In-app messaging is an often-overlooked but increasingly popular method for engaging users of modern apps and SaaS products, offering benefits such as increased product engagement and reduced churn without the drawbacks of email or push notifications. However, building an effective in-app messaging system requires considerable engineering resources, as teams need to manage real-time delivery, message persistence, and complex UI components while ensuring scalability and integration with internal teams like marketing and product management. This guide evaluates the top in-app messaging tools that enable faster implementation without sacrificing flexibility or performance. It highlights the shortcomings of no-code platforms like Pendo, Braze, Userpilot, and Chameleon, which often lack customization options, real-time capabilities, multi-channel support, and developer control, making them less ideal for engineering teams aiming to build robust notification systems. Among the evaluated tools, Knock stands out for its developer-friendly infrastructure, offering comprehensive APIs, SDKs, and customizable components that allow for seamless integration and multi-channel orchestration, making it suitable for teams looking to build versatile and scalable in-app messaging solutions.