Unable to use --cli-input-yaml with aws cloudformation create-stack

[gautaz]

Hello,

I am trying to wrap up arguments for the command aws cloudformation create-stack in a YAML file to be then passed with --cli-input-yaml.

My current working command line is:

aws cloudformation create-stack --stack-name CfnGitSyncPrerequisites --template-body file://git-sync/prerequisites.yaml --capabilities CAPABILITY_NAMED_IAM

Hence reproducing the exact same arguments in a YAML file (git-sync-prerequisites-stack.yaml) would from my understanding be with the following content:

StackName: CfnGitSyncPrerequisites
TemplateBody: file://git-sync/prerequisites.yaml
Capabilities:
- CAPABILITY_NAMED_IAM

But when using this YAML file with --cli-input-yaml, it fails:

❯ aws cloudformation create-stack --cli-input-yaml file://git-sync-prerequisites-stack.yaml

An error occurred (ValidationError) when calling the CreateStack operation: Template format error: unsupported structure.

I have tried to compare the --debug logs of each commands but I was unable to spot the issue (apart that the content of the git-sync/prerequisites.yaml file is not displayed in the logs for the --cli-input-yaml command).

Any idea on how to debug this or make this work is welcome.

[uldyssian-sh]

--cli-input-yaml reads an API request structure from your YAML file, but it does not reliably expand file://... references inside that YAML into the file contents for TemplateBody.

So CloudFormation ends up receiving TemplateBody as the literal string file://git-sync/prerequisites.yaml (not the template), and then fails template parsing with:

Template format error: unsupported structure.

The two correct ways to make this work are:

Use --cli-input-yaml for everything EXCEPT the template body, and pass the template on the command line (CLI args override the input file):

aws cloudformation create-stack
--cli-input-yaml file://git-sync-prerequisites-stack.yaml
--template-body file://git-sync/prerequisites.yaml

Or, don’t use TemplateBody in the input file at all and use TemplateURL instead (upload the template to S3 and reference it):

StackName: CfnGitSyncPrerequisites
TemplateURL: https://s3.amazonaws.com//git-sync/prerequisites.yaml
Capabilities:

• CAPABILITY_NAMED_IAM

If you want to debug what is actually being sent, run with --debug and look for the serialized request parameters — you will see whether TemplateBody is the real template text or just the literal file://... string.

Summary: keep the template file reference on the command line (recommended), or use TemplateURL; embedding file://... under TemplateBody inside --cli-input-yaml is what triggers the error.

[uldyssian-sh]

Thanks! If this helps, please consider marking it as “Helpful” or “Accepted” 😊