Bring the mental model from RLHF: Reward Modeling + KL-Regularized Policy Optimization; this page will reuse it instead of restarting from zero.
Reward Hacking: Overoptimizing Preference Proxies
When an imperfect preference proxy is optimized past its validation regime, policy mass shifts toward reward-model errors; KL, ensembles, LCBs, and monitoring slow this down but do not make the proxy true.
01
Intuition
Build the mental picture first so the rest of the page has something to attach to.
Source map: Amodei et al. introduce reward hacking as a wrong-objective safety problem; Gao et al. study reward-model overoptimization in RLHF; Manheim and Garrabrant provide the Goodhart-variants taxonomy used near the end of this page.
A reward model can validate well on held-out comparisons. Why can the policy get worse when we keep optimizing it?
The problem is not that optimization is mysterious. It is doing exactly what we asked: put more probability on completions with high proxy reward. The failure is that the proxy reward is not the real target. When the policy is optimized hard enough, it selects for places where the reward model is wrong in the optimizer's favor.
The mechanism is:
One useful finite-action model of reward hacking is selected proxy error. Optimization increases expected proxy reward while shifting probability mass toward outputs whose true utility is lower than the proxy says.
KL anchors, uncertainty penalties, ensembles, early stopping, fresh labels, and adversarial evaluation are brakes: in this setup they reduce optimization pressure, expose uncertainty, or slow concentration on proxy exploits. They do not prove the proxy is the target.
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02
Math
Translate the story into symbols, assumptions, and a derivation you can inspect.
Let one prompt x have a finite set of candidate completions y1,…,yK.
The target, learned proxy, and proxy error are
A KL-regularized finite-action update shifts probability toward high proxy scores and selects proxy error:
In practice we do not know ui, but the toy keeps it visible so we can see the failure.
A KL-regularized RLHF-style update uses the direction
The unpenalized-proxy policy solves
Smaller β means stronger optimization pressure. The proxy objective can improve while the real target gets worse:
while
In this toy, reward hacking is visible when R(β) keeps increasing but U(β) peaks and then falls.
Optimization does not merely reveal random error; it selects for candidates where the proxy error is useful to the optimizer.
Conservative score
With an ensemble, define a mean reward μi and disagreement σi. A lower-confidence score is
Optimizing this score gives
In the conservative setting, the selected proxy error is still measured against the unpenalized proxy mean:
The lower-confidence score changes which completions become attractive to the optimizer. It does not make the proxy error disappear, and it is not scored as si(λ)−ui.
This can reduce concentration on uncertain proxy exploits. It does not prove si(λ)=ui; it only changes which errors are attractive.
Goodhart variants near this toy
| Variant | How to read it here | | --- | --- | | Regressional Goodhart | Directly modeled: selecting high proxy scores also selects positive proxy error μi−ui. | | Extremal Goodhart | Suggested, not proved: the exploit starts with low reference probability and high uncertainty, a stand-in for moving away from the reward model's reliable region. | | Adversarial Goodhart | Suggested, not dynamically modeled: the fixed "proxy exploit" represents an output pattern that scores well under the evaluator while failing the intended target. | | Causal Goodhart | Not modeled here. Causal Goodhart needs the optimization process to change the data-generating or evaluation process itself. |
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03
Code
Keep the implementation aligned with the notation so the algorithm is legible.
This witness implements the same finite-action object as the math and demo.
import numpy as np
def normalize(weights):
weights = np.asarray(weights, dtype=float)
return weights / weights.sum()
def kl(p, q):
return float(np.sum(p * (np.log(p) - np.log(q))))
def expected(p, values):
return float(np.sum(p * values))
labels = np.array([
"clear answer",
"safe refusal",
"thin answer",
"proxy exploit",
])
# One prompt, four possible completions.
# Shapes: all arrays are (K,).
pi_ref = np.array([0.28, 0.22, 0.42, 0.08])
# True utility is hidden in practice; visible here for diagnosis.
u = np.array([1.20, 0.70, -0.30, -0.80])
# Mean reward model score. The final candidate is a reward-model exploit:
# it is truly bad, but the proxy scores it highly.
mu = np.array([1.10, 0.60, -0.10, 1.90])
# Ensemble disagreement / uncertainty.
sigma = np.array([0.15, 0.15, 0.20, 1.00])
def policy(beta, lam=0.0):
score = mu - lam * sigma
logits = np.log(pi_ref) + score / beta
logits = logits - logits.max()
return normalize(np.exp(logits))
u_ref = expected(pi_ref, u)
print("reference true utility:", round(u_ref, 3))
print()
for beta in [3.0, 2.0, 1.0, 0.7, 0.45, 0.30]:
pi = policy(beta, lam=0.0)
print("beta:", beta)
print(" policy: ", dict(zip(labels, np.round(pi, 3))))
print(" proxy reward: ", round(expected(pi, mu), 3))
print(" true utility: ", round(expected(pi, u), 3))
print(" selected err: ", round(expected(pi, mu - u), 3))
print(" KL to ref: ", round(kl(pi, pi_ref), 3))
print()
print("With an uncertainty penalty:")
for beta in [0.7, 0.45, 0.30]:
pi = policy(beta, lam=1.0)
print("beta:", beta)
print(" policy: ", dict(zip(labels, np.round(pi, 3))))
print(" proxy reward: ", round(expected(pi, mu), 3))
print(" true utility: ", round(expected(pi, u), 3))
print(" KL to ref: ", round(kl(pi, pi_ref), 3))
print()
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04
Interactive Demo
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Live Concept Demo
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Manipulate one control and predict the visible change.
Choose what to inspect in Reward Hacking: Overoptimizing Preference Proxies. This shared fallback is an observation guide, not evidence of learning.
Use the demo to watch one prompt's probability mass move. Lower β increases optimization pressure. Turning on the proxy gap makes one bad completion look excellent to the reward model. Increasing the uncertainty penalty slows concentration on that exploit, but it does not make the proxy identical to true utility.
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Concept: Reward Hacking: Overoptimizing Preference Proxies
What is the smallest example that makes Reward Hacking: Overoptimizing Preference Proxies click without losing the math?
Object contextAlignment
concept:alignment/reward-hackingReward Hacking: Overoptimizing Preference Proxies
What is the smallest example that makes Reward Hacking: Overoptimizing Preference Proxies click without losing the math?
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When an imperfect preference proxy is optimized past its validation regime, policy mass shifts toward reward-model errors; KL, ensembles, LCBs, and monitoring slow this down but do not make the proxy true.
The next edge should feel earned: use the demo prediction here before following Process Reward Models: Step-Level Verifiers for Reasoning.
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When an imperfect preference proxy is optimized past its validation regime, policy mass shifts toward reward-model errors; KL, ensembles, LCBs, and monitoring slow this down but do not make the proxy true.

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When an imperfect preference proxy is optimized past its validation regime, policy mass shifts toward reward-model errors; KL, ensembles, LCBs, and monitoring slow this down but do not make the proxy true.
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What is the smallest example that makes Reward Hacking: Overoptimizing Preference Proxies click without losing the math?
concept:alignment/reward-hackingsources: amodei-2016-concrete-safety, gao-2022-reward-overoptimization, manheim-2018-goodhart
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Names reward hacking as an accident-risk problem caused by optimizing the wrong objective proxy.
Amodei grounds reward hacking as an objective that can be gamed or pervert designer intent. Gao grounds RLHF overoptimization of an imperfect reward-model proxy: proxy optimization can ra...
Toy finite-action witness only. Gao's KL result is mixed and hyperparameter-sensitive; the uncertainty/LCB penalty is page-local, not sourced as a sufficient mitigation. No direct human u...
Studies RLHF reward-model overoptimization: optimizing an imperfect proxy too much can reduce gold-standard reward-model score; includes KL-penalty effects.
Amodei grounds reward hacking as an objective that can be gamed or pervert designer intent. Gao grounds RLHF overoptimization of an imperfect reward-model proxy: proxy optimization can ra...
Toy finite-action witness only. Gao's KL result is mixed and hyperparameter-sensitive; the uncertainty/LCB penalty is page-local, not sourced as a sufficient mitigation. No direct human u...
Classifies Goodhart variants and explains metric/proxy overoptimization failure modes; supports the Goodhart table rather than the selected-error mechanism itself.
The selected object cites this source; inspect the exact claim before treating it as support.
Attached source metadata is a review boundary, not proof of the local explanation.
Provides the preference-learning setup where learned rewards can become overoptimized proxies.
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Claim Review
When an imperfect preference proxy is optimized past its validation regime, policy mass shifts toward reward-model errors; KL, ensembles, LCBs, and monitoring slow this down but do not make the proxy true.
What is the smallest example that makes Reward Hacking: Overoptimizing Preference Proxies click without losing the math?
concept:alignment/reward-hackingsources: amodei-2016-concrete-safety, gao-2022-reward-overoptimization, manheim-2018-goodhart
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1 structured claim check on this concept.
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Publisher-side editorial review is not independent replication. Claims without it still need exact source-support review. 4 references and 3 local witnesses are available for inspection.
Amodei grounds reward hacking as an objective that can be gamed or pervert designer intent. Gao grounds RLHF overoptimization of an imperfect reward-model proxy: proxy optimization can raise proxy reward whi...
Toy finite-action witness only. Gao's KL result is mixed and hyperparameter-sensitive; the uncertainty/LCB penalty is page-local, not sourced as a sufficient mitigation. No direct human utility access, unive...
Amodei supports reward hacking as a wrong-objective/gamed-reward problem. Gao supports RLHF reward-model overoptimization where proxy optimization can improve learned reward while hindering gold reward. The page's finite-action math/code/demo are acceptable toy witnesses for selected proxy error, KL-softmax shift, and local uncertainty braking.
Reviewer: codex+oracle; reviewed 2026-05-07Practice notebook
Use the idea, then test it somewhere new
When an imperfect preference proxy is optimized past its validation regime, policy mass shifts toward reward-model errors; KL, ensembles, LCBs, and monitoring slow this down but do not make the proxy true.
What is the smallest example that makes Reward Hacking: Overoptimizing Preference Proxies click without losing the math?
concept:alignment/reward-hackingsources: amodei-2016-concrete-safety, gao-2022-reward-overoptimization, manheim-2018-goodhart
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Reward Hacking: Overoptimizing Preference Proxies
What is the smallest example that makes Reward Hacking: Overoptimizing Preference Proxies click without losing the math?
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I am working in Continuous Function's research reading room. Object: concept - Reward Hacking: Overoptimizing Preference Proxies Object key: concept:alignment/reward-hacking Context: Alignment Anchor id: concept/concept-notebook/alignment/reward-hacking Open question: What is the smallest example that makes Reward Hacking: Overoptimizing Preference Proxies click without losing the math? Evidence to inspect: - Source ids to inspect: amodei-2016-concrete-safety, gao-2022-reward-overoptimization, manheim-2018-goodhart, christiano-2017-human-preferences - Definition, prerequisite, and contrast concept links - The equation or code witness that makes the concept operational - One demo state that shows the invariant instead of a slogan Deterministic role lenses for this object: - Boundary: fixed perspectives, not people, community contributions, or independent review - Source-checking summary: Treat this as a mechanism object: connect the definition to one equation, code witness, or demo before broadening the discussion. - Proposed experiment: Ask the learner to perturb one representation, then check whether the same invariant survives in math, code, and demo. - Teach/transfer move: Turn the mechanism into one sentence that predicts a neighboring concept. - Assumptions: - Source ids amodei-2016-concrete-safety, gao-2022-reward-overoptimization, manheim-2018-goodhart, christiano-2017-human-preferences must support the exact object, not just the surrounding topic. - The stable content-object key lets local drafts, prompts, and route memory attach without changing the source page. - The concept explanation is local atlas prose until checked against its math, code, and source support. - Prerequisite gaps should become a repair route, not a reason to leave the object vague. - Role-lens requests: - Learner: ask for "Ask what would make "Reward Hacking: Overoptimizing Preference Proxies" feel predictable rather than familiar." | assumption: Source ids amodei-2016-concrete-safety, gao-2022-reward-overoptimization, manheim-2018-goodhart, christiano-2017-human-preferences must support the exact object, not just the surrounding topic. | next action: The learner can state the mechanism in their own words - Researcher: ask for "Source ids to inspect: amodei-2016-concrete-safety, gao-2022-reward-overoptimization, manheim-2018-goodhart, christiano-2017-human-preferences" | assumption: The stable content-object key lets local drafts, prompts, and route memory attach without changing the source page. | next action: The learner can name the prerequisite that would repair confusion - Experimenter: ask for "Choose one variable or condition to perturb before asking for an explanation." | assumption: The concept explanation is local atlas prose until checked against its math, code, and source support. | next action: The learner can predict how the mechanism changes under one perturbation - Professor: ask for "Find the smallest transferable rule a learner could reuse without the AI." | assumption: Prerequisite gaps should become a repair route, not a reason to leave the object vague. | next action: Teach or transfer: Turn the mechanism into one sentence that predicts a neighboring concept. What would resolve this: - The learner can state the mechanism in their own words - The learner can name the prerequisite that would repair confusion - The learner can predict how the mechanism changes under one perturbation Answer as a careful research tutor: stay source-grounded, separate verified evidence from assumptions, name the relevant math objects, and end with one next action. Current deterministic role lens for this object: - Role lens: Learner - Evidence request: Ask what would make "Reward Hacking: Overoptimizing Preference Proxies" feel predictable rather than familiar. - Assumption to keep visible: Source ids amodei-2016-concrete-safety, gao-2022-reward-overoptimization, manheim-2018-goodhart, christiano-2017-human-preferences must support the exact object, not just the surrounding topic. - Proposed experiment: Ask the learner to perturb one representation, then check whether the same invariant survives in math, code, and demo. - Next action: The learner can state the mechanism in their own words
concept/concept-notebook/alignment/reward-hacking
concept:alignment/reward-hacking