Setting up LangSmith tracing in LangChain

Introduction

LangSmith tracing helps you watch what your LangChain app does step-by-step. It records details so you can understand and fix your app easily.

You want to see how your LangChain app processes inputs and outputs.

You need to find bugs or unexpected behavior in your LangChain workflows.

You want to save detailed logs of your app's actions for later review.

You are building a complex app and want to track each step clearly.

Syntax

LangChain

import os
os.environ["LANGCHAIN_TRACING_V2"] = "true"
os.environ["LANGCHAIN_API_KEY"] = "lsv2_your-key-here"

# Use your LangChain components as usual - tracing is automatic
from langchain.chat_models import ChatOpenAI

llm = ChatOpenAI()

Set LANGCHAIN_TRACING_V2="true" and LANGCHAIN_API_KEY environment variables to start tracing.

LangChain components automatically send traces to LangSmith when env vars are set.

Examples

This example sets up tracing and runs a simple prompt with tracing enabled.

LangChain

import os
os.environ["LANGCHAIN_TRACING_V2"] = "true"
os.environ["LANGCHAIN_API_KEY"] = "lsv2_your-key-here"
from langchain.chat_models import ChatOpenAI

llm = ChatOpenAI()
response = llm.invoke("Hello!")

Shows how to use tracing with a chain to trace the whole workflow automatically.

LangChain

import os
os.environ["LANGCHAIN_TRACING_V2"] = "true"
os.environ["LANGCHAIN_API_KEY"] = "lsv2_your-key-here"
from langchain.chat_models import ChatOpenAI
from langchain.prompts import PromptTemplate
from langchain.chains import LLMChain

llm = ChatOpenAI()
prompt = PromptTemplate.from_template("Echo: {input}")
chain = LLMChain(llm=llm, prompt=prompt)
result = chain.invoke({"input": "Input text"})["text"]

Sample Program

This program sets up LangSmith tracing for a ChatOpenAI LLM by configuring environment variables. It runs a prompt asking for the capital of France and prints the response. View the trace in the LangSmith dashboard.

LangChain

import os
from langchain.chat_models import ChatOpenAI

# Set environment variables for LangSmith tracing
os.environ["LANGCHAIN_TRACING_V2"] = "true"
os.environ["LANGCHAIN_API_KEY"] = "lsv2_your-key-here"

# Create a ChatOpenAI LLM with tracing enabled automatically
llm = ChatOpenAI()

# Run a prompt
response = llm.invoke("What is the capital of France?")

print("Response:", response.content)

Important Notes

Make sure you have langchain, langsmith, and openai installed. Get your API key from LangSmith.

Tracing can slow down your app slightly because it records extra details.

You can view traces in LangSmith's dashboard or export them for analysis.

Summary

LangSmith tracing helps you watch your LangChain app's steps clearly.

Enable tracing by setting LANGCHAIN_TRACING_V2="true" and LANGCHAIN_API_KEY.

Use tracing to debug, understand, and improve your LangChain workflows.

Practice

1. What is the main purpose of setting up LangSmith tracing in a LangChain application?

easy

A. To encrypt data passed between LangChain components

B. To speed up the execution of LangChain components

C. To automatically fix errors in your LangChain code

D. To monitor and visualize the steps of your LangChain workflows

Solution

1. Step 1: Understand LangSmith tracing purpose

   LangSmith tracing is designed to help watch and understand the steps your LangChain app takes.

2. Step 2: Identify the correct purpose

   It does not speed up execution, fix errors automatically, or encrypt data, but helps monitor and visualize workflows.

3. Final Answer:

   To monitor and visualize the steps of your LangChain workflows -> Option D

4. Quick Check:

   Tracing = Monitoring steps [OK]

Hint: Tracing means watching steps clearly in LangChain [OK]

Common Mistakes:

• Thinking tracing speeds up code
• Assuming tracing fixes bugs automatically
• Confusing tracing with data encryption

2. Which of the following is the correct way to create a LangChainTracer for LangSmith tracing?

easy

A. tracer = LangChainTracer()

B. tracer = createTracer()

C. tracer = new LangSmithTracer()

D. tracer = LangSmith.createTracer()

Solution

1. Step 1: Recall LangChainTracer creation syntax

   The LangChainTracer is created by calling its constructor directly: LangChainTracer()

2. Step 2: Check options for correct syntax

   Options B, C, and D use incorrect function or class names or syntax not used in LangChain.

3. Final Answer:

   tracer = LangChainTracer() -> Option A

4. Quick Check:

   Constructor call = LangChainTracer() [OK]

Hint: Use LangChainTracer() constructor to create tracer [OK]

Common Mistakes:

• Using wrong class names like LangSmithTracer
• Calling non-existent functions like createTracer()
• Using 'new' keyword which is not Python syntax

3. Given the code snippet below, what will be the effect of passing the tracer to the LLM?

from langchain.chat_models import ChatOpenAI
from langchain.callbacks import LangChainTracer

tracer = LangChainTracer()
llm = ChatOpenAI(callbacks=[tracer])
response = llm.chat([{'role': 'user', 'content': 'Hello!'}])

medium

A. The LLM will log its steps to LangSmith for tracing

B. The LLM will run without any tracing or logging

C. The code will raise a syntax error due to wrong callback usage

D. The LLM will ignore the tracer and produce no output

Solution

1. Step 1: Understand passing tracer as callback

   Passing LangChainTracer in callbacks enables tracing of LLM steps.

2. Step 2: Analyze code behavior

   The LLM will send its internal steps to LangSmith via the tracer, enabling monitoring.

3. Final Answer:

   The LLM will log its steps to LangSmith for tracing -> Option A

4. Quick Check:

   Callbacks with tracer = tracing enabled [OK]

Hint: Callbacks=[tracer] enables LangSmith tracing [OK]

Common Mistakes:

• Assuming no tracing happens without explicit start call
• Thinking callbacks cause syntax errors here
• Believing tracer disables output

4. Identify the error in this LangSmith tracing setup code:

from langchain.chat_models import ChatOpenAI
from langchain.callbacks import LangChainTracer

tracer = LangChainTracer
llm = ChatOpenAI(callbacks=[tracer])

medium

A. LangChainTracer is not imported correctly

B. LangChainTracer is assigned without parentheses, missing instantiation

C. Callbacks list should be empty for tracing

D. ChatOpenAI does not accept callbacks parameter

Solution

1. Step 1: Check LangChainTracer assignment

   tracer = LangChainTracer misses parentheses, so tracer is a class, not an instance.

2. Step 2: Analyze usage in callbacks

   Passing callbacks=[tracer] passes the class instead of an instance, causing a runtime error when callbacks are used.

3. Final Answer:

   LangChainTracer is assigned without parentheses, missing instantiation -> Option B

4. Quick Check:

   Instantiate with () to create tracer object [OK]

Hint: Always instantiate classes with () before use [OK]

Common Mistakes:

• Assigning class instead of instance
• Calling instance as function
• Ignoring callbacks parameter usage

5. You want to trace both an LLM and a chain in LangChain using LangSmith. Which setup correctly enables tracing for both components?

hard

A. llm = ChatOpenAI()
chain = SomeChain(llm=llm)
tracer = LangChainTracer()
tracer.start()

B. tracer = LangChainTracer
llm = ChatOpenAI(callbacks=tracer)
chain = SomeChain(llm=llm, callbacks=tracer)

C. tracer = LangChainTracer()
llm = ChatOpenAI(callbacks=[tracer])
chain = SomeChain(llm=llm, callbacks=[tracer])

D. tracer = LangChainTracer()
llm = ChatOpenAI()
chain = SomeChain(llm=llm)

Solution

1. Step 1: Instantiate LangChainTracer correctly

   Use tracer = LangChainTracer() to create the tracer instance.

2. Step 2: Pass tracer in callbacks for both LLM and chain

   Both components accept callbacks as a list; passing [tracer] enables tracing on both.

3. Step 3: Evaluate options

   tracer = LangChainTracer() llm = ChatOpenAI(callbacks=[tracer]) chain = SomeChain(llm=llm, callbacks=[tracer]) correctly instantiates tracer and passes it as a list to both components. Others miss instantiation, use wrong types, or omit callbacks.

4. Final Answer:

   tracer = LangChainTracer() llm = ChatOpenAI(callbacks=[tracer]) chain = SomeChain(llm=llm, callbacks=[tracer]) -> Option C

5. Quick Check:

   Instantiate tracer and pass as list to callbacks [OK]

Hint: Instantiate tracer and pass as list to all callbacks [OK]

Common Mistakes:

• Not instantiating tracer with ()
• Passing tracer directly instead of in a list
• Forgetting to add callbacks to chain