Return Method not found for methods of undeclared capabilities

lib/mcp/server.rb

@@ -648,7 +648,14 @@ def handle_request(request, method, session: nil, related_request_id: nil, auth_
         # Re-raise through RequestHandlerError, the one channel whose message is
         # deliberately surfaced to clients: `JsonRpcHandler`'s blind `rescue StandardError`
         # no longer echoes exception messages into the error `data` member (CWE-209).
-        raise RequestHandlerError.new(e.message, request, error_type: :internal_error, original_error: e)
+        # A server that does not declare a capability does not implement its methods, hence Method not found.
+        raise RequestHandlerError.new(
+          e.message,
+          request,
+          error_type: :method_not_found,
+          error_code: JsonRpcHandler::ErrorCode::METHOD_NOT_FOUND,
+          original_error: e,
+        )
       end
 
       # `initialize` MUST NOT be cancelled (MCP spec 2025-11-25, cancellation item 2),

test/mcp/server/transports/streamable_http_transport_test.rb

@@ -6100,6 +6100,17 @@ def string
           end
         end
 
+        test "modern POST answers a method of an undeclared capability with 404 and -32601" do
+          @server.capabilities = { tools: {} }
+
+          response = @transport.handle_request(modern_rack_request(modern_body("prompts/list", {})))
+
+          assert_equal 404, response[0]
+          body = JSON.parse(response[2][0])
+          assert_equal(-32601, body.dig("error", "code"))
+          assert_equal "Server does not support prompts (required for prompts/list)", body.dig("error", "message")
+        end
+
         test "modern POST maps a missing client capability to 400 with -32021" do
           @server.define_tool(name: "guarded_tool") do |server_context:|
             server_context.require_client_capability!(:elicitation, :form)

test/mcp/server_test.rb

@@ -1880,23 +1880,26 @@ def read_resource_request(uri)
 
       assert_equal "2.0", response[:jsonrpc]
       assert_equal 1, response[:id]
-      assert_equal(-32603, response[:error][:code])
+      assert_equal(-32601, response[:error][:code])
       assert_includes response[:error][:data], "Server does not support logging"
     end
 
     test "#handle method with missing required top-level capability returns an error" do
       @server.capabilities = {}
 
       response = @server.handle({ jsonrpc: "2.0", method: "prompts/list", id: 1 })
+      assert_equal(-32601, response[:error][:code])
       assert_equal "Server does not support prompts (required for prompts/list)", response[:error][:data]
 
       response = @server.handle({ jsonrpc: "2.0", method: "resources/list", id: 1 })
+      assert_equal(-32601, response[:error][:code])
       assert_equal "Server does not support resources (required for resources/list)", response[:error][:data]
     end
 
     test "#handle method with missing required nested capability returns an error" do
       @server.capabilities = { resources: {} }
       response = @server.handle({ jsonrpc: "2.0", method: "resources/subscribe", id: 1 })
+      assert_equal(-32601, response[:error][:code])
       assert_equal "Server does not support resources.subscribe (required for resources/subscribe)",
         response[:error][:data]
     end