# Registering a Model

vLLM relies on a model registry to determine how to run each model.
A list of pre-registered architectures can be found [here](../../models/supported_models.md).

If your model is not on this list, you must register it to vLLM.
This page provides detailed instructions on how to do so.

## Built-in models

To add a model directly to the vLLM library, start by forking our [GitHub repository](https://github.com/vllm-project/vllm) and then [build it from source][build-from-source].
This gives you the ability to modify the codebase and test your model.

After you have implemented your model (see [tutorial](basic.md)), put it into the <gh-dir:vllm/model_executor/models> directory.
Then, add your model class to `_VLLM_MODELS` in <gh-file:vllm/model_executor/models/registry.py> so that it is automatically registered upon importing vLLM.
Finally, update our [list of supported models](../../models/supported_models.md) to promote your model!

!!! important
    The list of models in each section should be maintained in alphabetical order.

## Out-of-tree models

You can load an external model [using a plugin](../../design/plugin_system.md) without modifying the vLLM codebase.

To register the model, use the following code:

```python
# The entrypoint of your plugin
def register():
    from vllm import ModelRegistry
    from your_code import YourModelForCausalLM

    ModelRegistry.register_model("YourModelForCausalLM", YourModelForCausalLM)
```

If your model imports modules that initialize CUDA, consider lazy-importing it to avoid errors like `RuntimeError: Cannot re-initialize CUDA in forked subprocess`:

```python
# The entrypoint of your plugin
def register():
    from vllm import ModelRegistry

    ModelRegistry.register_model(
        "YourModelForCausalLM",
        "your_code:YourModelForCausalLM",
    )
```

!!! important
    If your model is a multimodal model, ensure the model class implements the [SupportsMultiModal][vllm.model_executor.models.interfaces.SupportsMultiModal] interface.
    Read more about that [here](multimodal.md).
