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

Author: JMLX42

crates/rmcp-macros/src/tool.rs

@@ -10,6 +10,8 @@ pub struct ToolAttribute {
     pub description: Option<String>,
     /// A JSON Schema object defining the expected parameters for the tool
     pub input_schema: Option<Expr>,
+    /// An optional JSON Schema object defining the structure of the tool's output
+    pub output_schema: Option<Expr>,
     /// Optional additional tool information.
     pub annotations: Option<ToolAnnotationsAttribute>,
 }
@@ -18,6 +20,7 @@ pub struct ResolvedToolAttribute {
     pub name: String,
     pub description: Option<String>,
     pub input_schema: Expr,
+    pub output_schema: Option<Expr>,
     pub annotations: Expr,
 }
 
@@ -27,19 +30,26 @@ impl ResolvedToolAttribute {
             name,
             description,
             input_schema,
+            output_schema,
             annotations,
         } = self;
         let description = if let Some(description) = description {
             quote! { Some(#description.into()) }
         } else {
             quote! { None }
         };
+        let output_schema = if let Some(output_schema) = output_schema {
+            quote! { Some(#output_schema) }
+        } else {
+            quote! { None }
+        };
         let tokens = quote! {
             pub fn #fn_ident() -> rmcp::model::Tool {
                 rmcp::model::Tool {
                     name: #name.into(),
                     description: #description,
                     input_schema: #input_schema,
+                    output_schema: #output_schema,
                     annotations: #annotations,
                 }
             }
@@ -89,6 +99,63 @@ fn none_expr() -> Expr {
     syn::parse2::<Expr>(quote! { None }).unwrap()
 }
 
+/// Check if a type is Json<T> and extract the inner type T
+fn extract_json_inner_type(ty: &syn::Type) -> Option<&syn::Type> {
+    if let syn::Type::Path(type_path) = ty {
+        if let Some(last_segment) = type_path.path.segments.last() {
+            if last_segment.ident == "Json" {
+                if let syn::PathArguments::AngleBracketed(args) = &last_segment.arguments {
+                    if let Some(syn::GenericArgument::Type(inner_type)) = args.args.first() {
+                        return Some(inner_type);
+                    }
+                }
+            }
+        }
+    }
+    None
+}
+
+/// Extract schema expression from a function's return type
+/// Handles patterns like Json<T> and Result<Json<T>, E>
+fn extract_schema_from_return_type(ret_type: &syn::Type) -> Option<Expr> {
+    // First, try direct Json<T>
+    if let Some(inner_type) = extract_json_inner_type(ret_type) {
+        return syn::parse2::<Expr>(quote! {
+            rmcp::handler::server::tool::cached_schema_for_type::<#inner_type>()
+        })
+        .ok();
+    }
+
+    // Then, try Result<Json<T>, E>
+    let type_path = match ret_type {
+        syn::Type::Path(path) => path,
+        _ => return None,
+    };
+
+    let last_segment = type_path.path.segments.last()?;
+
+    if last_segment.ident != "Result" {
+        return None;
+    }
+
+    let args = match &last_segment.arguments {
+        syn::PathArguments::AngleBracketed(args) => args,
+        _ => return None,
+    };
+
+    let ok_type = match args.args.first()? {
+        syn::GenericArgument::Type(ty) => ty,
+        _ => return None,
+    };
+
+    let inner_type = extract_json_inner_type(ok_type)?;
+
+    syn::parse2::<Expr>(quote! {
+        rmcp::handler::server::tool::cached_schema_for_type::<#inner_type>()
+    })
+    .ok()
+}
+
 // extract doc line from attribute
 fn extract_doc_line(existing_docs: Option<String>, attr: &syn::Attribute) -> Option<String> {
     if !attr.path().is_ident("doc") {
@@ -192,12 +259,22 @@ pub fn tool(attr: TokenStream, input: TokenStream) -> syn::Result<TokenStream> {
     } else {
         none_expr()
     };
+    // Handle output_schema - either explicit or generated from return type
+    let output_schema_expr = attribute.output_schema.or_else(|| {
+        // Try to generate schema from return type
+        match &fn_item.sig.output {
+            syn::ReturnType::Type(_, ret_type) => extract_schema_from_return_type(ret_type),
+            _ => None,
+        }
+    });
+
     let resolved_tool_attr = ResolvedToolAttribute {
         name: attribute.name.unwrap_or_else(|| fn_ident.to_string()),
         description: attribute
             .description
             .or_else(|| fn_item.attrs.iter().fold(None, extract_doc_line)),
         input_schema: input_schema_expr,
+        output_schema: output_schema_expr,
         annotations: annotations_expr,
     };
     let tool_attr_fn = resolved_tool_attr.into_fn(tool_attr_fn_ident)?;

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

@@ -6,6 +6,7 @@ use schemars::JsonSchema;
 use crate::{
     handler::server::tool::{
         CallToolHandler, DynCallToolHandler, ToolCallContext, schema_for_type,
+        validate_against_schema,
     },
     model::{CallToolResult, Tool, ToolAnnotations},
 };
@@ -242,7 +243,23 @@ where
             .map
             .get(context.name())
             .ok_or_else(|| crate::ErrorData::invalid_params("tool not found", None))?;
-        (item.call)(context).await
+
+        let result = (item.call)(context).await?;
+
+        // Validate structured content against output schema if present
+        if let Some(ref output_schema) = item.attr.output_schema {
+            // When output_schema is defined, structured_content is required
+            if result.structured_content.is_none() {
+                return Err(crate::ErrorData::invalid_params(
+                    "Tool with output_schema must return structured_content",
+                    None,
+                ));
+            }
+            // Validate the structured content against the schema
+            validate_against_schema(result.structured_content.as_ref().unwrap(), output_schema)?;
+        }
+
+        Ok(result)
     }
 
     pub fn list_all(&self) -> Vec<crate::model::Tool> {

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

@@ -1,3 +1,39 @@
+//! Tool handler traits and types for MCP servers.
+//!
+//! This module provides the infrastructure for implementing tools that can be called
+//! by MCP clients. Tools can return either unstructured content (text, images) or
+//! structured JSON data with schemas.
+//!
+//! # Structured Output
+//!
+//! Tools can return structured JSON data using the [`Json`] wrapper type.
+//! When using `Json<T>`, the framework will:
+//! - Automatically generate a JSON schema for the output type
+//! - Validate the output against the schema
+//! - Return the data in the `structured_content` field of [`CallToolResult`]
+//!
+//! # Example
+//!
+//! ```rust,ignore
+//! use rmcp::{tool, Json};
+//! use schemars::JsonSchema;
+//! use serde::{Serialize, Deserialize};
+//!
+//! #[derive(Serialize, Deserialize, JsonSchema)]
+//! struct AnalysisResult {
+//!     score: f64,
+//!     summary: String,
+//! }
+//!
+//! #[tool(name = "analyze")]
+//! async fn analyze(&self, text: String) -> Result<Json<AnalysisResult>, String> {
+//!     Ok(Json(AnalysisResult {
+//!         score: 0.95,
+//!         summary: "Positive sentiment".to_string(),
+//!     }))
+//! }
+//! ```
+
 use std::{
     any::TypeId, borrow::Cow, collections::HashMap, future::Ready, marker::PhantomData, sync::Arc,
 };
@@ -10,6 +46,7 @@ use tokio_util::sync::CancellationToken;
 pub use super::router::tool::{ToolRoute, ToolRouter};
 use crate::{
     RoleServer,
+    handler::server::wrapper::Json,
     model::{CallToolRequestParam, CallToolResult, IntoContents, JsonObject},
     schemars::generate::SchemaSettings,
     service::RequestContext,
@@ -30,6 +67,43 @@ pub fn schema_for_type<T: JsonSchema>() -> JsonObject {
     }
 }
 
+/// Validate that a JSON value conforms to basic type constraints from a schema.
+///
+/// Note: This is a basic validation that only checks type compatibility.
+/// For full JSON Schema validation, a dedicated validation library would be needed.
+pub fn validate_against_schema(
+    value: &serde_json::Value,
+    schema: &JsonObject,
+) -> Result<(), crate::ErrorData> {
+    // Basic type validation
+    if let Some(schema_type) = schema.get("type").and_then(|t| t.as_str()) {
+        let value_type = get_json_value_type(value);
+
+        if schema_type != value_type {
+            return Err(crate::ErrorData::invalid_params(
+                format!(
+                    "Value type does not match schema. Expected '{}', got '{}'",
+                    schema_type, value_type
+                ),
+                None,
+            ));
+        }
+    }
+
+    Ok(())
+}
+
+fn get_json_value_type(value: &serde_json::Value) -> &'static str {
+    match value {
+        serde_json::Value::Null => "null",
+        serde_json::Value::Bool(_) => "boolean",
+        serde_json::Value::Number(_) => "number",
+        serde_json::Value::String(_) => "string",
+        serde_json::Value::Array(_) => "array",
+        serde_json::Value::Object(_) => "object",
+    }
+}
+
 /// Call [`schema_for_type`] with a cache
 pub fn cached_schema_for_type<T: JsonSchema + std::any::Any>() -> Arc<JsonObject> {
     thread_local! {
@@ -97,8 +171,26 @@ pub trait FromToolCallContextPart<S>: Sized {
     ) -> Result<Self, crate::ErrorData>;
 }
 
+/// Trait for converting tool return values into [`CallToolResult`].
+///
+/// This trait is automatically implemented for:
+/// - Types implementing [`IntoContents`] (returns unstructured content)
+/// - `Result<T, E>` where both `T` and `E` implement [`IntoContents`]
+/// - [`Json<T>`](crate::handler::server::wrapper::Json) where `T` implements [`Serialize`] (returns structured content)
+/// - `Result<Json<T>, E>` for structured results with errors
+///
+/// The `#[tool]` macro uses this trait to convert tool function return values
+/// into the appropriate [`CallToolResult`] format.
 pub trait IntoCallToolResult {
     fn into_call_tool_result(self) -> Result<CallToolResult, crate::ErrorData>;
+
+    /// Returns the output schema for this type, if any.
+    ///
+    /// This is used by the macro to automatically generate output schemas
+    /// for tool functions that return structured data.
+    fn output_schema() -> Option<Arc<JsonObject>> {
+        None
+    }
 }
 
 impl<T: IntoContents> IntoCallToolResult for T {
@@ -125,6 +217,40 @@ impl<T: IntoCallToolResult> IntoCallToolResult for Result<T, crate::ErrorData> {
     }
 }
 
+// Implementation for Json<T> to create structured content
+impl<T: Serialize + JsonSchema + 'static> IntoCallToolResult for Json<T> {
+    fn into_call_tool_result(self) -> Result<CallToolResult, crate::ErrorData> {
+        let value = serde_json::to_value(self.0).map_err(|e| {
+            crate::ErrorData::internal_error(
+                format!("Failed to serialize structured content: {}", e),
+                None,
+            )
+        })?;
+
+        Ok(CallToolResult::structured(value))
+    }
+
+    fn output_schema() -> Option<Arc<JsonObject>> {
+        Some(cached_schema_for_type::<T>())
+    }
+}
+
+// Implementation for Result<Json<T>, E>
+impl<T: Serialize + JsonSchema + 'static, E: IntoContents> IntoCallToolResult
+    for Result<Json<T>, E>
+{
+    fn into_call_tool_result(self) -> Result<CallToolResult, crate::ErrorData> {
+        match self {
+            Ok(value) => value.into_call_tool_result(),
+            Err(error) => Ok(CallToolResult::error(error.into_contents())),
+        }
+    }
+
+    fn output_schema() -> Option<Arc<JsonObject>> {
+        Json::<T>::output_schema()
+    }
+}
+
 pin_project_lite::pin_project! {
     #[project = IntoCallToolResultFutProj]
     pub enum IntoCallToolResultFut<F, R> {

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

@@ -1,28 +1,22 @@
-use serde::Serialize;
+use std::borrow::Cow;
 
-use crate::model::IntoContents;
+use schemars::JsonSchema;
 
-/// Json wrapper
+/// Json wrapper for structured output
 ///
-/// This is used to tell the SDK to serialize the inner value into json
+/// When used with tools, this wrapper indicates that the value should be
+/// serialized as structured JSON content with an associated schema.
+/// The framework will place the JSON in the `structured_content` field
+/// of the tool result rather than the regular `content` field.
 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![]
-            }
-        }
+// Implement JsonSchema for Json<T> to delegate to T's schema
+impl<T: JsonSchema> JsonSchema for Json<T> {
+    fn schema_name() -> Cow<'static, str> {
+        T::schema_name()
+    }
+
+    fn json_schema(generator: &mut schemars::SchemaGenerator) -> schemars::Schema {
+        T::json_schema(generator)
     }
 }