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LLM-as-judge evaluation uses a language model to assess subjective qualities in agent outputs - things like helpfulness, tone, coherence, and factual accuracy. This approach bridges the gap between deterministic code checks and human judgment.

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When to Use LLM-as-Judge

Use LLM-as-judge for criteria that are:
  • Subjective - Helpfulness, professionalism, empathy
  • Semantic - Correctness beyond exact string matching
  • Contextual - Appropriateness given the question
  • Nuanced - Requires reasoning about content
LLM-as-judge is powerful but slower and more expensive than code-based evaluation. Use it for subjective criteria where code-based checks aren’t sufficient.

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Basic LLM-as-Judge

Here’s a simple evaluator using OpenAI:
simple_llm_judge.py
from openai import OpenAI

client = OpenAI()

def is_helpful_evaluator(outputs: dict, expected: dict) -> dict:
    """Use GPT to judge if response is helpful."""
    
    prompt = f"""
    You are evaluating a customer support response for helpfulness.
    
    Question: {expected['question']}
    Response: {outputs['answer']}
    
    Is this response helpful to the customer?
    Answer with just "yes" or "no".
    """
    
    response = client.chat.completions.create(
        model="gpt-5-nano",
        messages=[{"role": "user", "content": prompt}],
        temperature=0
    )
    
    verdict = response.choices[0].message.content.strip().lower()
    
    return {
        "score": 1 if verdict == "yes" else 0,
        "comment": f"LLM judged response as {'helpful' if verdict == 'yes' else 'not helpful'}"
    }

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Structured Prompts

Provide clear evaluation criteria in your prompt:
structured_prompt.py
from openai import OpenAI

client = OpenAI()

EVALUATION_PROMPT = """
You are evaluating customer support responses for quality.

Evaluate the response on these criteria:
1. **Accuracy**: Does it correctly answer the question?
2. **Completeness**: Does it provide all necessary information?
3. **Tone**: Is it professional and empathetic?
4. **Clarity**: Is it easy to understand?

**Question:** {question}

**Response:** {response}

**Expected Answer (reference):** {expected}

Provide your evaluation as a JSON object:
{{
  "score": 0-100,
  "reasoning": "Brief explanation of the score"
}}
"""

def comprehensive_evaluator(outputs: dict, expected: dict) -> dict:
    """Multi-criteria LLM evaluation."""
    
    response = client.chat.completions.create(
        model="gpt-5-nano",
        messages=[
            {"role": "system", "content": "You are an expert evaluator. Respond only with valid JSON."},
            {"role": "user", "content": EVALUATION_PROMPT.format(
                question=expected["question"],
                response=outputs["answer"],
                expected=expected.get("answer", "N/A")
            )}
        ],
        temperature=0,
        response_format={"type": "json_object"}
    )
    
    import json
    result = json.loads(response.choices[0].message.content)
    
    return {
        "score": result["score"] / 100,  # Normalize to 0-1
        "comment": result["reasoning"]
    }

​
Real-World Example: Correctness Check

Evaluate if the agent’s answer is factually correct:
eval_correctness.py
from openai import OpenAI
from langsmith import evaluate

client = OpenAI()

CORRECTNESS_PROMPT = """
You are evaluating whether an agent's response correctly answers the customer's question.

Compare the agent's response to the reference answer. The agent's response doesn't need to match word-for-word, but it should contain the same key information.

**Question:** {question}

**Reference Answer:** {reference}

**Agent Response:** {response}

Is the agent's response correct?
Respond with ONLY: "correct" or "incorrect"
"""

def correctness_evaluator(outputs: dict, expected: dict) -> dict:
    """Check if agent response is factually correct."""
    
    response = client.chat.completions.create(
        model="gpt-5-nano",
        messages=[{
            "role": "system",
            "content": "You are a precise evaluator. Respond with only 'correct' or 'incorrect'."
        }, {
            "role": "user",
            "content": CORRECTNESS_PROMPT.format(
                question=expected.get("question", "N/A"),
                reference=expected.get("answer", "N/A"),
                response=outputs["answer"]
            )
        }],
        temperature=0
    )
    
    verdict = response.choices[0].message.content.strip().lower()
    is_correct = "correct" in verdict
    
    return {
        "score": 1 if is_correct else 0,
        "comment": f"Response is {verdict}"
    }

# Use in experiment
results = evaluate(
    your_agent,
    data="officeflow-dataset",
    evaluators=[correctness_evaluator]
)
Using temperature=0 makes LLM evaluations more deterministic and reproducible.

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Using LangSmith’s Built-in Evaluators

LangSmith provides pre-built LLM evaluators:
langsmith_evaluators.py
from langsmith import evaluate
from langsmith.evaluation import LangChainStringEvaluator

# Use built-in evaluators
results = evaluate(
    your_agent,
    data="officeflow-dataset",
    evaluators=[
        LangChainStringEvaluator("qa"),           # Question-answering correctness
        LangChainStringEvaluator("helpfulness"),  # How helpful is the response
        LangChainStringEvaluator("relevance"),    # Is response relevant to question
    ]
)
Available built-in evaluators:
  • qa - Correctness for Q&A tasks
  • helpfulness - Overall helpfulness
  • relevance - Relevance to the question
  • coherence - Internal consistency
  • harmfulness - Detects harmful content

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Chain-of-Thought Evaluation

Improve evaluation quality by asking the LLM to reason step-by-step:
cot_evaluation.py
from openai import OpenAI

client = OpenAI()

COT_PROMPT = """
You are evaluating a customer support response.

**Question:** {question}

**Response:** {response}

Evaluate this response step-by-step:

1. **Identify the customer's core need**: What is the customer really asking for?

2. **Check completeness**: Does the response address all parts of the question?

3. **Assess accuracy**: Is the information provided correct?

4. **Evaluate tone**: Is the response professional and empathetic?

5. **Final verdict**: Based on the above, is this a good response?

Provide your analysis in this format:
Core need: [your analysis] Completeness: [your analysis] Accuracy: [your analysis] Tone: [your analysis] Verdict: PASS or FAIL 
"""

def cot_evaluator(outputs: dict, expected: dict) -> dict:
    """Chain-of-thought LLM evaluation."""
    
    response = client.chat.completions.create(
        model="gpt-5-nano",
        messages=[{"role": "user", "content": COT_PROMPT.format(
            question=expected["question"],
            response=outputs["answer"]
        )}],
        temperature=0
    )
    
    analysis = response.choices[0].message.content
    passed = "PASS" in analysis.upper()
    
    return {
        "score": 1 if passed else 0,
        "comment": analysis
    }
Chain-of-thought prompting often improves evaluation quality, especially for complex criteria, but increases latency and cost.

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Multi-Model Consensus

Use multiple models for higher confidence:
consensus_eval.py
from openai import OpenAI
from anthropic import Anthropic

openai_client = OpenAI()
anthropic_client = Anthropic()

def consensus_evaluator(outputs: dict, expected: dict) -> dict:
    """Get consensus from multiple LLMs."""
    
    prompt = f"""
    Is this response helpful and accurate?
    Question: {expected['question']}
    Response: {outputs['answer']}
    Answer: yes or no
    """
    
    # OpenAI evaluation
    openai_response = openai_client.chat.completions.create(
        model="gpt-5-nano",
        messages=[{"role": "user", "content": prompt}],
        temperature=0
    )
    openai_verdict = "yes" in openai_response.choices[0].message.content.lower()
    
    # Anthropic evaluation
    anthropic_response = anthropic_client.messages.create(
        model="claude-3-5-haiku-20241022",
        messages=[{"role": "user", "content": prompt}],
        max_tokens=10,
        temperature=0
    )
    anthropic_verdict = "yes" in anthropic_response.content[0].text.lower()
    
    # Consensus: both must agree
    consensus = openai_verdict and anthropic_verdict
    
    return {
        "score": 1 if consensus else 0,
        "comment": f"OpenAI: {openai_verdict}, Anthropic: {anthropic_verdict}"
    }

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Best Practices

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Use Specific Rubrics

Provide concrete criteria rather than vague instructions:
rubric_example.py
GOOD_PROMPT = """
Evaluate if the response is helpful:
- Does it directly answer the question? (yes/no)
- Does it provide actionable next steps? (yes/no)
- Is it concise (under 100 words)? (yes/no)

The response is helpful if all three criteria are met.
"""

VAGUE_PROMPT = """
Is this response good? Answer yes or no.
"""

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Calibrate Your Evaluator

Test on examples with known outcomes:
calibration.py
test_cases = [
    {
        "question": "What is your return policy?",
        "response": "You can return items within 30 days.",
        "expected_score": 1,  # Should pass
    },
    {
        "question": "What is your return policy?",
        "response": "We sell office supplies.",
        "expected_score": 0,  # Should fail
    }
]

for test in test_cases:
    result = your_evaluator({"answer": test["response"]}, {"question": test["question"]})
    actual_score = result["score"]
    
    if actual_score != test["expected_score"]:
        print(f"MISMATCH: Expected {test['expected_score']}, got {actual_score}")
        print(f"Comment: {result['comment']}")

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Set Temperature to 0

For reproducible evaluations:
reproducible_llm.py
response = client.chat.completions.create(
    model="gpt-5-nano",
    messages=[...],
    temperature=0,  # Deterministic sampling
    seed=42         # Further reproducibility (if supported)
)

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Use Structured Outputs

Request JSON for easier parsing:
structured_output.py
from openai import OpenAI
import json

client = OpenAI()

response = client.chat.completions.create(
    model="gpt-5-nano",
    messages=[{
        "role": "user",
        "content": "Evaluate this response and return JSON with 'score' (0-1) and 'reasoning' fields."
    }],
    response_format={"type": "json_object"},
    temperature=0
)

result = json.loads(response.choices[0].message.content)
score = result["score"]
reasoning = result["reasoning"]

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Cost and Performance

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Choose the Right Model

  • GPT-5-nano: Fast and cheap, good for simple judgments
  • GPT-4o-mini: Balanced cost/performance for most use cases
  • GPT-4o: Best quality for complex evaluations
  • Claude 3.5 Haiku: Fast and cost-effective alternative
  • Claude 3.5 Sonnet: High quality, good for detailed analysis

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Batch Evaluations

Use async for better throughput:
async_llm_eval.py
import asyncio
from openai import AsyncOpenAI

client = AsyncOpenAI()

async def evaluate_one(outputs: dict, expected: dict) -> dict:
    response = await client.chat.completions.create(
        model="gpt-5-nano",
        messages=[...],
        temperature=0
    )
    # ... process response ...
    return {"score": score, "comment": comment}

async def evaluate_batch(examples: list) -> list:
    tasks = [evaluate_one(ex["outputs"], ex["expected"]) for ex in examples]
    return await asyncio.gather(*tasks)

​
Limitations

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Bias and Inconsistency

LLMs can be inconsistent:
  • Position bias: May prefer first or last option
  • Length bias: May favor longer responses
  • Self-preference: May favor responses similar to their own style
Mitigate by:
  • Using multiple evaluators
  • Providing clear rubrics
  • Calibrating on known examples

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Cost at Scale

LLM evaluation is expensive:
  • Use code-based evaluators where possible
  • Sample a subset for LLM evaluation
  • Use cheaper models for simpler criteria

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Latency

LLM evaluation is slower:
  • Use async/parallel evaluation
  • Cache evaluation results
  • Consider running evaluations offline

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Next Steps

Pairwise Eval

Compare agent versions side-by-side

Code-based Eval

Fast deterministic checks

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