Function answer_parse_failure 

fn answer_parse_failure(
    rejection: JsonRejection,
    body: &[u8],
    headers: &McpHeaders,
    record_request: &dyn Fn(McpCallStatus),
) -> Response

Answers a 2026-07-28 request whose body did not parse, in JSON-RPC. One that passes check_modern_request gets 404 for a method 2026-07-28 does not define, and -32602 otherwise. Other rejections keep axum’s response.