fix(core): don't send a default project with agent-addressed runs

When LANGSMITH_AGENT_ID / LANGSMITH_AGENT_ENVIRONMENT address runs to a
LangSmith agent and no project is configured, LangChainTracer fell back to
the invented "default" project and assigned it to every run. The run then
carried both agent_id and session_name, which the API rejects with a 400,
dropping the trace.

With agent addressing active, fall back only to a project the user
configured. The agent helpers are looked up with getattr since they only
exist in langsmith>=0.14.0.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

libs/core/langchain_core/tracers/langchain.py

@@ -81,6 +81,27 @@ def get_client() -> Client:
     return rt.get_cached_client()
 
 
+def _get_default_project_name() -> str | None:
+    """Get the project to trace to when none is passed to the tracer.
+
+    When the environment addresses runs to a LangSmith agent
+    (`LANGSMITH_AGENT_ID` / `LANGSMITH_AGENT_ENVIRONMENT`), only a project the
+    user configured is returned. Falling back to the invented `'default'`
+    project would send it alongside the agent, and the API rejects a run
+    addressed to both.
+
+    Agent addressing requires `langsmith>=0.14.0`; older versions have no agent
+    to address, so the helpers are looked up rather than imported.
+    """
+    get_agent_id = getattr(ls_utils, "get_tracer_agent_id", None)
+    get_agent_environment = getattr(ls_utils, "get_tracer_agent_environment", None)
+    if (get_agent_id is not None and get_agent_id() is not None) or (
+        get_agent_environment is not None and get_agent_environment() is not None
+    ):
+        return ls_utils.get_tracer_project(return_default_value=False)
+    return ls_utils.get_tracer_project()
+
+
 def _get_executor() -> ThreadPoolExecutor:
     """Get the executor."""
     global _EXECUTOR  # noqa: PLW0603
@@ -168,7 +189,7 @@ class LangChainTracer(BaseTracer):
         self.example_id = (
             UUID(example_id) if isinstance(example_id, str) else example_id
         )
-        self.project_name = project_name or ls_utils.get_tracer_project()
+        self.project_name = project_name or _get_default_project_name()
         self.client = client or get_client()
         self.tags = tags or []
         self.latest_run: Run | None = None

libs/core/tests/unit_tests/tracers/test_langchain.py

@@ -8,6 +8,7 @@ from uuid import UUID
 
 import pytest
 from langsmith import Client
+from langsmith import utils as ls_utils
 from langsmith.run_trees import RunTree
 from langsmith.utils import get_env_var, get_tracer_project
 
@@ -154,6 +155,73 @@ def test_correct_get_tracer_project(
         assert projects == [expected_project_name]
 
 
+def _clear_tracer_env_caches() -> None:
+    getters: tuple[Any, ...] = (
+        get_env_var,
+        get_tracer_project,
+        getattr(ls_utils, "get_tracer_agent_id", None),
+        getattr(ls_utils, "get_tracer_agent_environment", None),
+    )
+    for getter in getters:
+        if hasattr(getter, "cache_clear"):
+            getter.cache_clear()
+
+
+@pytest.mark.parametrize(
+    ("envvars", "expected_project_name"),
+    [
+        ({"LANGSMITH_AGENT_ID": "my-agent"}, None),
+        ({"LANGSMITH_AGENT_ENVIRONMENT": "development"}, None),
+        (
+            {
+                "LANGSMITH_AGENT_ID": "my-agent",
+                "LANGSMITH_AGENT_ENVIRONMENT": "development",
+            },
+            None,
+        ),
+        (
+            {
+                "LANGSMITH_AGENT_ID": "my-agent",
+                "LANGSMITH_AGENT_ENVIRONMENT": "development",
+                "LANGSMITH_PROJECT": "configured",
+            },
+            "configured",
+        ),
+    ],
+    ids=[
+        "no 'default' project with agent id",
+        "no 'default' project with agent environment",
+        "no 'default' project with agent pair",
+        "keep a configured project with agent pair",
+    ],
+)
+def test_tracer_project_with_agent_addressing(
+    envvars: dict[str, str], expected_project_name: str | None
+) -> None:
+    if not hasattr(ls_utils, "get_tracer_agent_environment"):
+        pytest.skip("agent addressing requires langsmith>=0.14.0")
+    _clear_tracer_env_caches()
+    try:
+        with pytest.MonkeyPatch.context() as mp:
+            for k in (
+                "LANGSMITH_PROJECT",
+                "LANGCHAIN_PROJECT",
+                "LANGCHAIN_SESSION",
+                "HOSTED_LANGSERVE_PROJECT_NAME",
+                "LANGSMITH_AGENT_ID",
+                "LANGSMITH_AGENT_ENVIRONMENT",
+            ):
+                mp.delenv(k, raising=False)
+            for k, v in envvars.items():
+                mp.setenv(k, v)
+
+            tracer = LangChainTracer(client=unittest.mock.MagicMock(spec=Client))
+            assert tracer.project_name == expected_project_name
+    finally:
+        # The env getters are cached, so leave nothing behind for later tests.
+        _clear_tracer_env_caches()
+
+
 @pytest.mark.parametrize(
     ("generations", "expected"),
     [