Malformed
Thrown when a Client Auth response does not carry what its endpoint documents — either the { "data": … } envelope itself, or a field inside it that the flow cannot continue without.
Parameters
path
The request path, so the failure names the endpoint that produced it.
missing
Which field was absent. Defaults to the envelope. Pass a field name when data was present but incomplete: those two faults need different fixes, and a message that always blamed the envelope sent the reader to the wrong one.