diff --git a/libs/core/langchain_core/messages/base.py b/libs/core/langchain_core/messages/base.py
index daed44f5014..96c7665ec7b 100644
--- a/libs/core/langchain_core/messages/base.py
+++ b/libs/core/langchain_core/messages/base.py
@@ -98,8 +98,12 @@ class BaseMessageChunk(BaseMessage):
                 merged[k] = v
             elif merged[k] is None and v:
                 merged[k] = v
+            elif v is None:
+                continue
+            elif merged[k] == v:
+                continue
             elif type(merged[k]) != type(v):
-                raise ValueError(
+                raise TypeError(
                     f'additional_kwargs["{k}"] already exists in this message,'
                     " but with a different type."
                 )
@@ -107,8 +111,17 @@ class BaseMessageChunk(BaseMessage):
                 merged[k] += v
             elif isinstance(merged[k], dict):
                 merged[k] = self._merge_kwargs_dict(merged[k], v)
+            elif isinstance(merged[k], list):
+                merged[k] = merged[k].copy()
+                for i, e in enumerate(v):
+                    if isinstance(e, dict) and isinstance(e.get("index"), int):
+                        i = e["index"]
+                    if i < len(merged[k]):
+                        merged[k][i] = self._merge_kwargs_dict(merged[k][i], e)
+                    else:
+                        merged[k] = merged[k] + [e]
             else:
-                raise ValueError(
+                raise TypeError(
                     f"Additional kwargs key {k} already exists in this message."
                 )
         return merged
diff --git a/libs/core/tests/unit_tests/test_messages.py b/libs/core/tests/unit_tests/test_messages.py
index bcb8bc88ce4..95d60a52f2b 100644
--- a/libs/core/tests/unit_tests/test_messages.py
+++ b/libs/core/tests/unit_tests/test_messages.py
@@ -1,4 +1,5 @@
 import unittest
+from typing import List
 
 import pytest
 
@@ -203,3 +204,227 @@ def test_message_chunk_to_message() -> None:
     assert message_chunk_to_message(
         FunctionMessageChunk(name="hello", content="I am")
     ) == FunctionMessage(name="hello", content="I am")
+
+
+def test_tool_calls_merge() -> None:
+    chunks: List[dict] = [
+        dict(content=""),
+        dict(
+            content="",
+            additional_kwargs={
+                "tool_calls": [
+                    {
+                        "index": 0,
+                        "id": "call_CwGAsESnXehQEjiAIWzinlva",
+                        "function": {"arguments": "", "name": "person"},
+                        "type": "function",
+                    }
+                ]
+            },
+        ),
+        dict(
+            content="",
+            additional_kwargs={
+                "tool_calls": [
+                    {
+                        "index": 0,
+                        "id": None,
+                        "function": {"arguments": '{"na', "name": None},
+                        "type": None,
+                    }
+                ]
+            },
+        ),
+        dict(
+            content="",
+            additional_kwargs={
+                "tool_calls": [
+                    {
+                        "index": 0,
+                        "id": None,
+                        "function": {"arguments": 'me": ', "name": None},
+                        "type": None,
+                    }
+                ]
+            },
+        ),
+        dict(
+            content="",
+            additional_kwargs={
+                "tool_calls": [
+                    {
+                        "index": 0,
+                        "id": None,
+                        "function": {"arguments": '"jane"', "name": None},
+                        "type": None,
+                    }
+                ]
+            },
+        ),
+        dict(
+            content="",
+            additional_kwargs={
+                "tool_calls": [
+                    {
+                        "index": 0,
+                        "id": None,
+                        "function": {"arguments": ', "a', "name": None},
+                        "type": None,
+                    }
+                ]
+            },
+        ),
+        dict(
+            content="",
+            additional_kwargs={
+                "tool_calls": [
+                    {
+                        "index": 0,
+                        "id": None,
+                        "function": {"arguments": 'ge": ', "name": None},
+                        "type": None,
+                    }
+                ]
+            },
+        ),
+        dict(
+            content="",
+            additional_kwargs={
+                "tool_calls": [
+                    {
+                        "index": 0,
+                        "id": None,
+                        "function": {"arguments": "2}", "name": None},
+                        "type": None,
+                    }
+                ]
+            },
+        ),
+        dict(
+            content="",
+            additional_kwargs={
+                "tool_calls": [
+                    {
+                        "index": 1,
+                        "id": "call_zXSIylHvc5x3JUAPcHZR5GZI",
+                        "function": {"arguments": "", "name": "person"},
+                        "type": "function",
+                    }
+                ]
+            },
+        ),
+        dict(
+            content="",
+            additional_kwargs={
+                "tool_calls": [
+                    {
+                        "index": 1,
+                        "id": None,
+                        "function": {"arguments": '{"na', "name": None},
+                        "type": None,
+                    }
+                ]
+            },
+        ),
+        dict(
+            content="",
+            additional_kwargs={
+                "tool_calls": [
+                    {
+                        "index": 1,
+                        "id": None,
+                        "function": {"arguments": 'me": ', "name": None},
+                        "type": None,
+                    }
+                ]
+            },
+        ),
+        dict(
+            content="",
+            additional_kwargs={
+                "tool_calls": [
+                    {
+                        "index": 1,
+                        "id": None,
+                        "function": {"arguments": '"bob",', "name": None},
+                        "type": None,
+                    }
+                ]
+            },
+        ),
+        dict(
+            content="",
+            additional_kwargs={
+                "tool_calls": [
+                    {
+                        "index": 1,
+                        "id": None,
+                        "function": {"arguments": ' "ag', "name": None},
+                        "type": None,
+                    }
+                ]
+            },
+        ),
+        dict(
+            content="",
+            additional_kwargs={
+                "tool_calls": [
+                    {
+                        "index": 1,
+                        "id": None,
+                        "function": {"arguments": 'e": 3', "name": None},
+                        "type": None,
+                    }
+                ]
+            },
+        ),
+        dict(
+            content="",
+            additional_kwargs={
+                "tool_calls": [
+                    {
+                        "index": 1,
+                        "id": None,
+                        "function": {"arguments": "}", "name": None},
+                        "type": None,
+                    }
+                ]
+            },
+        ),
+        dict(content=""),
+    ]
+
+    final = None
+
+    for chunk in chunks:
+        msg = AIMessageChunk(**chunk)
+        if final is None:
+            final = msg
+        else:
+            final = final + msg
+
+    assert final == AIMessageChunk(
+        content="",
+        additional_kwargs={
+            "tool_calls": [
+                {
+                    "index": 0,
+                    "id": "call_CwGAsESnXehQEjiAIWzinlva",
+                    "function": {
+                        "arguments": '{"name": "jane", "age": 2}',
+                        "name": "person",
+                    },
+                    "type": "function",
+                },
+                {
+                    "index": 1,
+                    "id": "call_zXSIylHvc5x3JUAPcHZR5GZI",
+                    "function": {
+                        "arguments": '{"name": "bob", "age": 3}',
+                        "name": "person",
+                    },
+                    "type": "function",
+                },
+            ]
+        },
+    )