instructure/canvas-lms · error · ConversationsHelper::Error

not a participant

Error message

not a participant

What it means

Rescue translator: when an included_message references a message whose author/owner is not a participant of the conversation (from the caller's perspective), ConversationsHelper::InvalidMessageParticipantError is re-raised as a bad_request ConversationsHelper::Error with 'not a participant'.

Solutions

1. Ensure the current user is an active participant of the conversation before including its messages.
2. Drop included_messages ids for conversations the user is not a participant of.
3. Catch ConversationsHelper::Error (attribute included_messages, message 'not a participant') and show a friendly message.
4. Re-open the conversation in the UI so the participant list refreshes, then retry.

Example fix

// before
included_messages: Message.where(id: params[:included_messages])
// after
included_messages: current_user.conversations.find(conversation.id)
  .conversation_messages.where(id: params[:included_messages]).pluck(:id)

def participant?(conversation, user)
  conversation.conversation_participants.exists?(user_id: user.id)
end

begin
  add_message(...)
rescue ConversationsHelper::Error => e
  if e.message == I18n.t('not a participant')
    flash[:error] = t('you_are_no_longer_part_of_this_conversation')
  end
end

When it happens

Trigger: Conversations API call with included_messages pointing at a message the current user can access in another conversation context but who is not a conversation participant (e.g. observer/admin-visible message, or conversation the user was removed from).

Common situations: User removed from a group conversation still has cached message ids; forwarding a message received in a thread the user has since left; admin tools acting on behalf of a non-participant.

Understand the failure class

Background: Permission denied / not authorized / 403 Forbidden: access-control rejections when the caller lacks the required role, grant, or ownership — this error's family across 18 libraries.

Related errors

• not for this conversation
• can only forward one conversation at a time
• can't add participants if there are no messages
• can't add participants to a private conversation
• Course concluded, unable to send messages

AI-assisted analysis of instructure/canvas-lms@1c9f0bb801 (2026-09-15). Data as JSON: /api/errors/3300540fc3d04d90. Report an issue: GitHub.

Appendix: source

Thrown at app/helpers/conversations_helper.rb:144

        message.conversation_id = conversation.conversation_id
        message.created_at = Time.now.utc
        { message:, recipients_count: existing_participant_ids.size, status: :accepted }
      end
    elsif conversation.should_process_immediately?
      message = conversation.process_new_message(message_args, recipients, message_ids, tags)
      { message:, recipients_count: recipients ? recipients.count : 0, status: :ok }
    else
      conversation.delay(strand: "add_message_#{conversation.global_conversation_id}").process_new_message(message_args, recipients, message_ids, tags)
      message = Conversation.build_message(*message_args)
      message.id = 0
      message.conversation_id = conversation.conversation_id
      message.created_at = Time.now.utc
      { message:, recipients_count: recipients ? recipients.count : 0, status: :accepted }
    end
  rescue ConversationsHelper::InvalidMessageForConversationError
    raise ConversationsHelper::Error.new(message: I18n.t("not for this conversation"), status: :bad_request, attribute: "included_messages")
  rescue ConversationsHelper::InvalidMessageParticipantError
    raise ConversationsHelper::Error.new(message: I18n.t("not a participant"), status: :bad_request, attribute: "included_messages")
  end

  def contexts_for(audience, context_tags)
    result = { courses: {}, groups: {} }
    return result if audience.empty?

    context_tags.each do |tag|
      next unless tag =~ /\A(course|group)_(\d+)\z/

      result[:"#{$1}s"][$2.to_i] = []
    end

    if audience.size == 1 && include_private_conversation_enrollments
      enrollments = Shard.partition_by_shard(result[:courses].keys) do |course_ids|
        next unless audience.first.associated_shards.include?(Shard.current)

        Enrollment.where(course_id: course_ids, user_id: audience.first.id, workflow_state: "active").select([:course_id, :type]).to_a
      end

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