AWS CodeBuild S3 Caching not working

I think the CodeBuild feature is currently massively underdocumented. [1]

I am trying to cache a pnpm store path during an AWS-CDK CodePipeline synth step. Unfortunately only a 160 bytes large file is created in the S3 bucket.

It is totally unclear to me how to specify which files to cache when using the "Amazon S3 caching".

• It does not say from where it is selecting the objects.
• It does not say whether files are zipped before being uploaded to S3.
• It does not say whether the buildspec's cache property [2] can be used to select which files to cache.
• CodeBuild does not log what it is doing at all!! Just saying "Uploading S3 cache..."....

This feature is giving us 0% visibility into what it is doing and the docs give 0% guidance into how it is supposed to be used.
Could someone please clarify?

Here is another person having a similar issue: https://github.com/aws/aws-cdk/issues/21727

Thanks, Martin

[1] https://docs.aws.amazon.com/codebuild/latest/userguide/build-caching.html
[2] https://docs.aws.amazon.com/codebuild/latest/userguide/build-spec-ref.html#build-spec-ref-syntax