diff --git a/tests/unit_tests/test_utils.py b/tests/unit_tests/test_utils.py
index c2a6ce4..4bc4204 100644
--- a/tests/unit_tests/test_utils.py
+++ b/tests/unit_tests/test_utils.py
@@ -5,7 +5,8 @@
 from pydantic import ValidationError
 from typing_extensions import Annotated, TypedDict
 
-from trustcall._base import _convert_any_typed_dicts_to_pydantic
+from trustcall._base import _convert_any_typed_dicts_to_pydantic, _apply_message_ops, MessageOp
+from langchain_core.messages import AIMessage, HumanMessage, SystemMessage, ToolMessage
 
 
 def test_convert_any_typed_dicts_to_pydantic():
@@ -89,3 +90,114 @@ class RecursiveType(TypedDict):
     cyclic["next"] = cyclic
     with pytest.raises(ValueError):  # or RecursionError, depending on implementation
         model(**cyclic)
+
+
+def test_message_ops_update_tool_name():
+    """Test various scenarios for updating tool names in messages."""
+    
+    # Test case 1: Mixed message types
+    messages = [
+        SystemMessage(content="system message"),
+        HumanMessage(content="user message"),
+        AIMessage(
+            content="",
+            tool_calls=[{
+                "id": "tool1",
+                "name": "old_name",
+                "args": {"arg1": "value1"}
+            }]
+        ),
+        ToolMessage(
+            content="tool response",
+            tool_call_id="tool1",
+            name="old_name"
+        )
+    ]
+
+    message_ops = [
+        MessageOp(
+            op="update_tool_name",
+            target={
+                "id": "tool1",
+                "name": "new_name"
+            }
+        )
+    ]
+
+    result = _apply_message_ops(messages, message_ops)
+
+    # Verify message count and types
+    assert len(result) == 4, "All messages should be preserved"
+    assert isinstance(result[0], SystemMessage)
+    assert isinstance(result[1], HumanMessage)
+    assert isinstance(result[2], AIMessage)
+    assert isinstance(result[3], ToolMessage)
+    
+    # Verify content preservation
+    assert result[0].content == "system message"
+    assert result[1].content == "user message"
+    assert result[2].tool_calls[0]["name"] == "new_name"
+    assert result[3].content == "tool response"
+
+    # Test case 2: Multiple tool calls in single AIMessage
+    messages = [
+        AIMessage(
+            content="",
+            tool_calls=[
+                {
+                    "id": "tool1",
+                    "name": "old_name1",
+                    "args": {"arg1": "value1"}
+                },
+                {
+                    "id": "tool2",
+                    "name": "old_name2",
+                    "args": {"arg2": "value2"}
+                }
+            ]
+        )
+    ]
+
+    message_ops = [
+        MessageOp(
+            op="update_tool_name",
+            target={
+                "id": "tool1",
+                "name": "new_name1"
+            }
+        )
+    ]
+
+    result = _apply_message_ops(messages, message_ops)
+    
+    # Verify selective update
+    assert len(result) == 1
+    assert result[0].tool_calls[0]["name"] == "new_name1"  # Updated
+    assert result[0].tool_calls[1]["name"] == "old_name2"  # Unchanged
+
+    # Test case 3: No matching tool_id
+    messages = [
+        AIMessage(
+            content="",
+            tool_calls=[{
+                "id": "tool1",
+                "name": "old_name",
+                "args": {"arg1": "value1"}
+            }]
+        )
+    ]
+
+    message_ops = [
+        MessageOp(
+            op="update_tool_name",
+            target={
+                "id": "non_existent_tool",
+                "name": "new_name"
+            }
+        )
+    ]
+
+    result = _apply_message_ops(messages, message_ops)
+    
+    # Verify no changes for non-matching tool_id
+    assert result[0].tool_calls[0]["name"] == "old_name"
diff --git a/trustcall/_base.py b/trustcall/_base.py
index b4858dc..f2ce01b 100644
--- a/trustcall/_base.py
+++ b/trustcall/_base.py
@@ -1205,6 +1205,8 @@ def _apply_message_ops(
                         m = m.copy()
                         m.tool_calls = new
                     messages_.append(m)
+                else:
+                    messages_.append(m)
             messages = messages_
 
         else: