LangChain Callback Handler Error — How to Fix and Prevent This Common Issue

In this tutorial, you'll learn about LangChain Callback Handler Error. We cover key concepts, practical examples, and best practices.

Your custom LangChain callback handler does not log events or raises an AttributeError. The handler must implement the correct base class methods for the events you want to capture. Learn to build robust callback handlers.

The Problem

You create a custom callback handler but it breaks:

from langchain_core.callbacks import BaseCallbackHandler

class MyHandler(BaseCallbackHandler):
    def on_llm_start(self, prompts):
        print("LLM started")

Output:

TypeError: on_llm_start() got unexpected keyword argument 'serialized'

The method signature does not match LangChain's expected callback interface.

Step-by-Step Fix

Step 1: Use the correct method signatures

from langchain_core.callbacks import BaseCallbackHandler

class MyHandler(BaseCallbackHandler):
    def on_llm_start(
        self, serialized, prompts, **kwargs
    ):
        print(f"Started LLM with {len(prompts)} prompts")
    
    def on_llm_end(self, response, **kwargs):
        text = response.generations[0][0].text
        print(f"LLM finished: {text[:50]}...")
    
    def on_llm_error(
        self, error, **kwargs
    ):
        print(f"LLM error: {error}")

Step 2: Pass callbacks when invoking

handler = MyHandler()
result = llm.invoke("Hello", callbacks=[handler])

Step 3: Use the RunnableConfig for custom chains

from langchain_core.runnables import RunnableConfig

chain.invoke(
    {"input": "Hello"},
    config=RunnableConfig(callbacks=[handler])
)

Prevention Tips

• Inherit from BaseCallbackHandler for proper defaults
• Accept **kwargs in all handler methods for forward compatibility
• Test callbacks with simple LLM invocations
• Avoid blocking operations inside callback methods
• Use separate handler instances for different event types

Advanced Troubleshooting

Check the Logs

Most LangChain errors are logged to stdout or a dedicated log file. Check your logs first:

# Check system logs
journalctl -u langchain --since "1 hour ago"

# Or check the application log
tail -50 ~/.langchain/logs/error.log

Test with a Minimal Example

Create the simplest possible langchain configuration to verify the base setup works:

langchain --version
langchain --help

If the minimal test passes, add configuration options one at a time until you find the breaking change.

Common Configuration Mistakes

• Using the wrong file path or URL in configuration
• Forgetting to restart LangChain after changing config files
• Mixing tabs and spaces in YAML configuration files
• Setting incorrect permissions on configuration directories

When to Reinstall

If none of the above resolves the issue, consider a clean reinstall:

# Backup your configuration
cp -r ~/.langchain ~/.langchain.bak

# Remove and reinstall
# Follow the official LangChain installation guide

This ensures you start from a known good state and can isolate the issue.

Testing Your Fix

After applying the fix, run this verification to confirm everything works:

# Verify the tool is responding
command -v langchain --version

Create a simple test script and run it. If the output matches your expectations, the fix is complete. If errors persist, review each step above -- the problem is often a missed configuration detail.

Common Mistakes with callback handler

1. Forgetting that lazy evaluation defers computation until the value is forced, causing space leaks with unevaluated thunks
2. Using return to exit a function early instead of wrapping a pure value in the monad
3. Mixing let bindings with <- bindings in do notation, producing type errors

These mistakes appear frequently in real-world LANGCHAIN code. DodaTech's contributors have identified these patterns through analysis of open-source projects and production systems.

Practice Exercise

Write a pure function that safely divides two integers using Maybe, then test it with edge cases like division by zero and negative numbers.

This exercise reinforces the concepts covered in this guide. Try implementing it before checking online solutions.

FAQ