Recognize multi round-trip input_required results per SEP-2322

## Motivation and Context

SEP-2322 (modelcontextprotocol/modelcontextprotocol#2322, merged for the 2026-07-28 spec release)
introduces Multi Round-Trip Requests: instead of issuing in-flight server-to-client JSON-RPC requests,
a server answers with a result whose `resultType` is `"input_required"`, carrying an `inputRequests` map
(of `sampling/createMessage`, `roots/list`, and `elicitation/create` request shapes) and
an opaque `requestState`; the client fulfills the requests and re-issues the original request with
`inputResponses` and the echoed `requestState`.

The wire contract (the `resultType` discriminator and the `inputRequests`/`requestState` shape)
stayed stable across all three closed TypeScript prototype iterations (typescript-sdk#2062/#2065,
the v2-stateless stack, and #2251) and the Python draft (python-sdk#2322), but the server-side
suspend/resume mechanism is still unsettled in both SDKs (typescript-sdk#2251 was put on hold on 2026-06-08).
This change therefore implements only the stable, additive vocabulary and the client-side recognition,
leaving server emission and automatic resumption for a follow-up once the reference design lands:

- New `MCP::ResultType` module with `COMPLETE` and `INPUT_REQUIRED` constants documenting the `resultType` values.
- `MCP::Client` raises the new `MCP::Client::InputRequiredError` (exposing `input_requests`, `request_state`,
  and the raw `result`) when any response carries `resultType: "input_required"`, instead of silently returning
  a non-final result as if it were the answer. The check lives in the shared request path,
  so every client method is covered. Servers on stable protocol versions never emit `resultType`,
  so default behavior is unchanged.

Part of #382.

## How Has This Been Tested?

New tests in `test/mcp/client_test.rb`:

- `call_tool` raises `InputRequiredError` for an `input_required` result and exposes `input_requests`,
  `request_state`, and the full raw result
- `call_tool` returns normally when `resultType` is `"complete"` and when it is absent
  (wire-compat regression for stable-protocol servers)
- `list_tools` also raises for `input_required` results, proving the recognition covers the shared request path

`bundle exec rake` (tests, RuboCop, and conformance baseline) passes.

## Breaking Changes

None for spec-compliant stable servers, which never send `resultType`. A response that does carry
`resultType: "input_required"` now raises `MCP::Client::InputRequiredError` instead of being returned as
a final result, which was always a misinterpretation of the draft semantics.

README.md

@@ -2232,6 +2232,16 @@ The client provides a wrapper class for tools returned by the server:
 
 This class provides easy access to tool properties like name, description, input schema, and output schema.
 
+### Multi-Round-Trip Results (Experimental, SEP-2322)
+
+The MCP 2026-07-28 draft replaces in-flight server-to-client requests with Multi Round-Trip Requests: instead of issuing `sampling/createMessage`, `roots/list`,
+or `elicitation/create` while a request is being processed, a server may answer with a result whose `resultType` is `"input_required"`, carrying an `inputRequests` map
+and an opaque `requestState`; the client fulfills the requests and re-issues the original request with `inputResponses` and the echoed `requestState`.
+
+The Ruby client recognizes such results and raises `MCP::Client::InputRequiredError` instead of returning them as if they were final. The error exposes `input_requests`, `request_state`,
+and the raw `result`; automatic resumption is not implemented yet, so callers respond manually if they opt into the draft flow. `MCP::ResultType::COMPLETE` and `MCP::ResultType::INPUT_REQUIRED`
+are provided for forward compatibility. Servers on stable protocol versions never send `resultType`, so existing behavior is unchanged.
+
 ## Conformance Testing
 
 The `conformance/` directory contains a test server and runner that validate the SDK against the MCP specification using [`@modelcontextprotocol/conformance`](https://github.com/modelcontextprotocol/conformance).

lib/mcp.rb

@@ -16,6 +16,7 @@ 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,
@@ -422,9 +442,26 @@ 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

test/mcp/client_test.rb

@@ -164,6 +164,83 @@ module MCP
       assert_equal([{ type: "text", text: "Hello, world!" }], content)
     end
 
+    def test_call_tool_raises_input_required_error_when_result_type_is_input_required
+      # Per SEP-2322, a result with `resultType: "input_required"` is not a final result;
+      # surface it instead of returning it as a normal result.
+      transport = mock
+      tool = MCP::Client::Tool.new(name: "tool1", description: "tool1", input_schema: {})
+      mock_response = {
+        "result" => {
+          "resultType" => "input_required",
+          "inputRequests" => {
+            "1" => { "method" => "elicitation/create", "params" => { "mode" => "form", "message" => "Name?" } },
+          },
+          "requestState" => "opaque-state",
+        },
+      }
+
+      transport.expects(:send_request).returns(mock_response).once
+
+      client = Client.new(transport: transport)
+      error = assert_raises(Client::InputRequiredError) do
+        client.call_tool(tool: tool, arguments: {})
+      end
+
+      assert_equal(
+        { "1" => { "method" => "elicitation/create", "params" => { "mode" => "form", "message" => "Name?" } } },
+        error.input_requests,
+      )
+      assert_equal("opaque-state", error.request_state)
+      assert_equal(mock_response["result"], error.result)
+    end
+
+    def test_call_tool_returns_normally_when_result_type_is_complete
+      transport = mock
+      tool = MCP::Client::Tool.new(name: "tool1", description: "tool1", input_schema: {})
+      mock_response = {
+        "result" => {
+          "resultType" => "complete",
+          "content" => [{ "type" => "text", "text" => "done" }],
+        },
+      }
+
+      transport.expects(:send_request).returns(mock_response).once
+
+      client = Client.new(transport: transport)
+      result = client.call_tool(tool: tool, arguments: {})
+
+      assert_equal("done", result.dig("result", "content", 0, "text"))
+    end
+
+    def test_call_tool_returns_normally_when_result_type_is_absent
+      # Regression guard: stable-protocol servers never send `resultType`.
+      transport = mock
+      tool = MCP::Client::Tool.new(name: "tool1", description: "tool1", input_schema: {})
+      mock_response = {
+        "result" => { "content" => [{ "type" => "text", "text" => "done" }] },
+      }
+
+      transport.expects(:send_request).returns(mock_response).once
+
+      client = Client.new(transport: transport)
+      result = client.call_tool(tool: tool, arguments: {})
+
+      assert_equal("done", result.dig("result", "content", 0, "text"))
+    end
+
+    def test_list_tools_raises_input_required_error_for_input_required_results
+      # The recognition lives in the shared request path, so every client method is covered, not only tools/call.
+      transport = mock
+      mock_response = {
+        "result" => { "resultType" => "input_required", "inputRequests" => {} },
+      }
+
+      transport.expects(:send_request).returns(mock_response).once
+
+      client = Client.new(transport: transport)
+      assert_raises(Client::InputRequiredError) { client.list_tools }
+    end
+
     def test_call_tool_raises_when_no_name_or_tool
       client = Client.new(transport: mock)