# 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.