diff --git a/CHANGELOG.md b/CHANGELOG.md
index 804eae7..b555af5 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,6 +4,12 @@
 
 ### Fixed
 
+- **`docs_kit:page` injected the registry line once per group.** In a registry
+  laid out as blank-line-separated groups (the layout `docs_kit:install`
+  produces), the "page line not followed by a page line" Regexp anchor matched
+  the end of *every* group and Thor's `inject_into_file` replaced every match.
+  The anchor is now the file's last `page` line as a String, so one run adds
+  exactly one line, at the end of the last group.
 - **SEO `og:image` 404.** The og:image tag pointed at the raw config path
   (`https://site/og/og.png`), which isn't a served URL — Propshaft serves the
   digested asset under `/assets`. A relative `og_image` is now resolved through
@@ -18,6 +24,12 @@
 
 ### Added
 
+- **`DocsUI::Code` infers the language from `filename:`.**
+  `DocsUI::Code(<<~YAML, filename: "config/deploy.yml")` now highlights as YAML
+  instead of Ruby: with no `lexer:`, the lexer is guessed from Rouge's filename
+  globs (`*.yml` → yaml, `Dockerfile` → docker, `*.sh` → shell, …). An explicit
+  `lexer:` still wins, and a block with neither (or an unguessable filename)
+  stays Ruby, so existing output is unchanged.
 - **`DocsUI::Landing` hero logo (`c.landing.logo`).** The landing hero now takes
   an optional brand mark above the eyebrow, in two forms: an inline single-path
   SVG (`{ svg: "<path d>", viewbox:, label: }`, rendered with `fill: currentColor`
diff --git a/README.md b/README.md
index 6217c4b..aa56f2e 100644
--- a/README.md
+++ b/README.md
@@ -433,6 +433,10 @@ DocsUI::Section("Add the gem", id: "add", description: …)  # title positional
 DocsUI::Code(source, lexer: :ruby, filename: "Gemfile")   # source positional
 ```
 
+`DocsUI::Code` infers the language from `filename:` (`config/deploy.yml` →
+YAML, `Dockerfile` → docker, `bin/setup.sh` → shell); `lexer:` overrides the
+guess, and with neither the block is highlighted as Ruby.
+
 For the two wrappers that take **no** positional argument — prose and a
 multi-language example — `DocsUI::Page` gives you lowercase helpers so a block
 needs no parens:
diff --git a/app/components/docs_ui/code.rb b/app/components/docs_ui/code.rb
index c8b673d..af778b9 100644
--- a/app/components/docs_ui/code.rb
+++ b/app/components/docs_ui/code.rb
@@ -9,12 +9,16 @@ module DocsUI
   # stylesheet asset is required.
   #
   #   render DocsUI::Code.new(ruby_source)                          # ruby, no title
-  #   render DocsUI::Code.new(py, lexer: :python, filename: "a.py")  # any language
+  #   render DocsUI::Code.new(yaml, filename: "config/deploy.yml")  # yaml, inferred
+  #   render DocsUI::Code.new(py, lexer: :python, filename: "a.py")  # explicit wins
   #
-  # Any language Rouge knows (~200 lexers) works by its name or alias — python,
-  # go, rust, elixir, kotlin, swift, json, dockerfile, ... — no allowlist. Add
-  # friendly lexer aliases via DocsKit.configure (code_lexer_aliases). An unknown
-  # language falls back to plaintext (never raises). (Tab labels are a
+  # The language resolves in order: an explicit `lexer:`; a guess from the
+  # `filename:` (Rouge's own filename globs — *.yml → yaml, Dockerfile → docker,
+  # *.sh → shell, …); else ruby. Any language Rouge knows (~200 lexers) works by
+  # its name or alias — python, go, rust, elixir, kotlin, swift, json,
+  # dockerfile, ... — no allowlist. Add friendly lexer aliases via
+  # DocsKit.configure (code_lexer_aliases). An unknown language falls back to
+  # plaintext (never raises). (Tab labels are a
   # DocsUI::Example concern — set via code_language_labels, not here; Code has no
   # label, only a filename.)
   class Code < Phlex::HTML
@@ -22,7 +26,7 @@ class Code < Phlex::HTML
 
     FORMATTER = Rouge::Formatters::HTML.new
 
-    def initialize(source, lexer: :ruby, filename: nil)
+    def initialize(source, lexer: nil, filename: nil)
       @source = source.to_s.strip
       @lexer = lexer
       @filename = filename
@@ -66,11 +70,26 @@ def title_bar
       end
     end
 
-    # Resolve @lexer to a Rouge lexer instance. Order: an explicit Rouge::Lexer
-    # class/instance passed through; a configured friendly alias; Rouge's own
-    # registry (name/alias); then the configured fallback (plaintext).
+    # Resolve the lexer to a Rouge lexer instance. Order: an explicit Rouge::Lexer
+    # class/instance passed through; an explicit name via a configured friendly
+    # alias → Rouge's own registry → the configured fallback (plaintext); with no
+    # `lexer:` given, a guess from the filename; else the ruby default.
     def lexer
-      explicit_lexer || (find_lexer(@lexer.to_s) || Rouge::Lexers::PlainText).new
+      explicit_lexer ||
+        (@lexer && (find_lexer(@lexer.to_s) || Rouge::Lexers::PlainText).new) ||
+        guessed_lexer ||
+        Rouge::Lexers::Ruby.new
+    end
+
+    # A lexer inferred from @filename via Rouge's declared filename globs, or nil
+    # when there is no filename, nothing matches, or the match is ambiguous.
+    # (Lexer.guesses, not Lexer.guess: the latter answers PlainText for a
+    # no-match, which is indistinguishable from a real guess.)
+    def guessed_lexer
+      return nil if @filename.nil?
+
+      guesses = Rouge::Lexer.guesses(filename: @filename.to_s)
+      guesses.size == 1 ? guesses.first.new : nil
     end
 
     # A Rouge::Lexer instance passed directly (class or instance), else nil.
diff --git a/docs/app/views/docs/pages/authoring.rb b/docs/app/views/docs/pages/authoring.rb
index 1be8beb..62db325 100644
--- a/docs/app/views/docs/pages/authoring.rb
+++ b/docs/app/views/docs/pages/authoring.rb
@@ -154,7 +154,7 @@ def building_blocks_section
 
             md <<~'MD'
               The primary argument is always positional —
-              `Section("Title")`, `Code(source)`, `Header("Title")` — with
+              `Section("Title")`, `Code(source, filename:)` (the filename picks the language), `Header("Title")` — with
               modifiers as keywords (`description:`, `eyebrow:`).
 
               For the wrappers that take no argument, use the lowercase page
diff --git a/docs/app/views/docs/pages/components.rb b/docs/app/views/docs/pages/components.rb
index 6d669b2..e1ddb21 100644
--- a/docs/app/views/docs/pages/components.rb
+++ b/docs/app/views/docs/pages/components.rb
@@ -228,8 +228,8 @@ class User < ApplicationRecord
             render DocsUI::PropTable.new(
               [
                 [ "source", "String", "—", "The code to highlight." ],
-                [ "lexer", "Symbol", ":ruby", "Any Rouge language — :shell, :yaml, :erb, :python, :go, etc." ],
-                [ "filename", "String, nil", "nil", "Optional filename bar above the block." ]
+                [ "lexer", "Symbol", "inferred", "Any Rouge language — :shell, :yaml, :erb, :python, :go, etc. Overrides the filename guess; ruby when neither is given." ],
+                [ "filename", "String, nil", "nil", "Optional filename bar above the block. Also selects the language (*.yml → yaml, Dockerfile → docker, *.sh → shell)." ]
               ],
               headers: [ "Arg", "Type", "Default", "Description" ]
             )
diff --git a/lib/generators/docs_kit/install/templates/agents_md.erb b/lib/generators/docs_kit/install/templates/agents_md.erb
index f03b346..168e4e4 100644
--- a/lib/generators/docs_kit/install/templates/agents_md.erb
+++ b/lib/generators/docs_kit/install/templates/agents_md.erb
@@ -66,6 +66,9 @@ end
 - **The primary argument is positional; modifiers are keywords.**
   `Section("Title", description:)`, `Code(source, filename:)`,
   `Header("Title", eyebrow:)`.
+- **`Code`'s `filename:` selects the language** (`*.yml` → yaml, `Dockerfile`
+  → docker, `*.sh` → shell, …); pass `lexer:` only to override the guess or when
+  there is no filename (the default is ruby).
 - **Wrappers that take no positional arg use lowercase page helpers** so a block
   needs no parens: `md <<~'MD' … MD`, `prose { … }`, `example { |ex| … }`,
   `operation "operationId"`. (A bare `DocsUI::Prose do` is a Ruby SyntaxError; the
diff --git a/lib/generators/docs_kit/page/page_generator.rb b/lib/generators/docs_kit/page/page_generator.rb
index 963ad74..5c03f7e 100644
--- a/lib/generators/docs_kit/page/page_generator.rb
+++ b/lib/generators/docs_kit/page/page_generator.rb
@@ -101,13 +101,19 @@ def legacy_entries?(source)
         source.match?(/^\s*entries\s*\[/) && !source.match?(/^\s*page\s+["']/)
       end
 
-      # Inject after the last existing `page` line so ordering lands at the end
-      # of the group; else after view_namespace/path_prefix; else after the
-      # `extend DocsKit::Registry` line.
+      # Inject after the last existing `page` line of the FILE so ordering lands
+      # at the end of the last group; else after view_namespace/path_prefix;
+      # else after the `extend DocsKit::Registry` line.
+      #
+      # The `page` anchor is the last matching line as a String, not a Regexp:
+      # Thor's inject_into_file replaces EVERY match of a Regexp `after:`, so a
+      # "page line not followed by a page line" pattern fires once per group in
+      # a registry laid out with blank-line-separated groups (the layout
+      # docs_kit:install produces) and duplicates the entry.
       def registry_anchor(source)
         case source
         when /^\s*page\s+["']/
-          /^\s*page .*\n(?!\s*page )/
+          source.lines.grep(/^\s*page\s+["']/).last
         when /^\s*view_namespace\s/
           /^\s*view_namespace .*\n/
         when /^\s*path_prefix\s/
diff --git a/spec/docs_ui/code_spec.rb b/spec/docs_ui/code_spec.rb
index 43c86a8..3c5be59 100644
--- a/spec/docs_ui/code_spec.rb
+++ b/spec/docs_ui/code_spec.rb
@@ -123,6 +123,41 @@ def csp_nonce = "testnonce"
     end
   end
 
+  # `filename:` selects the language when `lexer:` is not given — the natural
+  # `DocsUI::Code(<<~YAML, filename: "config/deploy.yml")` highlights as YAML
+  # instead of silently lexing as Ruby.
+  describe "lexer inference from filename:" do
+    it "guesses the lexer from the filename extension" do
+      html = described_class.new("foo: bar\nbaz: [1, 2]", filename: "config/deploy.yml").call
+
+      expect(html).to include('data-md-lang="yaml"')
+      expect(html).not_to include('class="err"')
+    end
+
+    it "guesses from a well-known basename (Dockerfile)" do
+      resolved = described_class.new("FROM ruby", filename: "Dockerfile").send(:lexer)
+      expect(resolved).to be_a(Rouge::Lexers::Docker)
+    end
+
+    it "lets an explicit lexer: win over the filename" do
+      html = described_class.new("puts 'hi'", filename: "x.yml", lexer: :ruby).call
+
+      expect(html).to include('data-md-lang="ruby"')
+    end
+
+    it "falls back to ruby for an unguessable filename" do
+      html = described_class.new("puts 'hi'", filename: "notes").call
+
+      expect(html).to include('data-md-lang="ruby"')
+    end
+
+    it "still defaults to ruby with no filename and no lexer" do
+      html = described_class.new("puts 'hi'").call
+
+      expect(html).to include('data-md-lang="ruby"')
+    end
+  end
+
   it "falls back to plaintext for an unknown lexer" do
     html = described_class.new("anything", lexer: :nope).call
 
diff --git a/spec/generators/page_generator_spec.rb b/spec/generators/page_generator_spec.rb
index 174708a..72665cc 100644
--- a/spec/generators/page_generator_spec.rb
+++ b/spec/generators/page_generator_spec.rb
@@ -119,6 +119,49 @@ def silence_stream
     end
   end
 
+  describe "a grouped registry (blank lines + comments between page groups)" do
+    # The layout docs_kit:install itself produces: groups of `page` lines
+    # separated by a blank line and a `# comment`. The anchor must be the LAST
+    # page line of the FILE, not the last line of every group.
+    let(:grouped) do
+      <<~RUBY
+        # frozen_string_literal: true
+
+        class Doc
+          extend DocsKit::Registry
+          path_prefix "/docs"
+          view_namespace "Views::Docs::Pages"
+
+          # Guide
+          page "Installation", group: "Guide"
+          page "Configuration", group: "Guide"
+
+          # Deploying
+          page "Docker", group: "Deploying"
+          page "Tunnel", group: "Deploying"
+        end
+      RUBY
+    end
+
+    before { seed_registry(grouped) }
+
+    it "adds exactly one line, after the last page line of the file" do
+      run_generator(["New Page"], { "group" => "Deploying" })
+
+      doc = read("app/models/doc.rb")
+      expect(doc.scan(%(page "New Page")).size).to eq(1)
+      expect(doc.index(%(page "Tunnel"))).to be < doc.index(%(page "New Page"))
+      expect(doc.scan(/^\s*page /).size).to eq(5)
+    end
+
+    it "is idempotent on a second run" do
+      run_generator(["New Page"], { "group" => "Deploying" })
+      run_generator(["New Page"], { "group" => "Deploying", "skip" => true })
+
+      expect(read("app/models/doc.rb").scan(%(page "New Page")).size).to eq(1)
+    end
+  end
+
   describe "flag overrides" do
     before { seed_registry(registry_v2) }