Terraform "Reference to undeclared input variable" in CI

Every var.X reference must have a corresponding variable "X" block in the same module. When it does not, validate and plan stop with a reference to an undeclared input variable, pointing at the offending line.

What this error means

validate or plan fails with "Error: Reference to undeclared input variable" and "There is no variable named ..." at the file and line where var.X is used.

Terraform

Error: Reference to undeclared input variable

  on main.tf line 12, in resource "aws_instance" "web":
  12:   instance_type = var.instance_typ

There is no variable named "instance_typ".

Common causes

A typo or missing variable block

The name is misspelled, or the variable was used before its variable block was added to the module.

The variable lives in a different module

Variables are module-scoped; referencing a child module variable from the root (or vice versa) fails because it is not declared there.

How to fix it

Declare the variable in this module

Add a variable block matching the referenced name in the same module.
Pass its value via tfvars, -var, or the module call.
Run validate to confirm the reference resolves.

variables.tf

variable "instance_type" {
  type    = string
  default = "t3.micro"
}

Fix the reference name

Correct the typo so var.X matches an existing variable block exactly, since names are case sensitive.

How to prevent it

Run terraform validate in CI before plan to catch undeclared names.
Keep variable declarations in the same module that uses them.
Pass child module inputs explicitly through the module block.