has anyone build an actual agent using BAML, where you can define the system prompt, knowledge base, tools, etc once, and the function call with the prompt itself only includes the context for that call?

what do you mean by including only the context? the function parameters only include a small set of daata for what you need?

BAML gives you primitives that would allow you to do that. But for a standard ReAct style agents (or derivatives) I found BAML to not be the best tool. Something like LangChain gives you the perfect abstraction for "Giving the agent tools and memory and instructions then running to do a task" You could make a BAML function that accepts a dynamic input where you use a

TypeBuilder

to union together the tools you want the agent to have. It's doable, but not the most ergonomic developer experience. But... I highly reccomend you read 12 factor agents repo. Once you realize that "tools" are just structured output, and in fact everything is just structured output, your whole perspective change. Where BAML shines is because of the level of granular control you get. When building something custom to your specific domains requirements in order to achieve better reliability than a standard generalized ReAct agent could, that's where BAML shines.