diff --git a/libs/core/tests/unit_tests/messages/test_utils.py b/libs/core/tests/unit_tests/messages/test_utils.py
index 8777b2674c5..f0a5de99d4f 100644
--- a/libs/core/tests/unit_tests/messages/test_utils.py
+++ b/libs/core/tests/unit_tests/messages/test_utils.py
@@ -964,3 +964,46 @@ def test_convert_to_openai_messages_developer() -> None:
     ]
     result = convert_to_openai_messages(messages)
     assert result == [{"role": "developer", "content": "a"}] * 2
+
+def test_trim_messages_custom_counter() -> None:
+    def dummy_token_counter(messages: list[BaseMessage]) -> int:
+        # treat each message like it adds 3 default tokens at the beginning
+        # of the message and at the end of the message. 3 + 4 + 3 = 10 tokens
+        # per message.
+
+        default_content_len = 4
+        default_msg_prefix_len = 3
+        default_msg_suffix_len = 3
+
+        count = 0
+        for msg in messages:
+            if isinstance(msg.content, str):
+                str_len = int(len(msg.content) / 3)
+                count += default_msg_prefix_len + str_len + default_msg_suffix_len
+            if isinstance(msg.content, list):
+                content = int(len(msg.content[0]["text"]) / 3)
+                count += default_msg_prefix_len + content + default_msg_suffix_len
+        return count
+
+    messages = [
+        SystemMessage("This is a 4 token text."),
+        HumanMessage("This is a 4 token text.", id="first"),
+        AIMessage(
+            [
+                {"type": "text", "text": "This is the FIRST 4 token block."},
+                {"type": "text", "text": "This is the SECOND 4 token block."},
+            ],
+            id="second",
+        ),
+        HumanMessage("This is a 4 token text.", id="third"),
+        AIMessage("This is a 4 token text.", id="fourth"),
+    ]
+
+    new_messages = trim_messages(
+        messages,
+        max_tokens=10,
+        token_counter=dummy_token_counter,
+        strategy="last",
+        allow_partial=True,
+    )
+    raise ValueError(new_messages)