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Nathan Lambert

14 accepted papers

2026

RewardEval: Advancing Reward Model Evaluation

ICLR 2026poster

Reward models are used throughout the post-training of language models to capture nuanced signals from preference data and provide a training target for optimization across instruction following, reasoning, safety, and more domains. The community has begun establishing best practices for evaluating…

Cited by 0SourcecodeScholar
2026

Spurious Rewards: Rethinking Training Signals in RLVR

ICML 2026poster

We show that reinforcement learning with verifiable rewards (RLVR) can elicit strong mathematical reasoning in certain language models even with spurious rewards that have little, no, or outright negative correlation with the correct answer. For example, RLVR training with GRPO improves MATH-500 per…

Cited by 0SourcecodeScholar
2025

Generalizing Verifiable Instruction Following

NeurIPS 2025poster

A crucial factor for successful human and AI interaction is the ability of language models or chatbots to follow human instructions precisely. A common feature of instructions are output constraints like ``only answer with yes or no" or ``mention the word `abracadabra' at least 3 times" that the us…

Cited by 0SourceScholar
2025

M-RewardBench: Evaluating Reward Models in Multilingual Settings

ACL 2025long

Reward models (RMs) have driven the state-of-the-art performance of LLMs today by enabling the integration of human feedback into the language modeling process. However, RMs are primarily trained and evaluated in English, and their capabilities in multilingual settings remain largely understudied. I…

2025

Molmo and PixMo: Open Weights and Open Data for State-of-the-Art Vision-Language Models

CVPR 2025award

Today's most advanced vision-language models (VLMs) remain proprietary. The strongest open-weight models rely heavily on synthetic data from proprietary VLMs to achieve good performance, effectively distilling these closed VLMs into open ones. As a result, the community has been missing foundational…

2025

OLMoE: Open Mixture-of-Experts Language Models

ICLR 2025oral

We introduce OLMoE, a fully open, state-of-the-art language model leveraging sparse Mixture-of-Experts (MoE). OLMoE-1B-7B has 7 billion (B) parameters but uses only 1B per input token. We pretrain it on 5 trillion tokens and further adapt it to create OLMoE-1B-7B-Instruct. Our models outperform all…

2025

RewardBench: Evaluating Reward Models for Language Modeling

NAACL 2025findings

Reward models (RMs) are at the crux of successfully using RLHF to align pretrained models to human preferences, yet there has been relatively little study that focuses on evaluation of those models. Evaluating reward models presents an opportunity to understand the opaque technologies used for align…

2024

Dolma: an Open Corpus of Three Trillion Tokens for Language Model Pretraining Research

ACL 2024long

Information about pretraining corpora used to train the current best-performing language models is seldom discussed: commercial models rarely detail their data, and even open models are often released without accompanying training data or recipes to reproduce them. As a result, it is challenging to…

2024

OLMo: Accelerating the Science of Language Models

ACL 2024long

Language models (LMs) have become ubiquitous in both NLP research and in commercial product offerings. As their commercial importance has surged, the most powerful models have become closed off, gated behind proprietary interfaces, with important details of their training data, architectures, and de…

2024

Position: Social Choice Should Guide AI Alignment in Dealing with Diverse Human Feedback

ICML 2024poster

Foundation models such as GPT-4 are fine-tuned to avoid unsafe or otherwise problematic behavior, such as helping to commit crimes or producing racist text. One approach to fine-tuning, called reinforcement learning from human feedback, learns from humans’ expressed preferences over multiple outputs…

Cited by 29SourcePDFScholar
2024

Unpacking DPO and PPO: Disentangling Best Practices for Learning from Preference Feedback

NeurIPS 2024poster

Learning from preference feedback has emerged as an essential step for improving the generation quality and performance of modern language models (LMs). Despite its widespread use, the way preference-based learning is applied varies wildly, with differing data, learning algorithms, and evaluations u…

Cited by 45SourcePDFScholar
2024

WildGuard: Open One-stop Moderation Tools for Safety Risks, Jailbreaks, and Refusals of LLMs

NeurIPS 2024poster

We introduce WildGuard---an open, light-weight moderation tool for LLM safety that achieves three goals: (1) identifying malicious intent in user prompts, (2) detecting safety risks of model responses, and (3) determining model refusal rate. Together, WildGuard serves the increasing needs for automa…

2021

On the Importance of Hyperparameter Optimization for Model-based Reinforcement Learning

AISTATS 2021poster

Model-based Reinforcement Learning (MBRL) is a promising framework for learning control in a data-efficient manner. MBRL algorithms can be fairly complex due to the separate dynamics modeling and the subsequent planning algorithm, and as a result, they often possess tens of hyperparameters and archi…

2020

Learning Generalizable Locomotion Skills with Hierarchical Reinforcement Learning

ICRA 2020poster

Learning to locomote to arbitrary goals on hardware remains a challenging problem for reinforcement learning. In this paper, we present a hierarchical framework that improves sample-efficiency and generalizability of learned locomotion skills on real-world robots. Our approach divides the problem of…

Cited by 54SourceScholar