> ## 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/python/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:

```python theme={"theme":{"light":"catppuccin-latte","dark":"catppuccin-mocha"}}
class State(TypedDict):
    some_key: str  # [!code highlight]

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:

```python theme={"theme":{"light":"catppuccin-latte","dark":"catppuccin-mocha"}}
import operator
from typing import Annotated

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