[Fix #11164] Suppress server mode message with -f json (#11668)

changelog/fix_fix_rubocop_11164_suppress_server_mode_message.md

@@ -0,0 +1 @@
+* [#11164](https://github.com/rubocop/rubocop/issues/11164): Suppress server mode message with `-f json`. ([@jasondoc3][])

lib/rubocop/server/core.rb

@@ -100,7 +100,7 @@ def read_socket(socket)
 
       def use_json_format?
         return true if ARGV.include?('--format=json') || ARGV.include?('--format=j')
-        return false unless (index = ARGV.index('--format'))
+        return false unless (index = ARGV.index('--format') || ARGV.index('-f'))
 
         format = ARGV[index + 1]
 

spec/rubocop/server/rubocop_server_spec.rb

@@ -123,6 +123,34 @@
           expect(stdout).not_to start_with 'RuboCop server starting on '
         end
       end
+
+      context 'when `-f json`' do
+        it 'does not display the server start message' do
+          create_file('example.rb', <<~RUBY)
+            puts 0
+          RUBY
+
+          stdout, _stderr, _status = Open3.capture3(
+            'ruby', '-I', '.',
+            rubocop, '--server', '-f', 'json', '--stdin', 'example.rb', stdin_data: 'puts 0'
+          )
+          expect(stdout).not_to start_with 'RuboCop server starting on '
+        end
+      end
+
+      context 'when `-f j`' do
+        it 'does not display the server start message' do
+          create_file('example.rb', <<~RUBY)
+            puts 0
+          RUBY
+
+          stdout, _stderr, _status = Open3.capture3(
+            'ruby', '-I', '.',
+            rubocop, '--server', '-f', 'j', '--stdin', 'example.rb', stdin_data: 'puts 0'
+          )
+          expect(stdout).not_to start_with 'RuboCop server starting on '
+        end
+      end
     end
 
     context 'when using `--server` option after running server and updating configuration' do