fix(compaction): keep last valid cut point when recent-token budget overshoots

[code-yeongyu]

What

findCutPoint (compaction) defaulted cutIndex = cutPoints[0] and only reassigned it when a valid cut point sat at or after the budget-crossing entry. When the recent-token budget was exceeded entirely within trailing tool results (which are never valid cut points), no branch fired and cutIndex stayed at cutPoints[0] — so compaction silently kept the entire history instead of the recent window.

Fix: when no cut point is >= i, fall back to the last valid cut point (cutPoints[cutPoints.length - 1]).

Applied to both definitions:

• packages/coding-agent/src/core/compaction/compaction.ts
• packages/agent/src/harness/compaction/compaction.ts

(Ported as a senpi-native fix from gajae-code 534b4f0a; senpi keeps its estimateTokens char/4 estimator — gajae's native estimateEntryTokens was intentionally not adopted.)

Why

Without the fallback, a turn whose recent budget is consumed by large tool outputs after the last user/assistant message would compact to "keep everything", defeating compaction exactly when the context is largest.

Evidence (measured on this repo)

RED → GREEN (new tests, fail before / pass after):

• packages/coding-agent/test/compaction.test.ts > findCutPoint > falls back to the last valid cut point when the budget is exceeded beyond it — RED expected +0 to be 1, GREEN after fix.
• packages/agent/test/harness/compaction.test.ts > harness compaction > falls back to the last valid cut point when the budget overshoots it — RED expected +0 to be 1, GREEN after fix.

Regression (all green, no skips):

• coding-agent test/compaction.test.ts: 30/30
• agent test/harness/compaction.test.ts: 21/21
• coding-agent test/compaction/ policy suite: 18 files, 125/125

Gates: full npm run check green via pre-commit (biome, check:ts-imports, check:shrinkwrap, tsgo --noEmit, browser/web-ui smoke). Repo-root npm test runs in CI.

This is a correctness fix (no perf delta), so the improvement is the bug repro above rather than a benchmark number.

Plan: plans/port-gajae-3day-remaining-optimizations.md (WI-1)

---

Summary by cubic

Fix compaction to keep the last valid cut point when the recent-token budget is overshot within trailing tool results, instead of keeping the entire history. Applied to both packages/coding-agent and packages/agent implementations with tests.

• Bug Fixes
  • findCutPoint now falls back to the last valid cut point when no cut point exists at or after the budget-crossing entry.
  • Prevents full-history retention when large trailing tool outputs consume the recent budget; added tests covering the overshoot case.

^{Written for commit f154622. Summary will update on new commits.}