LangSmith - LLM Observability Platform

Development platform for debugging, evaluating, and monitoring language models and AI applications.

When to use LangSmith

Use LangSmith when:

• Debugging LLM application issues (prompts, chains, agents)
• Evaluating model outputs systematically against datasets
• Monitoring production LLM systems
• Building regression testing for AI features
• Analyzing latency, token usage, and costs
• Collaborating on prompt engineering

Key features:

• Tracing: Capture inputs, outputs, latency for all LLM calls
• Evaluation: Systematic testing with built-in and custom evaluators
• Datasets: Create test sets from production traces or manually
• Monitoring: Track metrics, errors, and costs in production
• Integrations: Works with OpenAI, Anthropic, LangChain, LlamaIndex

Use alternatives instead:

• Weights & Biases: Deep learning experiment tracking, model training
• MLflow: General ML lifecycle, model registry focus
• Arize/WhyLabs: ML monitoring, data drift detection

Quick start

Installation

pip install langsmith

# Set environment variables
export LANGSMITH_API_KEY="your-api-key"
export LANGSMITH_TRACING=true

Basic tracing with @traceable

from langsmith import traceable
from openai import OpenAI

client = OpenAI()

@traceable
def generate_response(prompt: str) -> str:
    response = client.chat.completions.create(
        model="gpt-4o",
        messages=[{"role": "user", "content": prompt}]
    )
    return response.choices[0].message.content

# Automatically traced to LangSmith
result = generate_response("What is machine learning?")

OpenAI wrapper (automatic tracing)

from langsmith.wrappers import wrap_openai
from openai import OpenAI

# Wrap client for automatic tracing
client = wrap_openai(OpenAI())

# All calls automatically traced
response = client.chat.completions.create(
    model="gpt-4o",
    messages=[{"role": "user", "content": "Hello!"}]
)

Core concepts

Runs and traces

A run is a single execution unit (LLM call, chain, tool). Runs form hierarchical traces showing the full execution flow.

from langsmith import traceable

@traceable(run_type="chain")
def process_query(query: str) -> str:
    # Parent run
    context = retrieve_context(query)  # Child run
    response = generate_answer(query, context)  # Child run
    return response

@traceable(run_type="retriever")
def retrieve_context(query: str) -> list:
    return vector_store.search(query)

@traceable(run_type="llm")
def generate_answer(query: str, context: list) -> str:
    return llm.invoke(f"Context: {context}\n\nQuestion: {query}")

Projects

Projects organize related runs. Set via environment or code:

import os
os.environ["LANGSMITH_PROJECT"] = "my-project"

# Or per-function
@traceable(project_name="my-project")
def my_function():
    pass

Client API

from langsmith import Client

client = Client()

# List runs
runs = list(client.list_runs(
    project_name="my-project",
    filter='eq(status, "success")',
    limit=100
))

# Get run details
run = client.read_run(run_id="...")

# Create feedback
client.create_feedback(
    run_id="...",
    key="correctness",
    score=0.9,
    comment="Good answer"
)

Datasets and evaluation

Create dataset

from langsmith import Client

client = Client()

# Create dataset
dataset = client.create_dataset("qa-test-set", description="QA evaluation")

# Add examples
client.create_examples(
    inputs=[
        {"question": "What is Python?"},
        {"question": "What is ML?"}
    ],
    outputs=[
        {"answer": "A programming language"},
        {"answer": "Machine learning"}
    ],
    dataset_id=dataset.id
)

Run evaluation

from langsmith import evaluate

def my_model(inputs: dict) -> dict:
    # Your model logic
    return {"answer": generate_answer(inputs["question"])}

def correctness_evaluator(run, example):
    prediction = run.outputs["answer"]
    reference = example.outputs["answer"]
    score = 1.0 if reference.lower() in prediction.lower() else 0.0
    return {"key": "correctness", "score": score}

results = evaluate(
    my_model,
    data="qa-test-set",
    evaluators=[correctness_evaluator],
    experiment_prefix="v1"
)

print(f"Average score: {results.aggregate_metrics['correctness']}")

Built-in evaluators

from langsmith.evaluation import LangChainStringEvaluator

# Use LangChain evaluators
results = evaluate(
    my_model,
    data="qa-test-set",
    evaluators=[
        LangChainStringEvaluator("qa"),
        LangChainStringEvaluator("cot_qa")
    ]
)

Advanced tracing

Tracing context

from langsmith import tracing_context

with tracing_context(
    project_name="experiment-1",
    tags=["production", "v2"],
    metadata={"version": "2.0"}
):
    # All traceable calls inherit context
    result = my_function()

Manual runs

from langsmith import trace

with trace(
    name="custom_operation",
    run_type="tool",
    inputs={"query": "test"}
) as run:
    result = do_something()
    run.end(outputs={"result": result})

Process inputs/outputs

def sanitize_inputs(inputs: dict) -> dict:
    if "password" in inputs:
        inputs["password"] = "***"
    return inputs

@traceable(process_inputs=sanitize_inputs)
def login(username: str, password: str):
    return authenticate(username, password)

Sampling

import os
os.environ["LANGSMITH_TRACING_SAMPLING_RATE"] = "0.1"  # 10% sampling

LangChain integration

from langchain_openai import ChatOpenAI
from langchain_core.prompts import ChatPromptTemplate

# Tracing enabled automatically with LANGSMITH_TRACING=true
llm = ChatOpenAI(model="gpt-4o")
prompt = ChatPromptTemplate.from_messages([
    ("system", "You are a helpful assistant."),
    ("user", "{input}")
])

chain = prompt | llm

# All chain runs traced automatically
response = chain.invoke({"input": "Hello!"})

Production monitoring

Hub prompts

from langsmith import Client

client = Client()

# Pull prompt from hub
prompt = client.pull_prompt("my-org/qa-prompt")

# Use in application
result = prompt.invoke({"question": "What is AI?"})

Async client

from langsmith import AsyncClient

async def main():
    client = AsyncClient()

    runs = []
    async for run in client.list_runs(project_name="my-project"):
        runs.append(run)

    return runs

Feedback collection

from langsmith import Client

client = Client()

# Collect user feedback
def record_feedback(run_id: str, user_rating: int, comment: str = None):
    client.create_feedback(
        run_id=run_id,
        key="user_rating",
        score=user_rating / 5.0,  # Normalize to 0-1
        comment=comment
    )

# In your application
record_feedback(run_id="...", user_rating=4, comment="Helpful response")

Testing integration

Pytest integration

from langsmith import test

@test
def test_qa_accuracy():
    result = my_qa_function("What is Python?")
    assert "programming" in result.lower()

Evaluation in CI/CD

from langsmith import evaluate

def run_evaluation():
    results = evaluate(
        my_model,
        data="regression-test-set",
        evaluators=[accuracy_evaluator]
    )

    # Fail CI if accuracy drops
    assert results.aggregate_metrics["accuracy"] >= 0.9, \
        f"Accuracy {results.aggregate_metrics['accuracy']} below threshold"

Best practices

1. Structured naming - Use consistent project/run naming conventions
2. Add metadata - Include version, environment, user info
3. Sample in production - Use sampling rate to control volume
4. Create datasets - Build test sets from interesting production cases
5. Automate evaluation - Run evaluations in CI/CD pipelines
6. Monitor costs - Track token usage and latency trends

Common issues

Traces not appearing:

import os
# Ensure tracing is enabled
os.environ["LANGSMITH_TRACING"] = "true"
os.environ["LANGSMITH_API_KEY"] = "your-key"

# Verify connection
from langsmith import Client
client = Client()
print(client.list_projects())  # Should work

High latency from tracing:

# Enable background batching (default)
from langsmith import Client
client = Client(auto_batch_tracing=True)

# Or use sampling
os.environ["LANGSMITH_TRACING_SAMPLING_RATE"] = "0.1"

Large payloads:

# Hide sensitive/large fields
@traceable(
    process_inputs=lambda x: {k: v for k, v in x.items() if k != "large_field"}
)
def my_function(data):
    pass

References

• Advanced Usage - Custom evaluators, distributed tracing, hub prompts
• Troubleshooting - Common issues, debugging, performance

Resources

• Documentation: https://docs.smith.langchain.com
• Python SDK: https://github.com/langchain-ai/langsmith-sdk
• Web App: https://smith.langchain.com
• Version: 0.2.0+
• License: MIT