fix: generate default schema for tools with no params (#446)

* fix: generate default schema for tools with no params

Author: vincent0426

crates/rmcp-macros/src/tool.rs

@@ -199,9 +199,13 @@ pub fn tool(attr: TokenStream, input: TokenStream) -> syn::Result<TokenStream> {
                 rmcp::handler::server::common::cached_schema_for_type::<#params_ty>()
             })?
         } else {
-            // if not found, use a simple empty JSON object
+            // if not found, use a default empty JSON schema object
+            // TODO: should be updated according to the new specifications
             syn::parse2::<Expr>(quote! {
-                std::sync::Arc::new(serde_json::Map::new())
+                std::sync::Arc::new(serde_json::json!({
+                    "type": "object",
+                    "properties": {}
+                }).as_object().unwrap().clone())
             })?
         }
     };

crates/rmcp/tests/test_tool_macros.rs

@@ -121,8 +121,14 @@ async fn test_tool_macros() {
 async fn test_tool_macros_with_empty_param() {
     let _attr = Server::empty_param_tool_attr();
     println!("{_attr:?}");
-    assert!(_attr.input_schema.get("type").is_none());
-    assert!(_attr.input_schema.get("properties").is_none());
+    assert_eq!(
+        _attr.input_schema.get("type"),
+        Some(&serde_json::Value::String("object".to_string()))
+    );
+    assert_eq!(
+        _attr.input_schema.get("properties"),
+        Some(&serde_json::Value::Object(serde_json::Map::new()))
+    );
 }
 
 #[tokio::test]