Function tool 

pub fn tool(
    name: impl Into<String>,
    description: impl Into<String>,
) -> ToolBuilder

Starts a ToolBuilder.

use open_agent::{tool, Error};
use serde_json::json;

let add = tool("add", "Add two numbers")
    .param("a", "number")
    .param("b", "number")
    .build(|args| async move {
        let a = args["a"].as_f64()
            .ok_or_else(|| Error::invalid_input("a must be a number"))?;
        let b = args["b"].as_f64()
            .ok_or_else(|| Error::invalid_input("b must be a number"))?;
        Ok(json!({"result": a + b}))
    });

Use ToolBuilder::schema for optional parameters or a complete JSON Schema, and ToolBuilder::build for an example with shared state.