I'm trying to use a similar scenario (although with Javascript SDK). But running into an error 405 - "MethodNotAllowed: The specified method is not allowed against this resource" when running s3.writeGetObjectResponse()

I have an IAM policy allowing s3-object-lambda:WriteGetObjectResponse for my lambda. And all resources belong to a single account.

Have you run into similar issue by any chance? ;)

Or maybe having ideas what can cause it?