How to delete a thread with MemorySaver in LangGraph

How to delete a thread with MemorySaver in LangGraph

TL;DR: there is no built-in delete. Write an empty checkpoint over the thread with checkpointer.put(config, empty_checkpoint, {}, {}) and the thread reads back blank. Keep a reference to your checkpointer (not just the compiled graph) so you can do this.

# no error. the problem: MemorySaver has no delete_thread() method,
# so finished sessions keep their checkpoint history forever.

When this applies

• You use MemorySaver (or InMemorySaver) as the checkpointer.
• A session/thread ended and you want its state gone (privacy, memory, clean slate).

When it does not

• Postgres/SQLite savers: delete the thread's rows from the checkpoints tables instead.
• If you just want a fresh conversation, start a new thread_id. That is simpler and always works.

Fix it

1. Keep the checkpointer reference

from langgraph.checkpoint.memory import MemorySaver

saver = MemorySaver()  # keep this, not just `graph`
graph = builder.compile(checkpointer=saver)

Expected: you can call saver.put(...) later. If you only kept graph, reach it via graph.checkpointer where available.

2. Write an empty checkpoint over the thread

import uuid
from datetime import datetime, timezone

def clear_thread(checkpointer, thread_id: str):
    config = {"configurable": {"thread_id": thread_id, "checkpoint_ns": ""}}
    empty = {
        "v": 1,
        "ts": datetime.now(timezone.utc).isoformat(),
        "id": str(uuid.uuid4()),
        "channel_values": {},
        "channel_versions": {"__start__": 1},
        "versions_seen": {"__input__": {}, "__start__": {}},
        "pending_sends": [],
    }
    checkpointer.put(config, empty, {}, {})

clear_thread(saver, "session-123")

Expected: graph.get_state({"configurable": {"thread_id": "session-123"}}) now returns empty values. The thread is effectively deleted.

3. Confirm it is gone

state = graph.get_state({"configurable": {"thread_id": "session-123"}})
print(state.values)  # {} -- blank

Expected: empty values. History for that thread_id reads back blank from here on.

Why it happens

MemorySaver is a plain in-memory dict of checkpoints with no delete API. Overwriting the thread's latest checkpoint with an empty one is the community-verified workaround: every read (get_state, get_state_history) resolves through the latest checkpoint, so a blank head means a blank thread.

Edge cases

• Old checkpoint versions for the thread may still sit in the saver's dict until the process restarts. For true erasure (compliance), drop the whole saver or filter its storage.
• InMemorySaver (1.x) has the same shape; the same workaround applies.
• Subgraph namespaces: if the thread used subgraphs, clear each checkpoint_ns too, or stale sub-states linger.
• In production, prefer per-session processes or a real database saver with row deletes over this trick.

Compatibility

langgraph 0.x (MemorySaver from langgraph.checkpoint.memory) and 1.x (InMemorySaver). Python and the equivalent JS MemorySaver accept the same empty-checkpoint shape.