Debug-Nullish-Cache-Hit-V2

The model receives the prompt (and optional system message). The run uses scorer rubric_json_metrics with the JSON configuration below. Pass/fail and partial credit are determined entirely by that scorer against the model output; no human grading.

Return JSON only with keys diagnosis, fix, tests. A memoized lookup returns cached misses as null and treats them as cache hits, so later populated values are never read. Identify the bug and propose a minimal JavaScript fix.

{
  "metrics": {
    "repro": {
      "checks": [
        {
          "contains": [
            "cached miss"
          ]
        },
        {
          "contains": [
            "null"
          ]
        },
        {
          "contains": [
            "later populated"
          ]
        },
        {
          "contains": [
            "cache hit"
          ]
        }
      ]
    },
    "hidden": {
      "checks": [
        {
          "contains": [
            "hasOwn"
          ]
        },
        {
          "contains": [
            "Map.has"
          ]
        },
        {
          "contains": [
            "undefined sentinel"
          ]
        }
      ]
    },
    "diagnose": {
      "checks": [
        {
          "contains": [
            "memoized lookup"
          ]
        },
        {
          "contains": [
            "truthy"
          ]
        },
        {
          "contains": [
            "miss"
          ]
        }
      ]
    }
  }
}