How do callbacks work in LangChain?

Callbacks in LangChain are hooks that fire at specific lifecycle events during chain, model, and agent execution. You implement a BaseCallbackHandler subclass and override only the methods you care about. Each method receives context about what just happened — which model was called, what the prompt was, what the response was, and how long it took.

Key callback methods (all have async equivalents prefixed with a):

• on_llm_start(serialized, prompts) — fired before an LLM call
• on_llm_end(response) — fired after an LLM call completes
• on_chain_start(serialized, inputs) — fired when a chain begins
• on_chain_end(outputs) — fired when a chain finishes
• on_tool_start(serialized, input_str) — fired before a tool executes
• on_tool_end(output) — fired after a tool returns
• on_agent_action(action) — fired each time an agent decides to use a tool

from langchain_core.callbacks import BaseCallbackHandler

class TokenLogger(BaseCallbackHandler):
    def on_llm_end(self, response, **kwargs):
        usage = response.llm_output.get('token_usage', {})
        print(f"Tokens used: {usage}")

chain.invoke({"input": "hello"}, config={"callbacks": [TokenLogger()]})

Callbacks can be attached per-invocation via config={"callbacks": [...]}, per-component via constructor arguments, or globally with set_global_handler(). LangSmith tracing itself is implemented as a callback handler.

Invest now in Acorns!!! 🚀
Join Acorns and get your $5 bonus!

Acorns is a micro-investing app that automatically invests your "spare change" from daily purchases into diversified, expert-built portfolios of ETFs. It is designed for beginners, allowing you to start investing with as little as $5. The service automates saving and investing. Disclosure: I may receive a referral bonus.

Invest now!!! Get Free equity stock (US, UK only)!

Use Robinhood app to invest in stocks. It is safe and secure. Use the Referral link to claim your free stock when you sign up!.

The Robinhood app makes it easy to trade stocks, crypto and more.

Webull! Receive free stock by signing up using the link: Webull signup.