Constant DEFAULT_MAX_LINE_BYTES 

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

Maximum bytes per JSONL line. Default covers real agent traces with long tool outputs + conversational context (observed p99 is ~50 KB per record); the ceiling catches runaway inputs early rather than OOMing deep inside read_line.

Tunable via Parser::with_max_line_bytes — callers that ingest legitimately bigger records (multimodal payloads, massive tool results) can raise it explicitly.