Errors

GraphQL error codes are mapped onto typefetch's shared ErrorKind, so one global handler covers every transport:

client.onError((error) => {
  if (error.kind === "unauthenticated") redirectToLogin();
});

fires for a GraphQL UNAUTHENTICATED exactly as it does for an HTTP 401.

`extensions.code`	`kind`
`UNAUTHENTICATED`	`unauthenticated`
`FORBIDDEN`	`permission_denied`
`BAD_USER_INPUT`, `GRAPHQL_VALIDATION_FAILED`, `GRAPHQL_PARSE_FAILED`	`invalid_argument`
`NOT_FOUND`	`not_found`
`TOO_MANY_REQUESTS`	`resource_exhausted`
`INTERNAL_SERVER_ERROR`	`internal`
`SERVICE_UNAVAILABLE`	`unavailable`

Both response media types are handled: the legacy application/json (errors delivered inside a 200) and application/graphql-response+json (errors with a real 4xx/5xx). extensions.http.status is honoured when present.

Typed error bodies

errors is keyed by extensions.code for a GraphQL endpoint, and isContractError narrows off it:

errors: {
  UNAUTHENTICATED: z.object({ code: z.literal("UNAUTHENTICATED"), realm: z.string() }),
}

try {
  await api.user.get({ id });
} catch (e) {
  if (isContractError(contracts.user.get, e, "UNAUTHENTICATED")) {
    e.data.realm; // fully typed
  }
}

Partial data

A response carrying both data and errors is the case every GraphQL client gets wrong in one direction or the other.

errorPolicy: "none" (default) — throw. A partially-failed response is a failed one, and silently returning half a result is how a null reaches a UI three screens from its cause.
errorPolicy: "all" — resolve the data and hand the errors to onPartialErrors, so the return type stays Promise<T> and nothing is dropped.

graphqlTransport({
  url: "…",
  errorPolicy: "all",
  onPartialErrors: (errors, { endpointId }) => log.warn(endpointId, errors),
});