Attribute Macro tool 

#[tool]

Marks an async function as an MCP tool and generates a companion {fn_name}_tool_def() function that returns a mcp::ToolDef.

Attributes

• description = "..." — human-readable description (required)
• name = "..." — tool name (defaults to the function name)

Example

use mcp_kit::prelude::*;

/// Add two numbers together.
#[tool(description = "Add two numbers")]
async fn add(a: f64, b: f64) -> String {
    format!("{}", a + b)
}

#[tokio::main]
async fn main() -> anyhow::Result<()> {
    // Register with the builder:
    let _server = McpServer::builder()
        .name("example")
        .version("1.0.0")
        .tool_def(add_tool_def())
        .build();
    Ok(())
}

Each function parameter must implement serde::Deserialize and will be extracted from the tool call’s arguments JSON object by field name.