# How to Support New Models

This document explains how to add support for new language models and vision‐language models (VLMs) in SGLang. It also covers how to test new models and register external implementations.

## How to Support a new Language Model

To support a new model in SGLang, you only need to add a single file under the [SGLang Models Directory](https://github.com/sgl-project/sglang/tree/main/python/sglang/srt/models). You can learn from existing model implementations and create a new file for your model. For most models, you should be able to find a similar model to start with (e.g., starting from Llama). Also refer how to [port a Model from vLLM to SGLang](#port-a-model-from-vllm-to-sglang)

## How to Support a new Vision-Language model

To support a new vision-language model (vLM) in SGLang, there are several key components in addition to the standard LLM support:

1. **Register your new model as multimodal**:
   Extend `is_multimodal_model` in [model_config.py](https://github.com/sgl-project/sglang/blob/main/python/sglang/srt/configs/model_config.py) to return `True` for your model.

2. **Process Images**:
   Define a new `Processor` class that inherits from `BaseProcessor` and register this processor as your model’s dedicated processor. See [multimodal_processor.py](https://github.com/sgl-project/sglang/blob/main/python/sglang/srt/managers/multimodal_processor.py) for more details.

3. **Handle Image Tokens**:
   Implement a `pad_input_ids` function for your new model. In this function, image tokens in the prompt should be expanded and replaced with image-hashes so that SGLang can recognize different images when using `RadixAttention`.

4. **Replace Vision Attention**:
   Replace the multi-headed `Attention` of ViT with SGLang’s `VisionAttention`.

You can refer to [Qwen2VL](https://github.com/sgl-project/sglang/blob/main/python/sglang/srt/models/qwen2_vl.py) or other vLM implementations. These models demonstrate how to correctly handle both multimodal and textual inputs.

You should test the new vLM locally against Hugging Face models. See the [`mmmu`](https://github.com/sgl-project/sglang/tree/main/benchmark/mmmu) benchmark for an example.

## Test the Correctness

### Interactive Debugging

For interactive debugging, compare the outputs of Hugging Face/Transformers and SGLang. The following two commands should give the same text output and very similar prefill logits:

- Get the reference output:
  ```bash
  python3 scripts/playground/reference_hf.py --model-path [new model] --model-type {text,vlm}
  ```
- Get the SGLang output:
  ```bash
  python3 -m sglang.bench_one_batch --correct --model [new model]
  ```

### Add the Model to the Test Suite

To ensure the new model is well maintained, add it to the test suite by including it in the `ALL_OTHER_MODELS` list in the [test_generation_models.py](https://github.com/sgl-project/sglang/blob/main/test/srt/models/test_generation_models.py) file, test the new model on your local machine and report the results on demonstrative benchmarks (GSM8K, MMLU, MMMU, MMMU-Pro, etc.) in your PR.

This is the command to test a new model on your local machine:

```bash
ONLY_RUN=Qwen/Qwen2-1.5B python3 -m unittest test_generation_models.TestGenerationModels.test_others
```

## Port a Model from vLLM to SGLang

The [vLLM Models Directory](https://github.com/vllm-project/vllm/tree/main/vllm/model_executor/models) is a valuable resource, as vLLM covers many models. SGLang reuses vLLM’s interface and some layers, making it easier to port models from vLLM to SGLang.

To port a model from vLLM to SGLang:

- Compare these two files for guidance:
  - [SGLang Llama Implementation](https://github.com/sgl-project/sglang/blob/main/python/sglang/srt/models/llama.py)
  - [vLLM Llama Implementation](https://github.com/vllm-project/vllm/blob/main/vllm/model_executor/models/llama.py)
- The major differences include:
  - **Replace vLLM’s `Attention` with `RadixAttention`** (ensure you pass `layer_id` to `RadixAttention`).
  - **Replace vLLM’s `LogitsProcessor` with SGLang’s `LogitsProcessor`.**
  - **Replace the multi-headed `Attention` of ViT with SGLang’s `VisionAttention`.**
  - **Replace other vLLM layers** (such as `RMSNorm`, `SiluAndMul`) with SGLang layers.
  - **Remove `Sample`.**
  - **Change the `forward()` functions** and add a `forward_batch()` method.
  - **Add `EntryClass`** at the end.
  - **Ensure that the new implementation uses only SGLang components** and does not rely on any vLLM components.

## Registering an External Model Implementation

In addition to the methods above, you can register your new model with the `ModelRegistry` before launching the server. This allows you to integrate your model without modifying the source code.

For example:

```python
from sglang.srt.models.registry import ModelRegistry
from sglang.srt.entrypoints.http_server import launch_server

# For a single model, add it to the registry:
ModelRegistry.models[model_name] = model_class

# For multiple models, you can imitate the import_model_classes() function:
from functools import lru_cache

@lru_cache()
def import_new_model_classes():
    model_arch_name_to_cls = {}
    # Populate model_arch_name_to_cls with your new model classes.
    ...
    return model_arch_name_to_cls

ModelRegistry.models.update(import_new_model_classes())

# Launch the server with your server arguments:
launch_server(server_args)
```

---

By following these guidelines, you can add support for new language models and vision-language models in SGLang and ensure they are thoroughly tested and easily integrated into the system.
