Repository navigation
update_state with Overwrite on a plain key persists the Overwrite wrapper instead of its payload (LastValue / LastValueAfterFinish / UntrackedValue) #9019
Description
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 typing_extensions import TypedDict
from langgraph.graph import StateGraph, START, END
from langgraph.checkpoint.memory import InMemorySaver
from langgraph.types import Overwrite
class S(TypedDict):
x: str
g = StateGraph(S)
g.add_node("n", lambda s: {"x": "from_node"})
g.add_edge(START, "n")
g.add_edge("n", END)
graph = g.compile(checkpointer=InMemorySaver())
cfg = {"configurable": {"thread_id": "t1"}}
graph.invoke({"x": "init"}, cfg)
graph.update_state(cfg, {"x": Overwrite("replaced")})
print(graph.get_state(cfg).values)
# {'x': Overwrite(value='replaced', type='__overwrite__')} <- the wrapper itselfDescription
- System information
- LangGraph
mainat230927fb3a9ac9b2893a30322b4dfea7cdea9a8f
- LangGraph
A replace-style state update (update_state with an Overwrite, i.e. the form used for replace-writes through the API server and by clients that wrap replace-style updates in Overwrite) on a plain (non-annotated) state key persists the Overwrite wrapper object into the checkpointed state instead of its payload. get_state().values then returns the wrapper, and any downstream consumer of the state fails (e.g. TypeError: 'Overwrite' object is not subscriptable).
Root cause: plain keys compile to LastValue, and LastValue.update (also LastValueAfterFinish.update and UntrackedValue.update) assigns values[-1] verbatim without the _get_overwrite check. BinaryOperatorAggregate (on its seeded path) and DeltaChannel both unwrap an Overwrite write, so the single-value channels are the inconsistent ones.
Expected: update_state(cfg, {"x": Overwrite("replaced")}) stores "replaced".
Channel-level repro for each affected class:
from langgraph.channels.last_value import LastValue
from langgraph.types import Overwrite
ch = LastValue(dict)
ch.key = "x"
ch.update([Overwrite({"a": 1})])
ch.get() # Overwrite(value={'a': 1}, type='__overwrite__') instead of {'a': 1}Error Message and Stack Trace (if applicable)
No error is raised at update time; the wrapper silently persists into state and surfaces at the consumer, e.g. TypeError: 'Overwrite' object is not subscriptable.
Activity
Rainmemery commentedon Sep 21, 2026
I'd like to fix this. The root cause is clear:
LastValue.update,LastValueAfterFinish.update, andUntrackedValue.updateassignvalues[-1]verbatim without running it through_get_overwrite, unlikeBinaryOperatorAggregate(on its seeded path) andDeltaChannel, which already unwrap anOverwritewrite. The fix is to unwrap through_get_overwritein those three single-value channels, so a replace-styleupdate_statepersists the payload (e.g."replaced") instead of theOverwritewrapper. I'll add channel-level regression tests for all three plus anupdate_stateend-to-end test.Could you assign me to #9019? Thanks!
Rainmemery commentedon Sep 21, 2026
The fix is ready: unwrap
Overwritewrites inLastValue.update,LastValueAfterFinish.update, andUntrackedValue.updatevia_get_overwrite(mirroringBinaryOperatorAggregate/DeltaChannel), with regression tests covering overwrite-on-plain-key withLastValue,LastValueAfterFinish, andUntrackedValue. All tests pass locally. May I get this assigned so I can open the PR?Rainmemery commentedon Sep 21, 2026
PR opened: #9032 —
LastValue/LastValueAfterFinish/UntrackedValuenow unwrap anOverwritewrite through_get_overwrite(mirroringBinaryOperatorAggregateandDeltaChannel), with regression tests for all three channels. Happy to adjust after review.Rainmemery commentedon Sep 21, 2026
cc maintainers (LangChain (@langchain-ai)) - could one of you assign me this issue? PR #9032 was auto-closed because external contributors need to be assigned first. The fix (unwrap Overwrite writes in the single-value channels + regression tests) is ready; once assigned the PR will be reopened automatically.
Rainmemery commentedon Sep 21, 2026
Sydney Runkle (@sydney-runkle) - could you assign me this issue (or another maintainer)? My PR #9032 (LastValue/LastValueAfterFinish/UntrackedValue unwrap Overwrite writes via _get_overwrite, with regression tests) was auto-closed pending assignment; once assigned the bot will reopen it automatically. Happy to adjust anything after review.
Iyanuoluwa007 commentedon Oct 21, 2026
Confirmed on current main (
3cd57c0):LastValue,LastValueAfterFinishandUntrackedValueall store anOverwritewrite verbatim, and the repro persists the wrapper. Unwrapping with_get_overwrite, asBinaryOperatorAggregateandDeltaChanneldo, fixes it without other test changes in the langgraph suite.Rain. (@Rainmemery) has this in #9032 (auto-closed only for the missing assignment), so assigning them seems simplest. One thing worth a maintainer's view:
_get_overwritealso unwraps the{"__overwrite__": v}and{"type": "__overwrite__", "value": v}dict forms, so a plain key that stores exactly one of those dicts would now be unwrapped. That matches reducer channels, but it's a small behaviour change.