🚨 OpenTofu Error: 'Missing required argument' — Cause, Fix, and Troubleshooting Guide

Overview

Provider schemas and module interfaces mark some arguments as required. If you omit one — a mandatory resource attribute, a nested-block field, or a module variable with no default — OpenTofu refuses to proceed and names exactly what is missing:

Error: Missing required argument

  on main.tf line 8, in resource "aws_instance" "web":
   8: resource "aws_instance" "web" {

The argument "instance_type" is required, but no definition was found.

Symptoms

• tofu validate/plan fails with Missing required argument.
• The message names the required argument and the block it belongs to.
• Common with new resources copied from partial examples, or module calls missing an input.
• Nested-block variants say the argument is required “in a X block.”

Common Root Causes

• Omitted required attribute — e.g., instance_type, bucket, name.
• Module input without a default not passed by the caller.
• Missing nested-block field — a required key inside ebs, setting, filter, etc.
• Conditional set to null — a ternary produced null for a required argument.
• Removed line during a refactor that dropped a required field.

How to diagnose

OpenTofu names the argument and location — go there first:

tofu validate

Confirm which arguments a resource requires via the schema:

tofu providers schema -json \
  | jq '.provider_schemas["registry.opentofu.org/hashicorp/aws"].resource_schemas["aws_instance"].block.attributes
        | to_entries | map(select(.value.required==true)) | map(.key)'

For a module call, list its required variables (those without a default):

grep -rnA4 'variable ' modules/network/ | grep -B3 -L 'default'

Fixes

Supply the required attribute:

resource "aws_instance" "web" {
  ami           = var.ami_id
  instance_type = "t3.micro"   # was missing
}

Pass the missing module input:

module "network" {
  source     = "./modules/network"
  cidr_block = "10.0.0.0/16"   # required variable, no default
}

Avoid null for a required argument — provide a real fallback:

instance_type = var.size != null ? var.size : "t3.micro"

Re-validate:

tofu validate && tofu plan

What to watch out for

• null counts as “not set” for required arguments — a ternary must yield a concrete value.
• The provider schema is authoritative about what is required; grep it rather than guessing from examples.
• Give module variables sensible defaults where appropriate so callers are not forced to set everything.
• Run tofu validate in CI to catch missing arguments before any API calls.

Related

• OpenTofu Error: ‘Unsupported argument’ — Cause, Fix, and Troubleshooting Guide
• OpenTofu Error: ‘Invalid function argument’ — Cause, Fix, and Troubleshooting Guide
• OpenTofu Error: ‘Unsuitable value type’ — Cause, Fix, and Troubleshooting Guide