firestore/internal/conformance/testdata/update-exists-precond.json

{
  "tests": [
    {
      "description": "update: Exists precondition is invalid",
      "comment": "The Update method does not support an explicit exists precondition.",
      "update": {
        "docRefPath": "projects/projectID/databases/(default)/documents/C/d",
        "precondition": {
          "exists": true
        },
        "jsonData": "{\"a\": 1}",
        "isError": true
      }
    }
  ]
}