diff --git a/libs/core/langchain_core/callbacks/base.py b/libs/core/langchain_core/callbacks/base.py
index 6f03a3f6709..6024a8fb2c3 100644
--- a/libs/core/langchain_core/callbacks/base.py
+++ b/libs/core/langchain_core/callbacks/base.py
@@ -6,7 +6,7 @@ import logging
 from typing import TYPE_CHECKING, Any
 
 if TYPE_CHECKING:
-    from collections.abc import Sequence
+    from collections.abc import Mapping, Sequence
     from uuid import UUID
 
     from tenacity import RetryCallState
@@ -948,6 +948,7 @@ class BaseCallbackManager(CallbackManagerMixin):
         inheritable_tags: list[str] | None = None,
         metadata: dict[str, Any] | None = None,
         inheritable_metadata: dict[str, Any] | None = None,
+        tracing_metadata: Mapping[str, str] | None = None,
     ) -> None:
         """Initialize callback manager.
 
@@ -959,6 +960,9 @@ class BaseCallbackManager(CallbackManagerMixin):
             inheritable_tags: The inheritable tags.
             metadata: The metadata.
             inheritable_metadata: The inheritable metadata.
+            tracing_metadata: Per-invocation default metadata merged into every run
+                started by this manager. Keys already present in a run's metadata
+                are not overwritten.
         """
         self.handlers: list[BaseCallbackHandler] = handlers
         self.inheritable_handlers: list[BaseCallbackHandler] = (
@@ -969,6 +973,7 @@ class BaseCallbackManager(CallbackManagerMixin):
         self.inheritable_tags = inheritable_tags or []
         self.metadata = metadata or {}
         self.inheritable_metadata = inheritable_metadata or {}
+        self.tracing_metadata: Mapping[str, str] | None = tracing_metadata
 
     def copy(self) -> Self:
         """Return a copy of the callback manager."""
@@ -980,6 +985,7 @@ class BaseCallbackManager(CallbackManagerMixin):
             inheritable_tags=self.inheritable_tags.copy(),
             metadata=self.metadata.copy(),
             inheritable_metadata=self.inheritable_metadata.copy(),
+            tracing_metadata=self.tracing_metadata,
         )
 
     def merge(self, other: BaseCallbackManager) -> Self:
diff --git a/libs/core/langchain_core/callbacks/manager.py b/libs/core/langchain_core/callbacks/manager.py
index 99bbc40e5eb..ace3a808e56 100644
--- a/libs/core/langchain_core/callbacks/manager.py
+++ b/libs/core/langchain_core/callbacks/manager.py
@@ -269,6 +269,8 @@ def handle_event(
         **kwargs: The keyword arguments to pass to the event handler
 
     """
+    # Pop tracing_metadata; only forwarded to handlers that opt in.
+    tracing_metadata = kwargs.pop("_tracing_metadata", None)
     coros: list[Coroutine[Any, Any, Any]] = []
 
     try:
@@ -278,7 +280,16 @@ def handle_event(
                 if ignore_condition_name is None or not getattr(
                     handler, ignore_condition_name
                 ):
-                    event = getattr(handler, event_name)(*args, **kwargs)
+                    handler_kwargs = (
+                        {
+                            **kwargs,
+                            "tracing_metadata": tracing_metadata,
+                        }
+                        if tracing_metadata
+                        and getattr(handler, "_accepts_tracing_metadata", False)
+                        else kwargs
+                    )
+                    event = getattr(handler, event_name)(*args, **handler_kwargs)
                     if asyncio.iscoroutine(event):
                         coros.append(event)
             except NotImplementedError as e:
@@ -370,6 +381,10 @@ async def _ahandle_event_for_handler(
     *args: Any,
     **kwargs: Any,
 ) -> None:
+    # Pop tracing_metadata; only forwarded to handlers that opt in.
+    tracing_metadata = kwargs.pop("_tracing_metadata", None)
+    if tracing_metadata and getattr(handler, "_accepts_tracing_metadata", False):
+        kwargs = {**kwargs, "tracing_metadata": tracing_metadata}
     try:
         if ignore_condition_name is None or not getattr(handler, ignore_condition_name):
             event = getattr(handler, event_name)
@@ -466,6 +481,7 @@ class BaseRunManager(RunManagerMixin):
         inheritable_tags: list[str] | None = None,
         metadata: dict[str, Any] | None = None,
         inheritable_metadata: dict[str, Any] | None = None,
+        tracing_metadata: Mapping[str, str] | None = None,
     ) -> None:
         """Initialize the run manager.
 
@@ -478,6 +494,8 @@ class BaseRunManager(RunManagerMixin):
             inheritable_tags: The list of inheritable tags.
             metadata: The metadata.
             inheritable_metadata: The inheritable metadata.
+            tracing_metadata: Per-invocation default metadata merged into runs
+                started by tracer handlers.
 
         """
         self.run_id = run_id
@@ -488,6 +506,7 @@ class BaseRunManager(RunManagerMixin):
         self.inheritable_tags = inheritable_tags or []
         self.metadata = metadata or {}
         self.inheritable_metadata = inheritable_metadata or {}
+        self.tracing_metadata: Mapping[str, str] | None = tracing_metadata
 
     @classmethod
     def get_noop_manager(cls) -> Self:
@@ -578,6 +597,7 @@ class ParentRunManager(RunManager):
         manager.set_handlers(self.inheritable_handlers)
         manager.add_tags(self.inheritable_tags)
         manager.add_metadata(self.inheritable_metadata)
+        manager.tracing_metadata = self.tracing_metadata
         if tag is not None:
             manager.add_tags([tag], inherit=False)
         return manager
@@ -662,6 +682,7 @@ class AsyncParentRunManager(AsyncRunManager):
         manager.set_handlers(self.inheritable_handlers)
         manager.add_tags(self.inheritable_tags)
         manager.add_metadata(self.inheritable_metadata)
+        manager.tracing_metadata = self.tracing_metadata
         if tag is not None:
             manager.add_tags([tag], inherit=False)
         return manager
@@ -1335,6 +1356,7 @@ class CallbackManager(BaseCallbackManager):
                 parent_run_id=self.parent_run_id,
                 tags=self.tags,
                 metadata=self.metadata,
+                _tracing_metadata=self.tracing_metadata,
                 **kwargs,
             )
 
@@ -1348,6 +1370,7 @@ class CallbackManager(BaseCallbackManager):
                     inheritable_tags=self.inheritable_tags,
                     metadata=self.metadata,
                     inheritable_metadata=self.inheritable_metadata,
+                    tracing_metadata=self.tracing_metadata,
                 )
             )
 
@@ -1389,6 +1412,7 @@ class CallbackManager(BaseCallbackManager):
                 parent_run_id=self.parent_run_id,
                 tags=self.tags,
                 metadata=self.metadata,
+                _tracing_metadata=self.tracing_metadata,
                 **kwargs,
             )
 
@@ -1402,6 +1426,7 @@ class CallbackManager(BaseCallbackManager):
                     inheritable_tags=self.inheritable_tags,
                     metadata=self.metadata,
                     inheritable_metadata=self.inheritable_metadata,
+                    tracing_metadata=self.tracing_metadata,
                 )
             )
 
@@ -1438,6 +1463,7 @@ class CallbackManager(BaseCallbackManager):
             parent_run_id=self.parent_run_id,
             tags=self.tags,
             metadata=self.metadata,
+            _tracing_metadata=self.tracing_metadata,
             **kwargs,
         )
 
@@ -1450,6 +1476,7 @@ class CallbackManager(BaseCallbackManager):
             inheritable_tags=self.inheritable_tags,
             metadata=self.metadata,
             inheritable_metadata=self.inheritable_metadata,
+            tracing_metadata=self.tracing_metadata,
         )
 
     @override
@@ -1498,6 +1525,7 @@ class CallbackManager(BaseCallbackManager):
             tags=self.tags,
             metadata=self.metadata,
             inputs=inputs,
+            _tracing_metadata=self.tracing_metadata,
             **kwargs,
         )
 
@@ -1510,6 +1538,7 @@ class CallbackManager(BaseCallbackManager):
             inheritable_tags=self.inheritable_tags,
             metadata=self.metadata,
             inheritable_metadata=self.inheritable_metadata,
+            tracing_metadata=self.tracing_metadata,
         )
 
     @override
@@ -1546,6 +1575,7 @@ class CallbackManager(BaseCallbackManager):
             parent_run_id=self.parent_run_id,
             tags=self.tags,
             metadata=self.metadata,
+            _tracing_metadata=self.tracing_metadata,
             **kwargs,
         )
 
@@ -1558,6 +1588,7 @@ class CallbackManager(BaseCallbackManager):
             inheritable_tags=self.inheritable_tags,
             metadata=self.metadata,
             inheritable_metadata=self.inheritable_metadata,
+            tracing_metadata=self.tracing_metadata,
         )
 
     def on_custom_event(
@@ -1615,7 +1646,7 @@ class CallbackManager(BaseCallbackManager):
         inheritable_metadata: dict[str, Any] | None = None,
         local_metadata: dict[str, Any] | None = None,
         *,
-        langsmith_metadata: Mapping[str, str] | None = None,
+        tracing_metadata: Mapping[str, str] | None = None,
     ) -> CallbackManager:
         """Configure the callback manager.
 
@@ -1627,8 +1658,8 @@ class CallbackManager(BaseCallbackManager):
             local_tags: The local tags.
             inheritable_metadata: The inheritable metadata.
             local_metadata: The local metadata.
-            langsmith_metadata: Default metadata applied to any
-                `LangChainTracer` handlers via `set_defaults`.
+            tracing_metadata: Default metadata merged into runs started by
+                tracer handlers. Existing run metadata keys are not overwritten.
 
         Returns:
             The configured callback manager.
@@ -1642,7 +1673,7 @@ class CallbackManager(BaseCallbackManager):
             inheritable_metadata,
             local_metadata,
             verbose=verbose,
-            langsmith_metadata=langsmith_metadata,
+            tracing_metadata=tracing_metadata,
         )
 
 
@@ -1831,6 +1862,7 @@ class AsyncCallbackManager(BaseCallbackManager):
                         parent_run_id=self.parent_run_id,
                         tags=self.tags,
                         metadata=self.metadata,
+                        _tracing_metadata=self.tracing_metadata,
                         **kwargs,
                     )
                 )
@@ -1846,6 +1878,7 @@ class AsyncCallbackManager(BaseCallbackManager):
                         parent_run_id=self.parent_run_id,
                         tags=self.tags,
                         metadata=self.metadata,
+                        _tracing_metadata=self.tracing_metadata,
                         **kwargs,
                     )
                 )
@@ -1860,6 +1893,7 @@ class AsyncCallbackManager(BaseCallbackManager):
                     inheritable_tags=self.inheritable_tags,
                     metadata=self.metadata,
                     inheritable_metadata=self.inheritable_metadata,
+                    tracing_metadata=self.tracing_metadata,
                 )
             )
 
@@ -1914,6 +1948,7 @@ class AsyncCallbackManager(BaseCallbackManager):
                     parent_run_id=self.parent_run_id,
                     tags=self.tags,
                     metadata=self.metadata,
+                    _tracing_metadata=self.tracing_metadata,
                     **kwargs,
                 )
                 if handler.run_inline:
@@ -1931,6 +1966,7 @@ class AsyncCallbackManager(BaseCallbackManager):
                     inheritable_tags=self.inheritable_tags,
                     metadata=self.metadata,
                     inheritable_metadata=self.inheritable_metadata,
+                    tracing_metadata=self.tracing_metadata,
                 )
             )
 
@@ -1975,6 +2011,7 @@ class AsyncCallbackManager(BaseCallbackManager):
             parent_run_id=self.parent_run_id,
             tags=self.tags,
             metadata=self.metadata,
+            _tracing_metadata=self.tracing_metadata,
             **kwargs,
         )
 
@@ -1987,6 +2024,7 @@ class AsyncCallbackManager(BaseCallbackManager):
             inheritable_tags=self.inheritable_tags,
             metadata=self.metadata,
             inheritable_metadata=self.inheritable_metadata,
+            tracing_metadata=self.tracing_metadata,
         )
 
     @override
@@ -2023,6 +2061,7 @@ class AsyncCallbackManager(BaseCallbackManager):
             parent_run_id=self.parent_run_id,
             tags=self.tags,
             metadata=self.metadata,
+            _tracing_metadata=self.tracing_metadata,
             **kwargs,
         )
 
@@ -2035,6 +2074,7 @@ class AsyncCallbackManager(BaseCallbackManager):
             inheritable_tags=self.inheritable_tags,
             metadata=self.metadata,
             inheritable_metadata=self.inheritable_metadata,
+            tracing_metadata=self.tracing_metadata,
         )
 
     async def on_custom_event(
@@ -2115,6 +2155,7 @@ class AsyncCallbackManager(BaseCallbackManager):
             parent_run_id=self.parent_run_id,
             tags=self.tags,
             metadata=self.metadata,
+            _tracing_metadata=self.tracing_metadata,
             **kwargs,
         )
 
@@ -2127,6 +2168,7 @@ class AsyncCallbackManager(BaseCallbackManager):
             inheritable_tags=self.inheritable_tags,
             metadata=self.metadata,
             inheritable_metadata=self.inheritable_metadata,
+            tracing_metadata=self.tracing_metadata,
         )
 
     @classmethod
@@ -2140,7 +2182,7 @@ class AsyncCallbackManager(BaseCallbackManager):
         inheritable_metadata: dict[str, Any] | None = None,
         local_metadata: dict[str, Any] | None = None,
         *,
-        langsmith_metadata: Mapping[str, str] | None = None,
+        tracing_metadata: Mapping[str, str] | None = None,
     ) -> AsyncCallbackManager:
         """Configure the async callback manager.
 
@@ -2152,8 +2194,8 @@ class AsyncCallbackManager(BaseCallbackManager):
             local_tags: The local tags.
             inheritable_metadata: The inheritable metadata.
             local_metadata: The local metadata.
-            langsmith_metadata: Default metadata applied to any
-                `LangChainTracer` handlers via `set_defaults`.
+            tracing_metadata: Default metadata merged into runs started by
+                tracer handlers. Existing run metadata keys are not overwritten.
 
         Returns:
             The configured async callback manager.
@@ -2167,7 +2209,7 @@ class AsyncCallbackManager(BaseCallbackManager):
             inheritable_metadata,
             local_metadata,
             verbose=verbose,
-            langsmith_metadata=langsmith_metadata,
+            tracing_metadata=tracing_metadata,
         )
 
 
@@ -2314,7 +2356,7 @@ def _configure(
     local_metadata: dict[str, Any] | None = None,
     *,
     verbose: bool = False,
-    langsmith_metadata: Mapping[str, str] | None = None,
+    tracing_metadata: Mapping[str, str] | None = None,
 ) -> T:
     """Configure the callback manager.
 
@@ -2327,8 +2369,8 @@ def _configure(
         inheritable_metadata: The inheritable metadata.
         local_metadata: The local metadata.
         verbose: Whether to enable verbose mode.
-        langsmith_metadata: Default metadata applied to any
-            `LangChainTracer` handlers via `set_defaults`.
+        tracing_metadata: Default metadata merged into runs started by
+            tracer handlers. Existing run metadata keys are not overwritten.
 
     Raises:
         RuntimeError: If `LANGCHAIN_TRACING` is set but `LANGCHAIN_TRACING_V2` is not.
@@ -2349,7 +2391,7 @@ def _configure(
     from langchain_core.tracers.stdout import ConsoleCallbackHandler  # noqa: PLC0415
 
     tracing_context = get_tracing_context()
-    tracing_metadata = tracing_context["metadata"]
+    context_metadata = tracing_context["metadata"]
     tracing_tags = tracing_context["tags"]
     run_tree: Run | None = tracing_context["parent"]
     parent_run_id = None if run_tree is None else run_tree.id
@@ -2386,6 +2428,7 @@ def _configure(
                 inheritable_tags=inheritable_callbacks.inheritable_tags.copy(),
                 metadata=inheritable_callbacks.metadata.copy(),
                 inheritable_metadata=inheritable_callbacks.inheritable_metadata.copy(),
+                tracing_metadata=(inheritable_callbacks.tracing_metadata),
             )
         local_handlers_ = (
             local_callbacks
@@ -2400,8 +2443,8 @@ def _configure(
     if inheritable_metadata or local_metadata:
         callback_manager.add_metadata(inheritable_metadata or {})
         callback_manager.add_metadata(local_metadata or {}, inherit=False)
-    if tracing_metadata:
-        callback_manager.add_metadata(tracing_metadata.copy())
+    if context_metadata:
+        callback_manager.add_metadata(context_metadata.copy())
     if tracing_tags:
         callback_manager.add_tags(tracing_tags.copy())
 
@@ -2492,10 +2535,8 @@ def _configure(
                 for handler in callback_manager.handlers
             ):
                 callback_manager.add_handler(var_handler, inheritable)
-    if langsmith_metadata:
-        for handler in callback_manager.handlers:
-            if isinstance(handler, LangChainTracer):
-                handler.set_defaults(metadata=langsmith_metadata)
+    if tracing_metadata:
+        callback_manager.tracing_metadata = tracing_metadata
     return callback_manager
 
 
diff --git a/libs/core/langchain_core/tracers/base.py b/libs/core/langchain_core/tracers/base.py
index eaf9c04359f..4e4a5535256 100644
--- a/libs/core/langchain_core/tracers/base.py
+++ b/libs/core/langchain_core/tracers/base.py
@@ -33,6 +33,9 @@ logger = logging.getLogger(__name__)
 class BaseTracer(_TracerCore, BaseCallbackHandler, ABC):
     """Base interface for tracers."""
 
+    # Opt in to receiving per-invocation tracing_metadata via handle_event.
+    _accepts_tracing_metadata = True
+
     @abstractmethod
     def _persist_run(self, run: Run) -> None:
         """Persist a run."""
@@ -59,6 +62,7 @@ class BaseTracer(_TracerCore, BaseCallbackHandler, ABC):
         parent_run_id: UUID | None = None,
         metadata: dict[str, Any] | None = None,
         name: str | None = None,
+        tracing_metadata: dict[str, str] | None = None,
         **kwargs: Any,
     ) -> Run:
         """Start a trace for a chat model run.
@@ -77,6 +81,8 @@ class BaseTracer(_TracerCore, BaseCallbackHandler, ABC):
             parent_run_id: The parent run ID.
             metadata: The metadata for the run.
             name: The name of the run.
+            tracing_metadata: Per-invocation default metadata to merge
+                into the run. Existing run metadata keys are not overwritten.
             **kwargs: Additional arguments.
 
         Returns:
@@ -92,6 +98,9 @@ class BaseTracer(_TracerCore, BaseCallbackHandler, ABC):
             name=name,
             **kwargs,
         )
+        if tracing_metadata:
+            existing = chat_model_run.extra.get("metadata") or {}
+            chat_model_run.extra["metadata"] = {**tracing_metadata, **existing}
         self._start_trace(chat_model_run)
         self._on_chat_model_start(chat_model_run)
         return chat_model_run
@@ -106,6 +115,7 @@ class BaseTracer(_TracerCore, BaseCallbackHandler, ABC):
         parent_run_id: UUID | None = None,
         metadata: dict[str, Any] | None = None,
         name: str | None = None,
+        tracing_metadata: dict[str, str] | None = None,
         **kwargs: Any,
     ) -> Run:
         """Start a trace for an LLM run.
@@ -118,6 +128,8 @@ class BaseTracer(_TracerCore, BaseCallbackHandler, ABC):
             parent_run_id: The parent run ID.
             metadata: The metadata for the run.
             name: The name of the run.
+            tracing_metadata: Per-invocation default metadata to merge
+                into the run. Existing run metadata keys are not overwritten.
             **kwargs: Additional arguments.
 
         Returns:
@@ -133,6 +145,9 @@ class BaseTracer(_TracerCore, BaseCallbackHandler, ABC):
             name=name,
             **kwargs,
         )
+        if tracing_metadata:
+            existing = llm_run.extra.get("metadata") or {}
+            llm_run.extra["metadata"] = {**tracing_metadata, **existing}
         self._start_trace(llm_run)
         self._on_llm_start(llm_run)
         return llm_run
@@ -260,6 +275,7 @@ class BaseTracer(_TracerCore, BaseCallbackHandler, ABC):
         metadata: dict[str, Any] | None = None,
         run_type: str | None = None,
         name: str | None = None,
+        tracing_metadata: dict[str, str] | None = None,
         **kwargs: Any,
     ) -> Run:
         """Start a trace for a chain run.
@@ -273,6 +289,8 @@ class BaseTracer(_TracerCore, BaseCallbackHandler, ABC):
             metadata: The metadata for the run.
             run_type: The type of the run.
             name: The name of the run.
+            tracing_metadata: Per-invocation default metadata to merge
+                into the run. Existing run metadata keys are not overwritten.
             **kwargs: Additional arguments.
 
         Returns:
@@ -289,6 +307,9 @@ class BaseTracer(_TracerCore, BaseCallbackHandler, ABC):
             name=name,
             **kwargs,
         )
+        if tracing_metadata:
+            existing = chain_run.extra.get("metadata") or {}
+            chain_run.extra["metadata"] = {**tracing_metadata, **existing}
         self._start_trace(chain_run)
         self._on_chain_start(chain_run)
         return chain_run
@@ -362,6 +383,7 @@ class BaseTracer(_TracerCore, BaseCallbackHandler, ABC):
         metadata: dict[str, Any] | None = None,
         name: str | None = None,
         inputs: dict[str, Any] | None = None,
+        tracing_metadata: dict[str, str] | None = None,
         **kwargs: Any,
     ) -> Run:
         """Start a trace for a tool run.
@@ -375,6 +397,8 @@ class BaseTracer(_TracerCore, BaseCallbackHandler, ABC):
             metadata: The metadata for the run.
             name: The name of the run.
             inputs: The inputs for the tool.
+            tracing_metadata: Per-invocation default metadata to merge
+                into the run. Existing run metadata keys are not overwritten.
             **kwargs: Additional arguments.
 
         Returns:
@@ -391,6 +415,9 @@ class BaseTracer(_TracerCore, BaseCallbackHandler, ABC):
             inputs=inputs,
             **kwargs,
         )
+        if tracing_metadata:
+            existing = tool_run.extra.get("metadata") or {}
+            tool_run.extra["metadata"] = {**tracing_metadata, **existing}
         self._start_trace(tool_run)
         self._on_tool_start(tool_run)
         return tool_run
@@ -451,6 +478,7 @@ class BaseTracer(_TracerCore, BaseCallbackHandler, ABC):
         tags: list[str] | None = None,
         metadata: dict[str, Any] | None = None,
         name: str | None = None,
+        tracing_metadata: dict[str, str] | None = None,
         **kwargs: Any,
     ) -> Run:
         """Run when the `Retriever` starts running.
@@ -463,6 +491,8 @@ class BaseTracer(_TracerCore, BaseCallbackHandler, ABC):
             tags: The tags for the run.
             metadata: The metadata for the run.
             name: The name of the run.
+            tracing_metadata: Per-invocation default metadata to merge
+                into the run. Existing run metadata keys are not overwritten.
             **kwargs: Additional arguments.
 
         Returns:
@@ -478,6 +508,9 @@ class BaseTracer(_TracerCore, BaseCallbackHandler, ABC):
             name=name,
             **kwargs,
         )
+        if tracing_metadata:
+            existing = retrieval_run.extra.get("metadata") or {}
+            retrieval_run.extra["metadata"] = {**tracing_metadata, **existing}
         self._start_trace(retrieval_run)
         self._on_retriever_start(retrieval_run)
         return retrieval_run
diff --git a/libs/core/langchain_core/tracers/langchain.py b/libs/core/langchain_core/tracers/langchain.py
index b8242a27b49..77c108893bf 100644
--- a/libs/core/langchain_core/tracers/langchain.py
+++ b/libs/core/langchain_core/tracers/langchain.py
@@ -201,6 +201,7 @@ class LangChainTracer(BaseTracer):
         parent_run_id: UUID | None = None,
         metadata: dict[str, Any] | None = None,
         name: str | None = None,
+        tracing_metadata: dict[str, str] | None = None,
         **kwargs: Any,
     ) -> Run:
         """Start a trace for an LLM run.
@@ -213,6 +214,7 @@ class LangChainTracer(BaseTracer):
             parent_run_id: The parent run ID.
             metadata: The metadata.
             name: The name.
+            tracing_metadata: Per-invocation default metadata.
             **kwargs: Additional keyword arguments.
 
         Returns:
@@ -233,6 +235,9 @@ class LangChainTracer(BaseTracer):
             tags=tags,
             name=name,
         )
+        if tracing_metadata:
+            existing = chat_model_run.extra.get("metadata") or {}
+            chat_model_run.extra["metadata"] = {**tracing_metadata, **existing}
         self._start_trace(chat_model_run)
         self._on_chat_model_start(chat_model_run)
         return chat_model_run
@@ -427,6 +432,7 @@ class LangChainTracer(BaseTracer):
 
 
 def _patch_missing_metadata(self: LangChainTracer, run: Run) -> None:
+    # Apply constructor-set tracing_metadata as defaults (fill-not-overwrite).
     if not self.tracing_metadata:
         return
     metadata = run.metadata
diff --git a/libs/core/tests/unit_tests/runnables/test_tracing_interops.py b/libs/core/tests/unit_tests/runnables/test_tracing_interops.py
index 2e5fdc77a29..0dd9f0bc2f3 100644
--- a/libs/core/tests/unit_tests/runnables/test_tracing_interops.py
+++ b/libs/core/tests/unit_tests/runnables/test_tracing_interops.py
@@ -681,42 +681,44 @@ class TestTracerMetadataThroughInvoke:
         assert md["config_key"] == "config_val"
 
 
-class TestLangsmithMetadataInConfigure:
-    """Tests for `langsmith_metadata` parameter in `CallbackManager.configure()`."""
+class TestTracingMetadataInConfigure:
+    """Tests for `tracing_metadata` parameter in `CallbackManager.configure()`."""
 
-    def test_langsmith_metadata_applied_via_configure(self) -> None:
-        """langsmith_metadata flows through configure to LangChainTracer."""
+    def test_tracing_metadata_applied_via_configure(self) -> None:
+        """tracing_metadata flows through configure to the CallbackManager."""
         tracer = _create_tracer_with_mocked_client()
         cm = CallbackManager.configure(
             inheritable_callbacks=[tracer],
-            langsmith_metadata={"env": "prod", "service": "api"},
+            tracing_metadata={"env": "prod", "service": "api"},
         )
-        # The tracer should have set_defaults called with the metadata
-        lc_tracers = [h for h in cm.handlers if isinstance(h, LangChainTracer)]
-        assert len(lc_tracers) == 1
-        assert lc_tracers[0].tracing_metadata == {"env": "prod", "service": "api"}
+        # Metadata is stored on the manager, not mutated on the shared tracer.
+        assert cm.tracing_metadata == {"env": "prod", "service": "api"}
+        # The shared tracer instance is NOT mutated.
+        assert tracer.tracing_metadata is None
 
-    def test_langsmith_metadata_does_not_overwrite_tracer_metadata(self) -> None:
-        """Tracer's own metadata takes precedence over langsmith_metadata."""
+    def test_tracing_metadata_does_not_overwrite_tracer_metadata(self) -> None:
+        """Tracer's own metadata takes precedence over tracing_metadata."""
         tracer = _create_tracer_with_mocked_client(metadata={"env": "staging"})
-        CallbackManager.configure(
+        cm = CallbackManager.configure(
             inheritable_callbacks=[tracer],
-            langsmith_metadata={"env": "prod", "service": "api"},
+            tracing_metadata={"env": "prod", "service": "api"},
         )
-        assert tracer.tracing_metadata == {"env": "staging", "service": "api"}
+        # The shared tracer instance is NOT mutated.
+        assert tracer.tracing_metadata == {"env": "staging"}
+        # Per-invocation defaults are on the manager.
+        assert cm.tracing_metadata == {"env": "prod", "service": "api"}
 
-    def test_langsmith_metadata_end_to_end(self) -> None:
-        """langsmith_metadata in configure propagates to posted runs."""
+    def test_tracing_metadata_end_to_end(self) -> None:
+        """tracing_metadata in configure propagates to posted runs."""
         tracer = _create_tracer_with_mocked_client()
 
         @RunnableLambda
         def my_func(x: int) -> int:
             return x
 
-        # Use langsmith_metadata through the config callbacks path
         cm = CallbackManager.configure(
             inheritable_callbacks=[tracer],
-            langsmith_metadata={"env": "prod"},
+            tracing_metadata={"env": "prod"},
         )
         my_func.invoke(1, {"callbacks": cm})
 
@@ -725,8 +727,8 @@ class TestLangsmithMetadataInConfigure:
         md = posts[0].get("extra", {}).get("metadata", {})
         assert md["env"] == "prod"
 
-    def test_langsmith_metadata_does_not_affect_non_tracer_handlers(self) -> None:
-        """langsmith_metadata only applies to LangChainTracer, not other handlers."""
+    def test_tracing_metadata_does_not_affect_non_tracer_handlers(self) -> None:
+        """tracing_metadata only applies to tracer handlers, not other handlers."""
         tracer = _create_tracer_with_mocked_client()
 
         received_metadata: list[dict[str, Any]] = []
@@ -738,7 +740,7 @@ class TestLangsmithMetadataInConfigure:
         capture = MetadataCapture()
         cm = CallbackManager.configure(
             inheritable_callbacks=[tracer, capture],
-            langsmith_metadata={"tracer_only": "yes"},
+            tracing_metadata={"tracer_only": "yes"},
         )
 
         @RunnableLambda
@@ -747,7 +749,7 @@ class TestLangsmithMetadataInConfigure:
 
         my_func.invoke(1, {"callbacks": cm})
 
-        # Non-tracer handler should NOT see langsmith_metadata
+        # Non-tracer handler should NOT see tracing_metadata
         assert len(received_metadata) >= 1
         for md in received_metadata:
             assert "tracer_only" not in md
@@ -759,11 +761,11 @@ class TestLangsmithMetadataInConfigure:
             post_md = post.get("extra", {}).get("metadata", {})
             assert post_md["tracer_only"] == "yes"
 
-    def test_no_langsmith_metadata_is_noop(self) -> None:
-        """Passing langsmith_metadata=None does not alter tracer state."""
+    def test_no_tracing_metadata_is_noop(self) -> None:
+        """Passing tracing_metadata=None does not alter tracer state."""
         tracer = _create_tracer_with_mocked_client()
         CallbackManager.configure(
             inheritable_callbacks=[tracer],
-            langsmith_metadata=None,
+            tracing_metadata=None,
         )
         assert tracer.tracing_metadata is None