Agent traces grow with the square of session length, because the prompt at step k is the whole conversation so far. On 400 public agent runs that is 36× the bytes of storing each message once. Stored three ways on Parquet and queried with DuckDB, typed columns beat raw JSON 91× on aggregates, while a message table keyed by hash replays 3.9× faster on half the disk and lets prompt text expire before the ids.