Codex and Claude Code harnesses handicap long-horizon tasks; custom Proximus harness lifts scores
nrehiew_ · x · 2026-09-03
Native harnesses not designed for long-horizon work — including Codex and Claude Code — artificially handicap model performance, so the team built Proximus on mini-swe-agent, seeing across-the-board gains in both score and how long models keep working. Long-horizon tasks also act as a proxy for out-of-distribution evaluation, and mass post-training is nearly infeasible when a single task takes 20 hours and 200M tokens.
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