Classification API

Classification API#

This document describes the /v1/classify API endpoint implementation in SGLang, which is compatible with vLLM’s classification API format.

Overview#

The classification API allows you to classify text inputs using classification models. This implementation follows the same format as vLLM’s 0.7.0 classification API.

API Endpoint#

POST /v1/classify

Request Format#

{
  "model": "model_name",
  "input": "text to classify"
}

Parameters#

  • model (string, required): The name of the classification model to use

  • input (string, required): The text to classify

  • user (string, optional): User identifier for tracking

  • rid (string, optional): Request ID for tracking

  • priority (integer, optional): Request priority

Response Format#

{
  "id": "classify-9bf17f2847b046c7b2d5495f4b4f9682",
  "object": "list",
  "created": 1745383213,
  "model": "jason9693/Qwen2.5-1.5B-apeach",
  "data": [
    {
      "index": 0,
      "label": "Default",
      "probs": [0.565970778465271, 0.4340292513370514],
      "num_classes": 2
    }
  ],
  "usage": {
    "prompt_tokens": 10,
    "total_tokens": 10,
    "completion_tokens": 0,
    "prompt_tokens_details": null
  }
}

Response Fields#

  • id: Unique identifier for the classification request

  • object: Always “list”

  • created: Unix timestamp when the request was created

  • model: The model used for classification

  • data: Array of classification results

    • index: Index of the result

    • label: Predicted class label

    • probs: Array of probabilities for each class

    • num_classes: Total number of classes

  • usage: Token usage information

    • prompt_tokens: Number of input tokens

    • total_tokens: Total number of tokens

    • completion_tokens: Number of completion tokens (always 0 for classification)

    • prompt_tokens_details: Additional token details (optional)

Example Usage#

Using curl#

curl -v "http://127.0.0.1:8000/v1/classify" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "jason9693/Qwen2.5-1.5B-apeach",
    "input": "Loved the new café—coffee was great."
  }'

Using Python#

import requests
import json

# Make classification request
response = requests.post(
    "http://127.0.0.1:8000/v1/classify",
    headers={"Content-Type": "application/json"},
    json={
        "model": "jason9693/Qwen2.5-1.5B-apeach",
        "input": "Loved the new café—coffee was great."
    }
)

# Parse response
result = response.json()
print(json.dumps(result, indent=2))

Supported Models#

The classification API works with any classification model supported by SGLang, including:

Classification Models (Multi-class)#

  • LlamaForSequenceClassification - Multi-class classification

  • Qwen2ForSequenceClassification - Multi-class classification

  • Qwen3ForSequenceClassification - Multi-class classification

  • BertForSequenceClassification - Multi-class classification

  • Gemma2ForSequenceClassification - Multi-class classification

Label Mapping: The API automatically uses the id2label mapping from the model’s config.json file to provide meaningful label names instead of generic class names. If id2label is not available, it falls back to LABEL_0, LABEL_1, etc., or Class_0, Class_1 as a last resort.

Reward Models (Single score)#

  • InternLM2ForRewardModel - Single reward score

  • Qwen2ForRewardModel - Single reward score

  • LlamaForSequenceClassificationWithNormal_Weights - Special reward model

Note: The /classify endpoint in SGLang was originally designed for reward models but now supports all non-generative models. Our /v1/classify endpoint provides a standardized vLLM-compatible interface for classification tasks.

Error Handling#

The API returns appropriate HTTP status codes and error messages:

  • 400 Bad Request: Invalid request format or missing required fields

  • 500 Internal Server Error: Server-side processing error

Error response format:

{
  "error": "Error message",
  "type": "error_type",
  "code": 400
}

Implementation Details#

The classification API is implemented using:

  1. Rust Model Gateway: Handles routing and request/response models in sgl-model-gateway/src/protocols/spec.rs

  2. Python HTTP Server: Implements the actual endpoint in python/sglang/srt/entrypoints/http_server.py

  3. Classification Service: Handles the classification logic in python/sglang/srt/entrypoints/openai/serving_classify.py

Testing#

Use the provided test script to verify the implementation:

python test_classify_api.py

Compatibility#

This implementation is compatible with vLLM’s classification API format, allowing seamless migration from vLLM to SGLang for classification tasks.

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