To describe the design, implementation, and plans to evaluate the Veterans Crisis Line (VCL) Caring Letters intervention. Veterans with VCL contact and VHA service utilization. Caring Letters is an evidence-based post-acute care suicide prevention intervention in which brief messages are mailed to individuals at high risk of suicide repeatedly over time to communicate that people care about them and are concerned for their well-being. An effectiveness-implementation hybrid type 1 trial using the RE-AIM evaluation framework is underway to examine the use of Caring Letters with veterans who contact the VCL. A team of suicide prevention subject matter experts, researchers, and operational partners from the VCL will evaluate the effects of Caring Letters on clinical outcomes and Department of Veterans Affairs - Veterans Health Administration (VHA) clinical utilization rates and examine facilitators and barriers to implementing the Caring Letters campaign. Veterans who contact the VCL are linked with national administrative VHA data. Semi-structured interviews were conducted as part of a qualitative formative evaluation. In the first 12 months of the intervention, Caring Letters have been sent to over 100,000 veterans with VCL contact (over 500,000 letters mailed). A formative qualitative evaluation early in implementation revealed a variety of positive veteran perspectives on the intervention. Partnered program design and evaluation with a high level of stakeholder engagement and participant feedback can result in a rigorous and feasible evaluation plan that improves implementation processes and produces actionable results. The initial results of this evaluation will be used to better inform care in the VHA and, specifically, the VCL.