diff --git a/libs/core/langchain_core/runnables/base.py b/libs/core/langchain_core/runnables/base.py
index e41ef5877d7..c47018f05bd 100644
--- a/libs/core/langchain_core/runnables/base.py
+++ b/libs/core/langchain_core/runnables/base.py
@@ -69,6 +69,7 @@ from langchain_core.runnables.utils import (
     accepts_config,
     accepts_context,
     accepts_run_manager,
+    adapt_first_streaming_chunk,
     create_model,
     gather_with_concurrency,
     get_function_first_arg_dict_keys,
@@ -1207,7 +1208,7 @@ class Runnable(Generic[Input, Output], ABC):
 
         for chunk in input:
             if not got_first_val:
-                final = _adapt_first_streaming_chunk(chunk)  # type: ignore
+                final = adapt_first_streaming_chunk(chunk)  # type: ignore
                 got_first_val = True
             else:
                 # Make a best effort to gather, for any type that supports `+`
@@ -1240,7 +1241,7 @@ class Runnable(Generic[Input, Output], ABC):
 
         async for chunk in input:
             if not got_first_val:
-                final = _adapt_first_streaming_chunk(chunk)  # type: ignore
+                final = adapt_first_streaming_chunk(chunk)  # type: ignore
                 got_first_val = True
             else:
                 # Make a best effort to gather, for any type that supports `+`
@@ -3731,7 +3732,7 @@ class RunnableLambda(Runnable[Input, Output]):
         final: Optional[Input] = None
         for ichunk in input:
             if final is None:
-                final = _adapt_first_streaming_chunk(ichunk)  # type: ignore
+                final = adapt_first_streaming_chunk(ichunk)  # type: ignore
             else:
                 try:
                     final = final + ichunk  # type: ignore[operator]
@@ -3815,7 +3816,7 @@ class RunnableLambda(Runnable[Input, Output]):
         final: Optional[Input] = None
         async for ichunk in input:
             if final is None:
-                final = _adapt_first_streaming_chunk(ichunk)
+                final = adapt_first_streaming_chunk(ichunk)
             else:
                 try:
                     final = final + ichunk  # type: ignore[operator]
@@ -4727,11 +4728,3 @@ def chain(
                 yield chunk
     """
     return RunnableLambda(func)
-
-
-def _adapt_first_streaming_chunk(chunk: Any) -> Any:
-    """This might transform the first chunk of a stream into an AddableDict."""
-    if isinstance(chunk, dict) and not isinstance(chunk, AddableDict):
-        return AddableDict(chunk)
-    else:
-        return chunk
diff --git a/libs/core/langchain_core/runnables/passthrough.py b/libs/core/langchain_core/runnables/passthrough.py
index 0f171795114..f3d4e5f78d5 100644
--- a/libs/core/langchain_core/runnables/passthrough.py
+++ b/libs/core/langchain_core/runnables/passthrough.py
@@ -40,6 +40,7 @@ from langchain_core.runnables.graph import Graph
 from langchain_core.runnables.utils import (
     AddableDict,
     ConfigurableFieldSpec,
+    adapt_first_streaming_chunk,
     create_model,
 )
 from langchain_core.utils.aiter import atee, py_anext
@@ -248,7 +249,7 @@ class RunnablePassthrough(RunnableSerializable[Other, Other]):
             for chunk in self._transform_stream_with_config(input, identity, config):
                 yield chunk
                 if final is None:
-                    final = chunk
+                    final = adapt_first_streaming_chunk(chunk)
                 else:
                     final = final + chunk
 
@@ -276,7 +277,7 @@ class RunnablePassthrough(RunnableSerializable[Other, Other]):
             ):
                 yield chunk
                 if final is None:
-                    final = chunk
+                    final = adapt_first_streaming_chunk(chunk)
                 else:
                     final = final + chunk
 
diff --git a/libs/core/langchain_core/runnables/utils.py b/libs/core/langchain_core/runnables/utils.py
index 8ff332173cc..40f52c18167 100644
--- a/libs/core/langchain_core/runnables/utils.py
+++ b/libs/core/langchain_core/runnables/utils.py
@@ -521,3 +521,11 @@ def _create_model_cached(
     return _create_model_base(
         __model_name, __config__=_SchemaConfig, **field_definitions
     )
+
+
+def adapt_first_streaming_chunk(chunk: Any) -> Any:
+    """This might transform the first chunk of a stream into an AddableDict."""
+    if isinstance(chunk, dict) and not isinstance(chunk, AddableDict):
+        return AddableDict(chunk)
+    else:
+        return chunk
diff --git a/libs/core/tests/unit_tests/runnables/test_runnable.py b/libs/core/tests/unit_tests/runnables/test_runnable.py
index fc63abd9af7..91cde649e77 100644
--- a/libs/core/tests/unit_tests/runnables/test_runnable.py
+++ b/libs/core/tests/unit_tests/runnables/test_runnable.py
@@ -5324,7 +5324,7 @@ def test_default_transform_with_dicts() -> None:
     assert list(runnable.transform(chunks)) == [{"foo": "an"}]
 
 
-async def test_defualt_atransform_with_dicts() -> None:
+async def test_default_atransform_with_dicts() -> None:
     """Test that default transform works with dicts."""
 
     class CustomRunnable(RunnableSerializable[Input, Output]):
@@ -5342,3 +5342,22 @@ async def test_defualt_atransform_with_dicts() -> None:
     chunks = [chunk async for chunk in runnable.atransform(chunk_iterator())]
 
     assert chunks == [{"foo": "an"}]
+
+
+def test_passthrough_transform_with_dicts() -> None:
+    """Test that default transform works with dicts."""
+    runnable = RunnablePassthrough(lambda x: x)
+    chunks = [chunk for chunk in runnable.transform(iter([{"foo": "a"}, {"foo": "n"}]))]
+    assert chunks == [{"foo": "a"}, {"foo": "n"}]
+
+
+async def test_passthrough_atransform_with_dicts() -> None:
+    """Test that default transform works with dicts."""
+    runnable = RunnablePassthrough(lambda x: x)
+
+    async def chunk_iterator() -> AsyncIterator[Dict[str, str]]:
+        yield {"foo": "a"}
+        yield {"foo": "n"}
+
+    chunks = [chunk async for chunk in runnable.atransform(chunk_iterator())]
+    assert chunks == [{"foo": "a"}, {"foo": "n"}]