langgraph: add support for BaseModel updates to Command (#2747)

Simple update that adds support for the `update` attribute of the
`Command` class to support Pydantic `BaseModel` type.

LangGraph already supports [Pydantic models for graph
states](https://langchain-ai.github.io/langgraph/how-tos/state-model/).

Extending support to the `update` attribute allows users to pass custom
BaseModel instances. Additionally, updates defined as `BaseModel` types
are type-validated when created.

https://github.com/langchain-ai/langgraph/issues/2804

---------

Co-authored-by: vbarda <vadym@langchain.dev>

libs/langgraph/langgraph/types.py

@@ -16,6 +16,7 @@ from typing import (
     TypeVar,
     Union,
     cast,
+    get_type_hints,
 )
 
 from langchain_core.runnables import Runnable, RunnableConfig
@@ -289,6 +290,8 @@ class Command(Generic[N], ToolOutputMixin):
             for t in self.update
         ):
             return self.update
+        elif hints := get_type_hints(type(self.update)):
+            return [(k, getattr(self.update, k)) for k in hints]
         elif self.update is not None:
             return [("__root__", self.update)]
         else:

libs/langgraph/tests/test_pregel.py

@@ -9,6 +9,7 @@ import warnings
 from collections import Counter, deque
 from concurrent.futures import ThreadPoolExecutor
 from contextlib import contextmanager
+from dataclasses import dataclass
 from random import randrange
 from typing import (
     Annotated,
@@ -5134,6 +5135,35 @@ def test_dict_mixed_return() -> None:
     assert graph.invoke({"foo": ""}) == {"foo": "ab"}
 
 
+def test_command_pydantic_dataclass() -> None:
+    from pydantic import BaseModel
+
+    class PydanticState(BaseModel):
+        foo: str
+
+    @dataclass
+    class DataclassState:
+        foo: str
+
+    for State in (PydanticState, DataclassState):
+
+        def node_a(state) -> Command[Literal["node_b"]]:
+            return Command(
+                update=State(foo="foo"),
+                goto="node_b",
+            )
+
+        def node_b(state):
+            return {"foo": state.foo + "bar"}
+
+        builder = StateGraph(State)
+        builder.add_edge(START, "node_a")
+        builder.add_node(node_a)
+        builder.add_node(node_b)
+        graph = builder.compile()
+        assert graph.invoke(State(foo="")) == {"foo": "foobar"}
+
+
 @pytest.mark.parametrize("checkpointer_name", ALL_CHECKPOINTERS_SYNC)
 def test_command_with_static_breakpoints(
     request: pytest.FixtureRequest, checkpointer_name: str