update

Author: mrbullwinkle

articles/ai-services/openai/includes/chat-completion.md

@@ -549,7 +549,11 @@ The token counting portion of the code demonstrated previously is a simplified v
 
 ### Do not use ChatML syntax with the Chat Completions endpoint
 
-We have found that some customers will try using the [legacy ChatML syntax](./chat-markup-language.md) with the chat completion endpoints and newer models. ChatML was a preview capability that only worked with the legacy completions endpoint with the `gpt-35-turbo` version 0301 model which is [slated for retirement](../concepts/model-retirements.md). Attempting to use ChatML syntax with newer models and the chat completions endpoint can result in unexpected model response behavior and is not recommended.  
+We have found that some customers will try using the [legacy ChatML syntax](./chat-markup-language.md) with the chat completion endpoints and newer models. ChatML was a preview capability that only worked with the legacy completions endpoint with the `gpt-35-turbo` version 0301 model which is [slated for retirement](../concepts/model-retirements.md). Attempting to use ChatML syntax with newer models and the chat completions endpoint can result errors as well as unexpected model response behavior, and is not recommended.  
+
+| Error |Cause | Solution |
+|---|---|---|
+| 400 - *Failed to generate output due to special tokens in the input.*  | Your prompt contains legacy ChatML tokens not recognized or supported by the model/endpoint. | Ensure that your prompt/messages array does not contain any legacy ChatML tokens. If you are upgrading from a legacy model, please exclude all special tokens before submitting an API request to the model.|
 
 ## Next steps