diff --git a/backend/open_webui/main.py b/backend/open_webui/main.py
index d959351d4c..73d0c075e1 100644
--- a/backend/open_webui/main.py
+++ b/backend/open_webui/main.py
@@ -1954,7 +1954,10 @@ async def generate_messages(
     # Convert Anthropic payload to OpenAI format
     requested_model = form_data.get('model', '')
 
-    openai_payload = convert_anthropic_to_openai_payload(form_data)
+    try:
+        openai_payload = convert_anthropic_to_openai_payload(form_data)
+    except (ValueError, NotImplementedError) as e:
+        raise HTTPException(status_code=400, detail=str(e))
 
     # Route through the existing chat_completion handler
     response = await chat_completion(request, openai_payload, user)