest, JsonRejection>

Parses a request body the way axum’s Json extractor does, so every rejection keeps its status and text. The one exception: an unknown method that carries params becomes McpMethod::Unknown instead of a 422 (see as_unknown_method). The body arrives through Bytes, so the route’s DefaultBodyLimit still applies.