The aux-window clamp installed by _lower_threshold_to_aux_context() was a one-time
assignment to threshold_tokens; ContextCompressor.update_model() recomputed the trigger
from the main model and discarded it, and the _compression_feasibility_checked latch was
never reset, so after a mid-session switch to a larger main model the trigger sat at the
main-model value (450K) while the pinned summariser accepted 272K (#114707).
- ContextCompressor holds the aux window as a durable _aux_context_ceiling that
_apply_threshold_tokens_cap() honours on every recomputation; update_model() voids it
only when the main runtime changes (an "auto" aux route follows the main model).
- revalidate_compression_feasibility(agent) resets the latch and re-probes eagerly at
every runtime change: switch_model (outside the rollback guard), fallback activation
and primary restore. Symmetric: a runtime whose aux fits restores the main trigger.
- Feasibility notices emit once per distinct verdict so /model --once restores and
fallback cycles do not re-announce an unchanged verdict.
- Rewrites the switch-time hunk salvaged from #114710: unconditional, outside the
rollback try, so a catalog hiccup never undoes a good switch. Test kept and extended.
Co-authored-by: KoNit-K <konit.block@protonmail.com>
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