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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

  # 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

  # 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.