New Nvidia paper shows scoring base models on every step of an agent run works worse than scoring them only on the edit that first makes the tests pass.
Post-training is expensive, so picking the right base model matters. Yet agent benchmarks score base models near zero, and simple coding tests barely predict later agent skill.
They replay a strong agent's successful run and rerun the tests after each edit to find the 1st edit that makes them pass. At this decisive step they test whether a base model favors that edit, picks it over failing edits, and can write a passing one.
How likely each base model was to write that exact edit ranked the same 10 models in line with their post-trained SWE-bench Verified scores, at 0.964.
When choosing a base model to post-train for agent work, run this check first. It needs only a benchmark's successful agent runs and its tests.