diff --git a/libs/langgraph/langgraph/stream/transformers.py b/libs/langgraph/langgraph/stream/transformers.py
index d08a47c1e..29e247b5e 100644
--- a/libs/langgraph/langgraph/stream/transformers.py
+++ b/libs/langgraph/langgraph/stream/transformers.py
@@ -419,6 +419,11 @@ class _TasksLifecycleBase(StreamTransformer):
         # `node:<task_id>` shares this task_id, so a subagent recovers the tool
         # call that spawned it (cross-payload).
         self._pending_tool_calls: dict[str, str] = {}
+        # `cause` for the current `_on_started` dispatch, set immediately before
+        # the call and read by overrides that surface it. Keeping `cause` off the
+        # `_on_started` signature means overrides predating it (e.g. deepagents'
+        # `SubagentTransformer`) don't break.
+        self._pending_cause: LifecycleCause | None = None
 
     # --- Template-method hooks (subclass overrides) ---
 
@@ -431,10 +436,11 @@ class _TasksLifecycleBase(StreamTransformer):
         ns: tuple[str, ...],
         graph_name: str | None,
         trigger_call_id: str | None,
-        *,
-        cause: LifecycleCause | None = None,
     ) -> None:
-        """Fired once per discovered namespace (first observed task event)."""
+        """Fired once per discovered namespace (first observed task event).
+
+        The triggering `cause` (if any) is available as `self._pending_cause`.
+        """
         raise NotImplementedError
 
     def _on_terminal(
@@ -532,7 +538,10 @@ class _TasksLifecycleBase(StreamTransformer):
             tool_call_id = self._pending_tool_calls.get(trigger_call_id)
             if tool_call_id:
                 cause = {"type": "toolCall", "tool_call_id": str(tool_call_id)}
-        self._on_started(ns, graph_name, trigger_call_id, cause=cause)
+        # Deliver `cause` via instance state, not the call signature, so
+        # `_on_started` stays backward-compatible with overrides predating it.
+        self._pending_cause = cause
+        self._on_started(ns, graph_name, trigger_call_id)
         if trigger_call_id is not None:
             self._open[ns] = trigger_call_id
 
@@ -631,8 +640,6 @@ class LifecycleTransformer(_TasksLifecycleBase):
         ns: tuple[str, ...],
         graph_name: str | None,
         trigger_call_id: str | None,
-        *,
-        cause: LifecycleCause | None = None,
     ) -> None:
         if trigger_call_id is None:
             # Without a task id we can't correlate a parent-result
@@ -643,6 +650,7 @@ class LifecycleTransformer(_TasksLifecycleBase):
         if graph_name:
             payload["graph_name"] = graph_name
         payload["trigger_call_id"] = trigger_call_id
+        cause = self._pending_cause
         if cause is not None:
             payload["cause"] = cause
         self._channel.push(payload)
@@ -707,8 +715,6 @@ class SubgraphTransformer(_TasksLifecycleBase):
         ns: tuple[str, ...],
         graph_name: str | None,
         trigger_call_id: str | None,
-        *,
-        cause: LifecycleCause | None = None,
     ) -> None:
         if self._mux is None:
             return
@@ -717,10 +723,9 @@ class SubgraphTransformer(_TasksLifecycleBase):
         except RuntimeError:
             return
         handle_cls = AsyncSubgraphRunStream if child_mux.is_async else SubgraphRunStream
-        # `cause` is intentionally ignored here: it is a wire/lifecycle-channel
-        # concern (carried on `LifecyclePayload`), not something the in-process
-        # subgraph navigation handle exposes. The argument is accepted only to
-        # keep the `_on_started` template signature uniform across transformers.
+        # The triggering `cause` (on `self._pending_cause`) is a wire/lifecycle
+        # concern carried on `LifecyclePayload`; the in-process subgraph
+        # navigation handle does not expose it.
         handle = handle_cls(
             mux=child_mux,
             path=ns,
diff --git a/libs/langgraph/tests/test_stream_lifecycle_transformer.py b/libs/langgraph/tests/test_stream_lifecycle_transformer.py
index 74bbc5560..893b0b3b7 100644
--- a/libs/langgraph/tests/test_stream_lifecycle_transformer.py
+++ b/libs/langgraph/tests/test_stream_lifecycle_transformer.py
@@ -25,6 +25,7 @@ from langgraph.stream._mux import StreamMux
 from langgraph.stream.transformers import (
     LifecyclePayload,
     LifecycleTransformer,
+    _TasksLifecycleBase,
 )
 
 TS = int(time.time() * 1000)
@@ -678,3 +679,34 @@ def test_lifecycle_subagent_terminal_roundtrip() -> None:
     started, _completed = subagent
     assert started["graph_name"] == "weather_agent"
     assert started["cause"] == {"type": "toolCall", "tool_call_id": "call_w"}
+
+
+def test_on_started_override_without_cause_is_backward_compatible() -> None:
+    """An `_on_started` override with the original 3-arg signature must work.
+
+    `cause` is delivered via `self._pending_cause`, not the call signature, so
+    older/third-party subclasses (e.g. deepagents' `SubagentTransformer`) that
+    override `_on_started(self, ns, graph_name, trigger_call_id)` keep working —
+    no `TypeError: _on_started() got an unexpected keyword argument 'cause'`.
+    """
+    seen: list[tuple] = []
+
+    class _LegacyTransformer(_TasksLifecycleBase):
+        def init(self) -> dict[str, Any]:
+            return {}
+
+        def _should_track(self, ns: tuple[str, ...]) -> bool:
+            return len(ns) == 1  # direct child of the root scope
+
+        # Deliberately omits `cause` — mirrors a pre-`cause` override.
+        def _on_started(self, ns, graph_name, trigger_call_id) -> None:  # type: ignore[override]
+            seen.append((ns, graph_name, trigger_call_id))
+
+        def _on_terminal(self, ns, status, error) -> None:
+            pass
+
+    transformer = _LegacyTransformer(scope=())
+    # Must not raise: the base calls `_on_started` without a `cause` kwarg.
+    transformer.process(_tasks_start(["agent:abc123"], task_id="abc123", name="agent"))
+
+    assert seen == [(("agent:abc123",), "agent", "abc123")]