Merge pull request #433 from koic/mrtr_result_type

Recognize multi round-trip `input_required` results per SEP-2322

lib/mcp.rb

@@ -17,6 +17,7 @@ module MCP
   autoload :Prompt, "mcp/prompt"
   autoload :Resource, "mcp/resource"
   autoload :ResourceTemplate, "mcp/resource_template"
+  autoload :ResultType, "mcp/result_type"
   autoload :Server, "mcp/server"
   autoload :ServerSession, "mcp/server_session"
   autoload :Tool, "mcp/tool"

lib/mcp/client.rb

@@ -5,6 +5,7 @@ require_relative "client/stdio"
 require_relative "client/http"
 require_relative "client/paginated_result"
 require_relative "client/tool"
+require_relative "result_type"
 
 module MCP
   class Client
@@ -34,6 +35,25 @@ module MCP
     # server-returned JSON-RPC error, which is raised as `ServerError`.
     class ValidationError < StandardError; end
 
+    # Raised when a server answers with a SEP-2322 Multi Round-Trip `input_required` result instead of
+    # a final result. The result is not an error on the wire: it asks the client to fulfill the server's
+    # `inputRequests` (a map of id => `{ "method" => ..., "params" => ... }` request objects with
+    # `sampling/createMessage`, `roots/list`, or `elicitation/create` shapes) and re-issue
+    # the original request with `inputResponses` plus the echoed opaque `requestState`.
+    # This SDK does not yet drive that resume loop automatically; callers can inspect `input_requests`
+    # and respond manually.
+    # https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2322
+    class InputRequiredError < StandardError
+      attr_reader :input_requests, :request_state, :result
+
+      def initialize(message, input_requests:, request_state: nil, result: nil)
+        super(message)
+        @input_requests = input_requests
+        @request_state = request_state
+        @result = result
+      end
+    end
+
     # Raised when the server responds 404 to a request containing a session ID,
     # indicating the session has expired. Inherits from `RequestHandlerError` for
     # backward compatibility with callers that rescue the generic error. Per spec,
@@ -434,9 +454,26 @@ module MCP
         raise ServerError.new(error["message"], code: error["code"], data: error["data"])
       end
 
+      raise_on_input_required(response)
+
       response
     end
 
+    # Recognizes a SEP-2322 `input_required` result and raises rather than returning it as if it were a final result.
+    # Servers on stable protocol versions never emit `resultType`, so this is a no-op for them.
+    def raise_on_input_required(response)
+      result = response.is_a?(Hash) ? response["result"] : nil
+      return unless result.is_a?(Hash) && result["resultType"] == ResultType::INPUT_REQUIRED
+
+      raise InputRequiredError.new(
+        "Server returned `input_required`; this SDK does not yet resume multi-round-trip requests (SEP-2322). " \
+          "Inspect `input_requests` to respond manually.",
+        input_requests: result["inputRequests"] || {},
+        request_state: result["requestState"],
+        result: result,
+      )
+    end
+
     # Generates a fresh JSON-RPC request id for an outgoing request.
     # Ids are an internal concern: the public API never accepts or exposes them, and cancellation is driven through
     # an `MCP::Cancellation` token instead.

lib/mcp/result_type.rb

@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+module MCP
+  # Values of the `resultType` result field introduced by SEP-2322 (Multi Round-Trip Requests)
+  # for the MCP 2026-07-28 draft.
+  #
+  # A result with `resultType: "input_required"` is not a final answer: it carries an `inputRequests` map
+  # of server-to-client requests (`sampling/createMessage`, `roots/list`, `elicitation/create` shapes) plus
+  # an opaque `requestState` string, and the client is expected to fulfill the requests and re-issue
+  # the original request with `inputResponses` and the echoed `requestState`. A missing `resultType`
+  # or `"complete"` is a final result.
+  #
+  # https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2322
+  module ResultType
+    COMPLETE = "complete"
+    INPUT_REQUIRED = "input_required"
+  end
+end