Attribute Macro tool 

#[tool]

Defines a tool handler.

The function signature should be:

async fn tool_name(ctx: &McpContext, args...) -> Result

Attributes

• name - Override the tool name (default: function name)
• description - Tool description (default: doc comment)
• tags - List of tool tags for filtering (tags = ["api", "read"])

Parameter Defaults

Rust has no default function arguments. For feature parity with Python FastMCP, #[tool] supports per-parameter defaults via defaults(...):

#[tool(defaults(title = "World"))]
fn greet(name: String, title: String) -> String {
    format!("Hello {title} {name}")
}

If the argument is omitted in the JSON-RPC call, the default value is used.