feat: Add support for Tool.outputSchema and CallToolResult.structuredContent (#316)

* feat: add output_schema field to Tool struct

- Add optional output_schema field to Tool struct for defining tool
output structure
- Update Tool::new() to initialize output_schema as None

* feat: add structured_content field to CallToolResult

- Add optional structured_content field for JSON object results
- Make content field optional to support either structured or
unstructured results
- Add CallToolResult::structured() and structured_error() constructor
methods

* feat: implement validation for mutually exclusive content/structuredContent

- Add validate() method to ensure content and structured_content are
mutually exclusive
- Implement custom Deserialize to enforce validation during
deserialization
- Update documentation to clarify the mutual exclusivity requirement

* feat: add output_schema support to #[tool] macro

- Add output_schema field to ToolAttribute and ResolvedToolAttribute
structs
- Implement automatic output schema generation from return types
- Support explicit output_schema attribute for manual specification
- Generate schemas for Result<T, E> where T is not CallToolResult
- Update tool generation to include output_schema in Tool struct

* feat: implement IntoCallToolResult for structured content

- Add Structured<T> wrapper type for explicit structured content
- Implement IntoCallToolResult for Structured<T> with JSON serialization
- Add support for Result<Structured<T>, E> conversions
- Enable tools to return structured content through the trait system

* fix: update simple-chat-client example for optional content field

- Handle Option<Vec<Content>> in CallToolResult.content
- Add proper unwrapping for the optional content field
- Fix compilation error in chat.rs

* fix: update examples and tests for optional content field

- Add output_schema field to Tool initialization in sampling_stdio
example
- Update test_tool_macros tests to handle Option<Vec<Content>>
- Use as_ref() before calling first() on optional content field

* feat: implement basic schema validation in conversion logic

- Add validate_against_schema function for basic type validation
- Add note that full JSON Schema validation requires dedicated library
- Document that actual validation should happen in tool handler

* feat: add structured output support for tools

- Add output_schema field to Tool struct for defining output JSON schemas
- Add structured_content field to CallToolResult (mutually exclusive with content)
- Implement Structured<T> wrapper for type-safe structured outputs
- Update #[tool] macro to automatically generate output schemas from return types
- Add validation of structured outputs against their schemas
- Update all examples and tests for breaking change (CallToolResult.content now Option)
- Add comprehensive documentation and rustdoc
- Add structured_output example demonstrating the feature

BREAKING CHANGE: CallToolResult.content is now Option<Vec<Content>> instead of Vec<Content>

Closes #312

* fix: correct structured output doctest to use Parameters wrapper

The #[tool] macro requires Parameters<T> wrapper for tool inputs.
This fixes the pre-existing broken doctest in the structured output
documentation example.

* feat: replace Structured<T> with Json<T> for structured output

- Remove Structured<T> type definition and implementations
- Reuse existing Json<T> wrapper for structured content
- Update IntoCallToolResult implementations to use Json<T>
- Add JsonSchema implementation for Json<T> delegating to T
- Update all examples and tests to use Json<T> instead of Structured<T>
- Update documentation and exports

BREAKING CHANGE: Structured<T> has been replaced with Json<T>. Users must update their code to use Json<T> for structured tool outputs.

* feat: add output_schema() method to IntoCallToolResult trait

- Add output_schema() method with default None implementation
- Implement output_schema() for Json<T> to return cached schema
- Implement output_schema() for Result<Json<T>, E> delegating to Json<T>
- Enable trait-based schema generation for structured outputs

* feat: update macro to detect Json<T> wrapper for output schemas

- Add extract_json_inner_type() helper to detect Json<T> types
- Update schema generation to only occur for Json<T> wrapped types
- Remove generic Result<T, E> detection in favor of specific Json<T> detection
- Add comprehensive tests to verify schema generation behavior

* feat: add builder methods to Tool struct for setting schemas

- Add with_output_schema<T>() method to set output schema from type
- Add with_input_schema<T>() method to set input schema from type
- Both methods use cached_schema_for_type internally
- Add comprehensive tests for builder methods

* fix: address clippy warnings

- Add Default implementation for StructuredOutputServer
- Fix collapsible else-if in simple-chat-client
- No functional changes

* style: apply cargo fmt

Apply automatic formatting changes to:
- examples/simple-chat-client/src/chat.rs - fix line wrapping
- crates/rmcp-macros/src/tool.rs - format method chaining
- examples/servers/src/structured_output.rs - reorder imports and format function signatures

* chore: fix formatting

* chore: fix rustdoc redundant link warning

* refactor: validate_against_schema

* feat: enforce structured_content usage when output_schema is defined

This commit implements strict validation to ensure tools with output_schema
consistently use structured_content for both success and error responses.

Changes:
- Enhanced ToolRouter::call() validation to require structured_content when output_schema is present
- Added validation that tools with output_schema cannot use regular content field
- Added comprehensive tests covering the new strict validation behavior
- Created example demonstrating proper structured output usage
- Updated TODO.md to track validation improvements

This ensures consistent response format and better type safety for MCP clients.

* chore: remove TODO.md

* refactor: simplify output schema extraction logic in tool macro

- Extract complex nested logic into dedicated helper function
- Replace deeply nested if-else chains with functional approach
- Use early returns and ? operator for cleaner code flow
- Reduce 46 lines to 7 lines in main logic while improving readability

* chore: run cargo fmt

* fix: enforce structured_content usage when output_schema is defined

Structured content is returned as a JSON object in the structuredContent
field of a result.For backwards compatibility, a tool that returns
structured content SHOULD also return the serialized JSON in a
TextContent block.

https://modelcontextprotocol.io/specification/2025-06-18/server/tools#structured-content

Tools may also provide an output schema for validation of structured
results. If an output schema is provided:

- Servers MUST provide structured results that conform to this schema.
- Clients SHOULD validate structured results against this schema.

https://modelcontextprotocol.io/specification/2025-06-18/server/tools#output-schema

* chore: cargo fmt

crates/rmcp/tests/test_json_schema_detection.rs

@@ -0,0 +1,114 @@
+//cargo test --test test_json_schema_detection --features "client server macros"
+use rmcp::{
+    Json, ServerHandler, handler::server::router::tool::ToolRouter, tool, tool_handler, tool_router,
+};
+use schemars::JsonSchema;
+use serde::{Deserialize, Serialize};
+
+#[derive(Serialize, Deserialize, JsonSchema)]
+pub struct TestData {
+    pub value: String,
+}
+
+#[tool_handler(router = self.tool_router)]
+impl ServerHandler for TestServer {}
+
+#[derive(Debug, Clone)]
+pub struct TestServer {
+    tool_router: ToolRouter<Self>,
+}
+
+impl Default for TestServer {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+#[tool_router(router = tool_router)]
+impl TestServer {
+    pub fn new() -> Self {
+        Self {
+            tool_router: Self::tool_router(),
+        }
+    }
+
+    /// Tool that returns Json<T> - should have output schema
+    #[tool(name = "with-json")]
+    pub async fn with_json(&self) -> Result<Json<TestData>, String> {
+        Ok(Json(TestData {
+            value: "test".to_string(),
+        }))
+    }
+
+    /// Tool that returns regular type - should NOT have output schema
+    #[tool(name = "without-json")]
+    pub async fn without_json(&self) -> Result<String, String> {
+        Ok("test".to_string())
+    }
+
+    /// Tool that returns Result with inner Json - should have output schema  
+    #[tool(name = "result-with-json")]
+    pub async fn result_with_json(&self) -> Result<Json<TestData>, rmcp::ErrorData> {
+        Ok(Json(TestData {
+            value: "test".to_string(),
+        }))
+    }
+
+    /// Tool with explicit output_schema attribute - should have output schema
+    #[tool(name = "explicit-schema", output_schema = rmcp::handler::server::tool::cached_schema_for_type::<TestData>())]
+    pub async fn explicit_schema(&self) -> Result<String, String> {
+        Ok("test".to_string())
+    }
+}
+
+#[tokio::test]
+async fn test_json_type_generates_schema() {
+    let server = TestServer::new();
+    let tools = server.tool_router.list_all();
+
+    // Find the with-json tool
+    let json_tool = tools.iter().find(|t| t.name == "with-json").unwrap();
+    assert!(
+        json_tool.output_schema.is_some(),
+        "Json<T> return type should generate output schema"
+    );
+}
+
+#[tokio::test]
+async fn test_non_json_type_no_schema() {
+    let server = TestServer::new();
+    let tools = server.tool_router.list_all();
+
+    // Find the without-json tool
+    let non_json_tool = tools.iter().find(|t| t.name == "without-json").unwrap();
+    assert!(
+        non_json_tool.output_schema.is_none(),
+        "Regular return type should NOT generate output schema"
+    );
+}
+
+#[tokio::test]
+async fn test_result_with_json_generates_schema() {
+    let server = TestServer::new();
+    let tools = server.tool_router.list_all();
+
+    // Find the result-with-json tool
+    let result_json_tool = tools.iter().find(|t| t.name == "result-with-json").unwrap();
+    assert!(
+        result_json_tool.output_schema.is_some(),
+        "Result<Json<T>, E> return type should generate output schema"
+    );
+}
+
+#[tokio::test]
+async fn test_explicit_schema_override() {
+    let server = TestServer::new();
+    let tools = server.tool_router.list_all();
+
+    // Find the explicit-schema tool
+    let explicit_tool = tools.iter().find(|t| t.name == "explicit-schema").unwrap();
+    assert!(
+        explicit_tool.output_schema.is_some(),
+        "Explicit output_schema attribute should work"
+    );
+}

crates/rmcp/tests/test_message_schema/server_json_rpc_message_schema.json

@@ -299,12 +299,15 @@
       }
     },
     "CallToolResult": {
-      "description": "The result of a tool call operation.\n\nContains the content returned by the tool execution and an optional\nflag indicating whether the operation resulted in an error.",
+      "description": "The result of a tool call operation.\n\nContains the content returned by the tool execution and an optional\nflag indicating whether the operation resulted in an error.\n\nNote: `content` and `structured_content` are mutually exclusive - exactly one must be provided.",
       "type": "object",
       "properties": {
         "content": {
           "description": "The content returned by the tool (text, images, etc.)",
-          "type": "array",
+          "type": [
+            "array",
+            "null"
+          ],
           "items": {
             "$ref": "#/definitions/Annotated"
           }
@@ -315,11 +318,11 @@
             "boolean",
             "null"
           ]
+        },
+        "structuredContent": {
+          "description": "An optional JSON object that represents the structured result of the tool call"
         }
-      },
-      "required": [
-        "content"
-      ]
+      }
     },
     "CancelledNotificationMethod": {
       "type": "string",
@@ -1580,6 +1583,14 @@
         "name": {
           "description": "The name of the tool",
           "type": "string"
+        },
+        "outputSchema": {
+          "description": "An optional JSON Schema object defining the structure of the tool's output",
+          "type": [
+            "object",
+            "null"
+          ],
+          "additionalProperties": true
         }
       },
       "required": [

crates/rmcp/tests/test_structured_output.rs

@@ -0,0 +1,233 @@
+//cargo test --test test_structured_output --features "client server macros"
+use rmcp::{
+    Json, ServerHandler,
+    handler::server::{router::tool::ToolRouter, tool::Parameters},
+    model::{CallToolResult, Content, Tool},
+    tool, tool_handler, tool_router,
+};
+use schemars::JsonSchema;
+use serde::{Deserialize, Serialize};
+use serde_json::json;
+
+#[derive(Serialize, Deserialize, JsonSchema)]
+pub struct CalculationRequest {
+    pub a: i32,
+    pub b: i32,
+}
+
+#[derive(Serialize, Deserialize, JsonSchema)]
+pub struct CalculationResult {
+    pub sum: i32,
+    pub product: i32,
+}
+
+#[derive(Serialize, Deserialize, JsonSchema)]
+pub struct UserInfo {
+    pub name: String,
+    pub age: u32,
+}
+
+#[tool_handler(router = self.tool_router)]
+impl ServerHandler for TestServer {}
+
+#[derive(Debug, Clone)]
+pub struct TestServer {
+    tool_router: ToolRouter<Self>,
+}
+
+impl Default for TestServer {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+#[tool_router(router = tool_router)]
+impl TestServer {
+    pub fn new() -> Self {
+        Self {
+            tool_router: Self::tool_router(),
+        }
+    }
+
+    /// Tool that returns structured output
+    #[tool(name = "calculate", description = "Perform calculations")]
+    pub async fn calculate(
+        &self,
+        params: Parameters<CalculationRequest>,
+    ) -> Result<Json<CalculationResult>, String> {
+        Ok(Json(CalculationResult {
+            sum: params.0.a + params.0.b,
+            product: params.0.a * params.0.b,
+        }))
+    }
+
+    /// Tool that returns regular string output
+    #[tool(name = "get-greeting", description = "Get a greeting")]
+    pub async fn get_greeting(&self, name: Parameters<String>) -> String {
+        format!("Hello, {}!", name.0)
+    }
+
+    /// Tool that returns structured user info
+    #[tool(name = "get-user", description = "Get user info")]
+    pub async fn get_user(&self, user_id: Parameters<String>) -> Result<Json<UserInfo>, String> {
+        if user_id.0 == "123" {
+            Ok(Json(UserInfo {
+                name: "Alice".to_string(),
+                age: 30,
+            }))
+        } else {
+            Err("User not found".to_string())
+        }
+    }
+}
+
+#[tokio::test]
+async fn test_tool_with_output_schema() {
+    let server = TestServer::new();
+    let tools = server.tool_router.list_all();
+
+    // Find the calculate tool
+    let calculate_tool = tools.iter().find(|t| t.name == "calculate").unwrap();
+
+    // Verify it has an output schema
+    assert!(calculate_tool.output_schema.is_some());
+
+    let schema = calculate_tool.output_schema.as_ref().unwrap();
+
+    // Check that the schema contains expected fields
+    let schema_str = serde_json::to_string(schema).unwrap();
+    assert!(schema_str.contains("sum"));
+    assert!(schema_str.contains("product"));
+}
+
+#[tokio::test]
+async fn test_tool_without_output_schema() {
+    let server = TestServer::new();
+    let tools = server.tool_router.list_all();
+
+    // Find the get-greeting tool
+    let greeting_tool = tools.iter().find(|t| t.name == "get-greeting").unwrap();
+
+    // Verify it doesn't have an output schema (returns String)
+    assert!(greeting_tool.output_schema.is_none());
+}
+
+#[tokio::test]
+async fn test_structured_content_in_call_result() {
+    // Test creating a CallToolResult with structured content
+    let structured_data = json!({
+        "sum": 7,
+        "product": 12
+    });
+
+    let result = CallToolResult::structured(structured_data.clone());
+
+    assert!(result.content.is_none());
+    assert!(result.structured_content.is_some());
+    assert_eq!(result.structured_content.unwrap(), structured_data);
+    assert_eq!(result.is_error, Some(false));
+}
+
+#[tokio::test]
+async fn test_structured_error_in_call_result() {
+    // Test creating a CallToolResult with structured error
+    let error_data = json!({
+        "error_code": "NOT_FOUND",
+        "message": "User not found"
+    });
+
+    let result = CallToolResult::structured_error(error_data.clone());
+
+    assert!(result.content.is_none());
+    assert!(result.structured_content.is_some());
+    assert_eq!(result.structured_content.unwrap(), error_data);
+    assert_eq!(result.is_error, Some(true));
+}
+
+#[tokio::test]
+async fn test_mutual_exclusivity_validation() {
+    // Test that content and structured_content are mutually exclusive
+    let content_result = CallToolResult::success(vec![Content::text("Hello")]);
+    let structured_result = CallToolResult::structured(json!({"message": "Hello"}));
+
+    // Verify the validation
+    assert!(content_result.validate().is_ok());
+    assert!(structured_result.validate().is_ok());
+
+    // Try to create an invalid result with both fields
+    let invalid_json = json!({
+        "content": [{"type": "text", "text": "Hello"}],
+        "structuredContent": {"message": "Hello"}
+    });
+
+    // The deserialization itself should fail due to validation
+    let deserialized: Result<CallToolResult, _> = serde_json::from_value(invalid_json);
+    assert!(deserialized.is_err());
+}
+
+#[tokio::test]
+async fn test_structured_return_conversion() {
+    // Test that Json<T> converts to CallToolResult with structured_content
+    let calc_result = CalculationResult {
+        sum: 7,
+        product: 12,
+    };
+
+    let structured = Json(calc_result);
+    let result: Result<CallToolResult, rmcp::ErrorData> =
+        rmcp::handler::server::tool::IntoCallToolResult::into_call_tool_result(structured);
+
+    assert!(result.is_ok());
+    let call_result = result.unwrap();
+
+    assert!(call_result.content.is_none());
+    assert!(call_result.structured_content.is_some());
+
+    let structured_value = call_result.structured_content.unwrap();
+    assert_eq!(structured_value["sum"], 7);
+    assert_eq!(structured_value["product"], 12);
+}
+
+#[tokio::test]
+async fn test_tool_serialization_with_output_schema() {
+    let server = TestServer::new();
+    let tools = server.tool_router.list_all();
+
+    let calculate_tool = tools.iter().find(|t| t.name == "calculate").unwrap();
+
+    // Serialize the tool
+    let serialized = serde_json::to_value(calculate_tool).unwrap();
+
+    // Check that outputSchema is included
+    assert!(serialized["outputSchema"].is_object());
+
+    // Deserialize back
+    let deserialized: Tool = serde_json::from_value(serialized).unwrap();
+    assert!(deserialized.output_schema.is_some());
+}
+
+#[tokio::test]
+async fn test_output_schema_requires_structured_content() {
+    // Test that tools with output_schema must use structured_content
+    let server = TestServer::new();
+    let tools = server.tool_router.list_all();
+
+    // The calculate tool should have output_schema
+    let calculate_tool = tools.iter().find(|t| t.name == "calculate").unwrap();
+    assert!(calculate_tool.output_schema.is_some());
+
+    // Directly call the tool and verify its result structure
+    let params = rmcp::handler::server::tool::Parameters(CalculationRequest { a: 5, b: 3 });
+    let result = server.calculate(params).await.unwrap();
+
+    // Convert the Json<CalculationResult> to CallToolResult
+    let call_result: Result<CallToolResult, rmcp::ErrorData> =
+        rmcp::handler::server::tool::IntoCallToolResult::into_call_tool_result(result);
+
+    assert!(call_result.is_ok());
+    let call_result = call_result.unwrap();
+
+    // Verify it has structured_content and no content
+    assert!(call_result.structured_content.is_some());
+    assert!(call_result.content.is_none());
+}

crates/rmcp/tests/test_tool_builder_methods.rs

@@ -0,0 +1,62 @@
+//cargo test --test test_tool_builder_methods --features "client server macros"
+use rmcp::model::{JsonObject, Tool};
+use schemars::JsonSchema;
+use serde::{Deserialize, Serialize};
+
+#[derive(Serialize, Deserialize, JsonSchema)]
+pub struct InputData {
+    pub name: String,
+    pub age: u32,
+}
+
+#[derive(Serialize, Deserialize, JsonSchema)]
+pub struct OutputData {
+    pub greeting: String,
+    pub is_adult: bool,
+}
+
+#[test]
+fn test_with_output_schema() {
+    let tool = Tool::new("test", "Test tool", JsonObject::new()).with_output_schema::<OutputData>();
+
+    assert!(tool.output_schema.is_some());
+
+    // Verify the schema contains expected fields
+    let schema_str = serde_json::to_string(tool.output_schema.as_ref().unwrap()).unwrap();
+    assert!(schema_str.contains("greeting"));
+    assert!(schema_str.contains("is_adult"));
+}
+
+#[test]
+fn test_with_input_schema() {
+    let tool = Tool::new("test", "Test tool", JsonObject::new()).with_input_schema::<InputData>();
+
+    // Verify the schema contains expected fields
+    let schema_str = serde_json::to_string(&tool.input_schema).unwrap();
+    assert!(schema_str.contains("name"));
+    assert!(schema_str.contains("age"));
+}
+
+#[test]
+fn test_chained_builder_methods() {
+    let tool = Tool::new("test", "Test tool", JsonObject::new())
+        .with_input_schema::<InputData>()
+        .with_output_schema::<OutputData>()
+        .annotate(rmcp::model::ToolAnnotations::new().read_only(true));
+
+    assert!(tool.output_schema.is_some());
+    assert!(tool.annotations.is_some());
+    assert_eq!(
+        tool.annotations.as_ref().unwrap().read_only_hint,
+        Some(true)
+    );
+
+    // Verify both schemas are set correctly
+    let input_schema_str = serde_json::to_string(&tool.input_schema).unwrap();
+    assert!(input_schema_str.contains("name"));
+    assert!(input_schema_str.contains("age"));
+
+    let output_schema_str = serde_json::to_string(tool.output_schema.as_ref().unwrap()).unwrap();
+    assert!(output_schema_str.contains("greeting"));
+    assert!(output_schema_str.contains("is_adult"));
+}

crates/rmcp/tests/test_tool_macros.rs

@@ -301,7 +301,8 @@ async fn test_optional_i64_field_with_null_input() -> anyhow::Result<()> {
 
     let result_text = result
         .content
-        .first()
+        .as_ref()
+        .and_then(|contents| contents.first())
         .and_then(|content| content.raw.as_text())
         .map(|text| text.text.as_str())
         .expect("Expected text content");
@@ -329,7 +330,8 @@ async fn test_optional_i64_field_with_null_input() -> anyhow::Result<()> {
 
     let some_result_text = some_result
         .content
-        .first()
+        .as_ref()
+        .and_then(|contents| contents.first())
         .and_then(|content| content.raw.as_text())
         .map(|text| text.text.as_str())
         .expect("Expected text content");