The lean tail budget is max(10K, min(25K, 2.5% of window)) and the boundary walk lets whole
rows overrun it by 1.5x. Neither term knew the window size, so on a small local model the
"protected" tail WAS the request: 10,636 tokens of a 8,192 window (129%), 64% of 16K. Every
compaction pass summarised six rows, kept 39 verbatim, and reclaimed nothing — a Titan RTX 27B
timed out before compaction ever changed anything, and protect_last_n read as an uncompressed
tail rather than a minimum.
TAIL_MAX_CONTEXT_FRACTION (0.20) now bounds both the budget (either tail_mode) and the walk /
pressure-demotion soft ceiling. Required last-user / last-assistant anchors and atomic tool
groups may still exceed it, so the retained tail lands at 22-25% on 8K-32K windows instead of
32-129%. Windows of 128K and above are unchanged (10K lean floor < 20%).
Probe (12 tool-heavy turns, 49 rows, 12.8K tokens):
ctx 8K: tail 10,636 tok / 39 rows -> 2,116 tok / 7 rows; window [4,10) -> [4,42)
ctx 16K: tail 10,636 tok / 39 rows -> 4,246 tok / 15 rows; window [4,10) -> [4,34)
ctx 32K: tail 10,636 tok / 39 rows -> 7,441 tok / 27 rows; window [4,10) -> [4,22)
ctx 128K: identical before/after
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