diff --git a/exe/referator b/exe/referator
index 23f5df6..f309a34 100755
--- a/exe/referator
+++ b/exe/referator
@@ -17,6 +17,14 @@ def parse_command(line)
   [command, rest].freeze
 end
 
+def success(data = nil)
+  puts "OK #{data.to_json}"
+end
+
+def failure(msg)
+  puts "ERR #{String(msg).lines.join(' ')}"
+end
+
 workdir = ARGV.first
 workdir = Dir.pwd if String(workdir).strip.empty?
 workdir = File.expand_path(workdir).freeze
@@ -25,39 +33,49 @@ config = Referator::Config.new workdir
 footnotes = Referator::Footnotes.new config
 
 while (line = $stdin.gets)
-  command, rest = parse_command line
-  case command
-  when 'REGISTER_FORMAT'
-    data = JSON.parse rest
-    config.formats.register(
-      String(data['name']).to_sym,
-      **data.except('name').transform_keys(&:to_sym),
-    )
-  when 'REGISTER_KIND'
-    data = JSON.parse rest
-    config.kinds.register String(data).to_sym
-  when 'ADD_REF'
-    data = JSON.parse rest
-    config.repo.add_ref(**data.transform_keys(&:to_sym))
-  when 'ADD_NOTE'
-    config.freeze
-    data = JSON.parse rest
-    puts footnotes.add_note(
-      String(data['kind']).to_sym,
-      String(data['id']),
-    ).to_h.to_json
-  when 'REF'
-    config.freeze
-    data = JSON.parse rest
-    puts config.repo[
-      String(data['kind']).to_sym,
-      String(data['id']),
-    ].to_h.to_json
-  when 'RENDER_FOOTNOTES'
-    config.freeze
-    data = JSON.parse rest
-    puts footnotes.render(String(data).to_sym).to_json
-  else
-    raise 'Invalid command'
+  begin
+    command, rest = parse_command line
+    case command
+    when 'EXIT'
+      success
+      exit
+    when 'REGISTER_FORMAT'
+      data = JSON.parse rest
+      config.formats.register(
+        String(data['name']).to_sym,
+        **data.except('name').transform_keys(&:to_sym),
+      )
+      success
+    when 'REGISTER_KIND'
+      data = JSON.parse rest
+      config.kinds.register String(data).to_sym
+      success
+    when 'ADD_REF'
+      data = JSON.parse rest
+      config.repo.add_ref(**data.transform_keys(&:to_sym))
+      success
+    when 'ADD_NOTE'
+      config.freeze
+      data = JSON.parse rest
+      success footnotes.add_note(
+        String(data['kind']).to_sym,
+        String(data['id']),
+      ).to_h
+    when 'REF'
+      config.freeze
+      data = JSON.parse rest
+      success config.repo[
+        String(data['kind']).to_sym,
+        String(data['id']),
+      ].to_h
+    when 'RENDER_FOOTNOTES'
+      config.freeze
+      data = JSON.parse rest
+      success String footnotes.render String(data).to_sym
+    else
+      raise 'Invalid command'
+    end
+  rescue => e
+    failure e.detailed_message
   end
 end
diff --git a/test.rb b/test.rb
new file mode 100755
index 0000000..2695fb9
--- /dev/null
+++ b/test.rb
@@ -0,0 +1,119 @@
+#!/usr/bin/env ruby
+# frozen_string_literal: true
+
+require 'json'
+require 'open3'
+
+EXE = File.expand_path('exe/referator', __dir__).freeze
+
+@stdin, @stdout, @wait_thr = Open3.popen2 EXE, chdir: __dir__
+
+@stdin.sync = true
+@stdout.sync = true
+
+at_exit do
+  @stdin.close
+  @stdout.close
+  raise 'Script error' unless @wait_thr.value.success?
+end
+
+def cmd(name, data)
+  @stdin.puts "#{name} #{data.to_json}"
+  result = @stdout.gets.chomp
+  if result.start_with? 'OK '
+    JSON.parse(result[3..]).freeze
+  elsif result.start_with? 'ERR '
+    raise "Error response: #{result[4..]}"
+  else
+    raise 'Invalid response'
+  end
+end
+
+cmd :REGISTER_KIND, :self
+cmd :REGISTER_KIND, :link
+
+cmd :REGISTER_FORMAT, { name: :html,
+                        args: ['test/footnotes.rb', { format: nil }],
+                        stdin: { notes: nil } }
+
+cmd :ADD_REF,
+    { kind: :self,
+      id: '/blog/foo',
+      slug: 'blog-foo',
+      text: 'Foo' }
+cmd :ADD_REF,
+    { kind: :self,
+      id: '/blog/bar',
+      slug: 'blog-bar',
+      text: 'Bar' }
+cmd :ADD_REF,
+    { kind: :link,
+      id: :example_com,
+      slug: 'example-com',
+      url: 'https://example.com',
+      text: 'Example Domain' }
+cmd :ADD_REF,
+    { kind: :link,
+      id: :causa_arcana_com,
+      slug: 'causa-arcana-com',
+      url: 'https://causa-arcana.com',
+      text: 'Causa Arcana' }
+
+cmd(:REF, { kind: :self, id: '/blog/foo' }).tap do |result|
+  expected = {
+    'kind' => 'self',
+    'id' => '/blog/foo',
+    'slug' => 'blog-foo',
+    'anchor' => 'self-blog-foo',
+    'fragment' => '#self-blog-foo',
+    'text' => 'Foo',
+  }
+
+  raise unless result == expected
+end
+cmd(:REF, { kind: :link, id: :example_com }).tap do |result|
+  expected = {
+    'kind' => 'link',
+    'id' => 'example_com',
+    'slug' => 'example-com',
+    'anchor' => 'link-example-com',
+    'fragment' => '#link-example-com',
+    'url' => 'https://example.com',
+    'text' => 'Example Domain',
+  }
+
+  raise unless result == expected
+end
+
+cmd :ADD_NOTE, { kind: :self, id: '/blog/foo' }
+cmd :ADD_NOTE, { kind: :link, id: :example_com }
+cmd :ADD_NOTE, { kind: :self, id: '/blog/bar' }
+cmd :ADD_NOTE, { kind: :link, id: :causa_arcana_com }
+
+cmd(:RENDER_FOOTNOTES, :html).tap do |result|
+  expected = <<~HTML.freeze
+    <h2>Self references</h2>
+
+    <ol>
+      <li id="self-blog-foo" value="1">
+        <a href="/blog/foo.html">Foo</a>
+      </li>
+      <li id="self-blog-bar" value="3">
+        <a href="/blog/bar.html">Bar</a>
+      </li>
+    </ol>
+
+    <h2>Link references</h2>
+
+    <ol>
+      <li id="link-example-com" value="2">
+        <a href="https://example.com">Example Domain</a>
+      </li>
+      <li id="link-causa-arcana-com" value="4">
+        <a href="https://causa-arcana.com">Causa Arcana</a>
+      </li>
+    </ol>
+  HTML
+
+  raise unless result == expected
+end
diff --git a/test/footnotes.html.erb b/test/footnotes.html.erb
new file mode 100644
index 0000000..0fe7170
--- /dev/null
+++ b/test/footnotes.html.erb
@@ -0,0 +1,24 @@
+<% if notes.any? { |note| note['kind'] == 'self' } -%>
+
+<h2>Self references</h2>
+
+<ol>
+<% notes.select { |note| note['kind'] == 'self' }.each do |note| -%>
+  <li id="<%= note['anchor'] %>" value="<%= note['index'] %>">
+    <a href="<%= note['id'] %>.html"><%= note['text'] %></a>
+  </li>
+<% end -%>
+</ol>
+<% end -%>
+<% if notes.any? { |note| note['kind'] == 'link' } -%>
+
+<h2>Link references</h2>
+
+<ol>
+<% notes.select { |note| note['kind'] == 'link' }.each do |note| -%>
+  <li id="<%= note['anchor'] %>" value="<%= note['index'] %>">
+    <a href="<%= note['url'] %>"><%= note['text'] %></a>
+  </li>
+<% end -%>
+</ol>
+<% end -%>
diff --git a/test/footnotes.rb b/test/footnotes.rb
new file mode 100755
index 0000000..44d5964
--- /dev/null
+++ b/test/footnotes.rb
@@ -0,0 +1,17 @@
+#!/usr/bin/env ruby
+# frozen_string_literal: true
+
+require 'erb'
+require 'json'
+require 'pathname'
+
+def template(filename)
+  pathname = Pathname.new(__dir__).join(filename).expand_path.freeze
+  ERB.new(pathname.read, trim_mode: '-').tap do |erb|
+    erb.filename = pathname.to_s.freeze
+  end.freeze
+end
+
+puts template("footnotes.#{ARGV.first}.erb").result_with_hash(
+  notes: JSON.parse($stdin.read),
+).strip