diff --git a/libs/langchain/langchain/chat_models/base.py b/libs/langchain/langchain/chat_models/base.py
index 83c6a45fa8a3b..f69c74f0407e8 100644
--- a/libs/langchain/langchain/chat_models/base.py
+++ b/libs/langchain/langchain/chat_models/base.py
@@ -11,7 +11,6 @@
     List,
     Optional,
     Sequence,
-    Union,
     cast,
 )
 
@@ -38,12 +37,10 @@
 from langchain.schema.language_model import BaseLanguageModel, LanguageModelInput
 from langchain.schema.messages import (
     AIMessage,
+    AnyMessage,
     BaseMessage,
     BaseMessageChunk,
-    ChatMessage,
-    FunctionMessage,
     HumanMessage,
-    SystemMessage,
 )
 from langchain.schema.output import ChatGenerationChunk
 from langchain.schema.runnable import RunnableConfig
@@ -79,7 +76,7 @@ async def _agenerate_from_stream(
     return ChatResult(generations=[generation])
 
 
-class BaseChatModel(BaseLanguageModel[BaseMessageChunk], ABC):
+class BaseChatModel(BaseLanguageModel[BaseMessage], ABC):
     """Base class for Chat models."""
 
     cache: Optional[bool] = None
@@ -116,9 +113,7 @@ class Config:
     @property
     def OutputType(self) -> Any:
         """Get the output type for this runnable."""
-        return Union[
-            HumanMessage, AIMessage, ChatMessage, FunctionMessage, SystemMessage
-        ]
+        return AnyMessage
 
     def _convert_input(self, input: LanguageModelInput) -> PromptValue:
         if isinstance(input, PromptValue):
@@ -140,23 +135,20 @@ def invoke(
         *,
         stop: Optional[List[str]] = None,
         **kwargs: Any,
-    ) -> BaseMessageChunk:
+    ) -> BaseMessage:
         config = config or {}
         return cast(
-            BaseMessageChunk,
-            cast(
-                ChatGeneration,
-                self.generate_prompt(
-                    [self._convert_input(input)],
-                    stop=stop,
-                    callbacks=config.get("callbacks"),
-                    tags=config.get("tags"),
-                    metadata=config.get("metadata"),
-                    run_name=config.get("run_name"),
-                    **kwargs,
-                ).generations[0][0],
-            ).message,
-        )
+            ChatGeneration,
+            self.generate_prompt(
+                [self._convert_input(input)],
+                stop=stop,
+                callbacks=config.get("callbacks"),
+                tags=config.get("tags"),
+                metadata=config.get("metadata"),
+                run_name=config.get("run_name"),
+                **kwargs,
+            ).generations[0][0],
+        ).message
 
     async def ainvoke(
         self,
@@ -165,7 +157,7 @@ async def ainvoke(
         *,
         stop: Optional[List[str]] = None,
         **kwargs: Any,
-    ) -> BaseMessageChunk:
+    ) -> BaseMessage:
         config = config or {}
         llm_result = await self.agenerate_prompt(
             [self._convert_input(input)],
@@ -176,9 +168,7 @@ async def ainvoke(
             run_name=config.get("run_name"),
             **kwargs,
         )
-        return cast(
-            BaseMessageChunk, cast(ChatGeneration, llm_result.generations[0][0]).message
-        )
+        return cast(ChatGeneration, llm_result.generations[0][0]).message
 
     def stream(
         self,
@@ -190,7 +180,9 @@ def stream(
     ) -> Iterator[BaseMessageChunk]:
         if type(self)._stream == BaseChatModel._stream:
             # model doesn't implement streaming, so use default implementation
-            yield self.invoke(input, config=config, stop=stop, **kwargs)
+            yield cast(
+                BaseMessageChunk, self.invoke(input, config=config, stop=stop, **kwargs)
+            )
         else:
             config = config or {}
             messages = self._convert_input(input).to_messages()
@@ -241,7 +233,9 @@ async def astream(
     ) -> AsyncIterator[BaseMessageChunk]:
         if type(self)._astream == BaseChatModel._astream:
             # model doesn't implement streaming, so use default implementation
-            yield self.invoke(input, config=config, stop=stop, **kwargs)
+            yield cast(
+                BaseMessageChunk, self.invoke(input, config=config, stop=stop, **kwargs)
+            )
         else:
             config = config or {}
             messages = self._convert_input(input).to_messages()
diff --git a/libs/langchain/tests/unit_tests/schema/runnable/__snapshots__/test_runnable.ambr b/libs/langchain/tests/unit_tests/schema/runnable/__snapshots__/test_runnable.ambr
index b4b607f3fa2ac..e78280380bf35 100644
--- a/libs/langchain/tests/unit_tests/schema/runnable/__snapshots__/test_runnable.ambr
+++ b/libs/langchain/tests/unit_tests/schema/runnable/__snapshots__/test_runnable.ambr
@@ -2163,19 +2163,19 @@
   dict({
     'anyOf': list([
       dict({
-        '$ref': '#/definitions/HumanMessage',
+        '$ref': '#/definitions/AIMessage',
       }),
       dict({
-        '$ref': '#/definitions/AIMessage',
+        '$ref': '#/definitions/HumanMessage',
       }),
       dict({
         '$ref': '#/definitions/ChatMessage',
       }),
       dict({
-        '$ref': '#/definitions/FunctionMessage',
+        '$ref': '#/definitions/SystemMessage',
       }),
       dict({
-        '$ref': '#/definitions/SystemMessage',
+        '$ref': '#/definitions/FunctionMessage',
       }),
     ]),
     'definitions': dict({