EurusPRM-Stage2

Links

• 📜 Paper
• 📜 Blog
• 🤗 PRIME Collection
• 🤗 Training Data

Introduction

EurusPRM-Stage2 is trained using Implicit PRM, which obtains free process rewards at no additional cost but just needs to simply train an ORM on the cheaper response-level labels. During inference, implicit process rewards are obtained by forward passing and calculating the log-likelihood ratio on each step.

The key ingredient of Implicit PRM is the reward representation, as demonstrated below:

Proposition: Consider an ORM where the reward is parameterized by the log-likelihood ratio of two causal LMs, i.e.

$$ r_\phi(\mathbf{y}) := \beta \log \frac{\pi_\phi(\mathbf{y})}{\pi_\text{ref}(\mathbf{y})}. $$

Define

$$ q_\phi^t(\mathbf{y}{<t}, y_t) := \sum{i=1}^{t} \beta \log \frac{\pi_\phi(y_{i}|\mathbf{y}{<i})}{\pi\text{ref}(y_{i}|\mathbf{y}_{<i})}. $$

is the exponential average of \(r_\theta\) at step \(t\).

$$ q_\phi^t(\mathbf{y}{<t}, y_t) = \beta \log \mathbb{E}{\pi\text{ref}(\mathbf{y}|\mathbf{y}{\leq t})} \left[ e^{\frac{1}{\beta} r\phi(\mathbf{y})} \right] $$

Hence, \(q_\theta^t\)represents an exact expectation of outcome reward \(r_\theta\) at step \(t\), i.e., the Q value.

The proposition indicates that when modeling

$$ r_\phi(\mathbf{y}) := \beta \log \frac{\pi_\phi(\mathbf{y})}{\pi_\text{ref}(\mathbf{y})} $$

to train an ORM with the standard pipeline, where \(\beta\) is a hyperparameter, \(\phi\) can implicitly learn a Q function. Hence, process reward \(r_\phi^t\) can be obtained by:

$$ r_\phi^t := q_\phi^t - q_\phi^{t-1} = \beta \log \frac{\pi_\phi(y_{t}|\mathbf{y}{<t})}{\pi\text{ref}(y_{t}|\mathbf{y}_{<t})}. $$

Therefore, we can indeed obtain PRMs simply by collecting response-level data and training an ORM, without any burden of annotating step labels.

The proposition is agnostic to specific choices of the training objective of ORMs. It can be instantiated with different objectives as vanilla ORM training, with the only difference being substituting the \(r_\phi \left( \mathbf{y} \right)\) with \(\beta \log \frac{\pi_\phi(\mathbf{y})}{\pi_\text{ref}(\mathbf{y})}\).

For example, DPO already meets our assumption and serves as a strong variant, while in this work, we instantiate our implicit PRM with cross entropy (CE) loss due to memory efficiency:

$$ \small \mathcal{L}{CE} = l \cdot \log \sigma \left( \beta \log \frac{\pi\phi(\mathbf{y})}{\pi_\text{ref}(\mathbf{y})} \right) + (1 - l) \cdot \log \left[ 1 - \sigma \left( \beta \log \frac{\pi_\phi(\mathbf{y})}{\pi_\text{ref}(\mathbf{y})} \right) \right] $$

We started the second-stage training on top of EurusPRM-Stage1 with fine-grained step-level labels. To obtain step-level labels, we employed Llama-3.1-70B-Inst and Qwen2.5-72B-Inst to insert nuance errors into correct solutions. We also mixed response-level data in this stage. The model was continually trained with \(L_{CE}\) with a learning rate of 5e-7 and a batch-size of 64.

Usage

Due to the training format of EurusPRM, a clearly separated answer should be provided to elicit a better performance. We strongly recommend using answers with each step starting with Step K.

We show an example leveraging EurusPRM-Stage2 below:

import torch
from transformers import AutoTokenizer,AutoModelForCausalLM
coef=0.001
d = {'query':'Convert the point $(0,3)$ in rectangular coordinates to polar coordinates.  Enter your answer in the form $(r,\\theta),$ where $r > 0$ and $0 \\le \\theta < 2 \\pi.$',
     'answer':[
            "Step 1: To convert the point (0,3) from rectangular coordinates to polar coordinates, we need to find the radius (r) and the angle theta (\u03b8).",
            "Step 2: Find the radius (r). The radius is the distance from the origin (0,0) to the point (0,3). Since the x-coordinate is 0, the distance is simply the absolute value of the y-coordinate. So, r = |3| = 3.",
            "Step 3: Find the angle theta (\u03b8). The angle theta is measured counterclockwise from the positive x-axis. Since the point (0,3) lies on the positive y-axis, the angle theta is 90 degrees or \u03c0\/2 radians.",
            "Step 4: Write the polar coordinates. The polar coordinates are (r, \u03b8), where r > 0 and 0 \u2264 \u03b8 < 2\u03c0. In this case, r = 3 and \u03b8 = \u03c0\/2.\n\nTherefore, the polar coordinates of the point (0,3) are (3, \u03c0\/2).\n\n\n\\boxed{(3,\\frac{\\pi}{2})}"
     ]
     }
model = AutoModelForCausalLM.from_pretrained('PRIME-RL/EurusPRM-Stage2')
tokenizer = AutoTokenizer.from_pretrained('PRIME-RL/EurusPRM-Stage2')
ref_model = AutoModelForCausalLM.from_pretrained('Qwen/Qwen2.5-Math-7B-Instruct')
input_ids = tokenizer.apply_chat_template([
    {"role": "user", "content": d["query"]},
    {"role": "assistant", "content": "\n\n".join(d["answer"])},
], tokenize=True, add_generation_prompt=False,return_tensors='pt')
attention_mask = input_ids!=tokenizer.pad_token_id
step_last_tokens = []
for step_num in range(0, len(d["answer"])+1):
    conv = tokenizer.apply_chat_template([
        {"role":"user", "content":d["query"]},
        {"role":"assistant", "content":"\n\n".join(d["answer"][:step_num])},
    ], tokenize=False, add_generation_prompt=False)
    conv = conv.strip()
    if step_num!=0 and step_num!=len(d['answer']):
        conv+='\n\n'
    currect_ids = tokenizer.encode(conv,add_special_tokens=False)
    step_last_tokens.append(len(currect_ids) - 2)


inputs = {'input_ids':input_ids,'attention_mask':attention_mask,'labels':input_ids}
label_mask = torch.tensor([[0]*step_last_tokens[0]+[1]*(input_ids.shape[-1]-step_last_tokens[0])])
step_last_tokens = torch.tensor([step_last_tokens])

def get_logps(model,inputs):
    logits = model(input_ids=inputs['input_ids'], attention_mask=inputs['attention_mask']).logits
    labels = inputs['labels'][:, 1:].clone().long()
    logits = logits[:, :-1, :]
    labels[labels == -100] = 0
    per_token_logps = torch.gather(logits.log_softmax(-1), dim=2, index=labels.unsqueeze(2)).squeeze(2)
    return per_token_logps

with torch.no_grad():
    per_token_logps = get_logps(model, inputs)
    ref_per_token_logps = get_logps(ref_model,inputs)

raw_reward = per_token_logps - ref_per_token_logps
beta_reward = coef * raw_reward * label_mask[:,1:]
beta_reward = beta_reward.cumsum(-1)
beta_reward = beta_reward.gather(dim=-1, index=step_last_tokens[:,1:])
print(beta_reward)

Evaluation

Evaluation Code

We use codes in Implicit PRM to evaluate the performance of EurusPRM. The reference model is Qwen2.5-Math-7B-Instruct.

Evaluation Base Model

For Best-of N Sampling, we adopt Eurus-2-7B-SFT, Qwen2.5-7B-Instruct and Llama-3.1-70B-Instruct as generation models to evaluate the performance of our implicit PRM. For all models, we set the sampling temperature as 0.5, p of the top-p sampling as 1.

For ProcessBench, we adopt Math-Shepherd-PRM-7B, RLHFlow-PRM-Mistral-8B, RLHFlow-PRM-Deepseek-8B, Skywork-PRM-7B, EurusPRM-Stage 1, and EurusPRM-Stage 2.

Best-of-N Sampling

We use Best-of-64 as our evaluation metric. The weighting methods are different for several PRMs below.

• For Skywork-o1-Open-PRM-Qwen-2.5-7B, we use simple average reward across all steps.
• For EurusPRM-Stage 1, we use the minimum reward across all steps.
• For EurusPRM-Stage 2, we use the accumulative rewards.

Eurus-2-7B-SFT

Llama-3.1-70B-Instruct

Qwen2.5-7B-Instruct

ProcessBench

We evaluate EurusPRM-Stage 1 and EurusPRM-Stage 2 on ProcessBench. The threshold is obtained by converting the original score of each step using sigmoid function and iterating to find the highest F1 on GSM8k sub-benchmark. The threshold for EurusPRM-Stage 1 and EurusPRM-Stage 2 is 0.5015 and 0.5005 respectively.

To leverage the capibility of EurusPRM better, we add Step K (where K is the actual index of the step) in front of each step in ProcessBench.

Citation

@article{cui2025process,
  title={Process reinforcement through implicit rewards},
  author={Cui, Ganqu and Yuan, Lifan and Wang, Zefan and Wang, Hanbin and Li, Wendi and He, Bingxiang and Fan, Yuchen and Yu, Tianyu and Xu, Qixin and Chen, Weize and others},
  journal={arXiv preprint arXiv:2502.01456},
  year={2025}
}

@article{yuan2024implicitprm,
  title={Free Process Rewards without Process Labels},
  author={Lifan Yuan and Wendi Li and Huayu Chen and Ganqu Cui and Ning Ding and Kaiyan Zhang and Bowen Zhou and Zhiyuan Liu and Hao Peng},
  journal={arXiv preprint arXiv:2412.01981},
  year={2024}
}