Terraform Stacks input variable validation

Introduced 2026-04-29 · score 0.95

HOW TO USE IT

When a Terraform Stack input variable needs a rule before planning proceeds, add a `validation` block to that variable; its `condition` must reference that variable as `var.<NAME>` and resolve to a boolean.

variable "prefix" {
  type = string

  validation {
    condition     = length(var.prefix) > 5
    error_message = "prefix must be longer than 5 characters."
  }
}

WHY THIS HELPS

Validate an input variable in a Terraform Stack before its value reaches a provider.

REQUIREMENTS AND LIMITATIONS

Version: 1.15.0
validation.condition (required): Expression that must evaluate to true for Terraform to proceed. For this Stack feature, it references the containing variable as `var.<NAME>` and resolves to a boolean.
validation.error_message (required): String displayed if the condition evaluates to false.
Terraform version (required): Use Terraform v1.15.0 or later for Stack input variable validation.
Prerequisite: Terraform v1.15.0 introduced input variable validation for Stacks.
The condition expression can reference the `var.<NAME>` value for the variable that contains it and resolves to a boolean.

FIRST-PARTY EVIDENCE

Check the installed version and the source before using it. Entry JSON

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Terraform Stacks input variable validation