For example, creating invoice payments return invoice contact summaries in responses.

If contact details are required on the client side the result is a subsequent API call to retrieve them.

In this case the overhead of the subsequent call is greater than the cost of returning the information in the invoice payment response. This issue is compounded due to Xero API's rate limiting on requests.

An optional parameter 'enableSummaries=false' would be nice.

I understand that for cases like retrieving multiple invoices that summaries are useful but for single result responses, such as invoice payments the current functionality seems inconsistent. The idea that invoice details in responses vary based on which endpoint i hit smells a bit.