diff --git a/litellm/llms/openai/chat/gpt_5_transformation.py b/litellm/llms/openai/chat/gpt_5_transformation.py
index 9ccb2e1c267..6923a3a4612 100644
--- a/litellm/llms/openai/chat/gpt_5_transformation.py
+++ b/litellm/llms/openai/chat/gpt_5_transformation.py
@@ -103,10 +103,16 @@ class OpenAIGPT5Config(OpenAIGPTConfig):
 
     @classmethod
     def is_model_gpt_5_4_plus_model(cls, model: str) -> bool:
-        """Check if the model is gpt-5.4 or newer (5.4, 5.5, 5.6, etc., including pro)."""
+        """Check if the model is gpt-5.4 or newer (5.4, 5.5, 5.6, etc., including pro).
+
+        Handles custom deployment names with prefixes (e.g. Azure deployments
+        like ``my-prefix-gpt-5.5``) by locating ``gpt-5.`` inside the name.
+        """
         model_name = model.split("/")[-1]
-        if not model_name.startswith("gpt-5."):
+        idx = model_name.find("gpt-5.")
+        if idx == -1:
             return False
+        model_name = model_name[idx:]
         try:
             version_str = model_name.replace("gpt-5.", "").split("-")[0]
             major = version_str.split(".")[0]
diff --git a/tests/test_litellm/llms/openai/test_gpt5_transformation.py b/tests/test_litellm/llms/openai/test_gpt5_transformation.py
index d279b119efe..ad0658615bd 100644
--- a/tests/test_litellm/llms/openai/test_gpt5_transformation.py
+++ b/tests/test_litellm/llms/openai/test_gpt5_transformation.py
@@ -342,6 +342,27 @@ def test_gpt5_4_pro_allows_reasoning_effort_xhigh(config: OpenAIConfig):
     assert params["reasoning_effort"] == "xhigh"
 
 
+def test_is_model_gpt_5_4_plus_model_prefixed_deployment_names(
+    gpt5_config: OpenAIGPT5Config,
+):
+    """Custom deployment names with prefixes (e.g. Azure) are recognized as gpt-5.4+."""
+    assert gpt5_config.is_model_gpt_5_4_plus_model("my-prefix-gpt-5.5")
+    assert gpt5_config.is_model_gpt_5_4_plus_model("azure/something-something-gpt-5.5")
+    assert gpt5_config.is_model_gpt_5_4_plus_model("my-prefix-gpt-5.4-pro")
+    assert gpt5_config.is_model_gpt_5_4_plus_model("gpt-5.4")
+    assert gpt5_config.is_model_gpt_5_4_plus_model("openai/gpt-5.6")
+
+
+def test_is_model_gpt_5_4_plus_model_rejects_non_5_4_plus_names(
+    gpt5_config: OpenAIGPT5Config,
+):
+    """Names below 5.4 or merely containing gpt-5 without a version must not match."""
+    assert not gpt5_config.is_model_gpt_5_4_plus_model("my-prefix-gpt-5.2")
+    assert not gpt5_config.is_model_gpt_5_4_plus_model("my-gpt-5-deployment")
+    assert not gpt5_config.is_model_gpt_5_4_plus_model("gpt-5")
+    assert not gpt5_config.is_model_gpt_5_4_plus_model("azure/prefix-gpt-5.x")
+
+
 def test_gpt5_4_mini_allows_reasoning_effort_xhigh(config: OpenAIConfig):
     """gpt-5.4-mini supports reasoning_effort='xhigh'."""
     params = config.map_openai_params(