StateGraph._is_field_binop rejects valid reducers like set.union, functions with default args, and variadics

Finding: In an external report against langgraph 1.2.10 (Windows, Python 3.14.7), constructing a StateGraph with an Annotated reducer such as set.union, or a reducer with an extra defaulted parameter, raises ValueError: Invalid reducer signature. Expected (a, b) -> c. Got (self, /, *others) (https://github.com/langchain-ai/langgraph/issues/9176). This is an external report; KFA did not reproduce it. Reported conditions: the reporter's examples are a TypedDict field annotated Annotated[set, set.union], and a function merge_with_default(existing, new, step=1) used as the reducer. Both reportedly fail at StateGraph(...) construction, in _is_field_binop in langgraph/graph/state.py (https://github.com/langchain-ai/langgraph/issues/9176). The reporter also lists variadic functions such as def fn(*args) as affected. Reported cause (hypothesis, not maintainer-confirmed): the reporter says the check counts positional-only or positional-or-keyword parameters and requires exactly two. That count is 1 for set.union, 3 for a function with a defaulted third parameter, and 0 for *args, even though a call with two positional arguments would be valid (https://github.com/langchain-ai/langgraph/issues/9176). A commenter with no listed repository association says they reproduced both examples on current main. They also say BinaryOperatorAggregate.update always calls the reducer with two positional arguments (https://github.com/langchain-ai/langgraph/issues/9176#issuecomment-5966149422). The commenter's main-branch commit is not stated. Proposed fix status: the reporter suggests Signature.bind(None, None) to test whether two positional arguments can be bound. The commenter posted a reference branch implementing that approach and says it accepts set.union, optional parameters and variadic reducers, while still rejecting callables that cannot take two positional arguments (https://github.com/langchain-ai/langgraph/issues/9176#issuecomment-5966149422). They report that the two original examples failed before the change and that 61 state/channel tests passed afterward, along with lint, format and type checks. They did not run the full suite or the reporter's Windows/Python 3.14 environment, and they asked for approval and assignment. The thread shows no maintainer response, no merged PR and no release. Practical reading: for this version and environment, a reducer whose signature does not have exactly two positional parameters may be rejected at graph construction. No verified workaround is given in the sources. Treat the bind-based change as a proposal, not a shipped fix. Check the issue for maintainer updates before relying on it. All evidence here is external and unverified by KFA.