feat: support markdown

Gemfile

@@ -44,6 +44,7 @@ gem "net-http-persistent"
 gem "kredis"
 gem "platform_agent"
 gem "thruster"
+gem "redcarpet"
 
 group :development, :test do
   gem "debug"

Gemfile.lock

@@ -288,6 +288,7 @@ GEM
       erb
       psych (>= 4.0.0)
       tsort
+    redcarpet (3.6.1)
     redis (5.4.1)
       redis-client (>= 0.22.0)
     redis-client (0.25.2)
@@ -428,6 +429,7 @@ DEPENDENCIES
   puma (~> 6.6)
   rails!
   rails_autolink
+  redcarpet
   redis (~> 5.4)
   resque (~> 2.7.0)
   resque-pool (~> 0.7.1)

app/helpers/content_filters.rb

@@ -1,3 +1,3 @@
 module ContentFilters
-  TextMessagePresentationFilters = ActionText::Content::Filters.new(RemoveSoloUnfurledLinkText, StyleUnfurledTwitterAvatars, SanitizeTags)
+  TextMessagePresentationFilters = ActionText::Content::Filters.new(MarkdownFilter, RemoveSoloUnfurledLinkText, StyleUnfurledTwitterAvatars, SanitizeTags)
 end

app/helpers/content_filters/markdown_filter.rb

@@ -0,0 +1,127 @@
+class ContentFilters::MarkdownFilter < ActionText::Content::Filter
+  # Markdown pattern detection
+  # NOTE: Underscore-based emphasis (__bold__ and _italic_) is intentionally not supported
+  # to avoid false positives with code identifiers like __init__ and my_variable_name
+  #
+  # NOTE: List patterns require at least 2 consecutive items to trigger detection.
+  # This prevents false positives like "* walks into room *" or "- Not sure about that"
+  # while still catching legitimate multi-item lists.
+  #
+  # NOTE: To type literal markdown characters, users have two options:
+  # 1. Use backticks for inline code: `**not bold**` → renders as <code>**not bold**</code>
+  # 2. Use backslash escaping: \*\*not bold\*\* → renders as **not bold** (plain text)
+  MARKDOWN_PATTERNS = [
+    /\*\*[^*]+\*\*/,                    # Bold: **text**
+    /(?<!^)\*[^*\s][^*]*\*/m,           # Italic: *text* (not at line start to avoid "* list items")
+    /`[^`]+`/,                          # Inline code: `code`
+    /```[\s\S]+?```/,                   # Code blocks: ```code```
+    /^[#]{1,6}\s/m,                     # Header levels 1-6: # Header
+    /(?:^\*\s.+$\n?){2,}/m,             # Unordered lists: requires 2+ items to avoid false positives
+    /(?:^-\s.+$\n?){2,}/m,              # Unordered lists alt: requires 2+ items to avoid false positives
+    /(?:^\d+\.\s.+$\n?){2,}/m,          # Ordered lists: requires 2+ items to avoid false positives
+    /\[.+?\]\(.+?\)/,                   # Links: [text](url)
+    /~~.+?~~/,                          # Strikethrough: ~~text~~
+    /^>\s/m,                            # Blockquotes: > quote
+    /^---+$/m                           # Horizontal rule: ---
+  ].freeze
+
+  def self.has_markdown?(text)
+    MARKDOWN_PATTERNS.any? { |pattern| text.match?(pattern) }
+  end
+
+  def applicable?
+    has_markdown? && !has_attachments?
+  end
+
+  def apply
+    # Pre-process text to ensure lists are properly separated from surrounding content
+    preprocessed_text = normalize_lists(plain_text_content)
+
+    # Convert markdown to HTML using Redcarpet
+    markdown_html = self.class.markdown_renderer.render(preprocessed_text)
+
+    # Replace the entire fragment with the rendered markdown
+    fragment.update do |source|
+      source.inner_html = markdown_html
+    end
+  end
+
+  private
+    def has_markdown?
+      self.class.has_markdown?(plain_text_content)
+    end
+
+    def has_attachments?
+      # Skip markdown rendering if ActionText attachments are present
+      # to avoid destroying them when we replace inner_html
+      fragment.find_all("action-text-attachment").any?
+    end
+
+    def plain_text_content
+      fragment.to_plain_text
+    end
+
+    def normalize_lists(text)
+      # Add blank lines before and after lists to ensure Redcarpet renders them correctly
+      # even when they're surrounded by other text.
+      # This handles cases like:
+      # - "Hello\n* item 1\n* item 2" -> "Hello\n\n* item 1\n* item 2"
+      # - "* item 1\n* item 2\nGoodbye" -> "* item 1\n* item 2\n\nGoodbye"
+
+      lines = text.split("\n", -1)
+      result = []
+      in_list = false
+      list_pattern = /^(\*|-|\d+\.)\s/
+
+      lines.each_with_index do |line, i|
+        is_list_item = line.match?(list_pattern)
+        prev_line = i > 0 ? lines[i - 1] : nil
+
+        # Starting a list: add blank line before if previous line exists and isn't blank
+        if is_list_item && !in_list
+          if prev_line && !prev_line.strip.empty?
+            result << ""
+          end
+          in_list = true
+        end
+
+        # Ending a list: add blank line after
+        if !is_list_item && in_list && !line.strip.empty?
+          result << ""
+          in_list = false
+        end
+
+        # Exiting list at blank line
+        if line.strip.empty?
+          in_list = false
+        end
+
+        result << line
+      end
+
+      result.join("\n")
+    end
+
+    def self.markdown_renderer
+      @markdown_renderer ||= Redcarpet::Markdown.new(
+        Redcarpet::Render::HTML.new(
+          filter_html: true,           # Strip HTML tags for security
+          safe_links_only: true,       # Block javascript: and data: URLs
+          no_styles: true,              # Remove inline styles
+          hard_wrap: true,              # Convert single newlines to <br> tags
+          link_attributes: { target: "_blank", rel: "noopener noreferrer" }
+        ),
+        autolink: true,                           # Convert plain URLs to links
+        disable_indented_code_blocks: true,       # Disable 4-space indented code blocks
+        fenced_code_blocks: true,                 # Allow ``` code blocks
+        footnotes: false,                         # Footnotes not needed in chat
+        highlight: false,                         # Syntax highlighting handled separately
+        quote: true,                              # Enable blockquotes with >
+        no_intra_emphasis: true,                  # Prevent emphasis_within_words
+        space_after_headers: true,                # Require space after # for headers
+        strikethrough: true,                      # Enable ~~strikethrough~~
+        tables: false,                            # Tables not supported in chat UI
+        underline: false                          # Underline not supported
+      )
+    end
+end

app/helpers/messages_helper.rb

@@ -57,7 +57,18 @@ def message_presentation(message)
     when "sound"
       message_sound_presentation(message)
     else
-      auto_link h(ContentFilters::TextMessagePresentationFilters.apply(message.body.body)), html: { target: "_blank" }
+      # Check for markdown ONCE before filtering (optimization to avoid double checking)
+      has_markdown = message.body.present? && ContentFilters::MarkdownFilter.has_markdown?(message.plain_text_body)
+
+      filtered_content = ContentFilters::TextMessagePresentationFilters.apply(message.body.body)
+
+      # Only apply auto_link if the message doesn't have markdown
+      # (markdown filter already processes links and sanitizes HTML)
+      if has_markdown
+        filtered_content.html_safe
+      else
+        auto_link h(filtered_content), html: { target: "_blank" }
+      end
     end
   rescue Exception => e
     Sentry.capture_exception(e, extra: { message: message })