diff --git a/python/packages/autogen-ext/src/autogen_ext/models/openai/_transformation/registry.py b/python/packages/autogen-ext/src/autogen_ext/models/openai/_transformation/registry.py
index 5c1187fb5224..c3148110d057 100644
--- a/python/packages/autogen-ext/src/autogen_ext/models/openai/_transformation/registry.py
+++ b/python/packages/autogen-ext/src/autogen_ext/models/openai/_transformation/registry.py
@@ -1,5 +1,5 @@
 from collections import defaultdict
-from typing import Any, Callable, Dict, List
+from typing import Any, Callable, Dict, List, get_args
 
 from autogen_core.models import LLMMessage, ModelFamily
 
@@ -87,10 +87,13 @@ def _find_model_family(api: str, model: str) -> str:
     Finds the best matching model family for the given model.
     Search via prefix matching (e.g. "gpt-4o" → "gpt-4o-1.0").
     """
+    len_family = 0
     family = ModelFamily.UNKNOWN
     for _family in MESSAGE_TRANSFORMERS[api].keys():
         if model.startswith(_family):
-            family = _family
+            if len(_family) > len_family:
+                family = _family
+                len_family = len(_family)
     return family
 
 
@@ -108,13 +111,14 @@ def get_transformer(api: str, model: str, model_family: str) -> TransformerMap:
     Keeping this as a function (instead of direct dict access) improves long-term flexibility.
     """
 
-    if model_family == ModelFamily.UNKNOWN:
+    if model_family not in set(get_args(ModelFamily.ANY)) or model_family == ModelFamily.UNKNOWN:
         # fallback to finding the best matching model family
         model_family = _find_model_family(api, model)
 
     transformer = MESSAGE_TRANSFORMERS.get(api, {}).get(model_family, {})
 
     if not transformer:
+        # Just in case, we should never reach here
         raise ValueError(f"No transformer found for model family '{model_family}'")
 
     return transformer
diff --git a/python/packages/autogen-ext/tests/models/test_openai_model_client.py b/python/packages/autogen-ext/tests/models/test_openai_model_client.py
index 97edb4dab054..60d5547d0302 100644
--- a/python/packages/autogen-ext/tests/models/test_openai_model_client.py
+++ b/python/packages/autogen-ext/tests/models/test_openai_model_client.py
@@ -30,6 +30,7 @@
     to_oai_type,
 )
 from autogen_ext.models.openai._transformation import TransformerMap, get_transformer
+from autogen_ext.models.openai._transformation.registry import _find_model_family  # pyright: ignore[reportPrivateUsage]
 from openai.resources.beta.chat.completions import (  # type: ignore
     AsyncChatCompletionStreamManager as BetaAsyncChatCompletionStreamManager,  # type: ignore
 )
@@ -2394,11 +2395,6 @@ def get_regitered_transformer(client: OpenAIChatCompletionClient) -> Transformer
     assert get_regitered_transformer(client1) == get_regitered_transformer(client2)
 
 
-def test_openai_model_registry_find_wrong() -> None:
-    with pytest.raises(ValueError, match="No transformer found for model family"):
-        get_transformer("openai", "gpt-7", "foobar")
-
-
 @pytest.mark.asyncio
 @pytest.mark.parametrize(
     "model",
@@ -2451,4 +2447,13 @@ def test_rstrip_railing_whitespace_at_last_assistant_content() -> None:
     assert result[-1].content == "foobar"
 
 
+def test_find_model_family() -> None:
+    assert _find_model_family("openai", "gpt-4") == ModelFamily.GPT_4
+    assert _find_model_family("openai", "gpt-4-latest") == ModelFamily.GPT_4
+    assert _find_model_family("openai", "gpt-4o") == ModelFamily.GPT_4O
+    assert _find_model_family("openai", "gemini-2.0-flash") == ModelFamily.GEMINI_2_0_FLASH
+    assert _find_model_family("openai", "claude-3-5-haiku-20241022") == ModelFamily.CLAUDE_3_5_HAIKU
+    assert _find_model_family("openai", "error") == ModelFamily.UNKNOWN
+
+
 # TODO: add integration tests for Azure OpenAI using AAD token.