judgekit

Bias probes for LLM judges, each with a bootstrap confidence interval: position, verbosity, self-preference, calibration and stability.

An LLM judge is a model too. It can prefer whichever answer came first, reward padding, favor its own outputs, hug the middle of the scale, and say "90% sure" while being right 70% of the time — all while agreeing with your annotators often enough to look fine. judgekit reads a JSONL of verdicts and measures each of these failure modes with a bootstrap confidence interval, then flags whatever leaves the range an innocent judge could occupy.

The audit in one table

Six synthetic judges, five implanted defects. Bold values are flags; each lands exactly where the defect was implanted, and the clean judge's row carries none.

The row to sit with is length-lover: quadratic-weighted kappa with humans of 0.80 — a number most eval writeups would celebrate — while carrying a verbosity bias of +0.82. Agreement is not an audit.

make demo regenerates this table, the full report and the figures from fixed seeds; CI rebuilds them from pinned dependencies and fails if a committed number differs from what the code produces.

Two real judges, audited

examples/audit_ollama.py builds a small adversarial arithmetic benchmark — one answer provably correct by construction, and half the time the wrong answer is the long, worked-through one — then asks a local Ollama model to judge every pair in both presentation orders. Verdicts and full reports are committed under examples/results.

Same task, different failure modes at different severities. Two judges follow slot order — aya picks whatever is presented first 91.0% of the time and flips 81.9% of pairs when the order is swapped; qwen sits at 76.8% and 56.8%. The llama judge grades effort instead, picking the longer answer 48 points more often than correctness warrants and agreeing with ground truth less often than a coin flip. Two of the three never once declare a tie between identical answers. n=105 pairs, adversarial by construction — an illustration of what the probes see, not a model leaderboard.

Install

pip install judgekit

Python 3.11+. Runtime dependencies: numpy, pydantic, matplotlib. The development version installs with pip install git+https://github.com/mohammadi-hadi/judgekit.

Quickstart

Write one JSON object per verdict. Three kinds exist, and every check switches on automatically when the fields it needs are present:

{"kind": "graded",   "item_id": "q101", "score": 4, "candidate_len": 512, "candidate_model": "gpt-4o"}
{"kind": "binary",   "item_id": "q102", "label": true, "confidence": 0.9}
{"kind": "pairwise", "item_id": "q103", "choice": "a", "swapped": true, "a_len": 210, "b_len": 480}

The reference file uses the same records (judge_id: "human"), aligned on item_id. For pairwise verdicts, choice is always in canonical space — "a" means canonical candidate A won regardless of which slot it was shown in — and swapped: true records that the pair was presented in reversed order. Judging every pair in both orders is what makes the position probes assumption-free.

from judgekit import load_verdicts, run_audit

audit = run_audit(
    load_verdicts("judge.jsonl"),
    load_verdicts("human.jsonl"),
    judge_model="gpt-4o",  # enables the self-preference probe
)
for flag in audit.flags:
    print(flag.name, "--", flag.detail)

Or from the shell, with an exit code CI can gate on:

judgekit report judge.jsonl --human human.jsonl --out audit --fail-on-flags

Alongside report.md the audit writes report.json for pipelines that gate on specific numbers. Any check that could not run is listed in the report with the exact fields to log to enable it — a skip describes the log file, not the judge.

What it checks

How the numbers are defended

• Cross-checked implementations. Cohen's kappa (all weightings) matches scikit-learn, Krippendorff's alpha matches the krippendorff package, and Spearman matches scipy to 1e-10 or better in the test suite. The library itself depends on none of them.
• Exact identities. The Brier score must equal reliability − resolution + uncertainty from its Murphy decomposition at machine precision; the tests assert it.
• Validation by implantation. The synthetic judges have defect dials, and the tests require each probe to recover its dial — a 0.4 position override must read as 0.7 first-slot preference — to stay monotone in it, and to stay silent on the clean judge, including a world where longer answers genuinely are better and the raw length correlation would convict an honest judge.
• Drift-checked results. Every number above and in results/report.md is regenerated by CI from pinned dependencies and diffed against the committed copy.

Design notes

• Innocent bands, not point nulls. With enough data a spread ratio of 1.04 excludes 1.0 while meaning nothing. Each triggering probe declares the band a defect-free judge could plausibly occupy — position 0.45–0.55, correlations ±0.10, self-preference ±0.20 scale points, spread 0.90–1.10, ECE ≤ 0.05 — and flags only when the whole 95% interval leaves it. The bands are opinions, stated in one place (bias.py) and easy to disagree with.
• Item-level bootstrap. Re-judgments and swapped presentations of an item are resampled together; treating correlated rows as independent evidence would shrink every interval.
• Bias triggers, instability informs. A fair-but-noisy judge flips genuinely close pairs under swap, so the flip rate and identical-pair decisiveness are reported without flagging; systematic preference is what position preference triggers on.
• Verbosity is a partial correlation. Longer answers are often genuinely better, so score-vs-length is confounded with quality; the probe rank-partials the human score out. When length never varies independently of quality the answer is nan, because no verdict is the honest verdict.
• Deterministic to the digit. Seeded PCG64 resampling, no BLAS in any statistic's path, and a pinned drift environment: the same verdicts give the same report on any machine.

Limitations

• An audit is bounded by what was logged. No lengths, no verbosity probe; no both-order presentations, weaker position probe; no re-judgments, no stability number.
• The human reference is treated as ground truth. Where annotators are noisy, judge-human disagreement is not all the judge's fault — with multiple raters, measure the human ceiling with agreement.krippendorff_alpha first.
• The innocent bands are judgment calls, not derivations.
• Implantation shows the probes detect the mechanisms simulated; real judges can fail in ways not simulated here.
• Verbosity control is linear in ranks of the human score; residual confounding remains possible.

Related work

The failure modes measured here were documented by, among others: MT-Bench's position, verbosity and self-enhancement analysis (Zheng et al., 2023), position bias specifically (Wang et al., 2023), adversarial judge evaluation (Zeng et al., 2024), self-recognition and self-preference (Panickssery et al., 2024), rubric-based judging (Liu et al., 2023) and the LLM-as-a-judge survey (Gu et al., 2024). judgekit's contribution is packaging the checks as one auditable tool with uncertainty on every number and validation by implantation.

Companion projects: trajectory-judge measures what outcome-only judges miss on agent trajectories; EvalMORAAL applies chain-of-thought judging to moral alignment across 20 models; arenakit audits the pairwise leaderboard a judge's verdicts add up to.

Citation

If judgekit is useful in your research, please cite it (see CITATION.cff):

@software{mohammadi_judgekit,
  author  = {Mohammadi, Hadi},
  title   = {judgekit: audit an LLM judge before you trust it},
  url     = {https://github.com/mohammadi-hadi/judgekit},
  doi     = {10.5281/zenodo.21802869},
  version = {0.2.0},
  year    = {2026}
}

License

MIT