Long agent runs accumulate state that no transcript records — edited files, a live dev server, installed packages, a warm prompt cache. When an agent misreads a traceback at step 10 and rewrites a correct file, patching forward burns tokens and restarting re-pays every call. Researchers at Northeastern University and Stanford University released Shepherd, an MIT-licensed Python runtime substrate that records every agent-environment interaction as a typed event in a Git-like execution trace. Each commit covers the agent process and filesystem together, copy-on-write, so a rewind restores live state instead of just files. The paper reports 5× faster forks than Docker, over 95% prompt-cache reuse on replay, and a live supervisor raising CooperBench pair-coding pass rates from 28.8% to 54.7%.
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