feat: support multiple extra LangSmith trace projects in SDK schema

Add an optional `project_names: list[str]` field to the `LangSmithTracing`
TypedDict so callers can request multiple extra trace replica projects on a
run, while keeping the singular `project_name` field working as before. The
Python SDK passes the tracing config straight through as `langsmith_tracer`,
so backward compatibility is preserved.

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

libs/sdk-py/langgraph_sdk/schema.py

@@ -112,7 +112,13 @@ class LangSmithTracing(TypedDict, total=False):
     """Configuration for LangSmith tracing."""
 
     project_name: str
-    """The LangSmith project name to trace to."""
+    """A single extra LangSmith project name to trace to."""
+    project_names: list[str]
+    """Multiple extra LangSmith project names to trace to.
+
+    Combined with `project_name` (if both are provided) to form the full set of
+    extra projects. The server also traces to its primary tracing project.
+    """
     example_id: str
     """The LangSmith example/dataset ID to associate with the trace."""
 

libs/sdk-py/tests/test_langsmith_tracing.py

@@ -144,3 +144,57 @@ class TestLangSmithTracingPayload:
         assert captured["json"]["langsmith_tracer"] == {
             "project_name": "my-project",
         }
+
+    def test_langsmith_tracing_project_names_only(self):
+        """Test that langsmith_tracing works with multiple project_names."""
+        from langgraph_sdk._sync.runs import SyncRunsClient
+
+        captured: dict[str, Any] = {}
+
+        def mock_post(_path, *, json=None, **_kwargs):
+            captured["json"] = json
+            return {"run_id": "r1", "status": "pending"}
+
+        http = MagicMock()
+        http.post = MagicMock(side_effect=mock_post)
+        client = SyncRunsClient(http)
+
+        client.create(
+            thread_id="t1",
+            assistant_id="a1",
+            langsmith_tracing={"project_names": ["first", "second"]},
+        )
+
+        assert captured["json"]["langsmith_tracer"] == {
+            "project_names": ["first", "second"],
+        }
+
+    def test_langsmith_tracing_project_name_and_project_names(self):
+        """Test that langsmith_tracing passes both project_name and project_names."""
+        from langgraph_sdk._sync.runs import SyncRunsClient
+
+        captured: dict[str, Any] = {}
+
+        def mock_post(_path, *, json=None, **_kwargs):
+            captured["json"] = json
+            return {"run_id": "r1", "status": "pending"}
+
+        http = MagicMock()
+        http.post = MagicMock(side_effect=mock_post)
+        client = SyncRunsClient(http)
+
+        client.create(
+            thread_id="t1",
+            assistant_id="a1",
+            langsmith_tracing={
+                "project_name": "first",
+                "project_names": ["second", "third"],
+                "example_id": "example-123",
+            },
+        )
+
+        assert captured["json"]["langsmith_tracer"] == {
+            "project_name": "first",
+            "project_names": ["second", "third"],
+            "example_id": "example-123",
+        }