Debugging failed chains in LangChain

Introduction

Debugging failed chains helps you find and fix errors in your LangChain workflows. It makes your chain work smoothly and correctly.

When your LangChain chain does not produce the expected output.

When you get error messages or exceptions running a chain.

When a chain stops working after adding new steps.

When you want to understand how data moves through your chain.

When you want to improve or optimize your chain by checking each part.

Syntax

LangChain

try:
    result = chain.run(input_data)
except Exception as e:
    print(f"Chain failed with error: {e}")
    # Optionally inspect intermediate steps or logs

Use try-except blocks to catch errors when running chains.

Check intermediate outputs or logs to find where the chain fails.

Examples

This example catches any error when running the chain and prints it.

LangChain

try:
    output = chain.run("Hello")
except Exception as error:
    print(f"Error: {error}")

Setting verbose = True shows detailed logs to help debug.

LangChain

chain.verbose = True
result = chain.run("Test input")

This example tracks token usage to debug cost or limits.

LangChain

from langchain.callbacks import get_openai_callback

with get_openai_callback() as cb:
    result = chain.run("Debug me")
    print(f"Tokens used: {cb.total_tokens}")

Sample Program

This example shows how to catch an error when a chain is misconfigured (empty chains list). The verbose flag helps show internal steps if chains existed.

LangChain

from langchain.chains import SimpleSequentialChain
from langchain.llms import OpenAI

# Create two simple chains
llm = OpenAI(temperature=0)

chain1 = SimpleSequentialChain(
    chains=[],  # Empty for example
    verbose=True
)

try:
    # This will fail because chains list is empty
    output = chain1.run("Hello")
except Exception as e:
    print(f"Chain failed with error: {e}")

Important Notes

Always check error messages carefully; they often tell you what went wrong.

Use verbose=True on chains to see detailed processing steps.

Try running smaller parts of your chain separately to isolate issues.

Summary

Use try-except blocks to catch and handle chain errors.

Enable verbose mode to get detailed logs for debugging.

Check intermediate outputs and error messages to find problems.