LangSmith Evaluator

Create evaluators to measure agent performance on your datasets. LangSmith supports two types: LLM as Judge (uses LLM to grade outputs) and Custom Code (deterministic logic).

Setup

Environment Variables

LANGSMITH_API_KEY=lsv2_pt_your_api_key_here          # Required
LANGSMITH_WORKSPACE_ID=your-workspace-id              # Optional: for org-scoped keys
OPENAI_API_KEY=your_openai_key                        # For LLM as Judge

Dependencies

pip install langsmith langchain-openai python-dotenv

Evaluator Format

Evaluators support two function signatures:

Method 1: Dict Parameters (For running evaluations locally):

def evaluator_name(inputs: dict, outputs: dict, reference_outputs: dict = None) -> dict:
    """Evaluate a single prediction."""
    user_query = inputs.get("query", "")
    agent_response = outputs.get("expected_response", "")
    expected = reference_outputs.get("expected_response", "") if reference_outputs else None

    return {
        "key": "metric_name",    # Metric identifier
        "score": 0.85,           # Number or boolean
        "comment": "Reason..."   # Optional explanation
    }

Method 2: Run/Example Parameters (For uploading to LangSmith):

def evaluator_name(run, example):
    """Evaluate using run/example dicts.

    Args:
        run: Dict with run["outputs"] containing agent outputs
        example: Dict with example["outputs"] containing expected outputs
    """
    agent_response = run["outputs"].get("expected_response", "")
    expected = example["outputs"].get("expected_response", "")

    return {
        "metric_name": 0.85,      # Metric name as key directly
        "comment": "Reason..."    # Optional explanation
    }

LLM as Judge Evaluators

Use structured output for reliable grading:

from typing import TypedDict, Annotated
from langchain_openai import ChatOpenAI

class AccuracyGrade(TypedDict):
    """Structured evaluation output."""
    reasoning: Annotated[str, ..., "Explain your reasoning"]
    is_accurate: Annotated[bool, ..., "True if response is accurate"]
    confidence: Annotated[float, ..., "Confidence 0.0-1.0"]

# Configure model with structured output
judge = ChatOpenAI(model="gpt-4o-mini", temperature=0).with_structured_output(
    AccuracyGrade, method="json_schema", strict=True
)

async def accuracy_evaluator(run, example):
    """Evaluate factual accuracy for LangSmith upload."""
    expected = example["outputs"].get('expected_response', '')
    agent_output = run["outputs"].get('expected_response', '')

    prompt = f"""Expected: {expected}
Agent Output: {agent_output}
Evaluate accuracy:"""

    grade = await judge.ainvoke([{"role": "user", "content": prompt}])

    return {
        "accuracy": 1 if grade["is_accurate"] else 0,
        "comment": f"{grade['reasoning']} (confidence: {grade['confidence']})"
    }

Common Metrics: Completeness, correctness, helpfulness, professionalism

Custom Code Evaluators

Exact Match

def exact_match_evaluator(run, example):
    """Check if output exactly matches expected."""
    output = run["outputs"].get("expected_response", "").strip().lower()
    expected = example["outputs"].get("expected_response", "").strip().lower()

    match = output == expected
    return {
        "exact_match": 1 if match else 0,
        "comment": f"Match: {match}"
    }

Trajectory Validation

def trajectory_evaluator(run, example):
    """Evaluate tool call sequence."""
    trajectory = run["outputs"].get("expected_trajectory", [])
    expected = example["outputs"].get("expected_trajectory", [])

    # Exact sequence match
    exact = trajectory == expected

    # All required tools used (order-agnostic)
    all_tools = set(expected).issubset(set(trajectory))

    # Efficiency: count extra steps
    extra_steps = len(trajectory) - len(expected)

    return {
        "trajectory_match": 1 if exact else 0,
        "comment": f"Exact: {exact}, All tools: {all_tools}, Extra: {extra_steps}"
    }

Single Step Validation

def single_step_evaluator(run, example):
    """Evaluate single node output."""
    output = run["outputs"].get("output", {})
    expected = example["outputs"].get("expected_output", {})
    node_name = run["outputs"].get("node_name", "")

    # For classification nodes
    if "classification" in node_name:
        classification = output.get("classification", "")
        expected_class = expected.get("classification", "")
        match = classification.lower() == expected_class.lower()

        return {
            "classification_correct": 1 if match else 0,
            "comment": f"Output: {classification}, Expected: {expected_class}"
        }

    # For other nodes
    match = output == expected
    return {
        "output_match": 1 if match else 0,
        "comment": f"Match: {match}"
    }

Running Evaluations

from langsmith import Client

client = Client()

# Define your agent function
def run_agent(inputs: dict) -> dict:
    """Your agent invocation logic."""
    result = your_agent.invoke(inputs)
    return {"expected_response": result}

# Run evaluation
results = await client.aevaluate(
    run_agent,
    data="Skills: Final Response",              # Dataset name
    evaluators=[
        exact_match_evaluator,
        accuracy_evaluator,
        trajectory_evaluator
    ],
    experiment_prefix="skills-eval-v1",
    max_concurrency=4
)

Upload Evaluators to LangSmith

The upload script is a utility tool to deploy your custom evaluators to LangSmith. Write evaluators specific to your use case, then upload them.

Navigate to skills/langsmith-evaluator/scripts/ to upload evaluators.

Important: LangSmith API requires evaluators to use (run, example) signature where:

• run: dict with run["outputs"] containing agent outputs
• example: dict with example["outputs"] containing expected outputs

Create Evaluator File

# my_project/evaluators/custom_evals.py

def my_custom_evaluator(run, example):
    """Your custom evaluation logic.

    Args:
        run: Dict with run["outputs"] - agent outputs
        example: Dict with example["outputs"] - expected outputs

    Returns:
        Dict with metric_name as key, score as value, optional comment
    """
    # Extract relevant data
    agent_output = run["outputs"].get("expected_trajectory", [])
    expected = example["outputs"].get("expected_trajectory", [])

    # Your custom logic here
    match = agent_output == expected

    return {
        "my_metric": 1 if match else 0,
        "comment": "Custom reasoning here"
    }

Upload

# List existing evaluators
python upload_evaluators.py list

# Upload evaluator
python upload_evaluators.py upload my_evaluators.py \
  --name "Trajectory Match" \
  --function trajectory_match \
  --dataset "Skills: Trajectory" \
  --replace

# Delete evaluator (will prompt for confirmation)
python upload_evaluators.py delete "Trajectory Match"

# Skip confirmation prompts (use with caution)
python upload_evaluators.py delete "Trajectory Match" --yes
python upload_evaluators.py upload my_evaluators.py \
  --name "Trajectory Match" \
  --function trajectory_match \
  --replace --yes

Options:

• --name - Display name in LangSmith
• --function - Function name to extract
• --dataset - Target dataset name
• --project - Target project name
• --sample-rate - Sampling rate (0.0-1.0)
• --replace - Replace if exists (will prompt for confirmation)
• --yes - Skip confirmation prompts for replace/delete operations

IMPORTANT - Safety Prompts:

• The script prompts for confirmation before any destructive operations (delete, replace)
• ALWAYS respect these prompts - wait for user input before proceeding
• NEVER use --yes flag unless the user explicitly requests it
• The --yes flag skips all safety prompts and should only be used in automated workflows when explicitly authorized by the user

Best Practices

1. Use structured output for LLM judges - More reliable than parsing free-text
2. Match evaluator to dataset type
   • Final Response → LLM as Judge for quality, Custom Code for format
   • Single Step → Custom Code for exact match
   • Trajectory → Custom Code for sequence/efficiency
3. Combine multiple evaluators - Run both subjective (LLM) and objective (code)
4. Use async for LLM judges - Enables parallel evaluation, much faster
5. Test evaluators independently - Validate on known good/bad examples first
6. Upload to LangSmith - Automatic evaluation on new runs

Example Workflow

# 1. Create evaluators file
cat > evaluators.py <<'EOF'
def exact_match(run, example):
    """Check if output exactly matches expected."""
    output = run["outputs"].get("expected_response", "").strip().lower()
    expected = example["outputs"].get("expected_response", "").strip().lower()
    match = output == expected
    return {
        "exact_match": 1 if match else 0,
        "comment": f"Match: {match}"
    }
EOF

# 2. Upload to LangSmith
python upload_evaluators.py upload evaluators.py \
  --name "Exact Match" \
  --function exact_match \
  --dataset "Skills: Final Response" \
  --replace

# 3. Evaluator runs automatically on new dataset runs

Resources

• LangSmith Evaluation Concepts
• Custom Code Evaluators
• OpenEvals - Readymade Evaluators

Related Skills

• Use langsmith-trace skill to query and export traces
• Use langsmith-dataset skill to generate evaluation datasets from traces