feat: route LangSmith traces from thread streams (#8723)

## Description
Expose the existing `langsmith_tracing` option on Python sync and async
thread-stream run starts and forward it through the protocol.

## Release Note
Python thread streams can route traces to an additional LangSmith
project per run.

## Test Plan
- [x] Verify sync and async run-start payloads include tracing settings

## Related PRs
- langchain-ai/agent-protocol#95
- langchain-ai/langgraphjs#2745
- langchain-ai/langgraph-api#4033

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Co-authored-by: open-swe[bot] <open-swe@users.noreply.github.com>

libs/sdk-py/langgraph_sdk/_async/stream.py

@@ -24,7 +24,7 @@ from langchain_core.language_models.chat_model_stream import AsyncChatModelStrea
 from langchain_protocol import Event, SubscribeParams
 
 from langgraph_sdk._async.http import HttpClient
-from langgraph_sdk.schema import QueryParamTypes
+from langgraph_sdk.schema import LangSmithTracing, QueryParamTypes
 from langgraph_sdk.stream.controller import _SeenEventIds
 from langgraph_sdk.stream.decoders import (
     DataDecoder,
@@ -172,6 +172,7 @@ class RunModule:
         input: Any = None,
         config: dict[str, Any] | None = None,
         metadata: dict[str, Any] | None = None,
+        langsmith_tracing: LangSmithTracing | None = None,
     ) -> dict[str, Any]:
         """Send `run.start` to the server. Returns the result (`{"run_id": ...}`)."""
         params: dict[str, Any] = {"assistant_id": self._owner.assistant_id}
@@ -181,6 +182,8 @@ class RunModule:
             params["config"] = config
         if metadata is not None:
             params["metadata"] = metadata
+        if langsmith_tracing is not None:
+            params["langsmith_tracer"] = langsmith_tracing
         loop = asyncio.get_running_loop()
         gate: asyncio.Future[None] = loop.create_future()
         self._owner._run_start_ready = gate

libs/sdk-py/langgraph_sdk/_sync/stream.py

@@ -23,7 +23,7 @@ from langchain_core.language_models.chat_model_stream import ChatModelStream
 from langchain_protocol import Event, SubscribeParams
 
 from langgraph_sdk._sync.http import SyncHttpClient
-from langgraph_sdk.schema import QueryParamTypes
+from langgraph_sdk.schema import LangSmithTracing, QueryParamTypes
 from langgraph_sdk.stream.decoders import (
     DataDecoder,
     Decoder,
@@ -215,6 +215,7 @@ class SyncRunModule:
         input: Any = None,
         config: dict[str, Any] | None = None,
         metadata: dict[str, Any] | None = None,
+        langsmith_tracing: LangSmithTracing | None = None,
     ) -> dict[str, Any]:
         """Send `run.start` to the server. Returns the result (`{"run_id": ...}`)."""
         params: dict[str, Any] = {"assistant_id": self._owner.assistant_id}
@@ -224,6 +225,8 @@ class SyncRunModule:
             params["config"] = config
         if metadata is not None:
             params["metadata"] = metadata
+        if langsmith_tracing is not None:
+            params["langsmith_tracer"] = langsmith_tracing
         result = self._owner._send_command("run.start", params)
         self._owner._run_seen = True
         controller = self._owner._controller

libs/sdk-py/tests/streaming/test_sync_thread_stream.py

@@ -426,11 +426,17 @@ def test_sync_run_start_sends_command():
     with httpx.Client(transport=fake.transport, base_url="http://test") as raw:
         threads = SyncThreadsClient(SyncHttpClient(raw))
         with threads.stream(thread_id="t-1", assistant_id="agent") as thread:
-            result = thread.run.start(input={"x": 1})
+            result = thread.run.start(
+                input={"x": 1},
+                langsmith_tracing={"project_name": "replica-project"},
+            )
 
     assert result == {"run_id": "run-1"}
     assert fake.received_commands[0]["method"] == "run.start"
     assert fake.received_commands[0]["params"]["assistant_id"] == "agent"
+    assert fake.received_commands[0]["params"]["langsmith_tracer"] == {
+        "project_name": "replica-project"
+    }
 
 
 def test_sync_events_iterates_raw_events():

libs/sdk-py/tests/streaming/test_thread_stream.py

@@ -287,7 +287,7 @@ async def test_command_ids_are_monotonic():
     assert [c["id"] for c in fake.received_commands] == [1, 2]
 
 
-async def test_run_start_forwards_config_and_metadata():
+async def test_run_start_forwards_config_metadata_and_langsmith_tracing():
     fake = FakeServer()
     transport = httpx.ASGITransport(app=fake.app)
     async with httpx.AsyncClient(transport=transport, base_url="http://test") as raw:
@@ -297,10 +297,18 @@ async def test_run_start_forwards_config_and_metadata():
                 input={"x": 1},
                 config={"recursion_limit": 5},
                 metadata={"trace": "abc"},
+                langsmith_tracing={
+                    "project_name": "replica-project",
+                    "example_id": "example-1",
+                },
             )
     params = fake.received_commands[0]["params"]
     assert params["config"] == {"recursion_limit": 5}
     assert params["metadata"] == {"trace": "abc"}
+    assert params["langsmith_tracer"] == {
+        "project_name": "replica-project",
+        "example_id": "example-1",
+    }
 
 
 async def test_run_start_raises_outside_context_manager():