Debug-Index-Off-By-One-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 for-loop uses i <= arr.length as its condition; on the final iteration the index equals arr.length and the access returns undefined, causing a processing error. Identify the off-by-one and fix it.

{
  "metrics": {
    "repro": {
      "checks": [
        {
          "contains": [
            "undefined"
          ]
        },
        {
          "contains": [
            "arr.length"
          ]
        },
        {
          "contains": [
            "last iteration"
          ]
        },
        {
          "contains": [
            "for loop"
          ]
        }
      ]
    },
    "hidden": {
      "checks": [
        {
          "contains": [
            "< arr.length"
          ]
        },
        {
          "contains": [
            "arr.length - 1"
          ]
        },
        {
          "contains": [
            "boundary"
          ]
        }
      ]
    },
    "diagnose": {
      "checks": [
        {
          "contains": [
            "off-by-one"
          ]
        },
        {
          "contains": [
            "out of bounds"
          ]
        },
        {
          "contains": [
            "<= arr.length"
          ]
        }
      ]
    }
  }
}