diff --git a/libs/langgraph/Makefile b/libs/langgraph/Makefile
index c5ce24a43..1ce5f4f6e 100644
--- a/libs/langgraph/Makefile
+++ b/libs/langgraph/Makefile
@@ -43,9 +43,15 @@ start-services:
 stop-services:
 	docker compose -f tests/compose-postgres.yml -f tests/compose-redis.yml down -v
 
+# The test matrix includes 3.10 and 3.14, but the locked in-memory server
+# dependencies are available on 3.11-3.13. Keep its interpreter separate.
+DEV_SERVER_PYTHON ?= 3.12
+DEV_SERVER_ENV = $(CURDIR)/.venvs/dev-server
+
 start-dev-server:
 	uv sync --frozen --group test --no-dev
-	@LOG_LEVEL=warning uv run --no-sync langgraph dev --config tests/example_app/langgraph.json --no-browser --no-reload & \
+	UV_PROJECT_ENVIRONMENT=$(DEV_SERVER_ENV) uv sync --frozen --group test --no-dev --python $(DEV_SERVER_PYTHON)
+	@LOG_LEVEL=warning UV_PROJECT_ENVIRONMENT=$(DEV_SERVER_ENV) uv run --no-sync --python $(DEV_SERVER_PYTHON) langgraph dev --config tests/example_app/langgraph.json --no-browser --no-reload & \
 	PID=$$!; echo "$$PID" > .devserver.pid; \
 	for ATTEMPT in $$(seq 1 60); do \
 		if ! kill -0 $$PID 2>/dev/null; then break; fi; \
diff --git a/libs/sdk-py/langgraph_sdk/_async/stream.py b/libs/sdk-py/langgraph_sdk/_async/stream.py
index bc7e68ff6..790a2fdcd 100644
--- a/libs/sdk-py/langgraph_sdk/_async/stream.py
+++ b/libs/sdk-py/langgraph_sdk/_async/stream.py
@@ -1972,6 +1972,10 @@ class AsyncThreadStream:
                 # knows a run exists (handles reattach without run.start).
                 self._run_seen = True
             elif phase in ("completed", "failed"):
+                # A child may finish before its parent commits the child's update.
+                # Only the root lifecycle can make thread.output terminal.
+                if isinstance(params, dict) and params.get("namespace"):
+                    return
                 # Why: interrupts describe current-run state. Clear on terminal
                 # lifecycle so a subsequent run.respond() can't fire against a
                 # stale prior-run interrupt_id. Acquire `_interrupts_lock` so
diff --git a/libs/sdk-py/langgraph_sdk/_sync/stream.py b/libs/sdk-py/langgraph_sdk/_sync/stream.py
index fabbff24d..3c3e0ce23 100644
--- a/libs/sdk-py/langgraph_sdk/_sync/stream.py
+++ b/libs/sdk-py/langgraph_sdk/_sync/stream.py
@@ -1614,6 +1614,10 @@ class SyncThreadStream:
             if phase in ("started", "running"):
                 self._run_seen = True
             elif phase in ("completed", "failed"):
+                # A child may finish before its parent commits the child's update.
+                # Only the root lifecycle can make thread.output terminal.
+                if isinstance(params, dict) and params.get("namespace"):
+                    return
                 self.interrupted = False
                 self.interrupts = []
                 run_done = self._run_done
diff --git a/libs/sdk-py/tests/streaming/test_lifecycle_namespace.py b/libs/sdk-py/tests/streaming/test_lifecycle_namespace.py
new file mode 100644
index 000000000..e22bb51f6
--- /dev/null
+++ b/libs/sdk-py/tests/streaming/test_lifecycle_namespace.py
@@ -0,0 +1,67 @@
+"""Child lifecycle completion must not finish the parent thread's output."""
+
+from __future__ import annotations
+
+import asyncio
+from typing import cast
+
+import httpx
+import pytest
+from langchain_protocol import Event
+
+from langgraph_sdk._async.http import HttpClient
+from langgraph_sdk._async.threads import ThreadsClient
+from langgraph_sdk._sync.http import SyncHttpClient
+from langgraph_sdk._sync.stream import _BlockingResult
+from langgraph_sdk._sync.threads import SyncThreadsClient
+from streaming._events import input_requested_event, lifecycle_event
+
+
+@pytest.mark.parametrize("phase", ["completed", "failed"])
+async def test_async_child_terminal_does_not_finish_root(phase):
+    async with httpx.AsyncClient(base_url="http://test") as raw:
+        thread = ThreadsClient(HttpClient(raw)).stream(
+            thread_id="t-1", assistant_id="agent"
+        )
+        thread._run_done = asyncio.get_running_loop().create_future()
+        await thread._apply_lifecycle_event(
+            cast(Event, input_requested_event(namespace=["sub:1"]))
+        )
+        await thread._apply_lifecycle_event(
+            cast(Event, lifecycle_event(namespace=["sub:1"], event=phase))
+        )
+        assert not thread._run_done.done()
+        assert thread.interrupted
+        assert thread.interrupts[0]["namespace"] == ["sub:1"]
+
+        await thread._apply_lifecycle_event(
+            cast(Event, lifecycle_event(event="completed"))
+        )
+        assert thread._run_done.done()
+        assert (await thread._run_done).error is None
+        assert not thread.interrupted
+        assert thread.interrupts == []
+
+
+@pytest.mark.parametrize("phase", ["completed", "failed"])
+def test_sync_child_terminal_does_not_finish_root(phase):
+    with httpx.Client(base_url="http://test") as raw:
+        thread = SyncThreadsClient(SyncHttpClient(raw)).stream(
+            thread_id="t-1", assistant_id="agent"
+        )
+        thread._run_done = _BlockingResult()
+        thread._apply_lifecycle_event(
+            cast(Event, input_requested_event(namespace=["sub:1"]))
+        )
+        thread._apply_lifecycle_event(
+            cast(Event, lifecycle_event(namespace=["sub:1"], event=phase))
+        )
+        assert not thread._run_done.done()
+        assert thread.interrupted
+        assert thread.interrupts[0]["namespace"] == ["sub:1"]
+
+        thread._apply_lifecycle_event(cast(Event, lifecycle_event(event="completed")))
+        assert thread._run_done.done()
+        assert thread._run_done.result().error is None
+        assert not thread.interrupted
+        assert thread.interrupts == []