Prepare canonical assistant and tool-result payloads once per append and retain their serialized bytes across mutation-conflict retries. Keep authoritative parent, sequence, idempotency, generation, and writer checks inside the transaction.
A 200 KiB SessionManager append benchmark reduces median write hold from 7.821 ms to 1.864 ms. Preserve code-mode source decisions, media normalization, public persistence interfaces, and exactly-once behavior. No schema or configuration changes.
