[Model] Automatic conversion of score (CrossEncoding) models

[noooop]

TL;DR

• vllm.model_executor.models.adapters.as_seq_cls_model
  • Theoretically, this PR could allow all *ForCausalLM models to automatically have *ForSequenceClassification implementation, and automatically support classify (classification) task and score (rerank) task.
  • simplify the code of the previous models
    • Qwen3ForSequenceClassification
  • New Model
    • GemmaForSequenceClassification
    • It should support any ForSequenceClassification models, but no suitable hf repository was found for testing.
• converting2seq_cls_models.py
  • Offline convert ForCausalLM into ForSequenceClassification model.
  • from_2_way_softmax
    • Qwen/Qwen3-Reranker-0.6B
    • Qwen/Qwen3-Reranker-4B
    • Qwen/Qwen3-Reranker-8B
  • no_post_processing
    • BAAI/bge-reranker-v2-gemma
• fix test_mteb_score randomness
  (Last week st scored 0.33437, today it's 0.33702. (╯‵□′)╯︵┻━┻. )
  It turned out that it was caused by PyStemmer resulting in slightly different stop words, leading to slightly different bm25 results.

PyStemmer==2.2.0.2 0.32082
PyStemmer-2.2.0.3 0.32082
PyStemmer-3.0.0 0.32102

• BAAI/bge-reranker-v2-gemma needs to directly concatenate the query and document without using pad_token to get exactly the same result as the official one.

Hope that after merging this pr, vllm can support more llms using the relevance generation method as classifiers and rerankers.

Usage

converting2seq_cls_models.py:

• for Qwen3-Reranker

python convert_model_to_seq_cls.py --model_name Qwen/Qwen3-Reranker-0.6B --classifier_from_tokens '["no", "yes"]' --method from_2_way_softmax --path ./Qwen3-Reranker-0.6B-seq-cls

PTAL #19260

• for BAAI/bge-reranker-v2-gemma

python convert_model_to_seq_cls.py --model_name BAAI/bge-reranker-v2-gemma --classifier_from_tokens '["Yes"]' --method no_post_processing --path ./bge-reranker-v2-gemma-seq-cls

Caution

"Yes" and "yes" are two different tokens

# v1 temporarily will report an error
VLLM_USE_V1=0 vllm serve ./bge-reranker-v2-gemma-seq-cls --task score --served-model-name BAAI/bge-reranker-v2-gemma

requests demo + formating query & document:

import requests

url = "http://127.0.0.1:8000/score"
MODEL_NAME = "BAAI/bge-reranker-v2-gemma"

# Please use the query_template and document_template to format the query and
# document for better reranker results.

prompt = "Given a query A and a passage B, determine whether the passage contains an answer to the query by providing a prediction of either 'Yes' or 'No'."
query_template = "A: {query}\n"
document_template = "B: {doc}\n{prompt}"

queries = [
    "What is the capital of China?",
    "Explain gravity",
]

documents = [
    "The capital of China is Beijing.",
    "Gravity is a force that attracts two bodies towards each other. It gives weight to physical objects and is responsible for the movement of planets around the sun.",
]

queries= [query_template.format(query=query) for query in queries ]
documents= [
    document_template.format(doc=doc, prompt=prompt)
    for doc in documents 
]

response = requests.post(url,
                         json={
                             "model": MODEL_NAME,
                             "text_1": queries,
                             "text_2": documents,
                             "truncate_prompt_tokens": -1,
                         }).json()

print(response)

expected output

{'id': 'score-30d577279e674c98ad1b0ee12f978b67', 'object': 'list', 'created': 1750361179, 'model': 'BAAI/bge-reranker-v2-gemma', 'data': [{'index': 0, 'object': 'score', 'score': 0.9998812675476074}, {'index': 1, 'object': 'score', 'score': 0.9997507929801941}], 'usage': {'prompt_tokens': 126, 'total_tokens': 126, 'completion_tokens': 0, 'prompt_tokens_details': None}}

If someone wants to implement an offline conversion from ForCausalLM to ForSequenceClassification support new methods or new models, please refer to

https://github.com/noooop/snippet/tree/main/converting2SequenceClassification

(I don't know where to place this code in vllm.)

Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.

Purpose

Follow-up #11469, Further improve #10674

• Automatic conversion of score (CrossEncoding) models

Test Plan

Test Result

(Optional) Documentation Update

Known issues

1. needs refactoring

• in get_model_architecture

    model_cls, arch = ModelRegistry.resolve_model_cls(architectures)
    if model_config.task == "embed":
        model_cls = as_embedding_model(model_cls)
    elif model_config.task in ["classify", "score"]:
        model_cls = as_seq_cls_model(model_cls)
    elif model_config.task == "reward":
        model_cls = as_reward_model(model_cls)

• _ModelRegistry.is_cross_encoder_model not considered as_seq_cls_model
• This pr actually allows all ForCausalLM to support corresponding ForSequenceClassification. Do we really need to list out all Auto-converted architectures?

vllm/vllm/model_executor/models/registry.py

Lines 160 to 161 in 6bc7b57

	# [Auto-converted (see adapters.py)]
	"Qwen2ForSequenceClassification": ("qwen2", "Qwen2ForCausalLM"),

2. default_softmax=True in conflict with what in documentation (default_softmax=False when the task is score).

In vllm, the score task only supports num_labels == 1, while models with num_labels == 1 in sentence-transformers use Sigmoid by default.

https://github.com/UKPLab/sentence-transformers/blob/910ed144dfc0a08f31517b0d01580302015fa408/sentence_transformers/cross_encoder/CrossEncoder.py#L485-L487

        if self.config.num_labels == 1:
            return nn.Sigmoid()

Perhaps we should update the documentation to set default_softmax=True when the task is score, consistent with the implementation in sentence-transformers. And we should pin the sentence-transformers version to >= 4.1.0.

3. change verify_and_update_config into a class method in the future and call it when initializing model_config.

4. Template aware prompt truncation to avoid cutting off important instructions.

5. Alibaba-NLP/gte-Qwen2-1.5B-instruct & Alibaba-NLP/gte-modernbert-base

NotImplementedError: Encoder self-attention and encoder/decoder cross-attention are not implemented for FlashAttentionImpl

Fix #19673

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[gemini-code-assist]

Summary of Changes

Hello @noooop, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request refactors the automatic model conversion mechanism to support both classification and scoring tasks under a unified as_seq_cls_model adapter. It updates the model loading utility to recognize the score task and applies the generic adapter. The adapter itself is refactored to handle pooling and scoring more flexibly, and the Qwen3 sequence classification model is updated to utilize this new generic adapter.

Highlights

• Score Task Support: The automatic model conversion logic in the model loader (vllm/model_executor/model_loader/utils.py) is updated to use the as_seq_cls_model adapter for models specified with the score task, in addition to the existing classify task.
• Adapter Refactoring: The as_seq_cls_model adapter in vllm/model_executor/models/adapters.py is refactored. The pooling and scoring logic is moved into a dedicated pooler method, allowing for more flexible handling of different pooling types within the adapter. It also adds a check to squeeze the output dimension for the score task.
• Qwen3 Reranker Integration: The specific Qwen3ForSequenceClassification implementation is updated to inherit from the new generic as_seq_cls_model adapter, simplifying its structure and leveraging the shared adapter logic. Specific Qwen3 reranker configuration verification is moved into a new config_verify method.

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[gemini-code-assist]

Code Review

This pull request focuses on enabling automatic conversion of score models by adapting the sequence classification model functionality. Key changes include:

• Renaming as_classification_model to as_seq_cls_model and updating its functionality to support both "classify" and "score" tasks. This is consistently applied across documentation, tests, and model loading utilities.
• Refactoring Qwen3ForSequenceClassification to leverage the new as_seq_cls_model adapter. This promotes code reuse and centralizes the classification/scoring logic.
• Introducing a config_verify method in the adapter pattern, allowing model-specific configurations, which is well-utilized by Qwen3ForSequenceClassification for its reranker variant.
• Ensuring that for "score" tasks, the model expects num_labels == 1 and the output is appropriately processed (squeezed).

The changes appear robust and improve the model adaptation framework. One area for potential clarification is the behavior of PoolingType.ALL within the as_seq_cls_model adapter, as noted in the specific comment.

Please also consider filling out the checklist in the PR description (Purpose, Test Plan, Test Result) for completeness.

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @noooop.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @noooop.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[noooop]

@DarkLight1337

Ready for review

[noooop]

@DarkLight1337

_ModelRegistry.is_cross_encoder_model not considered as_seq_cls_model

It seems I need to spend some more time fixing it.

[noooop]

@DarkLight1337

I find it difficult to fully fix is_cross_encoder, and I am not familiar with this part of code.

So I will use a temporary solution to fix it and document in Known issues.

    def is_cross_encoder(self) -> bool:
        # Temporary solution, See #19675
        return (self.registry.is_cross_encoder_model(self.architectures) or
                "forsequenceclassification" in self.architectures[0].lower())

---

how did this issues occur

• For the converted ./bge-reranker-v2-gemma-seq-cls model, its architecture is GemmaForSequenceClassification.

In vllm/model_executor/models/registry.py, it is routed to GemmaForCausalLM.

"GemmaForSequenceClassification": ("gemma", "GemmaForCausalLM"),

But _ModelRegistry.is_cross_encoder_model not considered as_seq_cls_model

so it derived that GemmaForSequenceClassification is_cross_encoder_model == False, which leads to a wrong calculation method

• For the converted ./Qwen3-Reranker-0.6B-seq-cls model, its architecture is Qwen3ForSequenceClassification.

In vllm/model_executor/models/registry.py, it is routed to Qwen3ForSequenceClassification.

"Qwen3ForSequenceClassification": ("qwen3", "Qwen3ForSequenceClassification"),

so it derived that GemmaForSequenceClassification is_cross_encoder_model == True

• Therefore, for any ForCausalLM, need add the following code in the model definition file.

XXXForSequenceClassification = as_seq_cls_model(XXXForCausalLM)

Add the following code in vllm/model_executor/models/registry.py

"XXXForSequenceClassification": ("xxx", "XXXForSequenceClassification"),

This is clearly too tedious.

[DarkLight1337]

Why can't is_cross_encoder_model return true for this model?

[noooop]

Because the code block was not run in ModelRegistry.is_cross_encoder_model

model_cls, arch = ModelRegistry.resolve_model_cls(architectures)
if model_config.task == "embed":
    model_cls = as_embedding_model(model_cls)
elif model_config.task in ["classify", "score"]:
    model_cls = as_seq_cls_model(model_cls)
elif model_config.task == "reward":
    model_cls = as_reward_model(model_cls)

_ModelRegistry.is_cross_encoder_model not considered as_seq_cls_model