application/problem+json with Cache-Control: no-store.
New members can be added to problem documents; ignore members you do not know.
Error types
The API never returns 504. New types can be added in v1; treat an unknown type by its status code.
Validation errors
invalid-request lists every problem with the request in errors. Each entry has a code, a detail, and either parameter or pointer:
parameter names a query parameter. pointer is a JSON Pointer into the batch request body. New codes can be added.