Simplify capability handling again

Author: kfischer-okarin

lib/mcp/server.rb

@@ -3,7 +3,6 @@
 require "json_rpc_handler"
 require_relative "instrumentation"
 require_relative "methods"
-require_relative "server/capabilities"
 
 module MCP
   class Server
@@ -23,8 +22,7 @@ def initialize(message, request, error_type: :internal_error, original_error: ni
 
     include Instrumentation
 
-    attr_writer :capabilities
-    attr_accessor :name, :version, :tools, :prompts, :resources, :server_context, :configuration, :transport
+    attr_accessor :name, :version, :tools, :prompts, :resources, :server_context, :configuration, :capabilities, :transport
 
     def initialize(
       name: "model_context_protocol",
@@ -47,10 +45,7 @@ def initialize(
       @resource_index = index_resources_by_uri(resources)
       @server_context = server_context
       @configuration = MCP.configuration.merge(configuration)
-      @capabilities = Capabilities.new(capabilities)
-      @capabilities.support_tools if tools.any?
-      @capabilities.support_prompts if prompts.any?
-      @capabilities.support_resources if resources.any? || resource_templates.any?
+      @capabilities = capabilities || default_capabilities
 
       @handlers = {
         Methods::RESOURCES_LIST => method(:list_resources),
@@ -72,10 +67,6 @@ def initialize(
       @transport = transport
     end
 
-    def capabilities
-      @capabilities.to_h
-    end
-
     def handle(request)
       JsonRpcHandler.handle(request) do |method|
         handle_request(request, method)
@@ -123,7 +114,6 @@ def notify_resources_list_changed
     end
 
     def resources_list_handler(&block)
-      @capabilities.support_resources
       @handlers[Methods::RESOURCES_LIST] = block
     end
 
@@ -132,12 +122,10 @@ def resources_read_handler(&block)
     end
 
     def resources_templates_list_handler(&block)
-      @capabilities.support_resources
       @handlers[Methods::RESOURCES_TEMPLATES_LIST] = block
     end
 
     def tools_list_handler(&block)
-      @capabilities.support_tools
       @handlers[Methods::TOOLS_LIST] = block
     end
 
@@ -146,7 +134,6 @@ def tools_call_handler(&block)
     end
 
     def prompts_list_handler(&block)
-      @capabilities.support_prompts
       @handlers[Methods::PROMPTS_LIST] = block
     end
 
@@ -194,6 +181,14 @@ def handle_request(request, method)
       }
     end
 
+    def default_capabilities
+      {
+        tools: { listChanged: true },
+        prompts: { listChanged: true },
+        resources: { listChanged: true },
+      }
+    end
+
     def server_info
       @server_info ||= {
         name:,

test/mcp/server/capabilities_test.rb

@@ -116,9 +116,9 @@ class ServerTest < ActiveSupport::TestCase
         "result": {
           "protocolVersion": "2024-11-05",
           "capabilities": {
-            "prompts": {},
-            "resources": {},
-            "tools": {},
+            "prompts": { listChanged: true },
+            "resources": { listChanged: true },
+            "tools": { listChanged: true },
           },
           "serverInfo": {
             "name": @server_name,
@@ -778,68 +778,6 @@ def call(message:, server_context: nil)
       assert_equal custom_version, response[:result][:protocolVersion]
     end
 
-    test "has tool capability only if tools or a tools_list_handler is defined" do
-      server_with_tools = Server.new(name: "test_server", tools: [@tool])
-
-      assert_includes server_with_tools.capabilities, :tools
-
-      server_with_handler = Server.new(name: "test_server")
-      server_with_handler.tools_list_handler do
-        [{ name: "test_tool", description: "Test tool" }]
-      end
-
-      assert_includes server_with_handler.capabilities, :tools
-
-      server_without_tools = Server.new(name: "test_server")
-
-      refute_includes server_without_tools.capabilities, :tools
-    end
-
-    test "has prompt capability only if prompts or a prompts_list_handler is defined" do
-      server_with_prompts = Server.new(name: "test_server", prompts: [@prompt])
-
-      assert_includes server_with_prompts.capabilities, :prompts
-
-      server_with_handler = Server.new(name: "test_server")
-      server_with_handler.prompts_list_handler do
-        [{ name: "test_prompt", description: "Test prompt" }]
-      end
-
-      assert_includes server_with_handler.capabilities, :prompts
-
-      server_without_prompts = Server.new(name: "test_server")
-
-      refute_includes server_without_prompts.capabilities, :prompts
-    end
-
-    test "has resources capability only if resources, template or custom handler is defined" do
-      server_with_resources = Server.new(name: "test_server", resources: [@resource])
-
-      assert_includes server_with_resources.capabilities, :resources
-
-      server_with_resource_template = Server.new(name: "test_server", resource_templates: [@resource_template])
-
-      assert_includes server_with_resource_template.capabilities, :resources
-
-      server_with_resources_list_handler = Server.new(name: "test_server")
-      server_with_resources_list_handler.resources_list_handler do
-        [{ uri: "test_resource", name: "Test resource", description: "Test resource" }]
-      end
-
-      assert_includes server_with_resources_list_handler.capabilities, :resources
-
-      server_with_resources_templates_list_handler = Server.new(name: "test_server")
-      server_with_resources_templates_list_handler.resources_templates_list_handler do
-        [{ uri_template: "test_resource/{id}", name: "Test resource", description: "Test resource" }]
-      end
-
-      assert_includes server_with_resources_templates_list_handler.capabilities, :resources
-
-      server_without_resources = Server.new(name: "test_server")
-
-      refute_includes server_without_resources.capabilities, :resources
-    end
-
     test "tools/call validates arguments against input schema when validate_tool_call_arguments is true" do
       server = Server.new(
         tools: [TestTool],