fix ChatMessageChunk concat error (#10174)

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- Description: fix `ChatMessageChunk` concat error 
- Issue: #10173 
- Dependencies: None
- Tag maintainer: @baskaryan, @eyurtsev, @rlancemartin
- Twitter handle: None

---------

Co-authored-by: wangshuai.scotty <wangshuai.scotty@bytedance.com>
Co-authored-by: Nuno Campos <nuno@boringbits.io>

libs/langchain/langchain/schema/messages.py

@@ -117,6 +117,14 @@ class BaseMessageChunk(BaseMessage):
             # If both are (subclasses of) BaseMessageChunk,
             # concat into a single BaseMessageChunk
 
+            if isinstance(self, ChatMessageChunk):
+                return self.__class__(
+                    role=self.role,
+                    content=self.content + other.content,
+                    additional_kwargs=self._merge_kwargs_dict(
+                        self.additional_kwargs, other.additional_kwargs
+                    ),
+                )
             return self.__class__(
                 content=self.content + other.content,
                 additional_kwargs=self._merge_kwargs_dict(
@@ -168,7 +176,22 @@ class AIMessage(BaseMessage):
 class AIMessageChunk(AIMessage, BaseMessageChunk):
     """A Message chunk from an AI."""
 
-    pass
+    def __add__(self, other: Any) -> BaseMessageChunk:  # type: ignore
+        if isinstance(other, AIMessageChunk):
+            if self.example != other.example:
+                raise ValueError(
+                    "Cannot concatenate AIMessageChunks with different example values."
+                )
+
+            return self.__class__(
+                example=self.example,
+                content=self.content + other.content,
+                additional_kwargs=self._merge_kwargs_dict(
+                    self.additional_kwargs, other.additional_kwargs
+                ),
+            )
+
+        return super().__add__(other)
 
 
 class SystemMessage(BaseMessage):
@@ -203,7 +226,22 @@ class FunctionMessage(BaseMessage):
 class FunctionMessageChunk(FunctionMessage, BaseMessageChunk):
     """A Function Message chunk."""
 
-    pass
+    def __add__(self, other: Any) -> BaseMessageChunk:  # type: ignore
+        if isinstance(other, FunctionMessageChunk):
+            if self.name != other.name:
+                raise ValueError(
+                    "Cannot concatenate FunctionMessageChunks with different names."
+                )
+
+            return self.__class__(
+                name=self.name,
+                content=self.content + other.content,
+                additional_kwargs=self._merge_kwargs_dict(
+                    self.additional_kwargs, other.additional_kwargs
+                ),
+            )
+
+        return super().__add__(other)
 
 
 class ChatMessage(BaseMessage):
@@ -221,7 +259,22 @@ class ChatMessage(BaseMessage):
 class ChatMessageChunk(ChatMessage, BaseMessageChunk):
     """A Chat Message chunk."""
 
-    pass
+    def __add__(self, other: Any) -> BaseMessageChunk:  # type: ignore
+        if isinstance(other, ChatMessageChunk):
+            if self.role != other.role:
+                raise ValueError(
+                    "Cannot concatenate ChatMessageChunks with different roles."
+                )
+
+            return self.__class__(
+                role=self.role,
+                content=self.content + other.content,
+                additional_kwargs=self._merge_kwargs_dict(
+                    self.additional_kwargs, other.additional_kwargs
+                ),
+            )
+
+        return super().__add__(other)
 
 
 def _message_to_dict(message: BaseMessage) -> dict:

libs/langchain/tests/unit_tests/schema/test_messages.py

@@ -1,4 +1,11 @@
-from langchain.schema.messages import AIMessageChunk, HumanMessageChunk
+import pytest
+
+from langchain.schema.messages import (
+    AIMessageChunk,
+    ChatMessageChunk,
+    FunctionMessageChunk,
+    HumanMessageChunk,
+)
 
 
 def test_message_chunks() -> None:
@@ -36,3 +43,54 @@ def test_message_chunks() -> None:
             }
         },
     ), "MessageChunk + MessageChunk should be a MessageChunk with merged additional_kwargs"  # noqa: E501
+
+
+def test_chat_message_chunks() -> None:
+    assert ChatMessageChunk(role="User", content="I am") + ChatMessageChunk(
+        role="User", content=" indeed."
+    ) == ChatMessageChunk(
+        role="User", content="I am indeed."
+    ), "ChatMessageChunk + ChatMessageChunk should be a ChatMessageChunk"
+
+    with pytest.raises(ValueError):
+        ChatMessageChunk(role="User", content="I am") + ChatMessageChunk(
+            role="Assistant", content=" indeed."
+        )
+
+    assert ChatMessageChunk(role="User", content="I am") + AIMessageChunk(
+        content=" indeed."
+    ) == ChatMessageChunk(
+        role="User", content="I am indeed."
+    ), "ChatMessageChunk + other MessageChunk should be a ChatMessageChunk with the left side's role"  # noqa: E501
+
+    assert AIMessageChunk(content="I am") + ChatMessageChunk(
+        role="User", content=" indeed."
+    ) == AIMessageChunk(
+        content="I am indeed."
+    ), "Other MessageChunk + ChatMessageChunk should be a MessageChunk as the left side"  # noqa: E501
+
+
+def test_function_message_chunks() -> None:
+    assert FunctionMessageChunk(name="hello", content="I am") + FunctionMessageChunk(
+        name="hello", content=" indeed."
+    ) == FunctionMessageChunk(
+        name="hello", content="I am indeed."
+    ), "FunctionMessageChunk + FunctionMessageChunk should be a FunctionMessageChunk"
+
+    with pytest.raises(ValueError):
+        FunctionMessageChunk(name="hello", content="I am") + FunctionMessageChunk(
+            name="bye", content=" indeed."
+        )
+
+
+def test_ani_message_chunks() -> None:
+    assert AIMessageChunk(example=True, content="I am") + AIMessageChunk(
+        example=True, content=" indeed."
+    ) == AIMessageChunk(
+        example=True, content="I am indeed."
+    ), "AIMessageChunk + AIMessageChunk should be a AIMessageChunk"
+
+    with pytest.raises(ValueError):
+        AIMessageChunk(example=True, content="I am") + AIMessageChunk(
+            example=False, content=" indeed."
+        )