When implementing configuration features (variables, flags, resources), thoroughly document their behavior in all scenarios, especially edge cases and interactions with dependencies. Configuration options should have well-defined and tested behaviors across all contexts where they might be used. For features like ephemeral resources, variable marks, or command flags like `-exclude`, ensure you clearly specify: - How marks propagate and interact with other systems ```hcl # When using ephemeral variables, document if and how the mark propagates variable "secret" { type = string ephemeral = true } locals { # Does this local automatically become ephemeral too? config = "${var.secret}_suffix" } ``` - Dependencies and cascading effects ```hcl # When implementing features like -exclude, document how dependencies are handled # For example, when excluding resource A that resource B depends on: resource "null_resource" "a" {} resource "null_resource" "b" { triggers = { a_id = null_resource.a.id } } # Document: Will `tofu plan -exclude=null_resource.a` also exclude resource B? ``` - Behavior during different execution phases (plan vs apply) - Edge cases like non-existent resources, empty collections, or conflicting settings When introducing new marks or configuration attributes, thoroughly test their interactions with existing marks to prevent subtle bugs. For example, ensure `sensitive()` doesn't accidentally strip ephemeral marks, and that marked values maintain their properties when transformed or combined.