What is BAML? — Boundary Documentation

The best way to understand BAML and its developer experience is to see it live in a demo (see below).

Demo video

Here we write a BAML function definition, and then call it from a Python script.

Examples

High-level Developer Flow

Write a BAML function definition

main.baml

1 class WeatherAPI {
2   city string @description("the user's city")
3   timeOfDay string @description("As an ISO8601 timestamp")
4 }
5 
6 function UseTool(user_message: string) -> WeatherAPI {
7   client "openai/gpt-4o"
8   prompt #"
9     Extract.... {# we will explain the rest in the guides #}
10   "#
11 }

Here you can run tests in the VSCode Playground.

Generate baml_client from those .baml files.

This is auto-generated code with all boilerplate to call the LLM endpoint, parse the output, fix broken JSON, and handle errors.

Call your function in any language

with type-safety, autocomplete, retry-logic, robust JSON parsing, etc..

1 import asyncio
2 from baml_client import b
3 from baml_client.types import WeatherAPI
4 
5 def main():
6     weather_info = b.UseTool("What's the weather like in San Francisco?")
7     print(weather_info)
8     assert isinstance(weather_info, WeatherAPI)
9     print(f"City: {weather_info.city}")
10     print(f"Time of Day: {weather_info.timeOfDay}")
11 
12 if __name__ == '__main__':
13     main()

Continue on to the Installation Guides for your language to setup BAML in a few minutes!

You don’t need to migrate 100% of your LLM code to BAML in one go! It works along-side any existing LLM framework.

1	class WeatherAPI {
2	city string @description("the user's city")
3	timeOfDay string @description("As an ISO8601 timestamp")
4	}
5
6	function UseTool(user_message: string) -> WeatherAPI {
7	client "openai/gpt-4o"
8	prompt #"
9	Extract.... {# we will explain the rest in the guides #}
10	"#
11	}

1	import asyncio
2	from baml_client import b
3	from baml_client.types import WeatherAPI
4
5	def main():
6	weather_info = b.UseTool("What's the weather like in San Francisco?")
7	print(weather_info)
8	assert isinstance(weather_info, WeatherAPI)
9	print(f"City: {weather_info.city}")
10	print(f"Time of Day: {weather_info.timeOfDay}")
11
12	if __name__ == '__main__':
13	main()