WriteFields in Firestore Rules only working in Sim

Question

A big problem I am having with Firestore Rules is that using the Simulator in the Firebase Console to simulate database accesses does not correspond to the behavior I am seeing from my clients. Now I have stumbled accross an issue that I cannot seem to fix.

Issue

The following syntax works fine in the simulator:

allow create: if request.writeFields.hasAll(["a", "b", "c"]);

When requesting with the exact same fields from my Android client, I get Error performing setData, PERMISSION_DENIED: Missing or insufficient permissions..

The request (simplified) looks as follows:

By setting allow create; (simplified) I have verified and identified that the problem has to be connected to request.writeFields because my requests, only from my real clients, fail as soon as I add logic regarding that.
The simulated data access is allowed in all my tests.

My reference is the List interface.

Question

I am wondering if I am either missing something about writeFields (potentially connected to allow create) and also how I am regularly seeing disparities between simulated and real accesses.
Keep in mind that my syntax works perfectly fine in the simulator, but a real request containing the exact same JSON data fails.

Answer 1

Recently the following error message was added to the Firebase Console Firestore Security Rules Simulator when trying to access writeFields:

That means that it does not work in the simulator anymore, which fixes this issue.