The legacy tail budget scales as threshold×target_ratio, which was designed
around 128K windows at a 50% trigger (~13K tail). On modern big-window
models with raised thresholds it silently hoards: a 1M-window session at
threshold 0.85 keeps a 170K-token verbatim tail (255K soft ceiling) out of
EVERY compaction, so a 540K manual /compress lands at ~290K and every
subsequent turn re-ships the hoard. Nobody chooses this; it is an artifact
of the formula outside its design envelope.
Lean mode (#87326, compaction-v2) was built for exactly this and its recall
was validated in the before/after eval (evals/compaction/results/): clamped
2.5%-of-window tail (10K floor / 25K cap), continuity carried by the
upgraded summary (digests, anchor index, verbatim user messages,
session_search recovery pointers). This flips the DEFAULT to lean; explicit
'tail_mode: legacy' in config keeps the old behavior exactly.
Also fixes a latent bug the flip exposed: update_model() re-assigned the
LEGACY formula directly when recomputing budgets, silently reverting a lean
compressor to the hoard on every mid-session model switch. The recompute
now routes through the mode-aware tail_token_budget property (regression
test included).
Surfaces: context_compressor.py defaults + getattr fallbacks, agent_init
parse default, DEFAULT_CONFIG, gateway _CACHE_BUSTING_CONFIG_KEYS gains
compression.tail_mode (mode changes now evict cached gateway agents like
target_ratio changes do), user + developer docs. Tests: 3 new default
contracts, legacy tests pinned explicitly, feasibility-skip scenario pinned
to legacy (under lean its payloads correctly become compressible).
E2E counterfactual (real imports, 1M window @ 0.85):
main default: legacy, tail 170,000 (ceiling 255,000)
head default: lean, tail 25,000 (ceiling 37,500)
head legacy: 170,000 (opt-out intact)
update_model to 400K: 10,000 (lean preserved across switch)
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