> ## Documentation Index
> Fetch the complete documentation index at: https://docs.langchain.com/llms.txt
> Use this file to discover all available pages before exploring further.

# INVALID_CONCURRENT_GRAPH_UPDATE

A LangGraph [`StateGraph`](https://langchain-ai.github.io/langgraph/reference/graphs/#langgraph.graph.state.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](/oss/javascript/langgraph/use-graph-api#map-reduce-and-the-send-api)
or other parallel execution in your graph and you have defined a graph like this:

```typescript theme={"theme":{"light":"catppuccin-latte","dark":"catppuccin-mocha"}}
import { StateGraph, StateSchema, START } from "@langchain/langgraph";
import { z } from "zod/v4";  // [!code highlight]

const State = new StateSchema({
  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();
```

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:

```typescript theme={"theme":{"light":"catppuccin-latte","dark":"catppuccin-mocha"}}
import { StateSchema, ReducedValue } from "@langchain/langgraph";
import { z } from "zod/v4";

const State = new StateSchema({  // [!code highlight]
  someKey: new ReducedValue(  // [!code highlight]
    z.array(z.string()).default(() => []),  // [!code highlight]
    {
      inputSchema: z.array(z.string()),
      reducer: (existing, update) => existing.concat(update),  // [!code highlight]
    }
  ),
});
```

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.

***

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