amazon web services – Aws sdk s3 to s3 copyObject checksum is null in CopyObjectResponse

When doing a s3 to s3 copy using aws sdk, I want to save the returned checksum from CopyObjectResponse, so that I can use it later to verify if file is modified or not
but checksum value returned is null

fun CopyObjectResponse.toCopyResponseModel(): CopyResponse =
    CopyResponse(
        etag = this.copyObjectResult().eTag(),
        isSuccessFull = this.sdkHttpResponse().isSuccessful,
        statusCode = this.sdkHttpResponse().statusCode(),
        checksum = this.copyObjectResult().checksumSHA256()
    )

how can i get checksum?

I can copy at max 5gb file in a singlePartCopy wondering can we use etag or not, will have to figure out multipart too since it changes on each copy even though the file is not modified