instructure/canvas-lms · error

can't add participants to a private conversation

Error message

can't add participants to a private conversation

What it means

Guard in Conversation#add_participants: attempting to add participants to a private (two-person) conversation, which Canvas doesn't support — private conversations cannot grow. Raised when users are added to a conversation whose privacy type is 'private'.

Solutions

1. Start a new group conversation that includes the original participants plus the new users instead of mutating the private one
2. Check conversation.private? before calling add_participants and skip/handle private ones
3. Convert workflows to use 'add to conversation' only for group conversations

Example fix

# before
conversation.add_participants(user, new_users)
# after
unless conversation.private?
  conversation.add_participants(user, new_users)
else
  Conversation.initiate([*conversation.participants.map(&:id), *new_users.map(&:id)], false)
end

raise 'cannot extend private conversation' if conversation.private?

null

begin
  conversation.add_participants(user, users)
rescue RuntimeError => e
  raise e unless e.message.include?('private conversation')
  # initiate a new group conversation including everyone
end

When it happens

Trigger: Calling add_participants on a conversation with private? == true (created as a single-pair conversation) with a non-empty list of new users; bulk scripts iterating conversations without checking private?.

Common situations: UI or API attempting to add people to a 1:1 conversation; legacy conversations data with private_type set; automation that assumes all conversations are group conversations.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

• can only forward one conversation at a time
• can't add participants if there are no messages
• Unauthorized, unable to add messages to conversation
• user doesn't have permission to forward these messages
• A new_id, '# ', referenced an existing # and the # with #…

AI-assisted analysis of instructure/canvas-lms@1c9f0bb801 (2026-09-15). Data as JSON: /api/errors/00d417014e1effeb. Report an issue: GitHub.

Appendix: source

Thrown at app/models/conversation.rb:172

      end
      conversation
    end
  end

  # conversations with more recipients than this should force individual messages
  def self.max_group_conversation_size
    Setting.get("max_group_conversation_size", 100).to_i
  end

  def can_add_participants?(users)
    (users.map(&:id) + conversation_participants.pluck(:user_id)).uniq.count <= self.class.max_group_conversation_size + 1
  end

  def add_participants(current_user, users, options = {})
    message = nil
    shard.activate do
      user_ids = users.map(&:id).uniq
      raise "can't add participants to a private conversation" if private?

      transaction do
        lock!
        user_ids -= conversation_participants.map(&:user_id)
        next if user_ids.empty?

        if options[:no_messages]
          bulk_insert_options = {
            workflow_state: "unread",
            message_count: 0,
            private_hash:
          }
        else
          last_message_at = conversation_messages.human.first.try(:created_at)
          raise "can't add participants if there are no messages" unless last_message_at

          num_messages = conversation_messages.human.size

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