BinaryOperatorAggregate stores the Overwrite wrapper itself when it is the first write to an empty channel #9017

Checked other resources

• This is a bug, not a usage question.
• I added a clear and descriptive title that summarizes this issue.
• I used the GitHub search to find a similar question and didn't find it.
• I am sure that this is a bug in LangGraph rather than my code.
• The bug is not resolved by updating to the latest stable version of LangGraph (or the specific integration package).
• This is not related to the langchain-community package.
• I posted a self-contained, minimal, reproducible example. A maintainer can copy it and run it AS IS.

Reproduction Steps / Example Code (Python)

from langgraph.channels.binop import BinaryOperatorAggregate
from langgraph.types import Overwrite

ch = BinaryOperatorAggregate(dict | None, lambda existing, new: new)
ch.key = "goal"
ch.update([Overwrite({"goal": "ship"})])
value = ch.get()
print(type(value).__name__, value)
# Overwrite Overwrite(value={'goal': 'ship'}, type='__overwrite__')

Description

• System information
  • LangGraph main at 230927fb3a9ac9b2893a30322b4dfea7cdea9a8f (the update implementation is unchanged in the latest release)

When the first write to an empty BinaryOperatorAggregate channel is an Overwrite, the empty-channel branch assigns values[0] verbatim, so the channel value — and the checkpointed state — holds the Overwrite wrapper itself instead of its payload. Every consumer that later reads the state then sees the wrapper object, typically failing downstream with something like TypeError: 'Overwrite' object is not subscriptable.

Union-typed channels (e.g. Annotated[dict | None, reducer]) have no constructible default, so they start out MISSING; a replace-style write into a fresh thread or a never-written channel is exactly this path.

The same write is handled correctly once the channel has a value:

ch = BinaryOperatorAggregate(dict | None, lambda existing, new: new).from_checkpoint({"seed": True})
ch.update([Overwrite({"goal": "ship"})])
ch.get()   # {'goal': 'ship'}  — unwrapped, as expected

DeltaChannel.update also unwraps in the same situation, so only the empty-channel first-write branch of BinaryOperatorAggregate skips the _get_overwrite check. Expected: Overwrite(x) as a first write stores x, matching the seeded-channel behavior.

Error Message and Stack Trace (if applicable)

The wrapper silently persists into the checkpointed state; the failure surfaces later at the consumer, e.g. TypeError: 'Overwrite' object is not subscriptable when the persisted state is read back.