According to the official Microsoft documentation for preparing data for fine-tuning Azure OpenAI models, the training and validation files must be in JSONL format. A critical requirement for these files is that they must be encoded in UTF-8 with a Byte Order Mark (BOM). This standard ensures that the wide range of characters and symbols potentially present in chat data from various languages are correctly interpreted by the fine-tuning process. Using any other encoding format will lead to a validation error when you attempt to upload the data.

B. UTF-16: The Azure OpenAI fine-tuning service explicitly requires UTF-8 encoding. Submitting a file with UTF-16 encoding will fail the data validation step.
C. ISO-8859-1: This is a limited, single-byte encoding that cannot represent the full range of Unicode characters required for modern language models. It is not the specified format.
D. ASCII: ASCII is a 7-bit character set and a subset of UTF-8. It is insufficient for handling multilingual data or special characters, and the service specifically mandates UTF-8.

1. Microsoft Learn. (2024). Customize a model with fine-tuning. Azure OpenAI Service Documentation.

Section: Prepare training and validation data

Quote: "The training and validation data files must be JSONL documents

where each line is a JSON object representing a single training example. Each file must be encoded in UTF-8 with a Byte Order Mark (BOM)."

2. Microsoft Learn. (2024). Fine-tuning tutorial. Azure OpenAI Service Documentation.

Section: Prepare data for fine-tuning

Quote: "All training files must be uploaded in JSONL format and encoded in UTF-8 with a Byte Order Mark (BOM)." This is consistently stated in the step-by-step guides for preparing data.