Talk therapy is a common, effective, and desirable form of mental health treatment. Yet, it is inaccessible to many people. Enabling peers to chat online using effective principles of talk therapy could help scale this form of mental health care. To understand how such chats could be designed, we conducted a two-week field experiment with 40 people experiencing mental illnesses comparing two types of online chats-chats guided by prompts, and unguided chats. Results show that anxiety was significantly reduced from pre-test to post-test. User feedback revealed that guided chats provided solutions to problems and new perspectives, and were perceived as "deep," while unguided chats offered personal connection on shared experiences and were experienced as "smooth." We contribute the design of an online guided chat tool and insights into the design of peer support chat systems that guide users to initiate, maintain, and reciprocate emotional support.