Agent Memory (Checkpointer)

Where we left off

Lesson 23 built an agent that automates the tool-call loop, but each .invoke() was still independent, no memory across calls. This lesson adds a checkpointer, so the agent remembers earlier turns automatically.

The checkpointer: real memory, not a list you manage

checkpointer=InMemorySaver()

In Lesson 17, history was a plain Python list, living only inside our running program, and we were responsible for appending to it and resending it every single time. A checkpointer does that same job, but automatically, and it's built to eventually save to a real database (so it survives your program restarting) instead of always living in memory.

InMemorySaver is the simplest version, it still forgets everything when the program ends, same limitation as Lesson 17, but the mechanism is now something a real production agent would just swap out for a persistent one (covered in Lesson 32), without changing any other code.

config = {"configurable": {"thread_id": "lesson-24-demo"}}
...
result = agent.invoke({"messages": [HumanMessage(user_input)]}, config)

thread_id is the key idea here. It's like a conversation ID. Every .invoke() call tagged with the same thread_id shares the same remembered history behind the scenes, the checkpointer looks up everything that happened before under that ID, and prepends it, before the agent ever sees your new message. Change the thread_id, and you'd get a completely fresh, unrelated conversation, even reusing the exact same agent object.

This is how a real app serves memory correctly for many different users or conversations at once, using one agent, distinguishing them purely by thread ID.

Notice what we send each turn now:

agent.invoke({"messages": [HumanMessage(user_input)]}, config)

Just the new message. Not the whole history, unlike Lesson 17. The checkpointer already has everything earlier, tagged under this thread_id.

What this project has built up to, so far

If any part of this lesson feels like magic, that's usually a sign it's worth going back to the earlier lesson where that exact mechanism was built by hand.