Fix stream_mode=updates for cases where one node returns multiple upd…

…ates for same key (#2903)

libs/langgraph/langgraph/pregel/io.py

@@ -1,3 +1,4 @@
+from collections import Counter
 from typing import Any, Iterator, Literal, Mapping, Optional, Sequence, TypeVar, Union
 from uuid import UUID
 
@@ -181,12 +182,27 @@ def map_output_updates(
                 (task.name, value) for chan, value in writes if chan == output_channels
             )
         elif any(chan in output_channels for chan, _ in writes):
-            updated.append(
-                (
-                    task.name,
-                    {chan: value for chan, value in writes if chan in output_channels},
+            counts = Counter(chan for chan, _ in writes)
+            if any(counts[chan] > 1 for chan in output_channels):
+                updated.extend(
+                    (
+                        task.name,
+                        {chan: value},
+                    )
+                    for chan, value in writes
+                    if chan in output_channels
+                )
+            else:
+                updated.append(
+                    (
+                        task.name,
+                        {
+                            chan: value
+                            for chan, value in writes
+                            if chan in output_channels
+                        },
+                    )
                 )
-            )
     grouped: dict[str, list[Any]] = {t.name: [] for t, _ in output_tasks}
     for node, value in updated:
         grouped[node].append(value)

libs/langgraph/tests/test_pregel.py

@@ -5242,3 +5242,54 @@ def second_node(state: State):
     # Verify the error was recorded in checkpoint
     failed_checkpoint = next(c for c in history if c.tasks and c.tasks[0].error)
     assert "RuntimeError('Simulated failure')" in failed_checkpoint.tasks[0].error
+
+
+def test_multiple_updates_root() -> None:
+    def node_a(state):
+        return [Command(update="a1"), Command(update="a2")]
+
+    def node_b(state):
+        return "b"
+
+    graph = (
+        StateGraph(Annotated[str, operator.add])
+        .add_sequence([node_a, node_b])
+        .add_edge(START, "node_a")
+        .compile()
+    )
+
+    assert graph.invoke("") == "a1a2b"
+
+    # only streams the last update from node_a
+    assert [c for c in graph.stream("", stream_mode="updates")] == [
+        {"node_a": ["a1", "a2"]},
+        {"node_b": "b"},
+    ]
+
+
+def test_multiple_updates() -> None:
+    class State(TypedDict):
+        foo: Annotated[str, operator.add]
+
+    def node_a(state):
+        return [Command(update={"foo": "a1"}), Command(update={"foo": "a2"})]
+
+    def node_b(state):
+        return {"foo": "b"}
+
+    graph = (
+        StateGraph(State)
+        .add_sequence([node_a, node_b])
+        .add_edge(START, "node_a")
+        .compile()
+    )
+
+    assert graph.invoke({"foo": ""}) == {
+        "foo": "a1a2b",
+    }
+
+    # only streams the last update from node_a
+    assert [c for c in graph.stream({"foo": ""}, stream_mode="updates")] == [
+        {"node_a": [{"foo": "a1"}, {"foo": "a2"}]},
+        {"node_b": {"foo": "b"}},
+    ]

libs/langgraph/tests/test_pregel_async.py

@@ -6618,3 +6618,54 @@ async def second_node(state: State):
         # Verify the error was recorded in checkpoint
         failed_checkpoint = next(c for c in history if c.tasks and c.tasks[0].error)
         assert "RuntimeError('Simulated failure')" in failed_checkpoint.tasks[0].error
+
+
+async def test_multiple_updates_root() -> None:
+    def node_a(state):
+        return [Command(update="a1"), Command(update="a2")]
+
+    def node_b(state):
+        return "b"
+
+    graph = (
+        StateGraph(Annotated[str, operator.add])
+        .add_sequence([node_a, node_b])
+        .add_edge(START, "node_a")
+        .compile()
+    )
+
+    assert await graph.ainvoke("") == "a1a2b"
+
+    # only streams the last update from node_a
+    assert [c async for c in graph.astream("", stream_mode="updates")] == [
+        {"node_a": ["a1", "a2"]},
+        {"node_b": "b"},
+    ]
+
+
+async def test_multiple_updates() -> None:
+    class State(TypedDict):
+        foo: Annotated[str, operator.add]
+
+    def node_a(state):
+        return [Command(update={"foo": "a1"}), Command(update={"foo": "a2"})]
+
+    def node_b(state):
+        return {"foo": "b"}
+
+    graph = (
+        StateGraph(State)
+        .add_sequence([node_a, node_b])
+        .add_edge(START, "node_a")
+        .compile()
+    )
+
+    assert await graph.ainvoke({"foo": ""}) == {
+        "foo": "a1a2b",
+    }
+
+    # only streams the last update from node_a
+    assert [c async for c in graph.astream({"foo": ""}, stream_mode="updates")] == [
+        {"node_a": [{"foo": "a1"}, {"foo": "a2"}]},
+        {"node_b": {"foo": "b"}},
+    ]