[1/2][feature] support openai like classification api (#11618)

docs/supported_models/classify_models.md

+# 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
+
+```json
+{
+  "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
+
+```json
+{
+  "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
+
+```bash
+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
+
+```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:
+```json
+{
+  "error": "Error message",
+  "type": "error_type",
+  "code": 400
+}
+```
+
+## Implementation Details
+
+The classification API is implemented using:
+
+1. **Rust Router**: Handles routing and request/response models in `sgl-router/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:
+
+```bash
+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.

python/sglang/srt/entrypoints/http_server.py

@@ -50,6 +50,7 @@ from sglang.srt.disaggregation.utils import FAKE_BOOTSTRAP_HOST, DisaggregationM
 from sglang.srt.entrypoints.engine import _launch_subprocesses
 from sglang.srt.entrypoints.openai.protocol import (
     ChatCompletionRequest,
+    ClassifyRequest,
     CompletionRequest,
     DetokenizeRequest,
     EmbeddingRequest,
@@ -62,6 +63,7 @@ from sglang.srt.entrypoints.openai.protocol import (
     V1RerankReqInput,
 )
 from sglang.srt.entrypoints.openai.serving_chat import OpenAIServingChat
+from sglang.srt.entrypoints.openai.serving_classify import OpenAIServingClassify
 from sglang.srt.entrypoints.openai.serving_completions import OpenAIServingCompletion
 from sglang.srt.entrypoints.openai.serving_embedding import OpenAIServingEmbedding
 from sglang.srt.entrypoints.openai.serving_rerank import OpenAIServingRerank
@@ -228,6 +230,9 @@ async def lifespan(fast_api_app: FastAPI):
     fast_api_app.state.openai_serving_embedding = OpenAIServingEmbedding(
         _global_state.tokenizer_manager, _global_state.template_manager
     )
+    fast_api_app.state.openai_serving_classify = OpenAIServingClassify(
+        _global_state.tokenizer_manager, _global_state.template_manager
+    )
     fast_api_app.state.openai_serving_score = OpenAIServingScore(
         _global_state.tokenizer_manager
     )
@@ -1082,6 +1087,18 @@ async def openai_v1_embeddings(request: EmbeddingRequest, raw_request: Request):
     )
 
 
+@app.post(
+    "/v1/classify",
+    response_class=ORJSONResponse,
+    dependencies=[Depends(validate_json_request)],
+)
+async def openai_v1_classify(request: ClassifyRequest, raw_request: Request):
+    """OpenAI-compatible classification endpoint."""
+    return await raw_request.app.state.openai_serving_classify.handle_request(
+        request, raw_request
+    )
+
+
 @app.post(
     "/v1/tokenize",
     response_class=ORJSONResponse,

python/sglang/srt/entrypoints/openai/protocol.py

@@ -761,6 +761,37 @@ class EmbeddingObject(BaseModel):
     object: str = "embedding"
 
 
+ClassifyInput = Union[str, List[str], List[int]]
+
+
+class ClassifyRequest(BaseModel):
+    # OpenAI-compatible classification request
+    model: str = DEFAULT_MODEL_NAME
+    input: ClassifyInput
+    user: Optional[str] = None
+
+    # The request id.
+    rid: Optional[Union[List[str], str]] = None
+    # Priority for the request
+    priority: Optional[int] = None
+
+
+class ClassifyData(BaseModel):
+    index: int
+    label: str
+    probs: List[float]
+    num_classes: int
+
+
+class ClassifyResponse(BaseModel):
+    id: str
+    object: str = "list"
+    created: int
+    model: str
+    data: List[ClassifyData]
+    usage: UsageInfo
+
+
 class EmbeddingResponse(BaseModel):
     data: List[EmbeddingObject]
     model: str
@@ -844,6 +875,7 @@ OpenAIServingRequest = Union[
     ChatCompletionRequest,
     CompletionRequest,
     EmbeddingRequest,
+    ClassifyRequest,
     ScoringRequest,
     V1RerankReqInput,
     TokenizeRequest,

python/sglang/srt/entrypoints/openai/serving_classify.py

+from __future__ import annotations
+
+import logging
+import time
+import uuid
+from typing import TYPE_CHECKING, Any, Dict, List, Optional, Union
+
+import torch
+import torch.nn.functional as F
+from fastapi import Request
+from fastapi.responses import ORJSONResponse
+
+from sglang.srt.entrypoints.openai.protocol import (
+    ClassifyRequest,
+    ClassifyResponse,
+    ErrorResponse,
+)
+from sglang.srt.entrypoints.openai.serving_base import OpenAIServingBase
+from sglang.srt.managers.io_struct import EmbeddingReqInput
+
+if TYPE_CHECKING:
+    from sglang.srt.managers.template_manager import TemplateManager
+    from sglang.srt.managers.tokenizer_manager import TokenizerManager
+
+logger = logging.getLogger(__name__)
+
+
+class OpenAIServingClassify(OpenAIServingBase):
+    """Handler for v1/classify requests"""
+
+    def __init__(
+        self,
+        tokenizer_manager: TokenizerManager,
+        template_manager: TemplateManager,
+    ):
+        super().__init__(tokenizer_manager)
+        self.template_manager = template_manager
+        self.id2label = self._get_id2label_mapping()
+        self.model_name = (
+            self.tokenizer_manager.served_model_name
+            if self.tokenizer_manager.served_model_name
+            else self.tokenizer_manager.server_args.model_path
+        )
+        if not self.id2label:
+            raise ValueError("id2label mapping is missing")
+
+    def _request_id_prefix(self) -> str:
+        return "classify-"
+
+    def _convert_to_internal_request(
+        self,
+        request: ClassifyRequest,
+        raw_request: Request = None,
+    ) -> tuple[EmbeddingReqInput, ClassifyRequest]:
+        """Convert OpenAI embedding request to internal format"""
+        prompt = request.input
+
+        if isinstance(prompt, str):
+            # Single string input
+            prompt_kwargs = {"text": prompt}
+        elif isinstance(prompt, list):
+            if len(prompt) > 0 and isinstance(prompt[0], str):
+                prompt_kwargs = {"text": prompt}
+            else:
+                # List of integers (token IDs) or empty list
+                prompt_kwargs = {"input_ids": prompt}
+        else:
+            # Other types (should not happen but handle gracefully)
+            prompt_kwargs = {"input_ids": prompt}
+
+        adapted_request = EmbeddingReqInput(
+            **prompt_kwargs,
+            rid=request.rid,
+            priority=request.priority,
+        )
+
+        return adapted_request, request
+
+    def _validate_request(self, request: ClassifyRequest) -> Optional[str]:
+        """Validate that the input is not empty or whitespace only."""
+        if not (input := request.input):
+            return "Input cannot be empty"
+
+        # Handle single string
+        if isinstance(input, str):
+            if not input.strip():
+                return "Input cannot be empty or whitespace only"
+            return None
+
+        # Handle list inputs
+        if isinstance(input, list):
+            # Check first element to determine type
+            first_item = input[0]
+
+            if isinstance(first_item, str):
+                # List of strings
+                for i, item in enumerate(input):
+                    if not isinstance(item, str):
+                        return f"All items in input list must be strings"
+                    if not item.strip():
+                        return f"Input at index {i} cannot be empty or whitespace only"
+            elif isinstance(first_item, int):
+                # List of integers (token IDs)
+                for i, item in enumerate(input):
+                    if not isinstance(item, int):
+                        return f"All items in input list must be integers"
+                    if item < 0:
+                        return f"Token ID at index {i} must be non-negative"
+        return None
+
+    def _get_id2label_mapping(self) -> Optional[Dict[int, str]]:
+        """Get id2label mapping from model config."""
+        try:
+            hf_config = self.tokenizer_manager.model_config.hf_config
+            # Check for id2label in hf_config
+            if hf_config.id2label:
+                return hf_config.id2label
+            # Check for num_labels and create default mapping if needed
+            if hasattr(hf_config, "num_labels") and hf_config.num_labels:
+                num_labels = hf_config.num_labels
+                # Create default mapping: {0: "LABEL_0", 1: "LABEL_1", ...}
+                return {i: f"LABEL_{i}" for i in range(num_labels)}
+
+        except Exception as e:
+            logger.warning(f"Failed to get id2label mapping: {e}")
+
+        return None
+
+    async def _handle_non_streaming_request(
+        self,
+        adapted_request: EmbeddingReqInput,
+        request: ClassifyRequest,
+        raw_request: Request,
+    ) -> Union[ClassifyResponse, ErrorResponse, ORJSONResponse]:
+        """Handle non-streaming classification request."""
+        # Generate request ID
+
+        try:
+            ret = await self.tokenizer_manager.generate_request(
+                adapted_request, raw_request
+            ).__anext__()
+        except ValueError as e:
+            return self.create_error_response(str(e))
+
+        if not isinstance(ret, list):
+            ret = [ret]
+
+        response = self._build_classify_response(ret)
+        return response
+
+    def _build_classify_response(self, ret: List[Dict[str, Any]]) -> ClassifyResponse:
+        request_id = f"{self._request_id_prefix()}{uuid.uuid4().hex}"
+        created_time = int(time.time())
+        classify_objects = []
+        prompt_tokens = 0
+        total_latency = 0.0
+
+        for i, item in enumerate(ret):
+            embedding = item.get("embedding", [])
+            meta_info = item.get("meta_info", {})
+
+            prompt_tokens += meta_info.get("prompt_tokens", 0)
+            total_latency += meta_info.get("e2e_latency", 0.0)
+
+            if embedding:
+                try:
+                    embedding_tensor = torch.tensor(embedding, dtype=torch.float32)
+                    probs = F.softmax(embedding_tensor, dim=0).tolist()
+
+                    predicted_class = torch.argmax(embedding_tensor).item()
+
+                    label = self.id2label[predicted_class]
+
+                except Exception as e:
+                    logger.error(f"Error processing embedding for item {i}: {e}")
+                    probs = [1.0]
+                    label = "Default"
+            else:
+                probs = [1.0]
+                label = "Default"
+
+            classify_obj = {
+                "index": i,
+                "label": label,
+                "probs": probs,
+                "num_classes": len(probs),
+            }
+            classify_objects.append(classify_obj)
+
+        response = {
+            "id": request_id,
+            "object": "list",
+            "created": created_time,
+            "model": self.model_name,
+            "data": classify_objects,
+            "usage": {
+                "prompt_tokens": prompt_tokens,
+                "total_tokens": prompt_tokens,
+                "completion_tokens": 0,
+                "prompt_tokens_details": None,
+            },
+        }
+
+        return ClassifyResponse(**response)