feat: add opt-in tool output validation

lib/mcp/configuration.rb

@@ -16,7 +16,7 @@ module MCP
     attr_writer :instrumentation_callback
 
     def initialize(exception_reporter: nil, around_request: nil, instrumentation_callback: nil, protocol_version: nil,
-      validate_tool_call_arguments: true)
+      validate_tool_call_arguments: true, validate_tool_call_results: false)
       @exception_reporter = exception_reporter
       @around_request = around_request
       @instrumentation_callback = instrumentation_callback
@@ -25,8 +25,10 @@ module MCP
         validate_protocol_version!(protocol_version)
       end
       validate_value_of_validate_tool_call_arguments!(validate_tool_call_arguments)
+      validate_value_of_validate_tool_call_results!(validate_tool_call_results)
 
       @validate_tool_call_arguments = validate_tool_call_arguments
+      @validate_tool_call_results = validate_tool_call_results
     end
 
     def protocol_version=(protocol_version)
@@ -41,6 +43,12 @@ module MCP
       @validate_tool_call_arguments = validate_tool_call_arguments
     end
 
+    def validate_tool_call_results=(validate_tool_call_results)
+      validate_value_of_validate_tool_call_results!(validate_tool_call_results)
+
+      @validate_tool_call_results = validate_tool_call_results
+    end
+
     def protocol_version
       @protocol_version || LATEST_STABLE_PROTOCOL_VERSION
     end
@@ -80,11 +88,16 @@ module MCP
     end
 
     attr_reader :validate_tool_call_arguments
+    attr_reader :validate_tool_call_results
 
     def validate_tool_call_arguments?
       !!@validate_tool_call_arguments
     end
 
+    def validate_tool_call_results?
+      !!@validate_tool_call_results
+    end
+
     def merge(other)
       return self if other.nil?
 
@@ -113,6 +126,7 @@ module MCP
       end
 
       validate_tool_call_arguments = other.validate_tool_call_arguments
+      validate_tool_call_results = other.validate_tool_call_results
 
       Configuration.new(
         exception_reporter: exception_reporter,
@@ -120,6 +134,7 @@ module MCP
         instrumentation_callback: instrumentation_callback,
         protocol_version: protocol_version,
         validate_tool_call_arguments: validate_tool_call_arguments,
+        validate_tool_call_results: validate_tool_call_results,
       )
     end
 
@@ -138,6 +153,12 @@ module MCP
       end
     end
 
+    def validate_value_of_validate_tool_call_results!(validate_tool_call_results)
+      unless validate_tool_call_results.is_a?(TrueClass) || validate_tool_call_results.is_a?(FalseClass)
+        raise ArgumentError, "validate_tool_call_results must be a boolean"
+      end
+    end
+
     def default_exception_reporter
       @default_exception_reporter ||= ->(exception, server_context) {}
     end

lib/mcp/server.rb

@@ -586,9 +586,11 @@ module MCP
 
       progress_token = request.dig(:_meta, :progressToken)
 
-      call_tool_with_args(
+      result = call_tool_with_args(
         tool, arguments, server_context_with_meta(request), progress_token: progress_token, session: session, related_request_id: related_request_id, cancellation: cancellation
       )
+      validate_tool_call_result!(tool, result)
+      result
     rescue RequestHandlerError, CancelledError
       # CancelledError is intentionally not wrapped so `handle_request` can turn it into
       # `JsonRpcHandler::NO_RESPONSE` per the MCP cancellation spec.
@@ -745,6 +747,14 @@ module MCP
       ).to_h
     end
 
+    def validate_tool_call_result!(tool, result)
+      return unless configuration.validate_tool_call_results
+      return unless tool.output_schema
+      return if result[:isError]
+
+      tool.output_schema.validate_result(result[:structuredContent])
+    end
+
     # Whether a tool/prompt handler opts in to receiving an `MCP::ServerContext`.
     # Recognizes `:keyrest` (`**kwargs`) because tools are invoked without a positional argument
     # (`tool.call(**args, server_context:)`), soa `**kwargs`-only signature safely captures `server_context:`.