# !aws.cloudformation.output

The `!aws.cloudformation.output` YAML function reads the deployed
[Outputs](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/outputs-section-structure.html)
of a native `aws/cloudformation` component directly in Atmos stack manifests,
by calling the CloudFormation `DescribeStacks` API. It is the Terraform↔CloudFormation
interop bridge: the sibling of [`!terraform.output`](/functions/yaml/terraform.output),
but for `aws/cloudformation` targets instead of Terraform/OpenTofu ones.

> ⚠️ Experimental

## Usage

The `!aws.cloudformation.output` function is called with either two or three parameters:

```yaml
  # Get the `output-key` Output of the `component` in the current stack
  !aws.cloudformation.output <component> <output-key>

  # Get the `output-key` Output of the `component` in the provided `stack`
  !aws.cloudformation.output <component> <stack> <output-key>
```

## Arguments

- **`component`**
  The 
  `aws/cloudformation`
   component name.
- **`stack`**
  (Optional) Atmos stack name. Defaults to the current stack when omitted.
- **`output-key`**

  The exact CloudFormation Output's logical key (the value of the `Outputs.<key>` entry
  in the target's template). Unlike [`!terraform.output`](/functions/yaml/terraform.output),
  this is a direct lookup by key, not a [YQ](https://mikefarah.gitbook.io/yq) expression —
  CloudFormation Outputs are always a flat key/value map, so there is nothing to query into.

:::tip
You can use [Atmos Stack Manifest Templating](/templates) in the `!aws.cloudformation.output` YAML function expressions.
Atmos processes the templates first, and then executes the `!aws.cloudformation.output` function, allowing you to provide the parameters to
the function dynamically.
:::

:::note Type-Aware Merging
Atmos supports type-aware merging of YAML functions and concrete values, allowing them to coexist in the inheritance chain without type conflicts.
See the full explanation: [YAML Function Merging](/reference/yaml-function-merging)
:::

## Requirements

- The target component must be a native `aws/cloudformation` component with a resolved `stack_name` — `!aws.cloudformation.output` errors if the target has no `stack_name` configured.
- The target stack must already be deployed. `!aws.cloudformation.output` calls the live `DescribeStacks` API; there is no local-state equivalent (`aws/cloudformation` has no Terraform/OpenTofu-style state file to read instead).
- Resolving the function requires AWS authentication — it uses the target component's own auth if it authenticates independently, otherwise the enclosing component's (propagated from `--identity`).

## Examples

**File:** `stack.yaml`

```yaml
components:
  terraform:
    my_lambda_component:
      vars:
        # Read the `VpcId` Output of the aws/cloudformation `vpc` component in the current stack
        vpc_id: !aws.cloudformation.output vpc VpcId

        # Read an Output from a different, hardcoded stack
        shared_bucket: !aws.cloudformation.output logging plat-ue2-audit LogBucketName

        # Use the `.stack` template identifier to reference the current stack explicitly
        subnet_ids: !aws.cloudformation.output vpc {{ .stack }} PrivateSubnetIds
```

## Caching the result of `!aws.cloudformation.output` function

Atmos caches (in memory) the results of the `!aws.cloudformation.output` function.

The cache is per Atmos CLI command execution, e.g., each new execution of a command like `atmos terraform plan`
or `atmos describe component` will create and use a new memory cache, re-invoking `DescribeStacks` on the next run.

If you define the function in stack manifests for the same component in a stack more than once, the first call will
produce the result and cache it, and all the consecutive calls will just use the cached data.

## Considerations

- `!aws.cloudformation.output` **is not allowed** in `metadata.tags` or `metadata.labels` values — the same restriction as [`!terraform.output`](/functions/yaml/terraform.output#considerations). Selectors (`--tags`/`--labels`) drive component-scoping decisions before evaluation, so they must be resolvable without authentication, API calls, or nondeterminism.

- Using `!aws.cloudformation.output` with secrets can expose sensitive data to standard output (stdout) in any commands that describe stacks or components.

- Consider cold-start scenarios: if the dependent `aws/cloudformation` stack has not yet been deployed, the function returns an error.

- Be mindful of disaster recovery (DR) implications when using it across regions.

## See Also

- [`!terraform.output`](/functions/yaml/terraform.output) — The Terraform/OpenTofu equivalent
- [`atmos.Component`](/functions/template/atmos.Component) — The template-function equivalent, which also resolves `.outputs` for `aws/cloudformation` targets
- [`atmos aws cloudformation output`](/cli/commands/aws/cloudformation/output) — Show a deployed stack's Outputs from the CLI