Is the `error` field in standard error responses required?

[zecakeh]

In the CS and IS APIs, the standard error response definition has this sentence:

Some errcodes define additional keys which should be present in the error response object, but the keys error and errcode MUST always be present.

But then in the OpenAPI definition of errors, the field is not marked as required.

Furthermore, it looks like the AS API never has this field in example error responses, but is the error response format supposed to be identical between specs anyway?