diff --git a/.github/workflows/ruby.yml b/.github/workflows/ruby.yml
index 7156f41b..79426b97 100644
--- a/.github/workflows/ruby.yml
+++ b/.github/workflows/ruby.yml
@@ -12,7 +12,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        ruby-version: ['2.6', '2.7', '3.0', '3.1', '3.2', '3.3']
+        ruby-version: ['2.6', '2.7', '3.0', '3.1', '3.2', '3.3', '4.0.7']
 
     steps:
     - uses: actions/checkout@v4
diff --git a/.rubocop.yml b/.rubocop.yml
index 80a91934..79cfac9b 100644
--- a/.rubocop.yml
+++ b/.rubocop.yml
@@ -6,7 +6,7 @@ inherit_gem:
   rubocop-jekyll: .rubocop.yml
 
 AllCops:
-  TargetRubyVersion: 2.5
+  TargetRubyVersion: 2.6
   SuggestExtensions: false
   Exclude:
     - vendor/**/*
@@ -18,7 +18,7 @@ Lint/EmptyInPattern:
   Enabled: false
   
 Metrics/AbcSize:
-  IgnoredMethods:
+  AllowedMethods:
   - generate # in generator.rb
 
 Naming/InclusiveLanguage:
diff --git a/.ruby-version b/.ruby-version
new file mode 100644
index 00000000..43beb400
--- /dev/null
+++ b/.ruby-version
@@ -0,0 +1 @@
+4.0.7
diff --git a/Gemfile b/Gemfile
index 1a27d6d2..04d9fb53 100644
--- a/Gemfile
+++ b/Gemfile
@@ -3,9 +3,12 @@
 source "https://rubygems.org"
 gemspec
 
+gem "benchmark" if RUBY_VERSION >= "4.0.0"
 gem "jekyll", ENV["JEKYLL_VERSION"] if ENV["JEKYLL_VERSION"]
 gem "kramdown-parser-gfm" if ENV["JEKYLL_VERSION"] == "~> 3.9"
+gem "ostruct" if RUBY_VERSION >= "4.0.0"
 gem "rss" if RUBY_VERSION >= "3.0.0"
+gem "tsort" if RUBY_VERSION >= "4.0.0"
 
 install_if -> { Gem.win_platform? } do
   gem "tzinfo", "~> 1.2"
diff --git a/jekyll-feed.gemspec b/jekyll-feed.gemspec
index 4ffbc6af..de170045 100644
--- a/jekyll-feed.gemspec
+++ b/jekyll-feed.gemspec
@@ -13,10 +13,9 @@ Gem::Specification.new do |spec|
 
   spec.files            = Dir["lib/**/*"]
   spec.extra_rdoc_files = Dir["README.md", "History.markdown", "LICENSE.txt"]
-  spec.test_files       = spec.files.grep(%r!^spec/!)
   spec.require_paths    = ["lib"]
 
-  spec.required_ruby_version = ">= 2.5.0"
+  spec.required_ruby_version = ">= 2.6.0"
 
   spec.add_dependency "jekyll", ">= 3.7", "< 5.0"
 
@@ -24,6 +23,7 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency "nokogiri", "~> 1.6"
   spec.add_development_dependency "rake", "~> 13.0"
   spec.add_development_dependency "rspec", "~> 3.0"
-  spec.add_development_dependency "rubocop-jekyll", "~> 0.12.0"
+  spec.add_development_dependency "rubocop-jekyll", "~> 0.14"
+  spec.add_development_dependency "simplecov", "~> 0.22"
   spec.add_development_dependency "typhoeus", ">= 0.7", "< 2.0"
 end
diff --git a/lib/jekyll-feed.rb b/lib/jekyll-feed.rb
index 60dd6078..88488827 100644
--- a/lib/jekyll-feed.rb
+++ b/lib/jekyll-feed.rb
@@ -4,6 +4,9 @@
 require "fileutils"
 require "jekyll-feed/generator"
 
+# Namespace for the jekyll-feed plugin: a Jekyll::Generator that builds Atom
+# feed(s) for a site, plus the {% feed_meta %} Liquid tag used to advertise
+# them in a page's <head>.
 module JekyllFeed
   autoload :MetaTag,          "jekyll-feed/meta-tag"
   autoload :PageWithoutAFile, "jekyll-feed/page-without-a-file.rb"
diff --git a/lib/jekyll-feed/generator.rb b/lib/jekyll-feed/generator.rb
index 7a959199..21dc67d0 100644
--- a/lib/jekyll-feed/generator.rb
+++ b/lib/jekyll-feed/generator.rb
@@ -1,6 +1,9 @@
 # frozen_string_literal: true
 
 module JekyllFeed
+  # Jekyll::Generator that adds one or more Atom feed pages to the site
+  # during the build, based on the `feed` key in the site's config
+  # (collections, categories, tags, and their respective output paths).
   class Generator < Jekyll::Generator
     safe true
     priority :lowest
@@ -60,7 +63,7 @@ def collections
 
       @collections = case config["collections"]
                      when Array
-                       config["collections"].map { |c| [c, {}] }.to_h
+                       config["collections"].to_h { |c| [c, {}] }
                      when Hash
                        config["collections"]
                      else
diff --git a/lib/jekyll-feed/meta-tag.rb b/lib/jekyll-feed/meta-tag.rb
index d143c728..1aefa415 100644
--- a/lib/jekyll-feed/meta-tag.rb
+++ b/lib/jekyll-feed/meta-tag.rb
@@ -1,10 +1,15 @@
 # frozen_string_literal: true
 
 module JekyllFeed
+  # Liquid tag that renders the <link> tag used for feed auto-discovery.
+  #
+  # Usage in a template: {% feed_meta %}
   class MetaTag < Liquid::Tag
     # Use Jekyll's native relative_url filter
     include Jekyll::Filters::URLFilters
 
+    # Renders the <link rel="alternate" ...> tag pointing at the site's feed,
+    # using `feed.path` (default "feed.xml") and `title`/`name` from the site config.
     def render(context)
       # Jekyll::Filters::URLFilters requires `@context` to be set in the environment.
       @context = context
diff --git a/lib/jekyll-feed/page-without-a-file.rb b/lib/jekyll-feed/page-without-a-file.rb
index b7d3278a..58a519b5 100644
--- a/lib/jekyll-feed/page-without-a-file.rb
+++ b/lib/jekyll-feed/page-without-a-file.rb
@@ -1,7 +1,13 @@
 # frozen_string_literal: true
 
 module JekyllFeed
+  # A Jekyll::Page whose content and data are set entirely in memory
+  # (e.g., via `file.content = ...` and `file.data.merge!(...)`) rather
+  # than read from a file on disk. Used to generate feed.xml without
+  # requiring a corresponding template file in the site source.
   class PageWithoutAFile < Jekyll::Page
+    # Overrides Jekyll::Page#read_yaml, which would otherwise try to read
+    # and parse front matter from a file that doesn't exist.
     def read_yaml(*)
       @data ||= {}
     end
diff --git a/spec/fixtures/feed/by_tag/ghost_tag.xml b/spec/fixtures/feed/by_tag/ghost_tag.xml
new file mode 100644
index 00000000..77a18594
--- /dev/null
+++ b/spec/fixtures/feed/by_tag/ghost_tag.xml
@@ -0,0 +1 @@
+This tag feed file already exists in the source directory and must not be overwritten.
diff --git a/spec/fixtures/feed/existing.xml b/spec/fixtures/feed/existing.xml
new file mode 100644
index 00000000..8cb0d068
--- /dev/null
+++ b/spec/fixtures/feed/existing.xml
@@ -0,0 +1 @@
+This feed file already exists in the source directory and must not be overwritten.
diff --git a/spec/jekyll-feed_spec.rb b/spec/jekyll-feed_spec.rb
index 6f5d58d0..4dbc8d2c 100644
--- a/spec/jekyll-feed_spec.rb
+++ b/spec/jekyll-feed_spec.rb
@@ -527,6 +527,23 @@ def to_s
       end
     end
 
+    context "when the feed file already exists in the source directory" do
+      let(:overrides) do
+        {
+          "collections" => {
+            "existing" => { "output" => true },
+          },
+          "feed"        => { "collections" => ["existing"] },
+        }
+      end
+
+      it "does not overwrite the existing file" do
+        expect(File.read(dest_dir("feed/existing.xml"))).to eql(
+          File.read(source_dir("feed/existing.xml"))
+        )
+      end
+    end
+
     context "with a custom path" do
       let(:overrides) do
         {
@@ -643,6 +660,18 @@ def to_s
       end
     end
 
+    context "when the tag feed file already exists in the source directory" do
+      let(:overrides) do
+        { "feed" => { "tags" => { "only" => ["ghost_tag"] } } }
+      end
+
+      it "does not overwrite the existing file" do
+        expect(File.read(dest_dir("feed/by_tag/ghost_tag.xml"))).to eql(
+          File.read(source_dir("feed/by_tag/ghost_tag.xml"))
+        )
+      end
+    end
+
     context "set alternate path" do
       let(:overrides) do
         {
diff --git a/spec/spec_helper.rb b/spec/spec_helper.rb
index 7ea33867..624a6c3d 100644
--- a/spec/spec_helper.rb
+++ b/spec/spec_helper.rb
@@ -1,5 +1,11 @@
 # frozen_string_literal: true
 
+require "simplecov"
+SimpleCov.start do
+  enable_coverage :branch
+  add_filter "/spec/"
+end
+
 require "jekyll"
 require "typhoeus" unless Gem.win_platform?
 require "nokogiri"