Add notification support

Co-authored-by: Kevin Fischer <kfischer_okarin@yahoo.co.jp>

README.md

@@ -34,6 +34,7 @@ It implements the Model Context Protocol specification, handling model context r
 - Supports prompt registration and execution
 - Supports resource registration and retrieval
 - Supports stdio & Streamable HTTP (including SSE) transports
+- Supports notifications for list changes (tools, prompts, resources)
 
 ### Supported Methods
 - `initialize` - Initializes the protocol and returns server capabilities
@@ -46,13 +47,46 @@ It implements the Model Context Protocol specification, handling model context r
 - `resources/read` - Retrieves a specific resource by name
 - `resources/templates/list` - Lists all registered resource templates and their schemas
 
+### Notifications
+
+The server supports sending notifications to clients when lists of tools, prompts, or resources change. This enables real-time updates without polling.
+
+#### Notification Methods
+
+The server provides three notification methods:
+- `notify_tools_list_changed()` - Send a notification when the tools list changes
+- `notify_prompts_list_changed()` - Send a notification when the prompts list changes  
+- `notify_resources_list_changed()` - Send a notification when the resources list changes
+
+#### Notification Format
+
+Notifications follow the JSON-RPC 2.0 specification and use these method names:
+- `notifications/tools/list_changed`
+- `notifications/prompts/list_changed`
+- `notifications/resources/list_changed`
+
+#### Transport Support
+
+- **HTTP Transport**: Notifications are sent as Server-Sent Events (SSE) to all connected sessions
+- **Stdio Transport**: Notifications are sent as JSON-RPC 2.0 messages to stdout
+
+#### Usage Example
+
+```ruby
+server = MCP::Server.new(name: "my_server")
+transport = MCP::Transports::HTTP.new(server)
+server.transport = transport
+
+# When tools change, notify clients
+server.define_tool(name: "new_tool") { |**args| { result: "ok" } }
+server.notify_tools_list_changed()
+```
+
 ### Unsupported Features ( to be implemented in future versions )
 
-- Notifications
 - Log Level
 - Resource subscriptions
 - Completions
-- Complete Streamable HTTP implementation with streaming responses
 
 ### Usage
 

lib/mcp/methods.rb

@@ -21,6 +21,11 @@ module MCP
 
     SAMPLING_CREATE_MESSAGE = "sampling/createMessage"
 
+    # Notification methods
+    NOTIFICATIONS_TOOLS_LIST_CHANGED = "notifications/tools/list_changed"
+    NOTIFICATIONS_PROMPTS_LIST_CHANGED = "notifications/prompts/list_changed"
+    NOTIFICATIONS_RESOURCES_LIST_CHANGED = "notifications/resources/list_changed"
+
     class MissingRequiredCapabilityError < StandardError
       attr_reader :method
       attr_reader :capability

lib/mcp/server.rb

@@ -98,6 +98,30 @@ module MCP
       @prompts[prompt.name_value] = prompt
     end
 
+    def notify_tools_list_changed
+      return unless @transport
+
+      @transport.send_notification(Methods::NOTIFICATIONS_TOOLS_LIST_CHANGED)
+    rescue => e
+      report_exception(e, { notification: "tools_list_changed" })
+    end
+
+    def notify_prompts_list_changed
+      return unless @transport
+
+      @transport.send_notification(Methods::NOTIFICATIONS_PROMPTS_LIST_CHANGED)
+    rescue => e
+      report_exception(e, { notification: "prompts_list_changed" })
+    end
+
+    def notify_resources_list_changed
+      return unless @transport
+
+      @transport.send_notification(Methods::NOTIFICATIONS_RESOURCES_LIST_CHANGED)
+    rescue => e
+      report_exception(e, { notification: "resources_list_changed" })
+    end
+
     def resources_list_handler(&block)
       @capabilities.support_resources
       @handlers[Methods::RESOURCES_LIST] = block

lib/mcp/server/transports/stdio_transport.rb

@@ -43,10 +43,18 @@ module MCP
           $stdout.flush
         end
 
-        def send_response(message)
+        def send_notification(method, params = nil)
-          json_message = message.is_a?(String) ? message : JSON.generate(message)
+          notification = {
-          $stdout.puts(json_message)
+            jsonrpc: "2.0",
-          $stdout.flush
+            method: method,
+          }
+          notification[:params] = params if params
+
+          send_response(notification)
+          true
+        rescue => e
+          MCP.configuration.exception_reporter.call(e, { error: "Failed to send notification" })
+          false
         end
       end
     end

test/mcp/server/transports/stdio_notification_integration_test.rb

@@ -0,0 +1,249 @@
+# frozen_string_literal: true
+
+require "test_helper"
+require "json"
+require "mcp/server"
+require "mcp/server/transports/stdio_transport"
+
+module MCP
+  class Server
+    module Transports
+      class StdioNotificationIntegrationTest < ActiveSupport::TestCase
+        class MockIO
+          attr_reader :output
+
+          def initialize
+            @output = []
+            @closed = false
+          end
+
+          def puts(message)
+            @output << message
+          end
+
+          def write(message)
+            @output << message
+            message.length
+          end
+
+          def gets
+            nil # Simulate end of input
+          end
+
+          def set_encoding(encoding)
+            # Mock implementation
+          end
+
+          def flush
+            # Mock implementation
+          end
+
+          def close
+            @closed = true
+          end
+
+          def closed?
+            @closed
+          end
+        end
+
+        setup do
+          @original_stdout = $stdout
+          @original_stdin = $stdin
+
+          @mock_stdout = MockIO.new
+          @mock_stdin = MockIO.new
+
+          $stdout = @mock_stdout
+          $stdin = @mock_stdin
+
+          @server = Server.new(
+            name: "test_server",
+            tools: [],
+            prompts: [],
+            resources: [],
+          )
+          @transport = StdioTransport.new(@server)
+          @server.transport = @transport
+        end
+
+        teardown do
+          $stdout = @original_stdout
+          $stdin = @original_stdin
+        end
+
+        test "server notification methods send JSON-RPC notifications through StdioTransport" do
+          # Test tools notification
+          @server.notify_tools_list_changed
+
+          # Test prompts notification
+          @server.notify_prompts_list_changed
+
+          # Test resources notification
+          @server.notify_resources_list_changed
+
+          # Check the notifications were sent
+          assert_equal 3, @mock_stdout.output.size
+
+          # Parse and verify each notification
+          notifications = @mock_stdout.output.map { |msg| JSON.parse(msg) }
+
+          assert_equal "2.0", notifications[0]["jsonrpc"]
+          assert_equal Methods::NOTIFICATIONS_TOOLS_LIST_CHANGED, notifications[0]["method"]
+          assert_nil notifications[0]["params"]
+
+          assert_equal "2.0", notifications[1]["jsonrpc"]
+          assert_equal Methods::NOTIFICATIONS_PROMPTS_LIST_CHANGED, notifications[1]["method"]
+          assert_nil notifications[1]["params"]
+
+          assert_equal "2.0", notifications[2]["jsonrpc"]
+          assert_equal Methods::NOTIFICATIONS_RESOURCES_LIST_CHANGED, notifications[2]["method"]
+          assert_nil notifications[2]["params"]
+        end
+
+        test "notifications include params when provided" do
+          # Test the transport's send_notification directly with params
+          result = @transport.send_notification("test/notification", { data: "test_value" })
+
+          assert result
+          assert_equal 1, @mock_stdout.output.size
+
+          notification = JSON.parse(@mock_stdout.output.first)
+          assert_equal "2.0", notification["jsonrpc"]
+          assert_equal "test/notification", notification["method"]
+          assert_equal({ "data" => "test_value" }, notification["params"])
+        end
+
+        test "server continues to work when stdout is closed" do
+          # Close stdout
+          @mock_stdout.close
+
+          # Server notifications should not raise errors
+          assert_nothing_raised do
+            @server.notify_tools_list_changed
+            @server.notify_prompts_list_changed
+            @server.notify_resources_list_changed
+          end
+        end
+
+        test "notifications work with dynamic tool additions" do
+          # Define a new tool
+          @server.define_tool(
+            name: "dynamic_tool",
+            description: "A dynamically added tool",
+          ) do |**args|
+            { result: "success" }
+          end
+
+          # Clear previous output
+          @mock_stdout.output.clear
+
+          # Manually trigger notification
+          @server.notify_tools_list_changed
+
+          # Check the notification was sent
+          assert_equal 1, @mock_stdout.output.size
+
+          notification = JSON.parse(@mock_stdout.output.first)
+          assert_equal Methods::NOTIFICATIONS_TOOLS_LIST_CHANGED, notification["method"]
+
+          # Verify the tool was added to the server
+          assert @server.tools.key?("dynamic_tool")
+        end
+
+        test "notifications are properly formatted JSON-RPC 2.0 messages" do
+          # Send a notification
+          @server.notify_prompts_list_changed
+
+          # Verify format
+          assert_equal 1, @mock_stdout.output.size
+          output = @mock_stdout.output.first
+
+          # Should be valid JSON
+          notification = JSON.parse(output)
+
+          # Should have required JSON-RPC 2.0 fields
+          assert_equal "2.0", notification["jsonrpc"]
+          assert notification.key?("method")
+          refute notification.key?("id") # Notifications should not have an id
+
+          # Method should be the expected notification type
+          assert_equal Methods::NOTIFICATIONS_PROMPTS_LIST_CHANGED, notification["method"]
+        end
+
+        test "multiple notifications are sent as separate JSON messages" do
+          # Send multiple notifications rapidly
+          5.times do
+            @server.notify_tools_list_changed
+          end
+
+          # Each should be a separate JSON message
+          assert_equal 5, @mock_stdout.output.size
+
+          # All should be parseable as JSON
+          @mock_stdout.output.each do |msg|
+            notification = JSON.parse(msg)
+            assert_equal "2.0", notification["jsonrpc"]
+            assert_equal Methods::NOTIFICATIONS_TOOLS_LIST_CHANGED, notification["method"]
+          end
+        end
+
+        test "transport handles errors gracefully" do
+          # Create a stdout that raises errors
+          error_stdout = Class.new(MockIO) do
+            def puts(message)
+              raise IOError, "Simulated IO error"
+            end
+          end.new
+
+          $stdout = error_stdout
+
+          # Notification should return false but not raise
+          result = @transport.send_notification("test/notification")
+          refute result
+        end
+
+        test "server notification flow works end-to-end with StdioTransport" do
+          # This test verifies the complete integration from server to transport
+
+          # Start with no output
+          assert_empty @mock_stdout.output
+
+          # Add a prompt and notify
+          @server.define_prompt(
+            name: "test_prompt",
+            description: "Test prompt",
+          ) do |args, server_context:|
+            MCP::PromptResponse.new(messages: [{ role: "user", content: "Test" }])
+          end
+
+          # Manually trigger notification
+          @server.notify_prompts_list_changed
+
+          # Verify notification was sent
+          assert_equal 1, @mock_stdout.output.size
+          notification = JSON.parse(@mock_stdout.output.first)
+          assert_equal Methods::NOTIFICATIONS_PROMPTS_LIST_CHANGED, notification["method"]
+
+          # Add a resource and notify
+          @server.resources = [
+            MCP::Resource.new(
+              uri: "test://resource",
+              name: "Test Resource",
+              description: "A test resource",
+              mime_type: "text/plain",
+            ),
+          ]
+
+          # Manually trigger notification
+          @server.notify_resources_list_changed
+
+          # Verify both notifications were sent
+          assert_equal 2, @mock_stdout.output.size
+          second_notification = JSON.parse(@mock_stdout.output.last)
+          assert_equal Methods::NOTIFICATIONS_RESOURCES_LIST_CHANGED, second_notification["method"]
+        end
+      end
+    end
+  end
+end

test/mcp/server/transports/streamable_http_notification_integration_test.rb

@@ -0,0 +1,249 @@
+# frozen_string_literal: true
+
+require "test_helper"
+require "json"
+require "rack"
+require "mcp/server"
+require "mcp/server/transports/streamable_http_transport"
+
+module MCP
+  class Server
+    module Transports
+      class StreamableHTTPNotificationIntegrationTest < ActiveSupport::TestCase
+        setup do
+          @server = Server.new(
+            name: "test_server",
+            tools: [],
+            prompts: [],
+            resources: [],
+          )
+          @transport = StreamableHTTPTransport.new(@server)
+          @server.transport = @transport
+        end
+
+        test "server notification methods send SSE notifications through HTTP transport" do
+          # Initialize a session
+          init_request = create_rack_request(
+            "POST",
+            "/",
+            { "CONTENT_TYPE" => "application/json" },
+            { jsonrpc: "2.0", method: "initialize", id: "init" }.to_json,
+          )
+          init_response = @transport.handle_request(init_request)
+          session_id = init_response[1]["Mcp-Session-Id"]
+
+          # Connect with SSE
+          io = StringIO.new
+          get_request = create_rack_request(
+            "GET",
+            "/",
+            { "HTTP_MCP_SESSION_ID" => session_id },
+          )
+          response = @transport.handle_request(get_request)
+          response[2].call(io) if response[2].is_a?(Proc)
+
+          # Give the stream time to set up
+          sleep(0.1)
+
+          # Test tools notification
+          @server.notify_tools_list_changed
+
+          # Test prompts notification
+          @server.notify_prompts_list_changed
+
+          # Test resources notification
+          @server.notify_resources_list_changed
+
+          # Check the notifications were received
+          io.rewind
+          output = io.read
+
+          assert_includes output, "data: #{Methods::NOTIFICATIONS_TOOLS_LIST_CHANGED}"
+          assert_includes output, "data: #{Methods::NOTIFICATIONS_PROMPTS_LIST_CHANGED}"
+          assert_includes output, "data: #{Methods::NOTIFICATIONS_RESOURCES_LIST_CHANGED}"
+        end
+
+        test "notifications are broadcast to all connected sessions" do
+          # Create two sessions
+          init_request1 = create_rack_request(
+            "POST",
+            "/",
+            { "CONTENT_TYPE" => "application/json" },
+            { jsonrpc: "2.0", method: "initialize", id: "123" }.to_json,
+          )
+          init_response1 = @transport.handle_request(init_request1)
+          session_id1 = init_response1[1]["Mcp-Session-Id"]
+
+          init_request2 = create_rack_request(
+            "POST",
+            "/",
+            { "CONTENT_TYPE" => "application/json" },
+            { jsonrpc: "2.0", method: "initialize", id: "456" }.to_json,
+          )
+          init_response2 = @transport.handle_request(init_request2)
+          session_id2 = init_response2[1]["Mcp-Session-Id"]
+
+          # Connect both sessions with SSE
+          io1 = StringIO.new
+          get_request1 = create_rack_request(
+            "GET",
+            "/",
+            { "HTTP_MCP_SESSION_ID" => session_id1 },
+          )
+          response1 = @transport.handle_request(get_request1)
+          response1[2].call(io1) if response1[2].is_a?(Proc)
+
+          io2 = StringIO.new
+          get_request2 = create_rack_request(
+            "GET",
+            "/",
+            { "HTTP_MCP_SESSION_ID" => session_id2 },
+          )
+          response2 = @transport.handle_request(get_request2)
+          response2[2].call(io2) if response2[2].is_a?(Proc)
+
+          # Give the streams time to set up
+          sleep(0.1)
+
+          # Send notification through server
+          @server.notify_tools_list_changed
+
+          # Check both sessions received the notification
+          io1.rewind
+          output1 = io1.read
+          assert_includes output1, "data: #{Methods::NOTIFICATIONS_TOOLS_LIST_CHANGED}"
+
+          io2.rewind
+          output2 = io2.read
+          assert_includes output2, "data: #{Methods::NOTIFICATIONS_TOOLS_LIST_CHANGED}"
+        end
+
+        test "server continues to work when SSE connection is closed" do
+          # Initialize a session
+          init_request = create_rack_request(
+            "POST",
+            "/",
+            { "CONTENT_TYPE" => "application/json" },
+            { jsonrpc: "2.0", method: "initialize", id: "init" }.to_json,
+          )
+          init_response = @transport.handle_request(init_request)
+          session_id = init_response[1]["Mcp-Session-Id"]
+
+          # Connect with SSE
+          io = StringIO.new
+          get_request = create_rack_request(
+            "GET",
+            "/",
+            { "HTTP_MCP_SESSION_ID" => session_id },
+          )
+          response = @transport.handle_request(get_request)
+          response[2].call(io) if response[2].is_a?(Proc)
+
+          # Give the stream time to set up
+          sleep(0.1)
+
+          # Close the stream
+          io.close
+
+          # Server notifications should not raise errors
+          assert_nothing_raised do
+            @server.notify_tools_list_changed
+            @server.notify_prompts_list_changed
+            @server.notify_resources_list_changed
+          end
+        end
+
+        test "notifications work with dynamic tool additions" do
+          # Initialize a session
+          init_request = create_rack_request(
+            "POST",
+            "/",
+            { "CONTENT_TYPE" => "application/json" },
+            { jsonrpc: "2.0", method: "initialize", id: "init" }.to_json,
+          )
+          init_response = @transport.handle_request(init_request)
+          session_id = init_response[1]["Mcp-Session-Id"]
+
+          # Connect with SSE
+          io = StringIO.new
+          get_request = create_rack_request(
+            "GET",
+            "/",
+            { "HTTP_MCP_SESSION_ID" => session_id },
+          )
+          response = @transport.handle_request(get_request)
+          response[2].call(io) if response[2].is_a?(Proc)
+
+          # Give the stream time to set up
+          sleep(0.1)
+
+          # Define a new tool (simulating dynamic tool addition)
+          @server.define_tool(
+            name: "dynamic_tool",
+            description: "A dynamically added tool",
+          ) do |**args|
+            { result: "success" }
+          end
+
+          # Manually trigger notification (since we removed the automatic triggers)
+          @server.notify_tools_list_changed
+
+          # Check the notification was received
+          io.rewind
+          output = io.read
+          assert_includes output, "data: #{Methods::NOTIFICATIONS_TOOLS_LIST_CHANGED}"
+
+          # Verify the tool was added to the server
+          assert @server.tools.key?("dynamic_tool")
+        end
+
+        test "SSE format is correct for notifications" do
+          # Initialize a session
+          init_request = create_rack_request(
+            "POST",
+            "/",
+            { "CONTENT_TYPE" => "application/json" },
+            { jsonrpc: "2.0", method: "initialize", id: "init" }.to_json,
+          )
+          init_response = @transport.handle_request(init_request)
+          session_id = init_response[1]["Mcp-Session-Id"]
+
+          # Connect with SSE
+          io = StringIO.new
+          get_request = create_rack_request(
+            "GET",
+            "/",
+            { "HTTP_MCP_SESSION_ID" => session_id },
+          )
+          response = @transport.handle_request(get_request)
+          response[2].call(io) if response[2].is_a?(Proc)
+
+          # Give the stream time to set up
+          sleep(0.1)
+
+          # Send a notification
+          @server.notify_tools_list_changed
+
+          # Check SSE format
+          io.rewind
+          output = io.read
+
+          # SSE format should be "data: <message>\n\n"
+          assert_match(/data: #{Regexp.escape(Methods::NOTIFICATIONS_TOOLS_LIST_CHANGED)}\n/, output)
+        end
+
+        private
+
+        def create_rack_request(method, path, headers, body = nil)
+          env = {
+            "REQUEST_METHOD" => method,
+            "PATH_INFO" => path,
+            "rack.input" => StringIO.new(body.to_s),
+          }.merge(headers)
+
+          Rack::Request.new(env)
+        end
+      end
+    end
+  end
+end

test/mcp/server_notification_test.rb

@@ -0,0 +1,136 @@
+# typed: true
+# frozen_string_literal: true
+
+require "test_helper"
+
+module MCP
+  class ServerNotificationTest < ActiveSupport::TestCase
+    include InstrumentationTestHelper
+
+    class MockTransport < Transport
+      attr_reader :notifications
+
+      def initialize(server)
+        super
+        @notifications = []
+      end
+
+      def send_notification(method, params = nil)
+        @notifications << { method: method, params: params }
+        true
+      end
+
+      def send_response(response); end
+      def open; end
+      def close; end
+      def handle_request(request); end
+    end
+
+    setup do
+      configuration = MCP::Configuration.new
+      configuration.instrumentation_callback = instrumentation_helper.callback
+      
+      @server = Server.new(
+        name: "test_server",
+        version: "1.0.0",
+        configuration: configuration
+      )
+      
+      @mock_transport = MockTransport.new(@server)
+      @server.transport = @mock_transport
+    end
+
+    test "#notify_tools_list_changed sends notification through transport" do
+      @server.notify_tools_list_changed
+
+      assert_equal 1, @mock_transport.notifications.size
+      notification = @mock_transport.notifications.first
+      assert_equal Methods::NOTIFICATIONS_TOOLS_LIST_CHANGED, notification[:method]
+      assert_nil notification[:params]
+    end
+
+    test "#notify_prompts_list_changed sends notification through transport" do
+      @server.notify_prompts_list_changed
+
+      assert_equal 1, @mock_transport.notifications.size
+      notification = @mock_transport.notifications.first
+      assert_equal Methods::NOTIFICATIONS_PROMPTS_LIST_CHANGED, notification[:method]
+      assert_nil notification[:params]
+    end
+
+    test "#notify_resources_list_changed sends notification through transport" do
+      @server.notify_resources_list_changed
+
+      assert_equal 1, @mock_transport.notifications.size
+      notification = @mock_transport.notifications.first
+      assert_equal Methods::NOTIFICATIONS_RESOURCES_LIST_CHANGED, notification[:method]
+      assert_nil notification[:params]
+    end
+
+    test "notification methods work without transport" do
+      server_without_transport = Server.new(name: "test_server")
+      
+      # Should not raise any errors
+      assert_nothing_raised do
+        server_without_transport.notify_tools_list_changed
+        server_without_transport.notify_prompts_list_changed
+        server_without_transport.notify_resources_list_changed
+      end
+    end
+
+    test "notification methods handle transport errors gracefully" do
+      # Create a transport that raises errors
+      error_transport = Class.new(MockTransport) do
+        def send_notification(method, params = nil)
+          raise StandardError, "Transport error"
+        end
+      end.new(@server)
+      
+      @server.transport = error_transport
+      
+      # Mock the exception reporter
+      expected_contexts = [
+        { notification: "tools_list_changed" },
+        { notification: "prompts_list_changed" },
+        { notification: "resources_list_changed" }
+      ]
+      
+      call_count = 0
+      @server.configuration.exception_reporter.expects(:call).times(3).with do |exception, context|
+        assert_kind_of StandardError, exception
+        assert_equal "Transport error", exception.message
+        assert_includes expected_contexts, context
+        call_count += 1
+        true
+      end
+      
+      # Should not raise errors to the caller
+      assert_nothing_raised do
+        @server.notify_tools_list_changed
+        @server.notify_prompts_list_changed
+        @server.notify_resources_list_changed
+      end
+      
+      assert_equal 3, call_count
+    end
+
+    test "multiple notification methods can be called in sequence" do
+      @server.notify_tools_list_changed
+      @server.notify_prompts_list_changed
+      @server.notify_resources_list_changed
+
+      assert_equal 3, @mock_transport.notifications.size
+      
+      notifications = @mock_transport.notifications
+      assert_equal Methods::NOTIFICATIONS_TOOLS_LIST_CHANGED, notifications[0][:method]
+      assert_equal Methods::NOTIFICATIONS_PROMPTS_LIST_CHANGED, notifications[1][:method]
+      assert_equal Methods::NOTIFICATIONS_RESOURCES_LIST_CHANGED, notifications[2][:method]
+    end
+
+    test "notification method constants are defined correctly" do
+      assert_equal "notifications/tools/list_changed", Methods::NOTIFICATIONS_TOOLS_LIST_CHANGED
+      assert_equal "notifications/prompts/list_changed", Methods::NOTIFICATIONS_PROMPTS_LIST_CHANGED
+      assert_equal "notifications/resources/list_changed", Methods::NOTIFICATIONS_RESOURCES_LIST_CHANGED
+    end
+  end
+end