The #80449 oversized-turn split only relaxes the USER anchor, and only
when that anchor actually pulls the cut back (user_anchored_cut <
cut_idx). In a long agentic turn whose assistant rows carry only
tool_calls, the latest user message sits inside the token-budget tail,
so the user anchor does not move, the split never fires, and
_ensure_last_assistant_message_in_tail then pulls the cut back to the
newest TEXT-BEARING assistant — which is the PREVIOUS turn's closer.
The whole oversized active turn rides the protected tail, the middle
collapses (middle_window_tokens=0), and every manual/auto compress
no-ops in ~30ms with failure_class=no_progress: the session is
permanently uncompressible (#131412, same wedge class as #80449/#105663).
Give the assistant anchor the same bound the user anchor got: when the
anchored region exceeds the tail soft ceiling (measured with the walk's
own accounting, #84371) and mid-turn splits are allowed, keep the walk's
tool-group-aligned cut instead of anchoring. Rolling micro-compaction
(allow_split_turn=False) keeps the old anchoring behavior, and the
N-user promise (#70250) is untouched.
(cherry picked from commit 9259ea925c1eb95b3da7bc2fb5ad4499c6afd46b)
a5e9238bb7