Reward Hacking Benchmark: Measuring Exploits in LLM Agents with Tool Use

Reinforcement learning (RL) trained language model agents with tool access are increasingly deployed in coding assistants, research tools, and autonomous systems. We introduce the Reward Hacking Benchmark (RHB), a suite of multi-step tasks requiring sequential tool operations with naturalistic shortcut opportunities such as skipping verification steps, inferring answers from task-adjacent metadata, or tampering with evaluation-relevant functions. RHB supports independent and chained task regimes, where chain length acts as a proxy for longer-horizon agent behavior. We evaluate 13 frontier models from OpenAI, Anthropic, Google, and DeepSeek. Exploit rates range from 0\% (Claude Sonnet 4.5) to 13.9\% (DeepSeek-R1-Zero), varying sharply by post-training style. A controlled sibling comparison (DeepSeek-V3 vs. DeepSeek-R1-Zero) shows RL post-training is associated with substantially higher reward hacking (0.6\% vs. 13.9\%), with consistent gaps across all four task families. We identify six exploit categories and find that 72\% of reward hacking episodes include explicit chain-of-thought rationale, suggesting models often frame exploits as legitimate problem-solving. Simple environmental hardening reduces exploit rates by 88\% without degrading task success. Models with near-zero exploit rates on standard tasks show elevated rates on harder variants, suggesting production post-training suppresses reward hacking only below a complexity threshold where honest solutions remain tractable.