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

TL;DR - This study measures how planning guidance and completion verification improve stateful LLM agents. Task-specific plans raise success, while inexpensive standalone verification delivers most of the protection against false passes.

• Fixed plans improved oracle-verified success by 7.17 percentage points across 265 matched cells, with larger gains on complex tasks.
• A read-only terminal verifier rejected 61% of oracle-invalid Retail episodes but also withheld 17% of correct episodes.
• Verification added less than $0.01 per episode, and its value increased when erroneous acceptance carried greater liability.
• At high false-acceptance costs, the standalone verifier captured nearly all the full planning-plus-verification stack’s false-pass benefit at substantially lower cost.