p2l-0.5b-bt-01132025

lmarena-ai/p2l-0.5b-bt-01132025

Large language model (LLM) evaluations typically rely on aggregated metrics like accuracy or human preference, averaging across users and prompts. This averaging obscures user- and prompt-specific variations in model performance. To address this, we propose Prompt-to-Leaderboard (P2L), a method that produces leaderboards specific to a prompt. The core idea is to train an LLM taking natural language prompts as input to output a vector of coefficients which are then used to predict the human preference vote. The resulting prompt-dependent leaderboards allow for unsupervised task-specific evaluation, optimal routing of queries to models, personalization, and automated evaluation of model strengths and weaknesses. Data from Chatbot Arena suggest that P2L better captures the nuanced landscape of language model performance than the averaged leaderboard.

Paper: Prompt-to-Leaderboard

Code: lmarena/p2l

This particular P2L model has a Bradley-Terry regression head, which we define below:

$$ \begin{equation} g_{\theta(z)}(y; x) = \begin{cases} \sigma(x^\top \theta^(z)) & y = 1, \ 1-\sigma(x^\top \theta^(z)) & y = 0. \end{cases} \end{equation} $$

More simply, given a prompt, P2L will output a vector of coefficients $\vec{\beta}$. Then the probability that model $i$ beats model $j$, $P(i \succ j) = \sigma(\vec{\beta}_i - \vec{\beta}_j)$.

See section 2.2 in our paper for more details on various regression heads.

Serving

To serve a P2L model, please see our documentation on GitHub: Serving P2L.

Note: the P2L model outputs with this structure:

class P2LOutputs(ModelOutput):
    coefs: torch.FloatTensor = None # "betas" as described above
    eta: Optional[torch.FloatTensor] = None # tie coefficent (not used for BT head)
    last_hidden_state: torch.FloatTensor = None # last hidden state from the transformer

To understand which coefficient index corresponds with which model, see the model_list.json found in the repo of each P2L model. As a general rule, the models will always be in sorted order.

The easiest way to get this list from inside code is with the following:

import json
from huggingface_hub import hf_hub_download

fname = hf_hub_download(
        repo_id="lmarena-ai/p2l-0.5b-bt-01132025", filename="model_list.json", repo_type="model"
    )

with open(fname) as fin:
    model_list = json.load(fin)

Loading from Pretrained

To define and load the model:

import torch
from transformers import (
    Qwen2Model,
    Qwen2PreTrainedModel,
    LlamaModel,
    LlamaPreTrainedModel,
    PreTrainedModel,
    AutoTokenizer,
)
from transformers import AutoTokenizer
from transformers.utils import ModelOutput
from dataclasses import dataclass
import torch.nn as nn
import torch.nn.functional as F
from typing import Dict, Tuple, Callable, Optional
from huggingface_hub import hf_hub_download
import json


@dataclass
class HeadOutputs(ModelOutput):
    coefs: torch.FloatTensor = None
    eta: Optional[torch.FloatTensor] = None
    gamma: Optional[torch.FloatTensor] = None


@dataclass
class P2LOutputs(ModelOutput):
    coefs: torch.FloatTensor = None
    eta: Optional[torch.FloatTensor] = None
    gamma: Optional[torch.FloatTensor] = None
    loss: Optional[torch.FloatTensor] = None
    last_hidden_state: torch.FloatTensor = None

class BTHead(nn.Module):
    def __init__(
        self, input_dim, output_dim, linear_head_downsize_factor=None, **kwargs
    ) -> None:
        super().__init__()

        if linear_head_downsize_factor:
            inner_dim = int(output_dim // linear_head_downsize_factor)
            self.head = nn.Sequential(
                nn.Linear(in_features=input_dim, out_features=inner_dim, bias=True),
                nn.Linear(in_features=inner_dim, out_features=output_dim, bias=True),
            )
        else:
            self.head = nn.Linear(
                in_features=input_dim, out_features=output_dim, bias=True
            )

    def forward(self, last_hidden_dim: torch.Tensor):
        coefs = self.head(last_hidden_dim)
        return HeadOutputs(coefs=coefs)

class P2LModel(Qwen2PreTrainedModel):
    def __init__(
        self,
        config,
        CLS_id,
        num_models,
        head_kwargs={},
        **kwargs,
    ):
        super().__init__(config)

        self.num_models = num_models
        self.cls_token_id = CLS_id

        self.model = Qwen2Model(config)

        self.head = BTHead(
            input_dim=config.hidden_size,
            output_dim=self.num_models,
            **head_kwargs,
        )

        self.post_init()

    def freeze_transformer(self):
        for param in self.model.parameters():
            param.requires_grad = False

    def get_input_embeddings(self):
        return self.model.embed_tokens

    def set_input_embeddings(self, value):
        self.model.embed_tokens = value

    def forward(self, input_ids, attention_mask, labels=None, weights=None):
        batch_size = input_ids.shape[0]

        hidden_outputs = self.model(
            input_ids=input_ids,
            attention_mask=attention_mask,
            output_hidden_states=False,
        ).last_hidden_state  # (bs, num_token, embed_dim)

        cls_mask = input_ids == self.cls_token_id

        # double check this is getting the current CLS token
        cls_hidden_dim = hidden_outputs[cls_mask]

        assert (
            cls_hidden_dim.shape[0] == batch_size
        ), f"input ids {input_ids.shape}, cls_mask {cls_mask.shape}, cls_logit {cls_hidden_dim.shape}"

        head_output = self.head(cls_hidden_dim)

    
        outputs = P2LOutputs(
            coefs=head_output.coefs,
            last_hidden_state=cls_hidden_dim,
            eta=head_output.eta,
            gamma=head_output.gamma,
        )

        return outputs


fname = hf_hub_download(
        repo_id="lmarena-ai/p2l-0.5b-bt-01132025", filename="model_list.json", repo_type="model"
    )

with open(fname) as fin:
    model_list = json.load(fin)

tokenizer = AutoTokenizer.from_pretrained("lmarena-ai/p2l-0.5b-bt-01132025")
model = P2LModel.from_pretrained(
    "lmarena-ai/p2l-0.5b-bt-01132025",
    CLS_id=tokenizer.cls_token_id,
    num_models=len(model_list),
    torch_dtype=torch.bfloat16,
)

Citation

@misc{frick2025prompttoleaderboard,
      title={Prompt-to-Leaderboard}, 
      author={Evan Frick and Connor Chen and Joseph Tennyson and Tianle Li and Wei-Lin Chiang and Anastasios N. Angelopoulos and Ion Stoica},
      year={2025},
      eprint={2502.14855},
      archivePrefix={arXiv},
      primaryClass={cs.LG},
      url={https://arxiv.org/abs/2502.14855}, 
}