LangChain has some built-in callback handlers, but you will often want to create your own handlers with custom logic.
To create a custom callback handler, we need to determine the event(s) we want our callback handler to handle as well as what we want our callback handler to do when the event is triggered. Then all we need to do is attach the callback handler to the object, for example via the constructor or at runtime.
In the example below, we'll implement streaming with a custom handler.
In our custom callback handler MyCustomHandler, we implement the on_llm_new_token handler to print the token we have just received. We then attach our custom handler to the model object as a constructor callback.
from langchain_anthropic import ChatAnthropic
from langchain_core.callbacks import BaseCallbackHandler
from langchain_core.prompts import ChatPromptTemplate
class MyCustomHandler(BaseCallbackHandler):
def on_llm_new_token(self, token: str, **kwargs) -> None:
print(f"My custom handler, token: {token}")
prompt = ChatPromptTemplate.from_messages(["Tell me a joke about {animal}"])
model = ChatAnthropic(
model="claude-3-7-sonnet-20250219", streaming=True, callbacks=[MyCustomHandler()]
)
chain = prompt | model
response = chain.invoke({"animal": "bears"})
You can see this reference page for a list of events you can handle. Note that the handle_chain_* events run for most LCEL runnables.
You've now learned how to create your own custom callback handlers.
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