llm-as-judge

LLM-as-Judge

Use a strong LLM to evaluate another LLM's output. Done right, it's fast, cheap, and correlates with human judgment. Done wrong, it's biased, inconsistent, and misleading.

When to Use

• Scaling eval beyond what humans can review
• Measuring open-ended outputs (summaries, code quality, helpfulness) where rule-based metrics fail
• Pairwise model comparison (A vs B on the same input)
• CI checks on agent outputs

When NOT to Use

• High-stakes decisions (medical, legal) — need humans
• When the judge is the same model as the generator — biased toward its own style
• Very short outputs where a rule can decide — exact_match is cheaper
• Tasks the judge can't do itself — if it can't write good code, it can't judge code well

Three Common Patterns

1. Rubric Scoring

Judge rates one output against explicit criteria on a 1-5 scale.

const prompt = `You are evaluating a response. Rate it 1-5 on each criterion.

<user_query>${query}</user_query>
<response>${response}</response>

Criteria:
- accuracy: factually correct?
- helpfulness: addresses what the user asked?
- conciseness: no unnecessary verbosity?

Return JSON: {"accuracy": N, "helpfulness": N, "conciseness": N, "reasoning": "..."}`;

const judgment = await client.messages.create({
  model: "claude-opus-4-6",
  max_tokens: 500,
  messages: [{ role: "user", content: prompt }],
});

Use a stronger model as judge than the one you're evaluating. Opus judges Sonnet; Sonnet judges Haiku.

2. Pairwise Comparison

Show two outputs, judge picks which is better. Most reliable pattern.

const prompt = `Compare two responses to the same query. Pick which is better overall.

<query>${query}</query>
<response_A>${responseA}</response_A>
<response_B>${responseB}</response_B>

Return JSON: {"winner": "A" | "B" | "tie", "reasoning": "..."}`;

To control for position bias, run each pair TWICE with order swapped. Average the judgments.

3. Reference-Based

Compare output to a gold-standard reference:

const prompt = `Is the generated answer equivalent to the reference answer?

<reference>${reference}</reference>
<generated>${generated}</generated>

"Equivalent" means factually consistent — wording can differ.
Return: {"equivalent": true|false, "reasoning": "..."}`;

Cheaper than rubric but requires good references.

Known Biases

Bias	Description	Mitigation
Position bias	Judge prefers first or second option	Randomize; run pairs twice with swapped order
Length bias	Judge prefers longer responses	Include "conciseness" in rubric; normalize by length
Self-preference	Judge prefers its own model's style	Use a DIFFERENT model family as judge
Verbosity bias	Judge prefers confident/flowery language	Rubric explicitly penalizes vagueness
Format bias	Prefers markdown/bullets over prose	Rubric targets content, not format

Name the biases in your judge prompt — it reduces them: "Do not prefer longer responses; judge only on accuracy."

Calibrating Against Humans

Don't trust judge scores in isolation. Calibrate:

1. Sample 50-200 outputs. Have humans label them.
2. Run the judge on the same set. Collect its scores.
3. Compute Cohen's kappa or Pearson correlation between human and judge.
4. If kappa > 0.6, the judge is reliable for this task. If < 0.4, rewrite the prompt.

from sklearn.metrics import cohen_kappa_score
kappa = cohen_kappa_score(human_labels, judge_labels)

Re-calibrate quarterly or whenever you change judge prompts or models.

Prompt Design for Judges

1. Role first — "You are a code review expert"
2. Criteria explicit — don't leave "quality" undefined
3. Output format constrained — JSON or a fixed label set
4. Reasoning field included — forces the model to justify, reduces careless judgments
5. Examples — 2-3 few-shots of correct judgments on your criteria

Structured Output

Get judgments as JSON so you can aggregate:

const judgment = await client.messages.create({
  model: "claude-opus-4-6",
  max_tokens: 500,
  messages: [
    { role: "user", content: judgePrompt },
    { role: "assistant", content: "{" },
  ],
});
const parsed = JSON.parse("{" + judgment.content[0].text);

Prefilling "{" nudges valid JSON. Validate with a schema before aggregating.

Aggregating at Scale

Per dataset:

• Mean rubric score per criterion — easy but loses variance
• Win rate in pairwise — e.g., "model B wins 62% of the time"
• Score distribution — detect regressions in the tail (a few catastrophic outputs even if mean is OK)

Report confidence intervals (bootstrap) — a 2% score gap on 100 samples is likely noise.

Cost Management

Judging is expensive. Reduce cost:

1. Sample — judge 200 items, not 10,000
2. Cheap judge first pass, expensive judge for disagreements
3. Cache judgments — if input didn't change, reuse the last score
4. Prompt caching on the judge prompt (see prompt-caching-ttl)

Anti-Patterns

1. Judge = generator — model evaluates itself; huge positive bias
2. No calibration — you're reporting numbers with no grounding
3. No position-bias control in pairwise
4. Vague rubric — "rate quality 1-5" — scores will be noisy
5. Single-run judgments — use multiple runs for critical evals; measure variance

Best Practices

1. Use a stronger, different model family as judge
2. Pairwise comparison > rubric when you have two outputs to compare
3. Always control for position bias — swap and average
4. Calibrate against human labels; target Cohen's kappa > 0.6
5. Name biases in the prompt to mitigate them
6. Return structured JSON with a reasoning field
7. Report confidence intervals, not just means
8. Cache and sample to keep costs bounded