Support elicitation/create per MCP specification

## Motivation and Context

The MCP specification (2025-11-25) defines elicitation as a server-to-client
request mechanism that allows servers to request user input during tool execution:
https://modelcontextprotocol.io/specification/2025-11-25/client/elicitation

This aligns with the Python SDK, which also exposes separate methods for form and URL mode elicitation.
Three methods are added following the Python SDK's API design:

- `create_form_elicitation`: collects structured data via JSON Schema
- `create_url_elicitation`: directs users to external URLs for sensitive
  interactions (OAuth, credentials, etc.)
- `notify_elicitation_complete`: notifies the client that a URL mode
  interaction has completed

`URLElicitationRequiredError` (error code `-32042`) is defined as a subclass of `RequestHandlerError`
with its own `error_code` and `error_data` attributes. `JsonRpcHandler#handle_request_error` is
extended to check for these attributes so that MCP-specific error codes can be propagated without
embedding MCP knowledge in the generic JSON-RPC layer.

## How Has This Been Tested?

- Unit tests for `ServerSession` form/URL mode, capability checks, nil
  capabilities, notification, and `URLElicitationRequiredError`
- `ServerContext` delegation tests for all three elicitation methods
- `JsonRpcHandler` tests for custom error code and fallback behavior
- `Methods` capability test for `NOTIFICATIONS_ELICITATION_COMPLETE`
- Conformance tests pass (40/40), including `tools-call-elicitation`,
  `elicitation-sep1034-defaults`, and `elicitation-sep1330-enums`
- All tests pass (`rake test`), RuboCop is clean
- Verified form mode elicitation end-to-end with a sample MCP server
  running in Claude Code

## Breaking Changes

None. This is a new feature addition.

lib/json_rpc_handler.rb

@@ -117,20 +117,27 @@ module JsonRpcHandler
   end
 
   def handle_request_error(error, id, id_validation_pattern)
-    error_type = error.respond_to?(:error_type) ? error.error_type : nil
+    if error.respond_to?(:error_code) && error.error_code
+      code = error.error_code
+      message = error.message
+    else
+      error_type = error.respond_to?(:error_type) ? error.error_type : nil
 
-    code, message = case error_type
-    when :invalid_request then [ErrorCode::INVALID_REQUEST, "Invalid Request"]
-    when :invalid_params then [ErrorCode::INVALID_PARAMS, "Invalid params"]
-    when :parse_error then [ErrorCode::PARSE_ERROR, "Parse error"]
-    when :internal_error then [ErrorCode::INTERNAL_ERROR, "Internal error"]
-    else [ErrorCode::INTERNAL_ERROR, "Internal error"]
+      code, message = case error_type
+      when :invalid_request then [ErrorCode::INVALID_REQUEST, "Invalid Request"]
+      when :invalid_params then [ErrorCode::INVALID_PARAMS, "Invalid params"]
+      when :parse_error then [ErrorCode::PARSE_ERROR, "Parse error"]
+      when :internal_error then [ErrorCode::INTERNAL_ERROR, "Internal error"]
+      else [ErrorCode::INTERNAL_ERROR, "Internal error"]
+      end
     end
 
+    data = error.respond_to?(:error_data) && error.error_data ? error.error_data : error.message
+
     error_response(id: id, id_validation_pattern: id_validation_pattern, error: {
       code: code,
       message: message,
-      data: error.message,
+      data: data,
     })
   end
 

lib/mcp/methods.rb

@@ -33,6 +33,7 @@ module MCP
     NOTIFICATIONS_MESSAGE = "notifications/message"
     NOTIFICATIONS_PROGRESS = "notifications/progress"
     NOTIFICATIONS_CANCELLED = "notifications/cancelled"
+    NOTIFICATIONS_ELICITATION_COMPLETE = "notifications/elicitation/complete"
 
     class MissingRequiredCapabilityError < StandardError
       attr_reader :method
@@ -79,8 +80,8 @@ module MCP
           require_capability!(method, capabilities, :sampling)
         when ELICITATION_CREATE
           require_capability!(method, capabilities, :elicitation)
-        when INITIALIZE, PING, NOTIFICATIONS_INITIALIZED, NOTIFICATIONS_PROGRESS, NOTIFICATIONS_CANCELLED
-          # No specific capability required for initialize, ping, progress or cancelled
+        when INITIALIZE, PING, NOTIFICATIONS_INITIALIZED, NOTIFICATIONS_PROGRESS, NOTIFICATIONS_CANCELLED, NOTIFICATIONS_ELICITATION_COMPLETE
+          # No specific capability required.
         end
       end
 

lib/mcp/server.rb

@@ -31,14 +31,27 @@ module MCP
     MAX_COMPLETION_VALUES = 100
 
     class RequestHandlerError < StandardError
-      attr_reader :error_type
-      attr_reader :original_error
+      attr_reader :error_type, :original_error, :error_code, :error_data
 
-      def initialize(message, request, error_type: :internal_error, original_error: nil)
+      def initialize(message, request, error_type: :internal_error, original_error: nil, error_code: nil, error_data: nil)
         super(message)
         @request = request
         @error_type = error_type
         @original_error = original_error
+        @error_code = error_code
+        @error_data = error_data
+      end
+    end
+
+    class URLElicitationRequiredError < RequestHandlerError
+      def initialize(elicitations)
+        super(
+          "URL elicitation required",
+          nil,
+          error_type: :url_elicitation_required,
+          error_code: -32042,
+          error_data: { elicitations: elicitations },
+        )
       end
     end
 
@@ -114,7 +127,6 @@ module MCP
         # No op handlers for currently unsupported methods
         Methods::RESOURCES_SUBSCRIBE => ->(_) { {} },
         Methods::RESOURCES_UNSUBSCRIBE => ->(_) { {} },
-        Methods::ELICITATION_CREATE => ->(_) {},
       }
       @transport = transport
     end

lib/mcp/server_context.rb

@@ -43,6 +43,42 @@ module MCP
       end
     end
 
+    # Delegates to the session so the request is scoped to the originating client.
+    # Falls back to `@context` (via `method_missing`) when `@notification_target`
+    # does not support elicitation.
+    def create_form_elicitation(**kwargs)
+      if @notification_target.respond_to?(:create_form_elicitation)
+        @notification_target.create_form_elicitation(**kwargs, related_request_id: @related_request_id)
+      elsif @context.respond_to?(:create_form_elicitation)
+        @context.create_form_elicitation(**kwargs, related_request_id: @related_request_id)
+      else
+        raise NoMethodError, "undefined method 'create_form_elicitation' for #{self}"
+      end
+    end
+
+    # Delegates to the session so the request is scoped to the originating client.
+    # Falls back to `@context` when `@notification_target` does not support URL mode elicitation.
+    def create_url_elicitation(**kwargs)
+      if @notification_target.respond_to?(:create_url_elicitation)
+        @notification_target.create_url_elicitation(**kwargs, related_request_id: @related_request_id)
+      elsif @context.respond_to?(:create_url_elicitation)
+        @context.create_url_elicitation(**kwargs, related_request_id: @related_request_id)
+      else
+        raise NoMethodError, "undefined method 'create_url_elicitation' for #{self}"
+      end
+    end
+
+    # Delegates to the session so the notification is scoped to the originating client.
+    def notify_elicitation_complete(**kwargs)
+      if @notification_target.respond_to?(:notify_elicitation_complete)
+        @notification_target.notify_elicitation_complete(**kwargs)
+      elsif @context.respond_to?(:notify_elicitation_complete)
+        @context.notify_elicitation_complete(**kwargs)
+      else
+        raise NoMethodError, "undefined method 'notify_elicitation_complete' for #{self}"
+      end
+    end
+
     def method_missing(name, ...)
       if @context.respond_to?(name)
         @context.public_send(name, ...)

lib/mcp/server_session.rb

@@ -47,6 +47,35 @@ module MCP
       send_to_transport_request(Methods::SAMPLING_CREATE_MESSAGE, params, related_request_id: related_request_id)
     end
 
+    # Sends an `elicitation/create` request (form mode) scoped to this session.
+    def create_form_elicitation(message:, requested_schema:, related_request_id: nil)
+      unless client_capabilities&.dig(:elicitation)
+        raise "Client does not support elicitation. " \
+          "The client must declare the `elicitation` capability during initialization."
+      end
+
+      params = { mode: "form", message: message, requestedSchema: requested_schema }
+      send_to_transport_request(Methods::ELICITATION_CREATE, params, related_request_id: related_request_id)
+    end
+
+    # Sends an `elicitation/create` request (URL mode) scoped to this session.
+    def create_url_elicitation(message:, url:, elicitation_id:, related_request_id: nil)
+      unless client_capabilities&.dig(:elicitation, :url)
+        raise "Client does not support URL mode elicitation. " \
+          "The client must declare the `elicitation.url` capability during initialization."
+      end
+
+      params = { mode: "url", message: message, url: url, elicitationId: elicitation_id }
+      send_to_transport_request(Methods::ELICITATION_CREATE, params, related_request_id: related_request_id)
+    end
+
+    # Sends an elicitation complete notification scoped to this session.
+    def notify_elicitation_complete(elicitation_id:)
+      send_to_transport(Methods::NOTIFICATIONS_ELICITATION_COMPLETE, { elicitationId: elicitation_id })
+    rescue => e
+      @server.report_exception(e, notification: "elicitation_complete")
+    end
+
     # Sends a progress notification to this session only.
     def notify_progress(progress_token:, progress:, total: nil, message: nil, related_request_id: nil)
       params = {