GraphQL/Relay - How to pass a mutation's input variable into a query on the mutation response?

Given a mutation like this (see how nextArticle needs an ID passing):

mutation markArticleCompleteMutation(
    $input: markArticleCompleteInput!
) {
    markArticleComplete(input: $input) {
        user {
            nextArticle(excludeArticleId: $excludeArticleId) {
                id
            }
        }
    }
}

// Input looks like:

{
  "input": {"completedId": "id123"}
}

how can I pass this from one of the input props?

I'd ideally like to write something like nextArticle(excludeArticleId: $input.completedId) but this doesn't seem to work

AFAIK you cannot access it like this. You have to define a separate variable and use it.

Notice the excludeArticleId is added to the list of variables of your document and it is defined separately in the variables map.

mutation markArticleCompleteMutation(
    $input: markArticleCompleteInput!,
    $excludeArticleId: String!
) {
    markArticleComplete(input: $input) {
        user {
            nextArticle(excludeArticleId: $excludeArticleId) {
                id
            }
        }
    }
}

Then the variables should be like this:

{
    "input": {"completedId": "id123"},
    "excludeArticleId": "id123"
}