How Do Agent Harnesses Create Value? Planning Information and Release Control in Stateful LLM Agents

RESUMO

Agent harnesses supply planning guidance, organize execution, and check completion. We study how these components affect success, erroneous acceptance, and cost in two Retail experiments and an Airline pilot τ²-bench. The primary comparison pairs prewritten task-specific plans (Fixed) with shuffled policy text matched in word count (Sham), isolating the contribution of guidance content. Across 265 cells, Fixed improves oracle-verified success by 7.17 percentage points (90% task-clustered bootstrap interval, 1.15–13.36 points), with gains concentrated in higher-complexity tasks. A read-only terminal verifier rejects 61% of oracle-invalid episodes while withholding 17% of correct ones, at less than one cent additional cost per episode.