From 682814e9443e4921f704d93bd4e3f9aa6b27b4be Mon Sep 17 00:00:00 2001
From: Sydney Runkle <54324534+sydney-runkle@users.noreply.github.com>
Date: Thu, 12 Mar 2026 11:58:04 -0400
Subject: [PATCH] fix: stream part generic order (#7134)

this is technically breaking but is a fix given dependency of output t
on state t
---
 libs/langgraph/langgraph/pregel/main.py     | 8 ++++----
 libs/langgraph/langgraph/pregel/protocol.py | 4 ++--
 libs/langgraph/langgraph/types.py           | 2 +-
 libs/langgraph/tests/test_stream_v2.py      | 2 +-
 4 files changed, 8 insertions(+), 8 deletions(-)

diff --git a/libs/langgraph/langgraph/pregel/main.py b/libs/langgraph/langgraph/pregel/main.py
index d4190a0e6..3ecefc1dc 100644
--- a/libs/langgraph/langgraph/pregel/main.py
+++ b/libs/langgraph/langgraph/pregel/main.py
@@ -2456,7 +2456,7 @@ class Pregel(
         debug: bool | None = None,
         version: Literal["v2"],
         **kwargs: Unpack[DeprecatedKwargs],
-    ) -> Iterator[StreamPart[OutputT, StateT]]: ...
+    ) -> Iterator[StreamPart[StateT, OutputT]]: ...
 
     @overload
     def stream(
@@ -2787,7 +2787,7 @@ class Pregel(
         debug: bool | None = None,
         version: Literal["v2"],
         **kwargs: Unpack[DeprecatedKwargs],
-    ) -> AsyncIterator[StreamPart[OutputT, StateT]]: ...
+    ) -> AsyncIterator[StreamPart[StateT, OutputT]]: ...
 
     @overload
     def astream(
@@ -3194,7 +3194,7 @@ class Pregel(
         durability: Durability | None = None,
         version: Literal["v2"],
         **kwargs: Any,
-    ) -> list[StreamPart[OutputT, StateT]]: ...
+    ) -> list[StreamPart[StateT, OutputT]]: ...
 
     @overload
     def invoke(
@@ -3364,7 +3364,7 @@ class Pregel(
         durability: Durability | None = None,
         version: Literal["v2"],
         **kwargs: Any,
-    ) -> list[StreamPart[OutputT, StateT]]: ...
+    ) -> list[StreamPart[StateT, OutputT]]: ...
 
     @overload
     async def ainvoke(
diff --git a/libs/langgraph/langgraph/pregel/protocol.py b/libs/langgraph/langgraph/pregel/protocol.py
index e5e957f50..259554ddb 100644
--- a/libs/langgraph/langgraph/pregel/protocol.py
+++ b/libs/langgraph/langgraph/pregel/protocol.py
@@ -117,7 +117,7 @@ class PregelProtocol(Runnable[InputT, Any], Generic[StateT, ContextT, InputT, Ou
         interrupt_after: All | Sequence[str] | None = None,
         subgraphs: bool = False,
         version: Literal["v2"],
-    ) -> Iterator[StreamPart[OutputT, StateT]]: ...
+    ) -> Iterator[StreamPart[StateT, OutputT]]: ...
 
     @overload
     @abstractmethod
@@ -161,7 +161,7 @@ class PregelProtocol(Runnable[InputT, Any], Generic[StateT, ContextT, InputT, Ou
         interrupt_after: All | Sequence[str] | None = None,
         subgraphs: bool = False,
         version: Literal["v2"],
-    ) -> AsyncIterator[StreamPart[OutputT, StateT]]: ...
+    ) -> AsyncIterator[StreamPart[StateT, OutputT]]: ...
 
     @overload
     @abstractmethod
diff --git a/libs/langgraph/langgraph/types.py b/libs/langgraph/langgraph/types.py
index 547513498..d04d82da7 100644
--- a/libs/langgraph/langgraph/types.py
+++ b/libs/langgraph/langgraph/types.py
@@ -335,7 +335,7 @@ StreamPart = TypeAliasType(
     | CheckpointStreamPart[StateT]
     | TasksStreamPart
     | DebugStreamPart[StateT],
-    type_params=(OutputT, StateT),
+    type_params=(StateT, OutputT),
 )
 """A discriminated union of all v2 stream part types.
 
diff --git a/libs/langgraph/tests/test_stream_v2.py b/libs/langgraph/tests/test_stream_v2.py
index 50fb83031..29d34e678 100644
--- a/libs/langgraph/tests/test_stream_v2.py
+++ b/libs/langgraph/tests/test_stream_v2.py
@@ -1129,7 +1129,7 @@ _OutputT = TypeVar("_OutputT")
 _StateT = TypeVar("_StateT")
 
 
-def _check_type_narrowing(part: StreamPart[_OutputT, _StateT]) -> None:
+def _check_type_narrowing(part: StreamPart[_StateT, _OutputT]) -> None:
     """Compile-time type narrowing checks — never called at runtime."""
     if part["type"] == "values":
         assert_type(part, ValuesStreamPart[_OutputT])