A long-lived gateway DM rots because a turn where hygiene did not commit
(turn-hold expiry, idle/ceiling timeout, cooldown, or one already in flight)
fed the model the FULL uncompressed transcript — watermark-fenced summaries
can land late, but a turn with none had no fail-closed bound at all.
bound_model_input_without_hygiene() keeps the leading system/session_meta
setup rows plus the newest tail, total <= hygiene_hard_message_limit, and
never starts the kept tail on an orphaned tool result. It is deterministic
and payload-only: the stored transcript is untouched, so the agent's durable
prefix slice (history_offset) and the disk rows are unchanged, and the
hygiene compressor still sees the full transcript.
Applied at the three exits of _hmwa_run_session_hygiene where hygiene did
not land. A landed compression publishes a NEW list on attempt.history, so
object identity (`attempt.history is history`) decides — an adopted summary
is passed through byte-identical, and below the limit the bound returns the
same list object (no copy, no behaviour change).
finn763
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