In the future-selves and implementation intentions dialog, the conversational agent will first tell the participant to imagine themselves in a future in which they successfully quit smoking (i.e., desired future self). The conversational agent can send text messages such as: "Please think about yourself in the future. Imagine that you have quit smoking successfully. Think about the person you will be. What do you look like? What does your life look like? Consider this future image as well as you can.". This procedure will be repeated for a future in which the participant continued smoking (i.e., the undesired future self). Finally, the conversational agent will explain to the participant that it is important to think about how to resist smoking in HRSs (i.e., implementation intentions) to ensure that the participant comes closer to becoming their desired future self as non-smoker and to avoid their undesired future self as a smoker.

In the identity-related positive self-talk dialog, the conversational agent will tell the participant to use positive self-talk focused on their identity to motivate themselves to successfully resist smoking. For example, the conversational agent will send text messages such as: "When you feel tempted to smoke, it can help to say positive things to yourself and motivate yourself to resist the temptation. What positive things can you say about yourself to motivate yourself to resist smoking? For instance, 'I am a strong person who can resist smoking' or 'I am a person who is persistent and has control over the urge to smoke'. Now say the sentence(s) you find motivating or your own motivating words to yourself, by thinking it, and repeating it in your head or out loud.".

The neutral dialog will act as an attention control condition. In this dialog, the conversational agent will not provide support using relapse prevention strategies. Instead, the conversational agent will start with a short introduction (e.g., "Hi, how are you doing?"), ask questions to show interest (e.g., "How are you feeling today?") and will talk about a neutral topic (e.g., animals; "What is your favorite animal?"), and then closes the conversation (e.g., "It was nice speaking with you. I hope you have a good rest of the day!").

The level of positive and negative affect is assessed by having participants indicate to what extent they experienced the four basic emotions on a scale from 1 (not at all) to 10 (very much; Versluis et al., 2018). The scores on anger, anxiety and sadness will be averaged to represent negative affect. The average score on negative affect ranges from 3 to 30, with a higher score indicating stronger feelings of negative affect. The happiness-rating will represent positive affect. The average score on positive affect ranges from 1 to 10, with a higher score indicating stronger feelings of positive affect.

Acceptance of the conversational agent will be assessed using a set of six items. Two example items are: "How satisfied were you with the virtual coach?" and "How easy was it talking to the virtual coach?". Answers are given on a 7-point scale. The questionnaire was originally used in an intervention study (Provoost et al., 2020). Three of the items of the questionnaire had to be adjusted to make them applicable to the experimental set-up of this study. For example, the item "How much would you like to continue working with the virtual coach if the course continued?" was changed into "How much would you like to continue working with the virtual coach if it was available on your smartphone?". Total scores range from 6 to 42, with a higher score indicating higher acceptance of the conversational agent.

The usability of the conversational agent will be measured with a number of items of the System Usability Scale (Brooke, 1996). The total questionnaire consists of ten items and items are rated on a 5-point Likert scale ranging from 0 (strongly disagree) to 4 (strongly agree). Four questions of the original questionnaire are used in this study. In addition, to get a better in-depth understanding of the participants' user experience with the virtual coach, the following open-ended question will be asked: "Would it be useful to have a virtual coach available on your phone to assist with smoking cessation? Please explain.".

General information about the participants will be obtained, namely gender (i.e., male, female, gender neutral, I would rather not say), age (birth year) and level of education (i.e., primary education, vmbo, havo, vwo, mbo, hbo, wo; as measure for SEP).

Participants familiarity with VR was assessed using the following question: "Have you ever used virtual reality before". If participants answers this question with yes, they will be asked how frequently they have used VR.

Participants indicate whether they intend to quit in the future. Answer categories are based on the stages of change of the Transtheoretical model (Prochaska & DiClemente, 2005) and include: within the next month (1), between 1 to 6 months from now (2), sometime in the future, beyond 6 months (3) or not planning to quit (4; Hummel et al., 2018). Participants that choose answer category four will be categorized as 'non-intenders'. This variable will be recoded so that higher scores indicate stronger intention to quit.

Trait tobacco craving will be assessed using the craving subscale of the Wisconsin Smoking Withdrawal Scale (Welsch et al., 1999). The subscale consists of four items that are scored on a 5-point scale ranging from 0 (strongly disagree) to 4 (strongly agree). An example item is "I have had frequent urges to smoke".

Physical nicotine dependence will be assessed at baseline using the Dutch version of the Fagerström Test of Nicotine Dependence (Heatherton et al., 1991; Penfornis et al., 2021).The questionnaire consists of six items, of which four items are scored on a dichotomous scale (yes / no), one item is scored using four answer categories, and one item has an open answer format (asking participants about the number of cigarettes smoked per day). An example item is "Do you smoke more in the morning than during the rest of the day?". Total scores can range from 5 to 12, with a higher score indicating higher nicotine dependence.

To assess the time since last exposure to nicotine, participants were asked how long ago they had their last cigarette. Participants will be able to specify both the number of hours and/or minutes.