diff --git a/libs/core/langchain_core/runnables/base.py b/libs/core/langchain_core/runnables/base.py
index 280ecd25eb3..4846648ffaa 100644
--- a/libs/core/langchain_core/runnables/base.py
+++ b/libs/core/langchain_core/runnables/base.py
@@ -452,6 +452,10 @@ class Runnable(Generic[Input, Output], ABC):
 
     def assign(
         self,
+        *args: Union[
+            Runnable[Dict[str, Any], Dict[str, Any]],
+            Callable[[Dict[str, Any]], Dict[str, Any]],
+        ],
         **kwargs: Union[
             Runnable[Dict[str, Any], Any],
             Callable[[Dict[str, Any]], Any],
@@ -465,7 +469,14 @@ class Runnable(Generic[Input, Output], ABC):
         Returns a new runnable."""
         from langchain_core.runnables.passthrough import RunnableAssign
 
-        return self | RunnableAssign(RunnableParallel(kwargs))
+        if args and kwargs:
+            raise ValueError
+        elif args and len(args) > 1:
+            raise ValueError
+        elif args:
+            return self | RunnableAssign(coerce_to_runnable(args[0]))
+        else:
+            return self | RunnableAssign(RunnableParallel(kwargs))
 
     """ --- Public API --- """
 
@@ -1689,7 +1700,12 @@ def _seq_input_schema(
                 **{
                     k: (v.annotation, v.default)
                     for k, v in next_input_schema.__fields__.items()
-                    if k not in first.mapper.steps
+                    if k
+                    not in (
+                        first.mapper.steps
+                        if isinstance(first.mapper, RunnableParallel)
+                        else {}
+                    )
                 },
                 __config__=_SchemaConfig,
             )
diff --git a/libs/core/langchain_core/runnables/passthrough.py b/libs/core/langchain_core/runnables/passthrough.py
index 1f4367d9d5e..6590234731c 100644
--- a/libs/core/langchain_core/runnables/passthrough.py
+++ b/libs/core/langchain_core/runnables/passthrough.py
@@ -26,6 +26,7 @@ from langchain_core.runnables.base import (
     Runnable,
     RunnableParallel,
     RunnableSerializable,
+    coerce_to_runnable,
 )
 from langchain_core.runnables.config import (
     RunnableConfig,
@@ -183,6 +184,10 @@ class RunnablePassthrough(RunnableSerializable[Other, Other]):
     @classmethod
     def assign(
         cls,
+        *args: Union[
+            Runnable[Dict[str, Any], Dict[str, Any]],
+            Callable[[Dict[str, Any]], Dict[str, Any]],
+        ],
         **kwargs: Union[
             Runnable[Dict[str, Any], Any],
             Callable[[Dict[str, Any]], Any],
@@ -201,7 +206,14 @@ class RunnablePassthrough(RunnableSerializable[Other, Other]):
             A runnable that merges the Dict input with the output produced by the
             mapping argument.
         """
-        return RunnableAssign(RunnableParallel(kwargs))
+        if args and kwargs:
+            raise ValueError
+        elif args and len(args) > 1:
+            raise ValueError
+        elif args:
+            return RunnableAssign(coerce_to_runnable(args[0]))
+        else:
+            return RunnableAssign(RunnableParallel(kwargs))
 
     def invoke(
         self, input: Other, config: Optional[RunnableConfig] = None, **kwargs: Any
@@ -313,9 +325,11 @@ class RunnableAssign(RunnableSerializable[Dict[str, Any], Dict[str, Any]]):
     A runnable that assigns key-value pairs to Dict[str, Any] inputs.
     """
 
-    mapper: RunnableParallel[Dict[str, Any]]
+    mapper: Runnable[Dict[str, Any], Dict[str, Any]]
 
-    def __init__(self, mapper: RunnableParallel[Dict[str, Any]], **kwargs: Any) -> None:
+    def __init__(
+        self, mapper: Runnable[Dict[str, Any], Dict[str, Any]], **kwargs: Any
+    ) -> None:
         super().__init__(mapper=mapper, **kwargs)
 
     @classmethod
@@ -330,9 +344,11 @@ class RunnableAssign(RunnableSerializable[Dict[str, Any], Dict[str, Any]]):
     def get_name(
         self, suffix: Optional[str] = None, *, name: Optional[str] = None
     ) -> str:
-        name = (
-            name or self.name or f"RunnableAssign<{','.join(self.mapper.steps.keys())}>"
-        )
+        if isinstance(self.mapper, RunnableParallel):
+            default_name = f"RunnableAssign<{','.join(self.mapper.steps.keys())}>"
+        else:
+            default_name = "RunnableAssign"
+        name = name or self.name or default_name
         return super().get_name(suffix, name=name)
 
     def get_input_schema(
@@ -450,7 +466,11 @@ class RunnableAssign(RunnableSerializable[Dict[str, Any], Dict[str, Any]]):
         **kwargs: Any,
     ) -> Iterator[Dict[str, Any]]:
         # collect mapper keys
-        mapper_keys = set(self.mapper.steps.keys())
+        mapper_keys = (
+            set(self.mapper.steps.keys())
+            if isinstance(self.mapper, RunnableParallel)
+            else {}
+        )
         # create two streams, one for the map and one for the passthrough
         for_passthrough, for_map = safetee(input, 2, lock=threading.Lock())
 
@@ -506,7 +526,11 @@ class RunnableAssign(RunnableSerializable[Dict[str, Any], Dict[str, Any]]):
         **kwargs: Any,
     ) -> AsyncIterator[Dict[str, Any]]:
         # collect mapper keys
-        mapper_keys = set(self.mapper.steps.keys())
+        mapper_keys = (
+            set(self.mapper.steps.keys())
+            if isinstance(self.mapper, RunnableParallel)
+            else {}
+        )
         # create two streams, one for the map and one for the passthrough
         for_passthrough, for_map = atee(input, 2, lock=asyncio.Lock())
         # create map output stream