Structured tool output + schema

[crepererum]

Is your feature request related to a problem? Please describe.
Currently it seems that tools that are created via the #[tool] macro can only return unstructured output without a schema:

#[derive(Debug, serde::Deserialize, schemars::JsonSchema)]
struct WeatherRequest {
    #[schemars(description = "longitude")]
    long: f64,

    #[schemars(description = "latitude")]
    lat: f64,
}

#[derive(Debug, serde::Serialize, schemars::JsonSchema)]
struct WeatherResult {
    #[schemars(description = "temperature in degrees Kelvin")]
    temp: f64,

    #[schemars(description = "humidity in percentage")]
    humidity: f64,
}

#[tool_router]
struct WeatherService {
    #[tool(description = "get weather for given location")]
    async fn get_weather(
        &self,
        Parameters(WeatherRequest { lat, long }): Parameters<WeatherRequest>,
    ) -> Result<CallToolResult, McpError> {
        // unstructured result w/o a schema
        Ok(CallToolResult::success(vec![
            Content::json(WeatherResult {
                temp: 4.2,
                humidity: 50.0,
            }).unwrap()
        ]))
    }
}

Describe the solution you'd like
It seems that the macro already reads the input schema from the parameters. Maybe we can directly return a structured result and have the schema registered for that in the tool metadata?

// ... WeatherRequest and WeatherResult as above

#[tool_router]
struct WeatherService {
    #[tool(description = "get weather for given location")]
    async fn get_weather(
        &self,
        Parameters(WeatherRequest { lat, long }): Parameters<WeatherRequest>,
    ) -> Result<WeatherResult, McpError> {
        // structured result w/ a schema
        Ok(WeatherResult {
            temp: 4.2,
            humidity: 50.0,
        })
    }
}

Describe alternatives you've considered
Not supporting structured content or hand-writing the respective code.

Additional context

• outputSchema in tools request: https://modelcontextprotocol.io/specification/2025-06-18/server/tools#tool
• structured content docs: https://modelcontextprotocol.io/specification/2025-06-18/server/tools#structured-content
• output schema docs: https://modelcontextprotocol.io/specification/2025-06-18/server/tools#output-schema

[4t145]

This feature was introduced in revision 2025-06-18, we haven't supported it yet!