diff --git a/Dockerfile.development b/Dockerfile.development
index 52b1bbc..72896e4 100644
--- a/Dockerfile.development
+++ b/Dockerfile.development
@@ -1,7 +1,7 @@
 ARG RUBY_VERSION=
 FROM ruby:$RUBY_VERSION
 
-RUN apt-get update && apt-get install less -y
+RUN apt-get update && apt-get install apt-transport-https netcat less -y
 RUN groupadd --gid 1000 ruby && useradd --uid 1000 --gid ruby --shell /bin/bash --create-home ruby
 RUN mkdir /app /vendor && chown -R ruby:ruby /app /vendor
 
diff --git a/docker-compose.override.yml b/docker-compose.override.yml
index 177ac10..b8d4f02 100644
--- a/docker-compose.override.yml
+++ b/docker-compose.override.yml
@@ -14,3 +14,6 @@ services:
     <<: *app
   ruby-2-2:
     <<: *app
+  selenium-vscode:
+    volumes:
+      - ./docker-selenium-vscode/vscode_wrapper:/opt/vscode_wrapper
diff --git a/docker-compose.yml b/docker-compose.yml
index a46cc29..390c972 100644
--- a/docker-compose.yml
+++ b/docker-compose.yml
@@ -11,6 +11,8 @@ services:
     volumes:
       - vendor:/vendor
       - home:/home/ruby
+    depends_on:
+      - selenium-vscode
   ruby-2-5:
     <<: *app
   ruby-2-4:
@@ -31,6 +33,11 @@ services:
       <<: *app-build
       args:
         RUBY_VERSION: 2.2.9
+  selenium-vscode:
+    build:
+      context: docker-selenium-vscode
+    ports:
+      - 5900
 volumes:
   home:
   vendor:
diff --git a/docker-selenium-vscode/Dockerfile b/docker-selenium-vscode/Dockerfile
new file mode 100644
index 0000000..c855eaa
--- /dev/null
+++ b/docker-selenium-vscode/Dockerfile
@@ -0,0 +1,16 @@
+FROM selenium/standalone-chrome-debug:3.7.1
+
+USER root
+RUN apt-get update && apt-get install apt-transport-https -y && \
+  wget https://packages.microsoft.com/keys/microsoft.asc -q -O - | gpg --dearmor > microsoft.gpg && \
+  mv microsoft.gpg /etc/apt/trusted.gpg.d/microsoft.gpg && \
+  sh -c 'echo "deb [arch=amd64] https://packages.microsoft.com/repos/vscode stable main" > /etc/apt/sources.list.d/vscode.list' && \
+  apt-get update && apt-get install code netcat -y
+
+USER seluser
+
+RUN wget https://mtsmfm.gallery.vsassets.io/_apis/public/gallery/publisher/mtsmfm/extension/ruby-lsc/0.1.1/assetbyname/Microsoft.VisualStudio.Services.VSIXPackage -O /tmp/ruby-lsc.vsix && \
+  /usr/bin/code --install-extension /tmp/ruby-lsc.vsix && \
+  rm /tmp/ruby-lsc.vsix
+
+COPY vscode_wrapper /opt/vscode_wrapper
diff --git a/docker-selenium-vscode/vscode_wrapper b/docker-selenium-vscode/vscode_wrapper
new file mode 100755
index 0000000..b9f0bf3
--- /dev/null
+++ b/docker-selenium-vscode/vscode_wrapper
@@ -0,0 +1,33 @@
+#! /usr/bin/env bash
+
+set -e
+tmpdir="/tmp/vscode-test-$(date +'%s')"
+mkdir $tmpdir
+cd $tmpdir
+mkdir .vscode
+
+args=()
+
+for opt in "$@"
+do
+    case $opt in
+        --before-boot=* )
+            before_boot=$(echo "$1" | sed -e "s/^--before-boot='//" -e "s/'$//")
+            echo $before_boot
+            ;;
+        'data:,' )
+            # Ignore
+            ;;
+        * )
+            args+=( $1 )
+            ;;
+    esac
+    shift
+done
+
+if [ "$before_boot" ]; then
+    eval "$before_boot"
+fi
+
+/usr/bin/code --skip-getting-started --disable-updates --disable-crash-reporter --wait ${args[*]} .
+rm -rf $tmpdir
diff --git a/language_server.gemspec b/language_server.gemspec
index 61a4ef5..9dcbf80 100644
--- a/language_server.gemspec
+++ b/language_server.gemspec
@@ -30,6 +30,7 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency "awesome_print"
   spec.add_development_dependency "benchmark-ips"
   spec.add_development_dependency "bundler", "~> 1.14"
+  spec.add_development_dependency "capybara"
   spec.add_development_dependency "guard"
   spec.add_development_dependency "guard-minitest"
   spec.add_development_dependency "m"
@@ -38,4 +39,5 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency "minitest-power_assert"
   spec.add_development_dependency "pry-byebug"
   spec.add_development_dependency "rake", "~> 12.2"
+  spec.add_development_dependency "selenium-webdriver"
 end
diff --git a/lib/language_server.rb b/lib/language_server.rb
index e92b932..d8d5684 100644
--- a/lib/language_server.rb
+++ b/lib/language_server.rb
@@ -88,10 +88,14 @@ def on(method, &callback)
         text_document_sync: Protocol::Interface::TextDocumentSyncOptions.new(
           change: Protocol::Constant::TextDocumentSyncKind::FULL,
         ),
-        completion_provider: Protocol::Interface::CompletionOptions.new(
-          resolve_provider: true,
-          trigger_characters: %w[.],
-        ),
+        completion_provider: if LanguageServer.adhoc_enabled?
+                               Protocol::Interface::CompletionOptions.new(
+                                 resolve_provider: true,
+                                 trigger_characters: %w[.],
+                               )
+                             else
+                               false
+                             end,
         definition_provider: LanguageServer.adhoc_enabled?,
       ),
     )
diff --git a/test/language_server_test.rb b/test/language_server_test.rb
new file mode 100644
index 0000000..021ffa6
--- /dev/null
+++ b/test/language_server_test.rb
@@ -0,0 +1,16 @@
+require "test_helper"
+require "open3"
+
+class LanguageServerTest < IntegrationTest
+  def test_syntax_check
+    create_and_open_new_file "a.rb"
+
+    input_contents "def hi"
+    assert_error_message_on "hi", message: "unexpected end-of-input, expecting ';' or '\\n'"
+
+    input_contents :enter
+    input_contents "end"
+
+    assert_no_error_message
+  end
+end
diff --git a/test/support/capybara.rb b/test/support/capybara.rb
new file mode 100644
index 0000000..55dd09b
--- /dev/null
+++ b/test/support/capybara.rb
@@ -0,0 +1,18 @@
+require "capybara/minitest"
+
+Capybara.register_driver(:vscode) do |app|
+  options = { browser: :remote, url: "http://selenium-vscode:4444/wd/hub" }
+
+  options[:desired_capabilities] = Selenium::WebDriver::Remote::Capabilities.chrome(
+    chrome_options: {
+      binary: "/opt/vscode_wrapper",
+      args: ["before-boot='echo \"{ \\\"ruby-lsc.commandWithArgs\\\": [ \\\"nc\\\", \\\"#{Socket.gethostname}\\\", \\\"12345\\\" ] }\" > .vscode/settings.json'"],
+    },
+  )
+
+  Capybara::Selenium::Driver.new(app, options)
+end
+
+Capybara.run_server = false
+Capybara.default_driver = :vscode
+Capybara.default_max_wait_time = 5
diff --git a/test/support/test_server.rb b/test/support/test_server.rb
new file mode 100644
index 0000000..52a7196
--- /dev/null
+++ b/test/support/test_server.rb
@@ -0,0 +1,49 @@
+class TestServer
+  attr_reader :stdin, :stdout, :stderr, :thread, :port
+
+  def initialize(port = 12345)
+    @stdin, @stdout, @stderr = Open3.popen3("bin/language_server-ruby")
+    @port = port
+    @thread = start_thread
+    @thread.abort_on_exception = true
+  end
+
+  private
+
+    def start_thread
+      Thread.new do
+        socket = TCPServer.open(port).accept
+
+        socket_buffer = ""
+        stdout_buffer = ""
+
+        loop do
+          readable_ios, writable_ios = IO.select([socket, stdout], [socket, stdin])
+
+          readable_ios.each do |io|
+            c = io.readchar
+
+            case io
+            when socket
+              socket_buffer += c
+            when stdout
+              stdout_buffer += c
+            end
+          end
+
+          next if readable_ios.any?
+
+          writable_ios.each do |io|
+            case io
+            when stdin
+              io.write(socket_buffer)
+              socket_buffer.clear
+            when socket
+              io.write(stdout_buffer)
+              stdout_buffer.clear
+            end
+          end
+        end
+      end
+    end
+end
diff --git a/test/test_helper.rb b/test/test_helper.rb
index 17c3ede..9b96a36 100644
--- a/test/test_helper.rb
+++ b/test/test_helper.rb
@@ -12,3 +12,53 @@ def strip_heredoc
     gsub(/^[ \t]{#{indent}}/, "")
   end
 end
+
+Dir.glob(File.join(__dir__, "support", "**", "*.rb")).each { |f| require f }
+
+class IntegrationTest < Minitest::Test
+  include Capybara::DSL
+  include Capybara::Minitest::Assertions
+
+  def setup
+    @server = TestServer.new
+
+    switch_to_window(windows.last)
+
+    assert_selector ".tab-label", text: /\AWelcome\z/, wait: 10
+  end
+
+  def teardown
+    evaluate_script "electron.remote.app.quit()"
+
+    puts @server.stderr.read unless passed?
+  end
+
+  def create_and_open_new_file(filename)
+    find(".explorer-viewlet").click
+    find(".action-label.new-file").click
+    find(".explorer-item input").send_keys(filename)
+    find(".explorer-item input").send_keys(:enter)
+  end
+
+  def input_contents(*contents)
+    find(".monaco-editor textarea").send_keys(*contents)
+  end
+
+  def focus_text(text)
+    input_contents :escape
+
+    find("span", text: /\A#{Regexp.escape(text)}\z/).hover
+  end
+
+  def assert_error_message_on(text, message: nil)
+    focus_text text
+
+    options = message ? { text: /\A#{Regexp.escape(message)}\z/ } : {}
+
+    assert_selector ".monaco-tokenized-source", options
+  end
+
+  def assert_no_error_message
+    assert_selector ".task-statusbar-item-label-error+.task-statusbar-item-label-counter", text: /\A0\z/
+  end
+end