This error occurs when you attempt to reference other variables or objects in a validation block before Terraform 1.9. Validation blocks in earlier versions could only reference the variable being validated. The fix depends on your Terraform version: upgrade to 1.9+ for cross-variable validation or use preconditions as a workaround.
The "Invalid reference in variable validation" error occurs when a variable's validation block tries to reference another variable, local value, data source, or resource. Prior to Terraform 1.9, validation blocks could only reference the variable being validated using self-reference (e.g., var.env). Any attempt to reference other objects would fail with this error. This limitation prevented authors from writing complex validation rules that depended on relationships between multiple input variables. Terraform 1.9 introduced a significant enhancement: validation blocks can now reference other input variables, local values, data sources, and resources, greatly expanding validation capabilities.
terraform versionIf you are running Terraform 1.9.0 or later, you can use cross-variable references directly in validation blocks. If you are on an earlier version, you will need to either upgrade or use a workaround.
variable "create_cluster" {
type = bool
default = false
}
variable "cluster_endpoint" {
type = string
default = ""
validation {
condition = var.create_cluster == false ? length(var.cluster_endpoint) > 0 : true
error_message = "You must specify a value for cluster_endpoint if create_cluster is false."
}
}This validation now checks if cluster_endpoint is provided when create_cluster is false, referencing both variables in the condition.
resource "null_resource" "validate_cluster_config" {
lifecycle {
precondition {
condition = var.create_cluster == false ? length(var.cluster_endpoint) > 0 : true
error_message = "You must specify a value for cluster_endpoint if create_cluster is false."
}
}
}This moves the cross-variable validation logic out of the variable block and into a resource-level precondition, which is supported in earlier Terraform versions.
locals {
# This validation happens at apply time
validation_check = (
var.create_cluster == false && length(var.cluster_endpoint) == 0
? file("ERROR: cluster_endpoint required when create_cluster is false")
: "OK"
)
}This approach will cause Terraform to fail at apply time if conditions are not met, though it is less elegant than native validation.
If possible, upgrade to Terraform 1.9 or later to use the native cross-variable validation feature. This is the recommended approach as it provides clear, built-in validation semantics.
Cross-variable validation is particularly useful for conditional input logic:
- Conditional requirements: Require certain variables only when other variables have specific values
- Interdependent variables: Validate relationships between multiple inputs without external modules
- Resource-level constraints: Reference resource attributes in validation (plan-phase attributes only)
- Data source lookups: Validate against data source outputs (use with caution for plan-time validation)
When referencing resources in validation blocks, be aware that only attributes known at plan time can be used. Computed values from apply-time operations cannot be referenced.
The precondition workaround for versions <1.9 executes at plan time but provides less clear error messaging than variable validation blocks.
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