The conversational capabilities of Large Language Models (LLMs) suggest that they may be able to perform as automated talk therapists. It is crucial to know if these systems would be effective and adhere to known standards. We present a counsellor chatbot that focuses on motivating tobacco smokers to quit smoking. It uses a state-of-the-art LLM and a widely applied therapeutic approach called Motivational Interviewing (MI), and was evolved in collaboration with clinician-scientists with expertise in MI. We also describe and validate an automated assessment of both the chatbot’s adherence to MI and client responses. The chatbot was tested on 106 participants, and their confidence that they could succeed in quitting smoking was measured before the conversation and one week later. Participants’ confidence increased by an average of 1.7 on a 0-10 scale. The automated assessment of the chatbot showed adherence to MI standards in 98% of utterances, higher than human counsellors. The chatbot scored well on a participant-reported metric of perceived empathy but lower than typical human counsellors. Furthermore, participants’ language indicated a good level of motivation to change, a key goal in MI. These results suggest that the automation of talk therapy with a modern LLM has promise.

Motivational Interviewing (MI) is a widely-used talk therapy approach employed by clinicians to guide clients toward healthy behaviour change. Both the automation of MI itself and the evaluation of human counsellors can benefit from high-quality automated classification of counsellor and client utterances. We show how to perform this ``coding'' of utterances using LLMs, by first performing utterance-level parsing and then hierarchical classification of counsellor and client language. Our system achieves an overall accuracy of 82% for the upper (coarse-grained) hierarchy of the counsellor codes and 88% for client codes. The lower (fine-grained) hierarchy scores at 68% and 76% respectively. We also show that these codes can be used to predict the session-level quality of a widely-used MI transcript dataset at 87% accuracy. As a demonstration of practical utility, we show that the slope of the amount of change/sustain talk in client speech across 106 MI transcripts from a human study has significant correlation with an independently surveyed week-later treatment outcome (r=0.28, p<0.005). Finally, we show how the codes can be used to visualize the trajectory of client motivation over a session alongside counsellor codes. The source code and several datasets of annotated MI transcripts are released.