feat: relax outputSchema to accept non-object JSON Schema types (SEP-2106)

crates/rmcp-macros/src/tool.rs

@@ -28,13 +28,6 @@ fn extract_schema_from_return_type(ret_type: &syn::Type) -> Option<Expr> {
     if let Some(inner_type) = extract_json_inner_type(ret_type) {
         return syn::parse2::<Expr>(quote! {
             rmcp::handler::server::tool::schema_for_output::<#inner_type>()
-                .unwrap_or_else(|e| {
-                    panic!(
-                        "Invalid output schema for Json<{}>: {}",
-                        std::any::type_name::<#inner_type>(),
-                        e
-                    )
-                })
         })
         .ok();
     }
@@ -65,13 +58,6 @@ fn extract_schema_from_return_type(ret_type: &syn::Type) -> Option<Expr> {
 
     syn::parse2::<Expr>(quote! {
         rmcp::handler::server::tool::schema_for_output::<#inner_type>()
-            .unwrap_or_else(|e| {
-                panic!(
-                    "Invalid output schema for Result<Json<{}>, E>: {}",
-                    std::any::type_name::<#inner_type>(),
-                    e
-                )
-            })
     })
     .ok()
 }

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

@@ -106,32 +106,40 @@ pub fn schema_for_empty_input() -> Arc<JsonObject> {
     EMPTY.clone()
 }
 
-/// Generate a JSON schema for outputSchema (must have root type "object"; top-level "title" and "description" are removed)
-pub fn schema_for_output<T: JsonSchema + std::any::Any>() -> Result<Arc<JsonObject>, String> {
+/// Strip top-level `title` and `description` from a JSON schema for outputSchema.
+/// Unlike `validate_and_strip`, this performs no validation — output schemas are not
+/// restricted to `type: "object"` (per SEP-2106).
+fn strip_output(raw: &Arc<JsonObject>) -> Arc<JsonObject> {
+    let mut object = raw.as_ref().clone();
+    object.remove("title");
+    object.remove("description");
+    Arc::new(object)
+}
+
+/// Generate and strip a JSON schema for outputSchema (top-level "title" and
+/// "description" are removed; output schemas are not restricted to root type "object").
+pub fn schema_for_output<T: JsonSchema + std::any::Any>() -> Arc<JsonObject> {
     thread_local! {
-        static CACHE_FOR_OUTPUT: std::sync::RwLock<HashMap<TypeId, Result<Arc<JsonObject>, String>>> = Default::default();
+        static CACHE_FOR_OUTPUT: std::sync::RwLock<HashMap<TypeId, Arc<JsonObject>>> = Default::default();
     };
 
     CACHE_FOR_OUTPUT.with(|cache| {
-        // Try to get from cache first
-        if let Some(result) = cache
+        if let Some(schema) = cache
             .read()
             .expect("output schema cache lock poisoned")
             .get(&TypeId::of::<T>())
         {
-            return result.clone();
+            return schema.clone();
         }
 
-        // Generate, validate, and strip unnecessary top-level fields
-        let result = validate_and_strip(&schema_for_type::<T>(), "outputSchema");
+        let schema = strip_output(&schema_for_type::<T>());
 
-        // Cache the result (both success and error cases)
         cache
             .write()
             .expect("output schema cache lock poisoned")
-            .insert(TypeId::of::<T>(), result.clone());
+            .insert(TypeId::of::<T>(), schema.clone());
 
-        result
+        schema
     })
 }
 
@@ -305,32 +313,88 @@ mod tests {
         assert!(Arc::ptr_eq(&schema, &cloned));
     }
 
+    #[test]
+    fn test_schema_for_output_accepts_primitive() {
+        let schema = schema_for_output::<i32>();
+        assert_eq!(schema.get("type"), Some(&serde_json::json!("integer")));
+    }
+
+    #[test]
+    fn test_schema_for_output_strips_description_for_primitive() {
+        let schema = schema_for_output::<i32>();
+        assert!(!schema.contains_key("description"));
+    }
+
+    #[test]
+    fn test_schema_for_output_accepts_composition() {
+        let schema = schema_for_output::<Option<String>>();
+        let schema_str = serde_json::to_string(&schema).unwrap();
+        assert!(
+            schema_str.contains("anyOf")
+                || schema_str.contains("oneOf")
+                || schema_str.contains("null"),
+            "Expected composition schema for Option<String>, got: {schema_str}"
+        );
+    }
+
+    #[test]
+    fn test_schema_for_output_caches_result() {
+        let schema1 = schema_for_output::<i32>();
+        let schema2 = schema_for_output::<i32>();
+        assert!(Arc::ptr_eq(&schema1, &schema2));
+    }
+
+    #[test]
+    fn test_schema_for_input_rejects_array() {
+        let result = schema_for_input::<Vec<i32>>();
+        assert!(result.is_err());
+    }
+
+    #[test]
+    fn test_schema_for_output_accepts_unit() {
+        let _schema = schema_for_output::<()>();
+    }
+
+    #[test]
+    fn test_schema_for_output_accepts_object() {
+        let schema = schema_for_output::<TestObject>();
+        assert_eq!(schema.get("type"), Some(&serde_json::json!("object")));
+    }
+
+    #[test]
+    fn test_schema_for_output_strips_top_level_title() {
+        let schema = schema_for_output::<TestObject>();
+        assert!(!schema.contains_key("title"));
+    }
+
+    #[test]
+    fn test_schema_for_output_strips_top_level_description() {
+        let schema = schema_for_output::<TestObject>();
+        assert!(!schema.contains_key("description"));
+    }
+
     #[rstest]
-    #[case::output(schema_for_output::<i32>)]
     #[case::input(schema_for_input::<i32>)]
-    fn test_schema_for_object_wrappers_reject_primitives(
+    fn test_schema_for_input_rejects_primitives(
         #[case] schema_fn: fn() -> Result<Arc<JsonObject>, String>,
     ) {
         let result = schema_fn();
         assert!(result.is_err());
     }
 
     #[rstest]
-    #[case::output(schema_for_output::<TestObject>)]
     #[case::input(schema_for_input::<TestObject>)]
-    fn test_schema_for_object_wrappers_accept_objects(
+    fn test_schema_for_input_accepts_objects(
         #[case] schema_fn: fn() -> Result<Arc<JsonObject>, String>,
     ) {
         let result = schema_fn();
         assert!(result.is_ok());
     }
 
     #[rstest]
-    #[case::output_title(schema_for_output::<TestObject>, "title")]
-    #[case::output_description(schema_for_output::<TestObject>, "description")]
     #[case::input_title(schema_for_input::<TestObject>, "title")]
     #[case::input_description(schema_for_input::<TestObject>, "description")]
-    fn test_schema_for_object_wrappers_strip_top_level_metadata(
+    fn test_schema_for_input_strips_top_level_metadata(
         #[case] schema_fn: fn() -> Result<Arc<JsonObject>, String>,
         #[case] field: &str,
     ) {

crates/rmcp/src/handler/server/router/tool/tool_traits.rs

@@ -65,13 +65,7 @@ pub trait ToolBase {
     ///
     /// If the tool does not have any output, you should override this methods to return [`None`].
     fn output_schema() -> Option<Arc<JsonObject>> {
-        Some(schema_for_output::<Self::Output>().unwrap_or_else(|e| {
-            panic!(
-                "Invalid output schema for ToolBase::Output type `{0}`: {1}",
-                std::any::type_name::<Self::Output>(),
-                e,
-            );
-        }))
+        Some(schema_for_output::<Self::Output>())
     }
 
     fn annotations() -> Option<ToolAnnotations> {

crates/rmcp/src/model/tool.rs

@@ -315,15 +315,9 @@ impl Tool {
     }
 
     /// Set the output schema using a type that implements JsonSchema
-    ///
-    /// # Panics
-    ///
-    /// Panics if the generated schema does not have root type "object" as required by MCP specification.
     #[cfg(feature = "server")]
     pub fn with_output_schema<T: JsonSchema + 'static>(mut self) -> Self {
-        let schema = crate::handler::server::tool::schema_for_output::<T>()
-            .unwrap_or_else(|e| panic!("Invalid output schema for tool '{}': {}", self.name, e));
-        self.output_schema = Some(schema);
+        self.output_schema = Some(crate::handler::server::tool::schema_for_output::<T>());
         self
     }
 

crates/rmcp/tests/test_json_schema_detection.rs

@@ -60,6 +60,28 @@ impl TestServer {
     pub async fn explicit_schema(&self) -> Result<String, String> {
         Ok("test".to_string())
     }
+
+    /// Tool that returns Json<Vec<T>> - array output schema
+    #[tool(name = "with-json-array")]
+    pub async fn with_json_array(&self) -> Result<Json<Vec<TestData>>, String> {
+        Ok(Json(vec![TestData {
+            value: "test".to_string(),
+        }]))
+    }
+
+    /// Tool that returns Result<Json<Vec<T>>, ErrorData> - array output schema
+    #[tool(name = "result-with-json-array")]
+    pub async fn result_with_json_array(&self) -> Result<Json<Vec<TestData>>, rmcp::ErrorData> {
+        Ok(Json(vec![TestData {
+            value: "test".to_string(),
+        }]))
+    }
+
+    /// Tool that returns Json<String> - string output schema
+    #[tool(name = "with-json-string")]
+    pub async fn with_json_string(&self) -> Result<Json<String>, String> {
+        Ok(Json("test".to_string()))
+    }
 }
 
 #[tokio::test]
@@ -113,3 +135,54 @@ async fn test_explicit_schema_override() {
         "Explicit output_schema attribute should work"
     );
 }
+
+#[tokio::test]
+async fn test_json_array_type_generates_schema() {
+    let server = TestServer::new();
+    let tools = server.tool_router.list_all();
+
+    let array_tool = tools.iter().find(|t| t.name == "with-json-array").unwrap();
+    assert!(
+        array_tool.output_schema.is_some(),
+        "Json<Vec<T>> return type should generate output schema"
+    );
+    let schema = array_tool.output_schema.as_ref().unwrap();
+    assert_eq!(
+        schema.get("type").and_then(|v| v.as_str()),
+        Some("array"),
+        "Json<Vec<T>> should produce an array schema"
+    );
+}
+
+#[tokio::test]
+async fn test_result_with_json_array_generates_schema() {
+    let server = TestServer::new();
+    let tools = server.tool_router.list_all();
+
+    let result_array_tool = tools
+        .iter()
+        .find(|t| t.name == "result-with-json-array")
+        .unwrap();
+    assert!(
+        result_array_tool.output_schema.is_some(),
+        "Result<Json<Vec<T>>, ErrorData> return type should generate output schema"
+    );
+}
+
+#[tokio::test]
+async fn test_json_string_type_generates_schema() {
+    let server = TestServer::new();
+    let tools = server.tool_router.list_all();
+
+    let string_tool = tools.iter().find(|t| t.name == "with-json-string").unwrap();
+    assert!(
+        string_tool.output_schema.is_some(),
+        "Json<String> return type should generate output schema"
+    );
+    let schema = string_tool.output_schema.as_ref().unwrap();
+    assert_eq!(
+        schema.get("type").and_then(|v| v.as_str()),
+        Some("string"),
+        "Json<String> should produce a string schema"
+    );
+}

crates/rmcp/tests/test_structured_output.rs

@@ -93,6 +93,27 @@ impl TestServer {
             Err("User not found".to_string())
         }
     }
+
+    /// Tool that returns a list of calculation results
+    #[tool(
+        name = "calculate-list",
+        description = "Return a list of calculation results"
+    )]
+    pub async fn calculate_list(
+        &self,
+        params: Parameters<CalculationRequest>,
+    ) -> Result<Json<Vec<CalculationResult>>, String> {
+        Ok(Json(vec![CalculationResult {
+            sum: params.0.a + params.0.b,
+            product: params.0.a * params.0.b,
+        }]))
+    }
+
+    /// Tool that returns a count
+    #[tool(name = "get-count", description = "Return a count")]
+    pub async fn get_count(&self) -> Result<Json<i32>, String> {
+        Ok(Json(42))
+    }
 }
 
 #[tokio::test]
@@ -360,3 +381,38 @@ fn test_call_tool_result_deserialize_without_content() {
     assert!(result.content.is_empty());
     assert!(result.structured_content.is_some());
 }
+
+#[tokio::test]
+async fn test_tool_with_array_output_schema() {
+    let server = TestServer::new();
+    let tools = server.tool_router.list_all();
+
+    // Find the calculate-list tool
+    let calculate_list_tool = tools.iter().find(|t| t.name == "calculate-list").unwrap();
+
+    // Verify it has an output schema
+    assert!(calculate_list_tool.output_schema.is_some());
+
+    let schema = calculate_list_tool.output_schema.as_ref().unwrap();
+
+    // Check that the schema contains array type
+    let schema_str = serde_json::to_string(schema).unwrap();
+    assert!(schema_str.contains("array"));
+}
+
+#[tokio::test]
+async fn test_tool_with_primitive_output_schema() {
+    let server = TestServer::new();
+    let tools = server.tool_router.list_all();
+
+    // Find the get-count tool
+    let get_count_tool = tools.iter().find(|t| t.name == "get-count").unwrap();
+
+    // Verify it has an output schema
+    assert!(get_count_tool.output_schema.is_some());
+
+    let schema = get_count_tool.output_schema.as_ref().unwrap();
+
+    // Check that the schema contains integer type
+    assert_eq!(schema.get("type"), Some(&serde_json::json!("integer")));
+}