langgraph/docs/docs/troubleshooting/errors/INVALID_CONCURRENT_GRAPH_UPDATE.md

INVALID_CONCURRENT_GRAPH_UPDATE

A LangGraph StateGraph received concurrent updates to its state from multiple nodes to a state property that doesn't support it.

One way this can occur is if you are using a fanout or other parallel execution in your graph and you have defined a graph like this:

:::python

class State(TypedDict):
    some_key: str

def node(state: State):
    return {"some_key": "some_string_value"}

def other_node(state: State):
    return {"some_key": "some_string_value"}


builder = StateGraph(State)
builder.add_node(node)
builder.add_node(other_node)
builder.add_edge(START, "node")
builder.add_edge(START, "other_node")
graph = builder.compile()

:::

:::js

import { StateGraph, Annotation, START } from "@langchain/langgraph";
import { z } from "zod";

const State = z.object({
  someKey: z.string(),
});

const builder = new StateGraph(State)
  .addNode("node", (state) => {
    return { someKey: "some_string_value" };
  })
  .addNode("otherNode", (state) => {
    return { someKey: "some_string_value" };
  })
  .addEdge(START, "node")
  .addEdge(START, "otherNode");

const graph = builder.compile();

:::

:::python If a node in the above graph returns { "some_key": "some_string_value" }, this will overwrite the state value for "some_key" with "some_string_value". However, if multiple nodes in e.g. a fanout within a single step return values for "some_key", the graph will throw this error because there is uncertainty around how to update the internal state. :::

:::js If a node in the above graph returns { someKey: "some_string_value" }, this will overwrite the state value for someKey with "some_string_value". However, if multiple nodes in e.g. a fanout within a single step return values for someKey, the graph will throw this error because there is uncertainty around how to update the internal state. :::

To get around this, you can define a reducer that combines multiple values:

:::python

import operator
from typing import Annotated

class State(TypedDict):
    # The operator.add reducer fn makes this append-only
    some_key: Annotated[list, operator.add]

:::

:::js

import { withLangGraph } from "@langchain/langgraph";
import { z } from "zod";

const State = z.object({
  someKey: withLangGraph(z.array(z.string()), {
    reducer: {
      fn: (existing, update) => existing.concat(update),
    },
    default: () => [],
  }),
});

:::

This will allow you to define logic that handles the same key returned from multiple nodes executed in parallel.

Troubleshooting

The following may help resolve this error:

• If your graph executes nodes in parallel, make sure you have defined relevant state keys with a reducer.