Twilio has introduced guardrail, a framework for providing principled code-generation in type-safe languages and frameworks, aiming to improve API documentation and ensure conformance to published specifications. The goal of guardrail is to make it easier to generate accurate, maintainable, and self-documenting APIs by leveraging typesystems to ensure correctness. By providing an abstract interface with concrete types that insulate the business layer from the underlying HTTP layer, Twilio's guardrail helps developers avoid common pitfalls like undocumented routes and mistakes in API documentation. The framework is initially targeted at Scala and Lightbend's akka-http web framework, but experimental support for http4s clients has been added recently.