diff --git a/test/test_events.rb b/test/test_events.rb
index ed29bb34..c3c713d3 100644
--- a/test/test_events.rb
+++ b/test/test_events.rb
@@ -6,15 +6,16 @@ class TestEvents < Minitest::Test
   def test_null
     events = Puma::Events.null
 
-    assert_kind_of Puma::NullIO, events.stdout
-    assert_kind_of Puma::NullIO, events.stderr
+    assert_instance_of Puma::NullIO, events.stdout
+    assert_instance_of Puma::NullIO, events.stderr
+    assert_equal events.stdout, events.stderr
   end
 
   def test_strings
     events = Puma::Events.strings
 
-    assert_kind_of StringIO, events.stdout
-    assert_kind_of StringIO, events.stderr
+    assert_instance_of StringIO, events.stdout
+    assert_instance_of StringIO, events.stderr
   end
 
   def test_stdio
@@ -55,4 +56,106 @@ class TestEvents < Minitest::Test
 
     assert_equal true, obj.res
   end
+
+  def test_fire_callback_with_multiple_arguments
+    res = []
+
+    events = Puma::Events.null
+
+    events.register(:exec) { |*args| res.concat(args) }
+
+    events.fire(:exec, :foo, :bar, :baz)
+
+    assert_equal [:foo, :bar, :baz], res
+  end
+
+  def test_on_booted_callback
+    res = false
+
+    events = Puma::Events.null
+
+    events.on_booted { res = true }
+
+    events.fire_on_booted!
+
+    assert res
+  end
+
+  def test_log_writes_to_stdout
+    out, _ = capture_io do
+      Puma::Events.stdio.log("ready")
+    end
+
+    assert_equal "ready\n", out
+  end
+
+  def test_write_writes_to_stdout
+    out, _ = capture_io do
+      Puma::Events.stdio.write("ready")
+    end
+
+    assert_equal "ready", out
+  end
+
+  def test_debug_writes_to_stdout_if_env_is_present
+    original_debug, ENV["PUMA_DEBUG"] = ENV["PUMA_DEBUG"], "1"
+
+    out, _ = capture_io do
+      Puma::Events.stdio.debug("ready")
+    end
+
+    assert_equal "% ready\n", out
+  ensure
+    ENV["PUMA_DEBUG"] = original_debug
+  end
+
+  def test_debug_not_write_to_stdout_if_env_is_not_present
+    out, _ = capture_io do
+      Puma::Events.stdio.debug("ready")
+    end
+
+    assert_empty out
+  end
+
+  def test_error_writes_to_stderr_and_exits
+    did_exit = false
+
+    _, err = capture_io do
+      Puma::Events.stdio.error("interrupted")
+    end
+
+    assert_equal "ERROR: interrupted", err
+  rescue SystemExit
+    did_exit = true
+  ensure
+    assert did_exit
+  end
+
+  def test_pid_formatter
+    pid = Process.pid
+
+    out, _ = capture_io do
+      events = Puma::Events.stdio
+
+      events.formatter = Puma::Events::PidFormatter.new
+
+      events.write("ready")
+    end
+
+    assert_equal "[#{ pid }] ready", out
+  end
+
+  def test_custom_log_formatter
+    custom_formatter = proc { |str| "-> #{ str }" }
+
+    out, _ = capture_io do
+      events = Puma::Events.stdio
+
+      events.formatter = custom_formatter
+
+      events.write("ready")
+    end
+
+    assert_equal "-> ready", out
+  end
 end