• Checkpointer: Saves/loads graph state at every super-step
• Thread ID: Identifies separate checkpoint sequences (conversations)
• Store: Cross-thread memory for user preferences, facts

Two memory types:

• Short-term (checkpointer): Thread-scoped conversation history
• Long-term (store): Cross-thread user preferences, facts</overview>

Checkpointer Setup

class State(TypedDict): messages: Annotated[list, operator.add]

def add_message(state: State) -> dict: return {"messages": ["Bot response"]}

checkpointer = InMemorySaver()

graph = ( StateGraph(State) .add_node("respond", add_message) .add_edge(START, "respond") .add_edge("respond", END) .compile(checkpointer=checkpointer) # Pass at compile time )

ALWAYS provide thread_id

config = {"configurable": {"thread_id": "conversation-1"}}

result1 = graph.invoke({"messages": ["Hello"]}, config) print(len(result1["messages"])) # 2

result2 = graph.invoke({"messages": ["How are you?"]}, config) print(len(result2["messages"])) # 4 (previous + new)

</python>
<typescript>
Set up a basic graph with in-memory checkpointing and thread-based state persistence.
```typescript
import { MemorySaver, StateGraph, StateSchema, MessagesValue, START, END } from "@langchain/langgraph";
import { HumanMessage } from "@langchain/core/messages";

const State = new StateSchema({ messages: MessagesValue });

const addMessage = async (state: typeof State.State) => {
  return { messages: [{ role: "assistant", content: "Bot response" }] };
};

const checkpointer = new MemorySaver();

const graph = new StateGraph(State)
  .addNode("respond", addMessage)
  .addEdge(START, "respond")
  .addEdge("respond", END)
  .compile({ checkpointer });

// ALWAYS provide thread_id
const config = { configurable: { thread_id: "conversation-1" } };

const result1 = await graph.invoke({ messages: [new HumanMessage("Hello")] }, config);
console.log(result1.messages.length);  // 2

const result2 = await graph.invoke({ messages: [new HumanMessage("How are you?")] }, config);
console.log(result2.messages.length);  // 4 (previous + new)

with PostgresSaver.from_conn_string( "postgresql://user:pass@localhost/db" ) as checkpointer: checkpointer.setup() # only needed on first use to create tables graph = builder.compile(checkpointer=checkpointer)

</python>
<typescript>
Configure PostgreSQL-backed checkpointing for production deployments.
```typescript
import { PostgresSaver } from "@langchain/langgraph-checkpoint-postgres";

const checkpointer = PostgresSaver.fromConnString(
  "postgresql://user:pass@localhost/db"
);
await checkpointer.setup(); // only needed on first use to create tables

const graph = builder.compile({ checkpointer });

Thread Management

graph.invoke({"messages": ["Hi from Alice"]}, alice_config) graph.invoke({"messages": ["Hi from Bob"]}, bob_config)

Alice's state is isolated from Bob's

</python>
<typescript>
Demonstrate isolated state between different thread IDs.
```typescript
// Different threads maintain separate state
const aliceConfig = { configurable: { thread_id: "user-alice" } };
const bobConfig = { configurable: { thread_id: "user-bob" } };

await graph.invoke({ messages: [new HumanMessage("Hi from Alice")] }, aliceConfig);
await graph.invoke({ messages: [new HumanMessage("Hi from Bob")] }, bobConfig);

// Alice's state is isolated from Bob's

State History & Time Travel

result = graph.invoke({"messages": ["start"]}, config)

Browse checkpoint history

states = list(graph.get_state_history(config))

Replay from a past checkpoint

past = states[-2] result = graph.invoke(None, past.config) # None = resume from checkpoint

Or fork: update state at a past checkpoint, then resume

fork_config = graph.update_state(past.config, {"messages": ["edited"]}) result = graph.invoke(None, fork_config)

</python>
<typescript>
Time travel: browse checkpoint history and replay or fork from a past state.
```typescript
const config = { configurable: { thread_id: "session-1" } };

const result = await graph.invoke({ messages: ["start"] }, config);

// Browse checkpoint history (async iterable, collect to array)
const states: Awaited<ReturnType<typeof graph.getState>>[] = [];
for await (const state of graph.getStateHistory(config)) {
  states.push(state);
}

// Replay from a past checkpoint
const past = states[states.length - 2];
const replayed = await graph.invoke(null, past.config);  // null = resume from checkpoint

// Or fork: update state at a past checkpoint, then resume
const forkConfig = await graph.updateState(past.config, { messages: ["edited"] });
const forked = await graph.invoke(null, forkConfig);

Modify state before resuming

graph.update_state(config, {"data": "manually_updated"})

Resume with updated state

result = graph.invoke(None, config)

</python>
<typescript>
Manually update graph state before resuming execution.
```typescript
const config = { configurable: { thread_id: "session-1" } };

// Modify state before resuming
await graph.updateState(config, { data: "manually_updated" });

// Resume with updated state
const result = await graph.invoke(null, config);

Subgraph Checkpointer Scoping

When compiling a subgraph, the checkpointer parameter controls persistence behavior. This is critical for subgraphs that use interrupts, need multi-turn memory, or run in parallel.

When to use each mode

• checkpointer=False — Subgraph doesn't need interrupts or persistence. Simplest option, no checkpoint overhead.
• None (default / omit checkpointer) — Subgraph needs interrupt() but not multi-turn memory. Each invocation starts fresh but can pause/resume. Parallel execution works because each invocation gets a unique namespace.
• checkpointer=True — Subgraph needs to remember state across invocations (multi-turn conversations). Each call picks up where the last left off.

Warning: Stateful subgraphs (checkpointer=True) do NOT support calling the same subgraph instance multiple times within a single node — the calls write to the same checkpoint namespace and conflict.

Need interrupts but not cross-invocation persistence (default)

subgraph = subgraph_builder.compile()

Need cross-invocation persistence (stateful)

subgraph = subgraph_builder.compile(checkpointer=True)

</python>
<typescript>
Choose the right checkpointer mode for your subgraph.
```typescript
// No interrupts needed — opt out of checkpointing
const subgraph = subgraphBuilder.compile({ checkpointer: false });

// Need interrupts but not cross-invocation persistence (default)
const subgraph = subgraphBuilder.compile();

// Need cross-invocation persistence (stateful)
const subgraph = subgraphBuilder.compile({ checkpointer: true });

Parallel subgraph namespacing

When multiple different stateful subgraphs run in parallel, wrap each in its own StateGraph with a unique node name for stable namespace isolation:

def create_sub_agent(model, *, name, **kwargs): """Wrap an agent with a unique node name for namespace isolation.""" agent = create_agent(model=model, name=name, **kwargs) return ( StateGraph(MessagesState) .add_node(name, agent) # unique name -> stable namespace .add_edge("start", name) .compile() )

fruit_agent = create_sub_agent( "gpt-4.1-mini", name="fruit_agent", tools=[fruit_info], prompt="...", checkpointer=True, ) veggie_agent = create_sub_agent( "gpt-4.1-mini", name="veggie_agent", tools=[veggie_info], prompt="...", checkpointer=True, )

</python>
<typescript>
```typescript
import { StateGraph, StateSchema, MessagesValue, START } from "@langchain/langgraph";

function createSubAgent(model: string, { name, ...kwargs }: { name: string; [key: string]: any }) {
  const agent = createAgent({ model, name, ...kwargs });
  return new StateGraph(new StateSchema({ messages: MessagesValue }))
    .addNode(name, agent)  // unique name -> stable namespace
    .addEdge(START, name)
    .compile();
}

const fruitAgent = createSubAgent("gpt-4.1-mini", {
  name: "fruit_agent", tools: [fruitInfo], prompt: "...", checkpointer: true,
});
const veggieAgent = createSubAgent("gpt-4.1-mini", {
  name: "veggie_agent", tools: [veggieInfo], prompt: "...", checkpointer: true,
});

Note: Subgraphs added as nodes (via add_node) already get name-based namespaces automatically and don't need this wrapper.

Long-Term Memory (Store)

store = InMemoryStore()

Save user preference (available across ALL threads)

store.put(("alice", "preferences"), "language", {"preference": "short responses"})

Node with store — access via runtime

from langgraph.runtime import Runtime

def respond(state, runtime: Runtime): prefs = runtime.store.get((state["user_id"], "preferences"), "language") return {"response": f"Using preference: {prefs.value}"}

Compile with BOTH checkpointer and store

graph = builder.compile(checkpointer=checkpointer, store=store)

Both threads access same long-term memory

graph.invoke({"user_id": "alice"}, {"configurable": {"thread_id": "thread-1"}}) graph.invoke({"user_id": "alice"}, {"configurable": {"thread_id": "thread-2"}}) # Same preferences!

</python>
<typescript>
Use a Store for cross-thread memory to share user preferences across conversations.
```typescript
import { MemoryStore } from "@langchain/langgraph";

const store = new MemoryStore();

// Save user preference (available across ALL threads)
await store.put(["alice", "preferences"], "language", { preference: "short responses" });

// Node with store — access via runtime
const respond = async (state: typeof State.State, runtime: any) => {
  const item = await runtime.store?.get(["alice", "preferences"], "language");
  return { response: `Using preference: ${item?.value?.preference}` };
};

// Compile with BOTH checkpointer and store
const graph = builder.compile({ checkpointer, store });

// Both threads access same long-term memory
await graph.invoke({ userId: "alice" }, { configurable: { thread_id: "thread-1" } });
await graph.invoke({ userId: "alice" }, { configurable: { thread_id: "thread-2" } });  // Same preferences!

store = InMemoryStore()

store.put(("user-123", "facts"), "location", {"city": "San Francisco"}) # Put item = store.get(("user-123", "facts"), "location") # Get results = store.search(("user-123", "facts"), filter={"city": "San Francisco"}) # Search store.delete(("user-123", "facts"), "location") # Delete

</python>
</ex-store-operations>

---

## Fixes

<fix-thread-id-required>
<python>
Always provide thread_id in config to enable state persistence.
```python
# WRONG: No thread_id - state NOT persisted!
graph.invoke({"messages": ["Hello"]})
graph.invoke({"messages": ["What did I say?"]})  # Doesn't remember!

# CORRECT: Always provide thread_id
config = {"configurable": {"thread_id": "session-1"}}
graph.invoke({"messages": ["Hello"]}, config)
graph.invoke({"messages": ["What did I say?"]}, config)  # Remembers!

// CORRECT: Always provide thread_id const config = { configurable: { thread_id: "session-1" } }; await graph.invoke({ messages: [new HumanMessage("Hello")] }, config); await graph.invoke({ messages: [new HumanMessage("What did I say?")] }, config); // Remembers!

</typescript>
</fix-thread-id-required>


<fix-inmemory-not-for-production>
<python>
Use PostgresSaver instead of InMemorySaver for production persistence.
```python
# WRONG: Data lost on process restart
checkpointer = InMemorySaver()  # In-memory only!

# CORRECT: Use persistent storage for production
from langgraph.checkpoint.postgres import PostgresSaver
with PostgresSaver.from_conn_string("postgresql://...") as checkpointer:
    checkpointer.setup()  # only needed on first use to create tables
    graph = builder.compile(checkpointer=checkpointer)

// CORRECT: Use persistent storage for production import { PostgresSaver } from "@langchain/langgraph-checkpoint-postgres"; const checkpointer = PostgresSaver.fromConnString("postgresql://..."); await checkpointer.setup(); // only needed on first use to create tables

</typescript>
</fix-inmemory-not-for-production>


<fix-update-state-with-reducers>
<python>
Use Overwrite to replace state values instead of passing through reducers.
```python
from langgraph.types import Overwrite

# State with reducer: items: Annotated[list, operator.add]
# Current state: {"items": ["A", "B"]}

# update_state PASSES THROUGH reducers
graph.update_state(config, {"items": ["C"]})  # Result: ["A", "B", "C"] - Appended!

# To REPLACE instead, use Overwrite
graph.update_state(config, {"items": Overwrite(["C"])})  # Result: ["C"] - Replaced

// State with reducer: items uses concat reducer // Current state: { items: ["A", "B"] }

// updateState PASSES THROUGH reducers await graph.updateState(config, { items: ["C"] }); // Result: ["A", "B", "C"] - Appended!

// To REPLACE instead, use Overwrite await graph.updateState(config, { items: new Overwrite(["C"]) }); // Result: ["C"] - Replaced

</typescript>
</fix-update-state-with-reducers>

<fix-store-injection>
<python>
Access store via the Runtime object in graph nodes.
```python
# WRONG: Store not available in node
def my_node(state):
    store.put(...)  # NameError! store not defined

# CORRECT: Access store via runtime
from langgraph.runtime import Runtime

def my_node(state, runtime: Runtime):
    runtime.store.put(...)  # Correct store instance

// CORRECT: Access store via runtime const myNode = async (state, runtime) => { await runtime.store?.put(...); // Correct store instance };

</typescript>
</fix-store-injection>

<boundaries>
### What You Should NOT Do

- Use `InMemorySaver` in production — data lost on restart; use `PostgresSaver`
- Forget `thread_id` — state won't persist without it
- Expect `update_state` to bypass reducers — it passes through them; use `Overwrite` to replace
- Run the same stateful subgraph (`checkpointer=True`) in parallel within one node — namespace conflict
- Access store directly in a node — use `runtime.store` via the `Runtime` param
</boundaries>