diff --git a/litellm/llms/vertex_ai/vertex_ai_partner_models/anthropic/experimental_pass_through/transformation.py b/litellm/llms/vertex_ai/vertex_ai_partner_models/anthropic/experimental_pass_through/transformation.py
index 5d338198c19..8566496bf9c 100644
--- a/litellm/llms/vertex_ai/vertex_ai_partner_models/anthropic/experimental_pass_through/transformation.py
+++ b/litellm/llms/vertex_ai/vertex_ai_partner_models/anthropic/experimental_pass_through/transformation.py
@@ -49,17 +49,15 @@ class VertexAIPartnerModelsAnthropicMessagesConfig(AnthropicMessagesConfig, Vert
         )
         headers["Authorization"] = f"Bearer {access_token}"
 
-        # Calculate api_base if not provided
-        if api_base is None:
-            api_base = self.get_complete_vertex_url(
-                custom_api_base=api_base,
-                vertex_location=vertex_ai_location,
-                vertex_project=vertex_ai_project,
-                project_id=project_id or "",
-                partner=VertexPartnerProvider.claude,
-                stream=optional_params.get("stream", False),
-                model=model,
-            )
+        api_base = self.get_complete_vertex_url(
+            custom_api_base=api_base,
+            vertex_location=vertex_ai_location,
+            vertex_project=vertex_ai_project,
+            project_id=project_id or "",
+            partner=VertexPartnerProvider.claude,
+            stream=optional_params.get("stream", False),
+            model=model,
+        )
 
         headers["content-type"] = "application/json"
 
diff --git a/tests/test_litellm/llms/vertex_ai/vertex_ai_partner_models/anthropic/test_vertex_ai_partner_models_anthropic_messages_config.py b/tests/test_litellm/llms/vertex_ai/vertex_ai_partner_models/anthropic/test_vertex_ai_partner_models_anthropic_messages_config.py
index d64b5c8d742..bfd37f73b2d 100644
--- a/tests/test_litellm/llms/vertex_ai/vertex_ai_partner_models/anthropic/test_vertex_ai_partner_models_anthropic_messages_config.py
+++ b/tests/test_litellm/llms/vertex_ai/vertex_ai_partner_models/anthropic/test_vertex_ai_partner_models_anthropic_messages_config.py
@@ -266,6 +266,73 @@ def test_validate_environment_always_refreshes_token_ignoring_stale_bearer():
         assert api_base == "https://mock-vertex-url"
 
 
+def test_validate_environment_appends_stream_raw_predict_with_custom_api_base():
+    """Regression: a custom api_base on /v1/messages must still get the endpoint
+    suffix appended. The old `if api_base is None` guard skipped
+    get_complete_vertex_url entirely, leaving the api_base without
+    `:streamRawPredict`."""
+    config = VertexAIPartnerModelsAnthropicMessagesConfig()
+    litellm_params = {
+        "vertex_ai_project": "test-project",
+        "vertex_ai_location": "us-central1",
+    }
+
+    with (
+        patch.object(
+            config,
+            "get_complete_vertex_url",
+            wraps=config.get_complete_vertex_url,
+        ) as spy_get_url,
+        patch.object(
+            config, "_ensure_access_token", return_value=("token", "test-project")
+        ),
+    ):
+        _, api_base = config.validate_anthropic_messages_environment(
+            headers={},
+            model="claude-sonnet-4",
+            messages=[],
+            optional_params={"stream": True},
+            litellm_params=litellm_params,
+            api_base="https://my-proxy.example.com",
+        )
+
+    spy_get_url.assert_called_once()
+    assert api_base is not None
+    assert ":streamRawPredict" in api_base
+
+
+def test_validate_environment_appends_raw_predict_with_custom_api_base():
+    """Regression: non-streaming custom api_base must end with `:rawPredict`."""
+    config = VertexAIPartnerModelsAnthropicMessagesConfig()
+    litellm_params = {
+        "vertex_ai_project": "test-project",
+        "vertex_ai_location": "us-central1",
+    }
+
+    with (
+        patch.object(
+            config,
+            "get_complete_vertex_url",
+            wraps=config.get_complete_vertex_url,
+        ) as spy_get_url,
+        patch.object(
+            config, "_ensure_access_token", return_value=("token", "test-project")
+        ),
+    ):
+        _, api_base = config.validate_anthropic_messages_environment(
+            headers={},
+            model="claude-sonnet-4",
+            messages=[],
+            optional_params={},
+            litellm_params=litellm_params,
+            api_base="https://my-proxy.example.com",
+        )
+
+    spy_get_url.assert_called_once()
+    assert api_base is not None
+    assert api_base.endswith(":rawPredict")
+
+
 def test_transform_anthropic_messages_request_removes_scope_from_cache_control():
     """Ensure scope field is removed from cache_control for Vertex AI (not supported)."""
     config = VertexAIPartnerModelsAnthropicMessagesConfig()