The context compactor computed split_point against convo_msgs (system
messages filtered out) but applied it directly to session.history which
includes the system messages. After compaction, the original system
prompt was dropped and replaced by an off-by-N slice of the full history.
This silently dropped the system prompt (preset, persona, RAG context)
from every compacted session — the model would lose persona, RAG, and
preset guidance on the next turn after a long conversation.
The split in maybe_compact does:
convo_msgs = [m for m in messages if m['role'] != 'system']
split_point = len(convo_msgs) // 2
so split_point is indexed against the system-stripped list. But the
helper _update_session_history took (session, split_point, summary) and
did session.history[split_point:]. session.history is the full list
including the leading system messages, so this dropped the first
system_msg_count messages.
Fix: pass system_msg_count=len(system_msgs) into _update_session_history
and use session.history[system_msg_count + split_point:] as the recent
slice, with session.history[:system_msg_count] prepended to preserve
persona/preset/RAG system messages.
Validated: tests/test_compactor_data_loss.py both tests now pass (were
failing). tests/test_context_compactor.py 12 pre-existing tests still
pass.
Symptom was: post-compaction history = [summary] + assistant_1 + user_2
+ assistant_2 (system_A was lost).
Co-authored-by: Ernest Hysa <ernest@example.com>
