feat(tool): allow tool call return a serializable value in json format (#75) (#78)

crates/rmcp/src/handler/server.rs

@@ -6,7 +6,7 @@ use crate::{
 
 mod resource;
 pub mod tool;
-
+pub mod wrapper;
 impl<H: ServerHandler> Service<RoleServer> for H {
     async fn handle_request(
         &self,

crates/rmcp/src/handler/server/wrapper.rs

@@ -0,0 +1,2 @@
+mod json;
+pub use json::*;

crates/rmcp/src/handler/server/wrapper/json.rs

@@ -0,0 +1,28 @@
+use serde::Serialize;
+
+use crate::model::IntoContents;
+
+/// Json wrapper
+///
+/// This is used to tell the SDK to serialize the inner value into json
+pub struct Json<T>(pub T);
+
+impl<T> IntoContents for Json<T>
+where
+    T: Serialize,
+{
+    fn into_contents(self) -> Vec<crate::model::Content> {
+        let result = crate::model::Content::json(self.0);
+        debug_assert!(
+            result.is_ok(),
+            "Json wrapped content should be able to serialized into json"
+        );
+        match result {
+            Ok(content) => vec![content],
+            Err(e) => {
+                tracing::error!("failed to convert json content: {e}");
+                vec![]
+            }
+        }
+    }
+}

examples/servers/src/common/calculator.rs

@@ -1,5 +1,6 @@
 use rmcp::{
     ServerHandler,
+    handler::server::wrapper::Json,
     model::{ServerCapabilities, ServerInfo},
     schemars, tool,
 };
@@ -28,8 +29,8 @@ impl Calculator {
         #[tool(param)]
         #[schemars(description = "the left hand side number")]
         b: i32,
-    ) -> String {
+    ) -> Json<i32> {
-        (a - b).to_string()
+        Json(a - b)
     }
 }