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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: 
import { StateGraph, Annotation, START } from "@langchain/langgraph";
import * as 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();
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: 
import { registry } from "@langchain/langgraph/zod";
import * as z from "zod";

const State = z.object({  
  someKey: z.array(z.string()).register(registry, {  
    reducer: {  
      fn: (existing, update) => existing.concat(update),  
    },
    default: () => [] as string[],
  }),
});
This will allow you to define logic that handles the same key returned from multiple nodes executed in parallel.

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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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