grpc/doc/http-grpc-status-mapping.md

HTTP to gRPC Status Code Mapping

Since intermediaries are a common part of HTTP infrastructure some responses to gRPC requests may be received that do not include the grpc-status header. In some cases mapping error codes from an intermediary allows the gRPC client to behave more appropriately to the error situation without overloading the semantics of either error code.

This table is to be used only for clients that received a response that did not include grpc-status. If grpc-status was provided, it must be used. Servers must not use this table to determine an HTTP status code to use; the mappings are neither symmetric nor 1-to-1.

HTTP Status Code	gRPC Status Code
400 Bad Request	INTERNAL
401 Unauthorized	UNAUTHENTICATED
403 Forbidden	PERMISSION_DENIED
404 Not Found	UNIMPLEMENTED
429 Too Many Requests	UNAVAILABLE
502 Bad Gateway	UNAVAILABLE
503 Service Unavailable	UNAVAILABLE
504 Gateway Timeout	UNAVAILABLE
All other codes	UNKNOWN

Technically, 1xx should have the entire header skipped and a subsequent header be read. See RFC 7540 §8.1.

200 is UNKNOWN because there should be a grpc-status in case of truly OK response.