It is most likely due to the missing content type. When sending a JSON body the receiver must also receive the content-type.
This should resolve your issues:
type Mutation {
auth(pass: String!, user: String!): Auth
@custom(
http: {
url: "http://localhost:8081/auth",
method: POST,
body: "{pass:$pass,user:$user}"
secretHeaders: ["Content-type"]
}
)
}
# Dgraph.Secret Content-type "application/json"
https://dgraph.io/docs/graphql/custom/directive/#dgraph-secret