From dcfaae85d2302fdb51fc1e841c387e8f7467b684 Mon Sep 17 00:00:00 2001
From: Aaron V <inf3rnus@users.noreply.github.com>
Date: Fri, 7 Feb 2025 19:43:36 -0500
Subject: [PATCH] Core: Fix __add__ for concatting two BaseMessageChunk's
 (#29531)

Description:

The change allows you to use the overloaded `+` operator correctly when
`+`ing two BaseMessageChunk subclasses. Without this you *must*
instantiate a subclass for it to work.

Which feels... wrong. Base classes should be decoupled from sub classes
and should have in no way a dependency on them.

Issue:

You can't `+` a BaseMessageChunk with a BaseMessageChunk

e.g. this will explode

```py
from langchain_core.outputs import (
    ChatGenerationChunk,
)
from langchain_core.messages import BaseMessageChunk


chunk1 = ChatGenerationChunk(
    message=BaseMessageChunk(
        type="customChunk",
        content="HI",
    ),
)

chunk2 = ChatGenerationChunk(
    message=BaseMessageChunk(
        type="customChunk",
        content="HI",
    ),
)

# this will throw
new_chunk = chunk1 + chunk2
```

In case anyone ran into this issue themselves, it's probably best to use
the AIMessageChunk:

a la

```py
from langchain_core.outputs import (
    ChatGenerationChunk,
)
from langchain_core.messages import AIMessageChunk


chunk1 = ChatGenerationChunk(
    message=AIMessageChunk(
        content="HI",
    ),
)

chunk2 = ChatGenerationChunk(
    message=AIMessageChunk(
        content="HI",
    ),
)

# No explosion!
new_chunk = chunk1 + chunk2
```

Dependencies:

None!

Twitter handle:
`aaron_vogler`

Keeping these for later if need be:
```
baskaryan
efriis
eyurtsev
ccurme
vbarda
hwchase17
baskaryan
efriis
```

Co-authored-by: Erick Friis <erick@langchain.dev>
---
 libs/core/langchain_core/messages/base.py | 1 +
 1 file changed, 1 insertion(+)

diff --git a/libs/core/langchain_core/messages/base.py b/libs/core/langchain_core/messages/base.py
index 9eab1ed431a..0c59f77e06e 100644
--- a/libs/core/langchain_core/messages/base.py
+++ b/libs/core/langchain_core/messages/base.py
@@ -198,6 +198,7 @@ class BaseMessageChunk(BaseMessage):
 
             return self.__class__(  # type: ignore[call-arg]
                 id=self.id,
+                type=self.type,
                 content=merge_content(self.content, other.content),
                 additional_kwargs=merge_dicts(
                     self.additional_kwargs, other.additional_kwargs