“Training a Reward Hacker Despite Perfect Labels” by ariana_azarbal, vgillioz, TurnTrout

LessWrong (Curated & Popular)

Summary: Perfectly labeled outcomes in training can still boost reward hacking tendencies in generalization. This can hold even when the train/test sets are drawn from the exact same distribution. We induce this surprising effect via a form of context distillation, which we call re-contextualization:

1. Generate model completions with a hack-encouraging system prompt + neutral user prompt.
2. Filter the completions to remove hacks.
3. Train on these prompt-completion pairs with the system prompt removed.

While we solely reinforce honest outcomes, the reasoning traces focus on hacking more than usual. We conclude that entraining hack-related reasoning boosts reward hacking. It's not enough to think about rewarding the right outcomes—we might also need to reinforce the right reasons.
Introduction
It's often thought that, if a model reward hacks on a task in deployment, then similar hacks were reinforced during training by a misspecified reward function.[1] In METR's report on reward hacking [...]
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Outline:
(01:05) Introduction
(02:35) Setup
(04:48) Evaluation
(05:03) Results
(05:33) Why is re-contextualized training on perfect completions increasing hacking?
(07:44) What happens when you train on purely hack samples?
(08:20) Discussion
(09:39) Remarks by Alex Turner
(11:51) Limitations
(12:16) Acknowledgements
(12:43) Appendix
The original text contained 6 footnotes which were omitted from this narration.
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First published:
August 14th, 2025
Source:
https://www.lesswrong.com/posts/dbYEoG7jNZbeWX39o/training-a-reward-hacker-despite-perfect-labels
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Narrated by TYPE III AUDIO.
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