Table 4.

Guiding principles and examples of how these were integrated in the chatbot prototype.

Core intervention objectives	Key features needed to achieve these objectives	Examples how this was included in the chatbot
Provide social support	• Unconstrained language input (i.e., free dialogue) • Use youth language • Give the chatbot a “human-like” look (e.g., empathic and humoristic) • Accurate answers • Realistic answers	• Using the software platform Dialogflow, the adolescents are free to ask any question they want to the chatbot, the conversation starts from the adolescent him- or herself. • Adapting language style (e.g., no scientific words, not too formal or too mature but simple and concise answers), splitting long pieces of text into shorter ones, using emoji's and adding “life quotes” to some chatbot responses. • Within the answers of the chatbot, care was taken to be empathic (“I understand what you mean,” “that must be annoying,” “that sounds like fun”) and to use some humor by adding jokes to the input. • Clustering the chatbot input according to certain health domains. A cluster consists of all kinds of different questions to which one joint answer could be given (see section Draft/Refine Intervention Materials). By having different training phrases within a cluster, there was less chance of the chatbot giving a wrong answer to a question. • The formulated chatbot answers were sent to stakeholders and experts within the theme for feedback.
Engagement	• Small talk • Attractive design with personalization options • Notifications	• Integrate standard small talk (e.g., exchanging greetings, how are you, who are you, how old are you, etc.). • Design in Messenger or WhatsApp style, for example when the chatbot is “still typing,” an image with three dots appears. • A settings page was created where users have the option to change the background of the chat, change the font color and the option to delete the entire conversation. • The chatbot weekly sends two messages to the adolescents (after school) so that they are drawn back to the app. One message motivates adolescents to set goals in the self-regulation app, the other gives the tip that a new episode of the narrative series is uploaded in the app.
Provide knowledge	• Giving tips • Referring to websites with the right information	• Website links were only added for extra information, but the chatbot answered the question itself as much as possible. This way, adolescents would have the choice if they wanted to read extra information on a website.
Act as a guide	• Referring to the appropriate organizations which can provide proper help	• Referrals to other resources were suggested only when really needed on the basis of frequently used terms which suggest mental difficulties (e.g., suicide, depression, self-mutilation, physical complaints, etc.).