The row-addressed repair stamped the decoded durable row on every
resolved message, so after our own rewrite the sync copied the lossy
durable projection (image parts -> "text\n[screenshot]") back onto the
live dict: multimodal user/tool messages lost their images and the
prompt-cache prefix changed. Adopt the DB row only when another writer
won (digest mismatch) or on the legacy assistant path, as BASE did.
The insert-time digest hashed Python bind values, but SQLite affinity
rewrites them on storage (int message_id -> TEXT, float token_count ->
INTEGER), so live and DB digests never matched and in-place edits were
silently dropped. Hash the stored rows instead, only on the
append_messages_batch flush path that reads the digest (one SELECT per
batch), incrementally (type tag + length prefix) instead of via JSON.
Also skip the no-op UPDATE, fix the _write_columns comment/spacing and
drop the duplicate top-level Optional import (F811).
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