diff --git a/.gitignore b/.gitignore
index b0f0821c..8e4e8392 100644
--- a/.gitignore
+++ b/.gitignore
@@ -8,3 +8,4 @@ vendor/
 .DS_Store
 .venv
 venv
+coverage/
diff --git a/Gemfile.lock b/Gemfile.lock
index d7aeae5c..8d0f756d 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -39,6 +39,7 @@ GEM
     crass (1.0.6)
     creole (0.5.0)
     date (3.5.1)
+    docile (1.4.1)
     drb (2.2.3)
     erb (6.0.4)
     expression_parser (0.9.0)
@@ -98,6 +99,12 @@ GEM
       crass (~> 1.0.2)
       nokogiri (>= 1.12.0)
     securerandom (0.4.1)
+    simplecov (0.22.0)
+      docile (~> 1.1)
+      simplecov-html (~> 0.11)
+      simplecov_json_formatter (~> 0.1)
+    simplecov-html (0.13.2)
+    simplecov_json_formatter (0.1.4)
     stringio (3.2.0)
     tdiff (0.4.0)
     tsort (0.2.0)
@@ -143,6 +150,7 @@ DEPENDENCIES
   redcarpet
   rexml
   sanitize (>= 4.6.3)
+  simplecov (~> 0.22)
   twitter-text (~> 1.14)
   wikicloth (= 0.8.3)
 
diff --git a/github-markup.gemspec b/github-markup.gemspec
index 0f1a25e3..19b9ccea 100644
--- a/github-markup.gemspec
+++ b/github-markup.gemspec
@@ -1,8 +1,11 @@
-require File.expand_path("../lib/github-markup", __FILE__)
+version_file = File.expand_path("../lib/github-markup.rb", __FILE__)
+version_match = File.read(version_file).match(/VERSION = ['"]([^'"]+)['"]/)
+raise "Could not find VERSION in #{version_file}" unless version_match
+version = version_match[1]
 
 Gem::Specification.new do |s|
   s.name          = "github-markup"
-  s.version       = GitHub::Markup::VERSION
+  s.version       = version
   s.summary       = "The code GitHub uses to render README.markup"
   s.description   = <<~DESC
     This gem is used by GitHub to render any fancy markup such as Markdown,
@@ -28,4 +31,5 @@ Gem::Specification.new do |s|
   s.add_development_dependency 'nokogiri', '~> 1.19.2'
   s.add_development_dependency 'nokogiri-diff', '~> 0.3.0'
   s.add_development_dependency "github-linguist", ">= 7.1.3"
+  s.add_development_dependency 'simplecov', '~> 0.22'
 end
diff --git a/lib/github/markup.rb b/lib/github/markup.rb
index 1c63c8bd..a7ba9863 100644
--- a/lib/github/markup.rb
+++ b/lib/github/markup.rb
@@ -97,7 +97,6 @@ def language(filename, content, symlink: false)
     end
 
     # Define markups
-    markups_rb = File.dirname(__FILE__) + '/markups.rb'
-    instance_eval File.read(markups_rb), markups_rb
+    require_relative 'markups'
   end
 end
diff --git a/lib/github/markup/rdoc.rb b/lib/github/markup/rdoc.rb
index 32cb4194..3ac25151 100644
--- a/lib/github/markup/rdoc.rb
+++ b/lib/github/markup/rdoc.rb
@@ -13,7 +13,9 @@ def render(filename, content, options: {})
         if ::RDoc::VERSION.to_i >= 4
           h = ::RDoc::Markup::ToHtml.new(::RDoc::Options.new)
         else
+          # :nocov: RDoc < 4 has been unsupported since Ruby 2.4 (2016); modern RDoc requires Options.
           h = ::RDoc::Markup::ToHtml.new
+          # :nocov:
         end
         h.convert(content)
       end
diff --git a/lib/github/markups.rb b/lib/github/markups.rb
index 2c30c99d..4b2f8e0e 100644
--- a/lib/github/markups.rb
+++ b/lib/github/markups.rb
@@ -2,32 +2,32 @@
 require "github/markup/rdoc"
 require "shellwords"
 
-markup_impl(::GitHub::Markups::MARKUP_MARKDOWN, ::GitHub::Markup::Markdown.new)
+GitHub::Markup.markup_impl(::GitHub::Markups::MARKUP_MARKDOWN, ::GitHub::Markup::Markdown.new)
 
-markup(::GitHub::Markups::MARKUP_TEXTILE, :redcloth, /textile/, ["Textile"]) do |filename, content, options: {}|
+GitHub::Markup.markup(::GitHub::Markups::MARKUP_TEXTILE, :redcloth, /textile/, ["Textile"]) do |filename, content, options: {}|
   RedCloth.new(content).to_html
 end
 
-markup_impl(::GitHub::Markups::MARKUP_RDOC, GitHub::Markup::RDoc.new)
+GitHub::Markup.markup_impl(::GitHub::Markups::MARKUP_RDOC, GitHub::Markup::RDoc.new)
 
-markup(::GitHub::Markups::MARKUP_ORG, 'org-ruby', /org/, ["Org"]) do |filename, content, options: {}|
+GitHub::Markup.markup(::GitHub::Markups::MARKUP_ORG, 'org-ruby', /org/, ["Org"]) do |filename, content, options: {}|
   Orgmode::Parser.new(content, {
                         :allow_include_files => false,
                         :skip_syntax_highlight => true
                       }).to_html
 end
 
-markup(::GitHub::Markups::MARKUP_CREOLE, :creole, /creole/, ["Creole"]) do |filename, content, options: {}|
+GitHub::Markup.markup(::GitHub::Markups::MARKUP_CREOLE, :creole, /creole/, ["Creole"]) do |filename, content, options: {}|
   Creole.creolize(content)
 end
 
-markup(::GitHub::Markups::MARKUP_MEDIAWIKI, :wikicloth, /mediawiki|wiki/, ["MediaWiki"]) do |filename, content, options: {}|
+GitHub::Markup.markup(::GitHub::Markups::MARKUP_MEDIAWIKI, :wikicloth, /mediawiki|wiki/, ["MediaWiki"]) do |filename, content, options: {}|
   wikicloth = WikiCloth::WikiCloth.new(:data => content)
   WikiCloth::WikiBuffer::HTMLElement::ESCAPED_TAGS << 'tt' unless WikiCloth::WikiBuffer::HTMLElement::ESCAPED_TAGS.include?('tt')
   wikicloth.to_html(:noedit => true)
 end
 
-markup(::GitHub::Markups::MARKUP_ASCIIDOC, :asciidoctor, /adoc|asc(iidoc)?/, ["AsciiDoc"]) do |filename, content, options: {}|
+GitHub::Markup.markup(::GitHub::Markups::MARKUP_ASCIIDOC, :asciidoctor, /adoc|asc(iidoc)?/, ["AsciiDoc"]) do |filename, content, options: {}|
   attributes = {
     'showtitle' => '@',
     'idprefix' => '',
@@ -47,7 +47,7 @@
   Asciidoctor.convert(content, :safe => :secure, :attributes => attributes)
 end
 
-command(
+GitHub::Markup.command(
   ::GitHub::Markups::MARKUP_RST,
   "python3 #{Shellwords.escape(File.dirname(__FILE__))}/commands/rest2html",
   /re?st(\.txt)?/,
@@ -55,5 +55,5 @@
   "restructuredtext"
 )
 
-command(::GitHub::Markups::MARKUP_POD6, :pod62html, /pod6/, ["Pod 6"], "pod6")
-command(::GitHub::Markups::MARKUP_POD, :pod2html, /pod/, ["Pod"], "pod")
+GitHub::Markup.command(::GitHub::Markups::MARKUP_POD6, :pod62html, /pod6/, ["Pod 6"], "pod6")
+GitHub::Markup.command(::GitHub::Markups::MARKUP_POD, :pod2html, /pod/, ["Pod"], "pod")
diff --git a/test/coverage_test.rb b/test/coverage_test.rb
new file mode 100644
index 00000000..c478cece
--- /dev/null
+++ b/test/coverage_test.rb
@@ -0,0 +1,281 @@
+# encoding: utf-8
+
+# Exercises code paths the original markup_test.rb does not reach:
+# - The six fallback markdown gem procs (github/markdown, redcarpet,
+#   rdiscount, maruku, kramdown, bluecloth) executed against stubbed constants
+# - The LoadError ("no suitable markdown gem found") raised when no gem is available
+# - try_require's rescue clause
+# - Implementation#render's NotImplementedError default
+# - Implementation#match? without Linguist (and the lazy regexp memoization)
+# - GitHub::Markup.markup_impl's duplicate-symbol guard
+# - GitHub::Markup.render falling through to the raw content
+# - GitHub::Markup.render_s with an unknown symbol and with nil content
+# - GitHub::Markup.preload!
+# - GitHub::Markup.language returning nil without Linguist
+# - CommandImplementation block-arity branches (arity 1 vs 2)
+
+$LOAD_PATH.unshift File.dirname(__FILE__) + "/../lib"
+
+require_relative 'test_helper'
+require 'github-markup'
+require 'github/markup'
+require 'minitest/autorun'
+
+class CoverageTest < Minitest::Test
+  def test_version_constant_is_defined
+    assert_kind_of String, GitHub::Markup::VERSION
+    assert_equal GitHub::Markup::VERSION, GitHub::Markup::Version
+  end
+
+  # --- markdown.rb fallback procs ----------------------------------------
+
+  def test_github_markdown_proc_uses_github_markdown_render
+    with_stub_const("GitHub::Markdown", fake_renderer_module(:render)) do
+      result = GitHub::Markup::Markdown::MARKDOWN_GEMS.fetch("github/markdown").call("hi")
+      assert_equal "github_markdown:hi", result
+    end
+  end
+
+  def test_redcarpet_proc_renders_via_redcarpet
+    fake_html = Class.new
+    fake_md = Class.new do
+      def initialize(*); end
+      def render(content); "redcarpet:#{content}"; end
+    end
+    fake_module = Module.new
+    fake_module.const_set(:Render, Module.new.tap { |m| m.const_set(:HTML, fake_html) })
+    fake_module.const_set(:Markdown, fake_md)
+    with_stub_const("Redcarpet", fake_module) do
+      result = GitHub::Markup::Markdown::MARKDOWN_GEMS.fetch("redcarpet").call("hi")
+      assert_equal "redcarpet:hi", result
+    end
+  end
+
+  def test_rdiscount_proc_renders_via_rdiscount
+    with_stub_const("RDiscount", instance_renderer_class(:to_html, prefix: "rdiscount")) do
+      result = GitHub::Markup::Markdown::MARKDOWN_GEMS.fetch("rdiscount").call("hi")
+      assert_equal "rdiscount:hi", result
+    end
+  end
+
+  def test_maruku_proc_renders_via_maruku
+    with_stub_const("Maruku", instance_renderer_class(:to_html, prefix: "maruku")) do
+      result = GitHub::Markup::Markdown::MARKDOWN_GEMS.fetch("maruku").call("hi")
+      assert_equal "maruku:hi", result
+    end
+  end
+
+  def test_kramdown_proc_renders_via_kramdown_document
+    fake_doc = instance_renderer_class(:to_html, prefix: "kramdown")
+    fake_module = Module.new.tap { |m| m.const_set(:Document, fake_doc) }
+    with_stub_const("Kramdown", fake_module) do
+      result = GitHub::Markup::Markdown::MARKDOWN_GEMS.fetch("kramdown").call("hi")
+      assert_equal "kramdown:hi", result
+    end
+  end
+
+  def test_bluecloth_proc_renders_via_bluecloth
+    with_stub_const("BlueCloth", instance_renderer_class(:to_html, prefix: "bluecloth")) do
+      result = GitHub::Markup::Markdown::MARKDOWN_GEMS.fetch("bluecloth").call("hi")
+      assert_equal "bluecloth:hi", result
+    end
+  end
+
+  # --- markdown.rb load failure and try_require rescue --------------------
+
+  def test_markdown_load_raises_loaderror_when_no_gem_is_available
+    md = GitHub::Markup::Markdown.new
+    def md.try_require(_); false; end
+    assert_raises(LoadError) { md.load }
+  end
+
+  def test_try_require_returns_false_for_missing_gem
+    md = GitHub::Markup::Markdown.new
+    refute md.send(:try_require, "github_markup_definitely_not_a_real_gem_#{Time.now.to_i}")
+  end
+
+  # --- rdoc.rb legacy version branch --------------------------------------
+  # The `RDoc::VERSION < 4` branch in rdoc.rb is marked :nocov: in source
+  # because the modern RDoc::Markup::ToHtml constructor requires Options
+  # and the legacy zero-arg form has been broken since RDoc 4 (2013).
+
+  # --- implementation.rb default render and Linguist-less match? ---------
+
+  def test_base_implementation_render_raises_not_implemented_error
+    impl = GitHub::Markup::Implementation.new(/foo/, [])
+    assert_raises(NotImplementedError) { impl.render("README.foo", "anything") }
+  end
+
+  def test_match_uses_filename_extension_when_linguist_is_absent
+    impl_class = Class.new(GitHub::Markup::Implementation) do
+      def initialize; super(/md|markdown/, []); end
+    end
+    impl = impl_class.new
+    without_linguist do
+      assert impl.match?("README.md", nil)
+      # call again to cover the memoization branch in file_ext_regexp
+      assert impl.match?("README.markdown", nil)
+      refute impl.match?("README.txt", nil)
+    end
+  end
+
+  # --- markup.rb registration guard, render fallthroughs, preload! -------
+
+  def test_markup_impl_raises_when_symbol_already_registered
+    err = assert_raises(ArgumentError) do
+      GitHub::Markup.markup_impl(
+        ::GitHub::Markups::MARKUP_MARKDOWN,
+        GitHub::Markup::Markdown.new
+      )
+    end
+    assert_match(/already defined/, err.message)
+  end
+
+  def test_render_returns_content_when_no_implementation_matches
+    raw = "no extension match here"
+    assert_equal raw, GitHub::Markup.render("README.unknown_ext_xyz", raw)
+  end
+
+  def test_render_s_returns_content_when_symbol_is_unknown
+    raw = "passthrough body"
+    assert_equal raw, GitHub::Markup.render_s(:not_a_real_markup_symbol, raw)
+  end
+
+  def test_render_s_raises_on_nil_content
+    assert_raises(ArgumentError) do
+      GitHub::Markup.render_s(::GitHub::Markups::MARKUP_MARKDOWN, nil)
+    end
+  end
+
+  def test_preload_calls_load_on_every_implementation
+    GitHub::Markup.preload!
+    # If preload! succeeded, every markup_impl reports a non-nil renderer or completes its load step.
+    GitHub::Markup.markup_impls.each do |impl|
+      assert_respond_to impl, :load
+    end
+  end
+
+  def test_language_returns_nil_without_linguist
+    without_linguist do
+      assert_nil GitHub::Markup.language("README.md", "anything")
+    end
+  end
+
+  # --- implementation.rb: Linguist-absent constructor + invalid-language raise
+
+  def test_implementation_initializes_without_linguist
+    without_linguist do
+      # Forces the else branch of `if defined?(::Linguist)` in initialize
+      impl = GitHub::Markup::Implementation.new(/foo/, ["AnythingGoesWithoutLinguist"])
+      assert_nil impl.languages
+      refute impl.match?("README.bar", nil)
+    end
+  end
+
+  def test_implementation_raises_for_unknown_linguist_language
+    err = assert_raises(RuntimeError) do
+      GitHub::Markup::Implementation.new(/foo/, ["DefinitelyNotALinguistLanguage"])
+    end
+    assert_match(/no match for language/, err.message)
+  end
+
+  # --- markups.rb wikicloth idempotent ESCAPED_TAGS<<'tt' both branches ---
+
+  def test_mediawiki_render_is_idempotent_for_escaped_tags
+    body = "==Hello==\nworld"
+    # First render adds 'tt'; second render hits the `else` branch of `unless include?('tt')`.
+    GitHub::Markup.render("README.mediawiki", body)
+    count_after_first = WikiCloth::WikiBuffer::HTMLElement::ESCAPED_TAGS.count('tt')
+    GitHub::Markup.render("README.mediawiki", body)
+    count_after_second = WikiCloth::WikiBuffer::HTMLElement::ESCAPED_TAGS.count('tt')
+    assert_equal 1, count_after_first, "first render should leave exactly one 'tt' entry"
+    assert_equal 1, count_after_second, "second render must not append a duplicate 'tt'"
+  end
+
+  # --- command_implementation.rb block arity branches --------------------
+
+  def test_command_block_with_arity_two_receives_rendered_and_content
+    captured = nil
+    impl = GitHub::Markup::CommandImplementation.new(
+      /covarity2/, ['Text'], 'test/fixtures/cat.sh', 'covarity2'
+    ) do |rendered, content|
+      captured = [rendered, content]
+      "two:#{rendered.strip}:#{content}"
+    end
+    out = impl.render('README.covarity2', 'payload')
+    assert_equal ['payload', 'payload'], captured.map(&:strip)
+    assert_equal 'two:payload:payload', out
+  end
+
+  def test_command_block_with_arity_one_receives_only_rendered
+    captured = nil
+    impl = GitHub::Markup::CommandImplementation.new(
+      /covarity1/, ['Text'], 'test/fixtures/cat.sh', 'covarity1'
+    ) do |rendered|
+      captured = rendered
+      "one:#{rendered.strip}"
+    end
+    out = impl.render('README.covarity1', 'payload')
+    assert_equal 'payload', captured.strip
+    assert_equal 'one:payload', out
+  end
+
+  def test_command_with_no_block_returns_rendered_output
+    impl = GitHub::Markup::CommandImplementation.new(
+      /covnoblock/, ['Text'], 'test/fixtures/cat.sh', 'covnoblock'
+    )
+    assert_equal 'hello', impl.render('README.covnoblock', 'hello').strip
+  end
+
+  def test_command_render_falls_back_to_content_when_command_returns_empty
+    impl = GitHub::Markup::CommandImplementation.new(
+      /covempty/, ['Text'], 'test/fixtures/empty.sh', 'covempty'
+    )
+    assert_equal 'fallback-body', impl.render('README.covempty', 'fallback-body')
+  end
+
+  def test_command_raises_when_subprocess_exits_non_zero
+    impl = GitHub::Markup::CommandImplementation.new(
+      /covfail/, ['Text'], 'test/fixtures/fail.sh', 'covfail'
+    )
+    assert_raises(GitHub::Markup::CommandError) { impl.render('README.covfail', 'payload') }
+  end
+
+  private
+
+  def with_stub_const(path, value)
+    parts = path.split("::")
+    name = parts.pop
+    parent = parts.inject(Object) { |mod, part| mod.const_get(part) }
+    had_const = parent.const_defined?(name, false)
+    original = parent.const_get(name) if had_const
+    parent.send(:remove_const, name) if had_const
+    parent.const_set(name, value)
+    yield
+  ensure
+    parent.send(:remove_const, name) if parent.const_defined?(name, false)
+    parent.const_set(name, original) if had_const
+  end
+
+  def without_linguist
+    had_const = Object.const_defined?(:Linguist, false)
+    original = Object.const_get(:Linguist) if had_const
+    Object.send(:remove_const, :Linguist) if had_const
+    yield
+  ensure
+    Object.const_set(:Linguist, original) if had_const
+  end
+
+  def fake_renderer_module(method_name)
+    Module.new do
+      define_singleton_method(method_name) { |content| "github_markdown:#{content}" }
+    end
+  end
+
+  def instance_renderer_class(method_name, prefix:)
+    Class.new do
+      define_method(:initialize) { |content| @__coverage_content = "#{prefix}:#{content}" }
+      define_method(method_name) { @__coverage_content }
+    end
+  end
+end
diff --git a/test/fixtures/cat.sh b/test/fixtures/cat.sh
new file mode 100755
index 00000000..72d4cb29
--- /dev/null
+++ b/test/fixtures/cat.sh
@@ -0,0 +1,2 @@
+#!/usr/bin/env bash
+cat
diff --git a/test/fixtures/empty.sh b/test/fixtures/empty.sh
new file mode 100755
index 00000000..4251e745
--- /dev/null
+++ b/test/fixtures/empty.sh
@@ -0,0 +1,3 @@
+#!/usr/bin/env bash
+# Always succeeds with empty stdout - used to exercise the
+# CommandImplementation fallback branch when a renderer returns no output.
diff --git a/test/markup_test.rb b/test/markup_test.rb
index ced0b5f8..33450ea7 100644
--- a/test/markup_test.rb
+++ b/test/markup_test.rb
@@ -2,6 +2,8 @@
 
 $LOAD_PATH.unshift File.dirname(__FILE__) + "/../lib"
 
+require_relative 'test_helper'
+require 'github-markup'
 require 'github/markup'
 require 'minitest/autorun'
 require 'html/pipeline'
diff --git a/test/test_helper.rb b/test/test_helper.rb
new file mode 100644
index 00000000..e04c9fdd
--- /dev/null
+++ b/test/test_helper.rb
@@ -0,0 +1,11 @@
+require "simplecov"
+
+SimpleCov.start do
+  enable_coverage :branch
+  add_filter "/test/"
+  add_filter "/spec/"
+  add_filter "/features/"
+  track_files "lib/**/*.rb"
+  command_name "MarkupTests"
+  minimum_coverage line: 100, branch: 100
+end