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JSONL Fine-Tuning Dataset Builder
Convert CSV or prompt/response pairs into chat-format JSONL for fine-tuning OpenAI, Mistral or Llama models, with an optional system message.
🔒 Processed locally in your browser — we don’t upload your input.
How to use the JSONL Dataset Builder
Fine-tuning a language model starts with training data in the right format, and most platforms expect JSON Lines (JSONL): one JSON object per line, usually in the chat format with a messages array of system, user and assistant turns. Building that by hand is tedious and error-prone. This builder converts simple input into valid JSONL in seconds.
Paste rows as CSV with two columns (prompt and completion) or as tab-separated pairs, choose whether the first row is a header, and optionally add a system message that will be included in every example. Choose the output style: chat messages format, used by OpenAI and many open-source trainers such as Axolotl and Unsloth, or the simple prompt/completion format used by some older and custom pipelines. Quotes, commas inside quoted fields and line breaks are handled correctly, and every line is properly JSON-escaped.
Download the result as a .jsonl file, then check it with the JSONL Validator and split it into training and validation sets with the Dataset Splitter. Because conversion runs in your browser, proprietary support tickets or internal documents used as training examples are never uploaded.
Frequently asked questions
Which format does OpenAI fine-tuning use?
The chat format: each line has a messages array with role and content fields.
Can I include a system prompt?
Yes, it is added as the first message of every example.
Does it handle commas and quotes in CSV?
Yes, standard quoted CSV fields are parsed correctly.
How many examples do I need?
Providers often suggest starting with 50–100 high-quality examples and scaling up.
About this tool
Convert CSV or prompt/response pairs into chat-format JSONL for fine-tuning OpenAI, Mistral or Llama models, with an optional system message. People also look for: jsonl generator, fine tuning dataset, openai fine-tune jsonl, csv to jsonl.
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