# INVALID_CONCURRENT_GRAPH_UPDATE

The broken shape:

```python
class State(TypedDict):
    some_key - str  # no reducer: parallel writes collide

builder.add_edge(START, "node")
builder.add_edge(START, "other_node")  # both return {"some_key": ...}
```

The fix: make the key append-only with a reducer:

```python
import operator
from typing import Annotated

class State(TypedDict):
    some_key - Annotated[list, operator.add]
```

## Rules

- Any state key written by parallel branches needs a reducer. Sequential writes are fine without one; the error only fires on concurrent writes to the same key.
- `operator.add` on a list is the standard append reducer. Custom reducers are just functions of (existing, update).
- If the two writes are actually the same logical value and order does not matter, the reducer still has to exist; the runtime will not pick a winner for you.