Every error reply carries a code. It is the part a client should branch on: the HTTP status is a rendering of it, and TCP and UDS callers never see a status line at all.

Code	Status	Means
invalid	400	the request is malformed or unacceptable — retrying it unchanged will never succeed
unauthenticated	401	no identity was established
forbidden	403	the caller is known and lacks the permission this action needs
not_found	404	the definition, instance, channel or task named does not exist
conflict	409	the request is well-formed and the target exists, but its current state forbids the operation; the identical request may succeed later
internal	500	anything unclassified, which is a server fault until proven otherwise
unsupported	501	the endpoint exists, but this server is not configured to serve it
unavailable	503	this server cannot serve requests right now, its database being unreachable — route elsewhere and retry