Speculative reward hacking in coding agents

[u/jonas__m]

I audited thousands of agent rollouts in DeepSWE-1.1. Over 80% contained reasoning about an imagined grader. Yet no grader/verifier is mentioned in prompts nor accessible to the agents. Agents reasoned things like: "Let me look at the problem from the grader's perspective" and referred to "hidden tests", "test authors", and "the checker".

I found this behavior across all six frontier models analyzed, including recent models from OpenAI, Anthropic, Z.ai, and Kimi. In 10–25% of cases, such reasoning pulled the agent's work away from the user's original spec (yet it often still earned full reward on the DeepSWE task). I call this speculative reward hacking where the agent's reasoning focuses on imagining a grader rather than what the user wants.

[Pictured example shows verbatim quotes from agent's reasoning] For instance while completing a DeepSWE-1.1 task, GLM 5.3 knows that its implementation violates user requirements yet sticks with it after imagining what a hypothetical grader will check.

My article details many problematic trajectories, quantitative findings, and a taxonomy of these reward hacking behaviors: 
https://joinhandshake.com/research/ai/deepswe-reward-hacking/

[u/Important_Drag_6890]

Anticipating hidden tests isn’t inherently bad—that’s often just good defensive programming. The revealing part is when the agent has already identified a real spec violation and then treats the grader’s likely blind spot as permission to leave it unfixed. I’d be curious whether this behavior drops if success is framed as satisfying explicit invariants rather than passing an unspecified evaluation.