Write your own ML backend

Use the Label Studio ML backend to integrate Label Studio with machine learning models. The Label Studio ML backend is an SDK that you can use to wrap your machine learning model code and turn it into a web server. The machine learning server uses uWSGI and supervisord, and handles background training jobs with RQ.

Follow the steps below to wrap custom machine learning model code with the Label Studio ML SDK, or see our library of example ML backends to integrate with popular machine learning frameworks and tools such as Huggingface’s Transformers, OpenAI, Langchain and others.

For information on using one of Label Studio’s example backends, see Set up an example ML backend

1. Install the ML backend repo

Download and install label-studio-ml-backend from the repository:

git clone https://github.com/HumanSignal/label-studio-ml-backend.git
cd label-studio-ml-backend/
pip install -e .

2. Create an empty ML backend

label-studio-ml create my_ml_backend

This creates the following directory structure, which you can modify to implement your own inference logic:

├── Dockerfile
├── .dockerignore
├── docker-compose.yml
├── model.py
├── _wsgi.py
├── README.md
├── requirements-base.txt
├── requirements-test.txt
├── requirements.txt
└── test_api.py


  • Dockerfile, docker-compose.yml and .dockerignore are used to run the ML backend with Docker.
  • model.py is the main file where you can implement your own training and inference logic.
  • _wsgi.py is a helper file that is used to run the ML backend with Docker (you don’t need to modify this).
  • README.md must contain instructions on how to run the ML backend.
  • requirements.txt is where you put your Python dependencies.
  • requirements_base.txt and requirements_test.txt are basic dependencies (you don’t need to modify this)
  • test_api.py is where you put your model tests

3. Implement prediction logic

In your model directory, locate the model.py file (for example, my_ml_backend/model.py).

The model.py file contains a class declaration inherited from LabelStudioMLBase. This class provides wrappers for the API methods that are used by Label Studio to communicate with the ML backend. You can override the methods to implement your own logic:

def predict(self, tasks, context, **kwargs):
    """Make predictions for the tasks."""
    return predictions

The predict method is used to make predictions for the tasks. It uses the following:

Once you implement the predict method, you can see predictions from the connected ML backend in Label Studio.

Support interactive pre-annotations in your ML backend

If you want to support interactive pre-annotations in your machine learning backend, write an inference call using the predict() method. For an example that does this for text labeling projects, see this code example for substring matching.

Complete the following steps:

  1. Define an inference call with the predict() method as outlined above. The predict() method takes task data and context data:
  • The tasks parameter contains details about the task being pre-annotated. See Label Studio tasks in JSON format.

  • The context parameter contains details about annotation actions performed in Label Studio, such as a text string highlighted sent in Label Studio annotation results format.

    context has the following properties.

    • annotation_id: The annotation ID.
    • draft_id: The draft annotation ID.
    • user_id: The user ID.
    • result: This is the annotation result, but it includes an is_positive: true flag that can be changed by the user. For example, by pressing the Alt key and using keypoints to interact with the image in the UI.
  1. With the task and context data, construct a prediction using the data received from Label Studio.
  2. Return a result in the Label Studio predictions format, which varies depending on the type of labeling being performed.

Refer to the code example linked above for more details about how this might be performed for a NER labeling project.

For more information about enabling pre-annotations, see Interactive pre-annotations.

4. Implement training logic (optional)

You can also implement the fit method to train your model. The fit method is typically used to train the model on the labeled data, although it can be used for any arbitrary operations that require data persistence (for example, storing labeled data in database, saving model weights, keeping LLM prompts history, etc).

By default, the fit method is called at any data action in Label Studio, like creating a new task or updating annotations. You can modify this behavior in using Webhooks.

To implement the fit method, you need to override the fit method in your model.py file:

def fit(self, event, data, **kwargs):
    """Train the model on the labeled data."""
    old_model = self.get('old_model')
    # write your logic to update the model
    self.set('new_model', new_model)


  • event: The event type. This can be 'ANNOTATION_CREATED', 'ANNOTATION_UPDATED', etc.
  • data: The payload received from the event (see the Webhook event reference).

Additionally, there are two helper methods that you can use to store and retrieve data from the ML backend:

  • self.set(key, value) - Store data in the ML backend
  • self.get(key) - Retrieve data from the ML backend

Both methods can be used elsewhere in the ML backend code, for example, in the predict method to get the new model weights.

Trigger training with webhooks

Starting in version 1.4.1 of Label Studio, when you add an ML backend to your project, Label Studio creates a webhook to your ML backend to send an event every time an annotation is created or updated.

By default, the payload of the webhook event does not contain the annotation itself. You can either modify the webhook event sent by Label Studio to send the full payload, or retrieve the annotation using the Label Studio API using the get annotation by its ID endpoint, SDK using the get task by ID method, or by retrieving it from target storage that you set up to store annotations.

See the annotation webhook event reference for more details about the webhook event.

Other methods and parameters

Other methods and parameters are available within the LabelStudioMLBase class:

  • self.label_interface - Returns the Label Studio Label Interface object that contains all information about the labeling task.
  • self.model_version - Returns the current model version.

4. Ensure the ML backend can access Label Studio data

If your data is stored in a cloud, local directory, or has been imported into Label Studio, you will need to set the LABEL_STUDIO_URL and LABEL_STUDIO_API_KEY environment variables.

For more information, see Allow the ML backend to access Label Studio data.

5. Run the ML backend server

To run with Docker Compose:

docker-compose up

The ML backend server is available at http://localhost:9090. You can use this URL when connecting the ML backend to Label Studio.


localhost is a special domain name that loops back directly to your local environment. In the instance of Docker-hosted containers, this loops back to the container itself, and not the machine the container is hosted on. Docker provides a special domain as a workaround for this, docker.host.internal. If you're hosting Label Studio and your ML Backend inside of Docker, try using that domain instead of localhost. (http://host.docker.internal:9090)

Run without Docker

To run without docker (for example, for debugging purposes), you can use the following command:

pip install -r my_ml_backend
label-studio-ml start my_ml_backend

Modify the host and port

To modify the host and port, use the following command line parameters:

label-studio-ml start my_ml_backend -p 9091 --host

Test your ML backend

Modify the my_ml_backend/test_api.py to ensure that your ML backend works as expected.

6. Connect the ML backend to Label Studio

You can use the API or Settings > Model. For more information, see Connect the model to Label Studio.

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