graphql @skip @include directives cannot query field

Question

I want to use graphql @include and/or @skip directives for compatibility between frontend and backend. Assume backend and frontend have a differing schema: backend:

type Foo {
  someOldProp: Int!
}

frontend:

type Foo {
  someOldProp: Int! @deprecated(reason: "Use someNewProp instead")
  someNewProp: Int!
}

now in frontend I define following fragment:

fragment FooFragment on Foo {
  someOldProp @skip(if: $hasNew)
  someNewProp @include(if: $hasNew)
}

When passing $hasNew as false in a query, the fragment should be resolved as:

fragment FooFragment on Foo {
  someOldProp
}

which would be a valid fragment to be used with the backend schema above. However, even in this case our graphql service complains about Cannot query field 'someOldProp' on type 'Foo'.

Reading the spec, it's not totally clear to me whether this is a bug in our graphql implementation (which should not attempt accessing a field when @include/@skip exclude the field), or whether these directives are only to be used when all underlying fields are available, but when a subset of them is not needed by the client.

In the latter case a graphql implementation would be free to query a field at first, and only erase it before handing it to the client which would explain the error-message.

Does anyone have some insight?