diff --git a/app/assets/javascripts/discourse/app/lib/text.js b/app/assets/javascripts/discourse/app/lib/text.js
index 6e7def0ecdb..635a5c44b2a 100644
--- a/app/assets/javascripts/discourse/app/lib/text.js
+++ b/app/assets/javascripts/discourse/app/lib/text.js
@@ -23,6 +23,7 @@ function getOpts(opts) {
       formatUsername,
       watchedWordsReplace: context.site.watched_words_replace,
       watchedWordsLink: context.site.watched_words_link,
+      additionalOptions: context.site.markdown_additional_options,
     },
     opts
   );
diff --git a/app/assets/javascripts/pretty-text/addon/pretty-text.js b/app/assets/javascripts/pretty-text/addon/pretty-text.js
index 6654f9cc244..80138a7ab39 100644
--- a/app/assets/javascripts/pretty-text/addon/pretty-text.js
+++ b/app/assets/javascripts/pretty-text/addon/pretty-text.js
@@ -40,6 +40,7 @@ export function buildOptions(state) {
     watchedWordsLink,
     featuresOverride,
     markdownItRules,
+    additionalOptions,
   } = state;
 
   let features = {};
@@ -80,6 +81,7 @@ export function buildOptions(state) {
     watchedWordsLink,
     featuresOverride,
     markdownItRules,
+    additionalOptions,
   };
 
   // note, this will mutate options due to the way the API is designed
diff --git a/app/models/site.rb b/app/models/site.rb
index d8ba6cf083c..4442a12e272 100644
--- a/app/models/site.rb
+++ b/app/models/site.rb
@@ -7,6 +7,20 @@ class Site
   cattr_accessor :preloaded_category_custom_fields
   self.preloaded_category_custom_fields = Set.new
 
+  ##
+  # Sometimes plugins need to have additional data or options available
+  # when rendering custom markdown features/rules that are not available
+  # on the default opts.discourse object. These additional options should
+  # be namespaced to the plugin adding them.
+  #
+  # ```
+  # Site.markdown_additional_options["chat"] = { limited_pretty_text_markdown_rules: [] }
+  # ```
+  #
+  # These are passed down to markdown rules on opts.discourse.additionalOptions.
+  cattr_accessor :markdown_additional_options
+  self.markdown_additional_options = {}
+
   def self.add_categories_callbacks(&block)
     categories_callbacks << block
   end
diff --git a/app/serializers/site_serializer.rb b/app/serializers/site_serializer.rb
index c65628748f3..7b16d6f1a64 100644
--- a/app/serializers/site_serializer.rb
+++ b/app/serializers/site_serializer.rb
@@ -32,7 +32,8 @@ class SiteSerializer < ApplicationSerializer
     :custom_emoji_translation,
     :watched_words_replace,
     :watched_words_link,
-    :categories
+    :categories,
+    :markdown_additional_options
   )
 
   has_many :archetypes, embed: :objects, serializer: ArchetypeSerializer
@@ -203,6 +204,10 @@ class SiteSerializer < ApplicationSerializer
     object.categories.map { |c| c.to_h }
   end
 
+  def markdown_additional_options
+    Site.markdown_additional_options
+  end
+
   private
 
   def ordered_flags(flags)
diff --git a/lib/pretty_text.rb b/lib/pretty_text.rb
index a5885ce22d6..558a1880ea6 100644
--- a/lib/pretty_text.rb
+++ b/lib/pretty_text.rb
@@ -201,6 +201,7 @@ module PrettyText
         __optInput.censoredRegexp = #{WordWatcher.word_matcher_regexp(:censor)&.source.to_json};
         __optInput.watchedWordsReplace = #{WordWatcher.word_matcher_regexps(:replace).to_json};
         __optInput.watchedWordsLink = #{WordWatcher.word_matcher_regexps(:link).to_json};
+        __optInput.additionalOptions = #{Site.markdown_additional_options.to_json};
       JS
 
       if opts[:topic_id]
diff --git a/spec/requests/api/schemas/json/site_response.json b/spec/requests/api/schemas/json/site_response.json
index b1a2a0c325b..a65eea3cf67 100644
--- a/spec/requests/api/schemas/json/site_response.json
+++ b/spec/requests/api/schemas/json/site_response.json
@@ -466,6 +466,9 @@
         "null"
       ]
     },
+    "markdown_additional_options" : {
+      "type": "object"
+    },
     "categories": {
       "type": "array",
       "items": [