Aws `default_tags` config - module inheritance?

I’m using an in house module to build aws ec2 instances, but I’m running into a weird issue with the aws provider’s default_tags feature. I can’t figure out if there’s a bug or I’m doing it wrong.

TL;DR: It looks as if the module must contain its own provider + default_tags configuration. It isn’t inheriting the default_tags from the caller/parent afaict.

The caller/parent has its provider configured thusly, and seems to work fine on resources defined directly in this project:

provider "aws" {
  region = "us-west-2"
  default_tags {
    tags = {
      created-by: "terraform"
      owned-by: "dept_42"
      terraform-source: "https://some.vcs_url"

As an example of the behavior in question, this project calls out to a module requesting to build a standardized ec2 instance:

module "green_instances" {
  source = "/path/to/module/source"
  instance_count   = var.instance_count
  ami              = {owner: module.base_config.aws_account_owner, id: "ami-123456789a0"}
  # ...a few other parameters here

The module appears to need to have its own provider block with the default_tags defined, or it won’t set the tags properly even though they’re defined in the caller’s provider. If the module is not configured as described, Terraform will remove these tags, despite them being declared in the caller’s provider config.

Is this expected behavior? thanks!

So are you meaning the module contains a provider block or just the root module?

The child module must contain a provider block with the default_tags or the tags won’t be set on the resources the child module creates, even if the default_tags are defined on the root module’s provider.

So you are saying that if there is no provider block at all in the module it seems to not set the default tags?

correct. this block exists in the root module:

provider "aws" {
  region = "us-west-2" # not included in child module declaration
  default_tags {
    tags = { ... some tags here ... }

However, this block must also appear in the child module. If it is removed from the child module, Terraform will remove the associated tags from the resources the child module creates (ec2 instances in my case). The anticipated behavior is configuring the provider once in the root module, and any children would inherit its settings.

So if you have absolutely no provider blocks in the module you are seeing this issue?