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After building your chat application, serving it is the next step before it can be used by others. To facilitate this and to offer a starting point for your custom implementation, LMQL includes a simple server implementation and corresponding client web UI, that communicate via the websocket protocol.

To access and use this implementation we offer two routes: (1) Use the lmql chat command directly with your .lmql file, or (2) use the from your own code.

Using the lmql chat command

To locally serve an LMQL chat endpoint and user interface, simply run the following command:

lmql chat <path-to-lmql-file>.lmql

This will serve a web interface on http://localhost:8089, and automatically open it in your default browser. You can now start chatting with your custom LMQL chat application. The internal trace on the right-hand side (shown below), always displays the complete (conversational) prompt, reflecting the current state of your chat application.

Note that changing the .lmql file will not automatically reload the server, so you will have to restart the server manually to see the changes.

A simple chatbot using the LMQL Chat UI.
A simple chatbot launched via lmql chat.

Using chatserver

Alternatively, to launch the LMQL chat server from your own code, you can use This c;ass takes the path of the query file or a query function as argument, and returns a chat server object, that can be launched using run():

from import chatserver


Note that when passing a query function directly, you have to always provide a async def function, which enables concurrent client serving.

@message Streaming

Chat relies on a decorator-based output streaming. More specifically, only model output variables that are annotated as @message are streamed and shown to the user in the chat interface. This allows for a clean separation of model output and chat output, and eneables hidden/internal reasoning.

To use @message with your custom output writer, make sure to inherit from's ChatMessageOutputWriter, which offers additional methods for specifically handling and streaming @message variables.

More Advanced Usage

For more advanced serving scenarios, e.g. when integrating Chat into your own web applications, please refer to the very minimal implementation of chatserver in src/lmql/ui/chat/ This implementation is very minimal and can be easily adapted to your own needs and infrastructure. The corresponding web UI is implemented in src/lmql/ui/chat/assets/ and offers a good starting point for your own implementation and UI adaptations on the client side.

For other forms of output streaming e.g. via HTTP or SSE, see also the chapter on Output Streaming

Disclaimer: The LMQL chat server is a simple code template that does not include any security features, authentication or cost control. It is intended for local development and testing only, and should not be used as-is in production environments. Before deploying your own chat application, make sure to implement the necessary security measures, cost control and authentication mechanisms.