The display_history_prefix calculation used by session.resume's
_live_session_payload was display_history[:len(display) - len(raw)].
This assumed the model (repaired) history is always a suffix of the
display history — i.e., repair_message_sequence only removes messages
from the tail. That assumption broke when verification candidates
(finish_reason=verification_required) were persisted to state.db (#65919):
- repair collapses consecutive assistant messages, removing the
verification candidate from the MODEL history
- the candidate stays in the DISPLAY history (it's real persisted content)
- the length gap (gap = len(display) - len(raw)) counts BOTH ancestor
messages AND repair-removed tip messages
- the prefix = display[:gap] grabs the first N display messages, which
are tip messages (not ancestors) when there are no compression ancestors
- _live_session_payload concatenates prefix + model_history, duplicating
the first N messages
On session 20260720_110036_a33889 (8 verification candidates), this
duplicated the first 8 messages in every warm-cache session.activate
response, producing visible duplicate user messages in the desktop.
Fix: add SessionDB.get_ancestor_display_prefix() which returns ONLY
genuine ancestor messages (rows where session_id != tip_session_id),
identified at the row level before _rows_to_conversation strips
session_id. Both resume paths (eager + deferred) now use this instead
of the length-slice heuristic.
Tests:
- test_get_ancestor_display_prefix_single_session_returns_empty
- test_get_ancestor_display_prefix_returns_ancestor_only_messages
- Updated 12 mock DBs across test_protocol.py + test_tui_gateway_server.py
- 848 passed (run_tests.sh), 0 regressions
3133af8215