fix: calculator example comply MCP spec

Author: DaleSeo

examples/servers/Cargo.toml

@@ -105,3 +105,7 @@ path = "src/completion_stdio.rs"
 [[example]]
 name = "servers_progress_demo"
 path = "src/progress_demo.rs"
+
+[[example]]
+name = "servers_calculator_stdio"
+path = "src/calculator_stdio.rs"

examples/servers/src/calculator_stdio.rs

@@ -0,0 +1,26 @@
+use anyhow::Result;
+use common::calculator::Calculator;
+use rmcp::{ServiceExt, transport::stdio};
+use tracing_subscriber::{self, EnvFilter};
+mod common;
+
+/// npx @modelcontextprotocol/inspector cargo run -p mcp-server-examples --example servers_calculator_stdio
+#[tokio::main]
+async fn main() -> Result<()> {
+    // Initialize the tracing subscriber with file and stdout logging
+    tracing_subscriber::fmt()
+        .with_env_filter(EnvFilter::from_default_env().add_directive(tracing::Level::DEBUG.into()))
+        .with_writer(std::io::stderr)
+        .with_ansi(false)
+        .init();
+
+    tracing::info!("Starting Calculator MCP server");
+
+    // Create an instance of our calculator router
+    let service = Calculator::new().serve(stdio()).await.inspect_err(|e| {
+        tracing::error!("serving error: {:?}", e);
+    })?;
+
+    service.waiting().await?;
+    Ok(())
+}

examples/servers/src/common/calculator.rs

@@ -2,10 +2,7 @@
 
 use rmcp::{
     ServerHandler,
-    handler::server::{
-        router::tool::ToolRouter,
-        wrapper::{Json, Parameters},
-    },
+    handler::server::{router::tool::ToolRouter, wrapper::Parameters},
     model::{ServerCapabilities, ServerInfo},
     schemars, tool, tool_handler, tool_router,
 };
@@ -44,8 +41,8 @@ impl Calculator {
     }
 
     #[tool(description = "Calculate the difference of two numbers")]
-    fn sub(&self, Parameters(SubRequest { a, b }): Parameters<SubRequest>) -> Json<i32> {
-        Json(a - b)
+    fn sub(&self, Parameters(SubRequest { a, b }): Parameters<SubRequest>) -> String {
+        (a - b).to_string()
     }
 }