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: 
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()
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. To get around this, you can define a reducer that combines multiple values: 
import operator
from typing import Annotated

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