diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 4cfd977..1c6ead0 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -48,6 +48,29 @@ jobs:
       - name: Lint code for consistent style
         run: bin/rubocop
 
+  herb:
+    name: Herb check
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+    steps:
+      - name: Install packages
+        run: sudo apt-get update && sudo apt-get install --no-install-recommends -y libvips
+
+      - name: Checkout code
+        uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+        with:
+          persist-credentials: false
+
+      - name: Set up Ruby
+        uses: ruby/setup-ruby@a0102e0972be65f351c307e2d64b9314a57c8073 # v1.324.0
+        with:
+          ruby-version: .ruby-version
+          bundler-cache: true
+
+      - name: Check that every HTML template compiles through Herb
+        run: bin/rails herb:check
+
   lint-actions:
     name: GitHub Actions audit
     runs-on: ubuntu-latest
diff --git a/Gemfile.lock b/Gemfile.lock
index 3d02eff..65641b8 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -32,7 +32,7 @@ GIT
 
 GIT
   remote: https://github.com/rails/rails.git
-  revision: 1a02651ac37fb64b4de2a2b73461d86acf9c98fb
+  revision: e3d5c569d217c56c022b04acb127a8c24dbe9685
   branch: main
   specs:
     actioncable (8.2.0.alpha)
@@ -63,10 +63,10 @@ GIT
       rack-session (>= 1.0.1)
       rack-test (>= 0.6.3)
       rails-dom-testing (~> 2.2)
-      rails-html-sanitizer (~> 1.6)
+      rails-html-sanitizer (~> 1.7)
       useragent (~> 0.16)
     actiontext (8.2.0.alpha)
-      action_text-trix (~> 2.1.15)
+      action_text-trix (~> 2.1.16)
       actionpack (= 8.2.0.alpha)
       activerecord (= 8.2.0.alpha)
       activestorage (= 8.2.0.alpha)
@@ -77,11 +77,12 @@ GIT
       activesupport (= 8.2.0.alpha)
       builder (~> 3.1)
       erubi (~> 1.11)
+      herb (>= 0.10)
       rails-dom-testing (~> 2.2)
-      rails-html-sanitizer (~> 1.6)
+      rails-html-sanitizer (~> 1.7)
     activejob (8.2.0.alpha)
       activesupport (= 8.2.0.alpha)
-      globalid (>= 0.3.6)
+      globalid (>= 1.4.0)
     activemodel (8.2.0.alpha)
       activesupport (= 8.2.0.alpha)
     activerecord (8.2.0.alpha)
@@ -93,7 +94,7 @@ GIT
       activejob (= 8.2.0.alpha)
       activerecord (= 8.2.0.alpha)
       activesupport (= 8.2.0.alpha)
-      marcel (~> 1.0)
+      marcel (~> 2.0)
     activesupport (8.2.0.alpha)
       base64
       bigdecimal
@@ -104,6 +105,8 @@ GIT
       json
       logger (>= 1.4.2)
       minitest (>= 5.1)
+      psych (>= 4)
+      ractor-dispatch (>= 0.3.0)
       securerandom (>= 0.3)
       tzinfo (~> 2.0, >= 2.0.5)
       uri (>= 0.13.1)
@@ -170,7 +173,7 @@ GEM
       irb (~> 1.10)
       reline (>= 0.3.8)
     drb (2.2.3)
-    erb (6.0.4)
+    erb (6.0.7)
     erubi (1.13.1)
     faker (3.8.0)
       i18n (>= 1.8.11, < 2)
@@ -182,17 +185,22 @@ GEM
     geared_pagination (1.2.0)
       activesupport (>= 5.0)
       addressable (>= 2.5.0)
-    globalid (1.3.0)
+    globalid (1.4.0)
       activesupport (>= 6.1)
     hashdiff (1.2.0)
+    herb (0.11.0-aarch64-linux-gnu)
+    herb (0.11.0-arm64-darwin)
+    herb (0.11.0-x86_64-darwin)
+    herb (0.11.0-x86_64-linux-gnu)
     i18n (1.15.2)
       concurrent-ruby (~> 1.0)
     image_processing (1.14.0)
       mini_magick (>= 4.9.5, < 6)
       ruby-vips (>= 2.0.17, < 3)
-    io-console (0.8.1)
-    irb (1.15.3)
+    io-console (0.9.4)
+    irb (1.18.0)
       pp (>= 0.6.0)
+      prism (>= 1.3.0)
       rdoc (>= 4.0.0)
       reline (>= 0.4.2)
     jbuilder (2.14.1)
@@ -219,7 +227,7 @@ GEM
       net-imap
       net-pop
       net-smtp
-    marcel (1.1.0)
+    marcel (2.1.0)
     matrix (0.4.3)
     mini_magick (5.3.1)
       logger
@@ -235,14 +243,14 @@ GEM
       ruby2_keywords (~> 0.0.1)
     net-http-persistent (4.0.6)
       connection_pool (~> 2.2, >= 2.2.4)
-    net-imap (0.6.4.1)
+    net-imap (0.6.7)
       date
       net-protocol
     net-pop (0.1.2)
       net-protocol
-    net-protocol (0.2.2)
+    net-protocol (0.4.0)
       timeout
-    net-smtp (0.5.1)
+    net-smtp (0.5.2)
       net-protocol
     nio4r (2.7.5)
     nokogiri (1.19.4-aarch64-linux-gnu)
@@ -262,11 +270,11 @@ GEM
     platform_agent (1.0.1)
       activesupport (>= 5.2.0)
       useragent (~> 0.16.3)
-    pp (0.6.3)
+    pp (0.6.4)
       prettyprint
     prettyprint (0.2.0)
     prism (1.9.0)
-    psych (5.2.6)
+    psych (5.5.0)
       date
       stringio
     public_suffix (6.0.2)
@@ -283,8 +291,9 @@ GEM
       rack (>= 3.0.0)
     rack-test (2.2.0)
       rack (>= 1.3)
-    rackup (2.2.1)
+    rackup (2.3.1)
       rack (>= 3)
+    ractor-dispatch (0.3.0)
     rails-dom-testing (2.3.0)
       activesupport (>= 5.0.0)
       minitest
@@ -297,19 +306,24 @@ GEM
       activesupport (> 3.1)
       railties (> 3.1)
     rainbow (3.1.1)
-    rake (13.3.1)
-    rdoc (6.16.1)
+    rake (13.4.2)
+    rbs (4.2.0)
+      logger
+      prism (>= 1.6.0)
+      tsort
+    rdoc (8.1.0)
       erb
-      psych (>= 4.0.0)
+      prism (>= 1.6.0)
+      rbs (>= 4.0.0)
       tsort
     redis (5.4.1)
       redis-client (>= 0.22.0)
-    redis-client (0.25.2)
+    redis-client (0.30.1)
       connection_pool
     redis-namespace (1.11.0)
       redis (>= 4)
     regexp_parser (2.11.2)
-    reline (0.6.3)
+    reline (0.7.0)
       io-console (~> 0.5)
     resque (2.7.0)
       mono_logger (~> 1)
@@ -384,8 +398,8 @@ GEM
     sqlite3 (2.9.6-x86_64-linux-gnu)
     stimulus-rails (1.3.4)
       railties (>= 6.0.0)
-    stringio (3.1.8)
-    thor (1.4.0)
+    stringio (3.2.0)
+    thor (1.5.0)
     thruster (0.1.23-aarch64-linux)
     thruster (0.1.23-arm64-darwin)
     thruster (0.1.23-x86_64-darwin)
@@ -414,7 +428,7 @@ GEM
     websocket-extensions (0.1.5)
     xpath (3.2.0)
       nokogiri (~> 1.8)
-    zeitwerk (2.7.3)
+    zeitwerk (2.8.3)
 
 PLATFORMS
   aarch64-linux
diff --git a/app/models/user/mentionable.rb b/app/models/user/mentionable.rb
index b028284..a48df75 100644
--- a/app/models/user/mentionable.rb
+++ b/app/models/user/mentionable.rb
@@ -11,6 +11,11 @@ module User::Mentionable
     "users/mention"
   end
 
+  # How a mention appears inside the editor, matching the prompt's editor template.
+  def to_editor_content_attachment_partial_path
+    "users/mention"
+  end
+
   def attachable_plain_text_representation(caption)
     "@#{name}"
   end
diff --git a/app/views/accounts/edit.html.erb b/app/views/accounts/edit.html.erb
index cfa750d..f2d1ae8 100644
--- a/app/views/accounts/edit.html.erb
+++ b/app/views/accounts/edit.html.erb
@@ -76,10 +76,9 @@
         <% end %>
 
         <label class="switch">
-          <input type="checkbox"
-                class="switch__input"
-                <%= "checked" if Current.account.settings.restrict_room_creation_to_administrators? %>
-                data-action="change->form#submit">
+          <%= tag.input type: "checkbox", class: "switch__input",
+                checked: Current.account.settings.restrict_room_creation_to_administrators?,
+                data: { action: "change->form#submit" } %>
           <span class="switch__btn round"></span>
           <span class="for-screen-reader">
             Must be admin to create new rooms
diff --git a/app/views/pwa/_browser_settings.html.erb b/app/views/pwa/_browser_settings.html.erb
index 70144f1..985f6f1 100644
--- a/app/views/pwa/_browser_settings.html.erb
+++ b/app/views/pwa/_browser_settings.html.erb
@@ -6,8 +6,8 @@
       <%= image_tag "disclosure.svg", aria: { hidden: "true" }, size: 10, class: "disclosure" %>
     </summary>
 
-    <% case
-      when platform.firefox? && platform.android? %>
+    <% case %>
+      <% when platform.firefox? && platform.android? %>
         <ol>
           <li>Tap <em><%= image_tag "lock.svg", alt: "the View site information button", size: 20 %></em> in the address bar.</li>
           <li>Tap <em>Notification</em> to change to <em>Allowed</em>.</li>
diff --git a/app/views/pwa/_install_instructions.html.erb b/app/views/pwa/_install_instructions.html.erb
index b82ddb2..fcfe889 100644
--- a/app/views/pwa/_install_instructions.html.erb
+++ b/app/views/pwa/_install_instructions.html.erb
@@ -6,8 +6,8 @@
       <%= image_tag "disclosure.svg", aria: { hidden: "true" }, size: 10, class: "disclosure" %>
     </summary>
 
-    <% case
-      when platform.edge? %>
+    <% case %>
+      <% when platform.edge? %>
         <ol>
           <li>Click <em><%= image_tag "external/install-edge.svg", alt: "the app available - install Campfire chat button", size: 16 %></em>in the address bar.</li>
           <li>Click <em>Install</em>.</li>
diff --git a/app/views/pwa/_system_settings.html.erb b/app/views/pwa/_system_settings.html.erb
index 6c6dc7b..3edccda 100644
--- a/app/views/pwa/_system_settings.html.erb
+++ b/app/views/pwa/_system_settings.html.erb
@@ -5,8 +5,8 @@
     <%= image_tag "disclosure.svg", aria: { hidden: "true" }, size: 10, class: "disclosure" %>
   </summary>
 
-  <% case
-    when platform.firefox? && platform.android? %>
+  <% case %>
+    <% when platform.firefox? && platform.android? %>
       <ol>
         <li>Tap the <em><%= image_tag "menu-dots-vertical.svg", alt: "More options", size: 16 %></em> menu button.</li>
         <li>Tap <em>Settings</em>.</li>
diff --git a/config/environments/development.rb b/config/environments/development.rb
index 7994eb0..a25cb62 100644
--- a/config/environments/development.rb
+++ b/config/environments/development.rb
@@ -80,5 +80,5 @@ Rails.application.configure do
   config.action_controller.raise_on_missing_callback_actions = true
 
   # Visit /rails/locks to see the locks
-  config.middleware.insert_before Rack::Sendfile, ActionDispatch::DebugLocks
+  config.middleware.insert_before ActionDispatch::Executor, ActionDispatch::DebugLocks
 end
diff --git a/config/initializers/sentry.rb b/config/initializers/sentry.rb
index 2706bc1..2d32885 100644
--- a/config/initializers/sentry.rb
+++ b/config/initializers/sentry.rb
@@ -6,3 +6,11 @@ if Rails.env.production? && ENV["SKIP_TELEMETRY"].blank?
     config.release = ENV["GIT_REVISION"]
   end
 end
+
+# sentry-rails declares its Action Cable handle_open and handle_close wrappers private,
+# but Rails 8.2 calls them from outside the connection, so every /cable connection fails.
+# Remove once sentry-rails ships getsentry/sentry-ruby#2972 (issue #2975).
+ActiveSupport.on_load(:action_cable_connection) do
+  require "sentry/rails/action_cable"
+  Sentry::Rails::ActionCableExtensions::Connection.send(:public, :handle_open, :handle_close)
+end
diff --git a/config/initializers/time_formats.rb b/config/initializers/time_formats.rb
index 07e300e..9360926 100644
--- a/config/initializers/time_formats.rb
+++ b/config/initializers/time_formats.rb
@@ -1,2 +1,2 @@
 # Used to match JavaScripts (new Date).getTime() for sorting
-Time::DATE_FORMATS[:epoch] = ->(time) { (time.to_f * 1000).to_i }
+ActiveSupport::TimeFormats.register(:epoch, ->(time) { (time.to_f * 1000).to_i })
diff --git a/test/channels/unread_rooms_channel_test.rb b/test/channels/unread_rooms_channel_test.rb
index 4444eb1..98cd236 100644
--- a/test/channels/unread_rooms_channel_test.rb
+++ b/test/channels/unread_rooms_channel_test.rb
@@ -8,7 +8,7 @@ class UnreadRoomsChannelTest < ActionCable::Channel::TestCase
 
     assert subscription.confirmed?
     assert_has_stream "user_#{users(:jz).id}_unreads"
-    assert_not_includes subscription.streams, "unread_rooms"
+    assert_not_includes subscription.stream_names, "unread_rooms"
   end
 
   test "an outsider is not told about activity in a room they can't see" do
@@ -37,7 +37,7 @@ class UnreadRoomsChannelTest < ActionCable::Channel::TestCase
       stub_connection(current_user: user)
       subscribe
 
-      stream = subscription.streams.sole
+      stream = subscription.stream_names.sole
       before = ActionCable.server.pubsub.broadcasts(stream).size
 
       yield
diff --git a/test/controllers/accounts_controller_test.rb b/test/controllers/accounts_controller_test.rb
index 8292c6f..8cbe222 100644
--- a/test/controllers/accounts_controller_test.rb
+++ b/test/controllers/accounts_controller_test.rb
@@ -10,6 +10,16 @@ class AccountsControllerTest < ActionDispatch::IntegrationTest
     assert_response :ok
   end
 
+  test "edit shows whether room creation is restricted to administrators" do
+    get edit_account_url
+    assert_select "label.switch input.switch__input[type=checkbox]:not([checked])"
+
+    accounts(:signal).update!(settings: { restrict_room_creation_to_administrators: true })
+
+    get edit_account_url
+    assert_select "label.switch input.switch__input[type=checkbox][checked][data-action='change->form#submit']"
+  end
+
   test "edit groups administrators separately from members with a divider" do
     get edit_account_url
 
diff --git a/test/controllers/rooms_controller_test.rb b/test/controllers/rooms_controller_test.rb
index 62edf72..dfba3bb 100644
--- a/test/controllers/rooms_controller_test.rb
+++ b/test/controllers/rooms_controller_test.rb
@@ -15,6 +15,21 @@ class RoomsControllerTest < ActionDispatch::IntegrationTest
     assert_response :success
   end
 
+  test "show renders notification help for each platform" do
+    {
+      "Firefox on Android" => "Mozilla/5.0 (Android 14; Mobile; rv:131.0) Gecko/131.0 Firefox/131.0",
+      "Chrome on Android" => "Mozilla/5.0 (Linux; Android 14; Pixel 8) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/141.0.0.0 Mobile Safari/537.36",
+      "Firefox on desktop" => "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:131.0) Gecko/20100101 Firefox/131.0",
+      "Chrome on desktop" => "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/141.0.0.0 Safari/537.36",
+      "Safari on macOS" => "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/18.0 Safari/605.1.15"
+    }.each do |platform, user_agent|
+      get room_url(users(:david).rooms.last), headers: { "User-Agent" => user_agent }
+
+      assert_response :success, platform
+      assert_select ".notifications-help ol", { minimum: 1 }, platform
+    end
+  end
+
   test "shows records the last room visited in a cookie" do
     get room_url(users(:david).rooms.last)
     assert response.cookies[:last_room] = users(:david).rooms.last.id
diff --git a/test/test_helper.rb b/test/test_helper.rb
index 5c4ac31..5a998e9 100644
--- a/test/test_helper.rb
+++ b/test/test_helper.rb
@@ -2,7 +2,6 @@ ENV["RAILS_ENV"] ||= "test"
 require_relative "../config/environment"
 
 require "rails/test_help"
-require "minitest/unit"
 require "mocha/minitest"
 require "webmock/minitest"
 require "turbo/broadcastable/test_helper"