The legacy rotation branch in agent/conversation_compression.py flushes the
current turn to the OLD session before ending it (#47202) via
_flush_messages_to_session_db(messages) with no conversation_history boundary.
On the first turn after a cold Desktop resume, the restored transcript rows
live in the message list as plain dicts that have not yet been stamped with
_DB_PERSISTED_MARKER — the normal turn flush that stamps them runs after
preflight compression. With no boundary, _flush_messages_to_session_db builds
an empty history_ids set and treats every restored row as new, durably
re-appending the whole transcript to the parent session. Repeated
restart/resume + threshold compression keeps growing the parent transcript.
Pass messages[:_persist_user_message_idx] (the already-durable prefix that
turn_context anchors before preflight runs, guarded for int/bounds) as
conversation_history so the flush skips the persisted rows by identity and
writes only the current turn's new messages.
Adds a regression test that pre-populates SQLite, cold-loads the transcript,
appends one current user row, and forces rotating compression: it fails before
this change (parent grows to 5 rows) and passes after (parent holds the two
originals plus the single new turn).
PRATHAMESH75
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