What output formats are supported?

Two — the modern chat format with a messages array (optional system, plus user and assistant turns), and the classic prompt/completion pair. Pick the one your fine-tuning provider expects.

Does it handle quoted CSV fields?

Yes. The parser respects double-quoted fields, escaped quotes ("") inside them, and commas or newlines within quotes, so messy exports convert cleanly.

What is JSONL and why fine-tuning uses it?

JSONL (JSON Lines) puts one JSON object per line. Fine-tuning APIs stream the file line by line, so each line is one independent training example — exactly one conversation or prompt/completion pair.

Is my data uploaded anywhere?

No. Parsing, conversion, and download all happen in your browser. Nothing is sent to a server or stored.

CSV → JSONL Converter for LLM Training

Convert a CSV dataset into fine-tuning JSONL

The CSV → JSONL Converter maps the columns of a CSV dataset onto the JSON Lines format that LLM fine-tuning APIs expect. Choose between the modern chat messages format and the classic prompt/completion pair, map your columns, preview the result, and download a clean .jsonl file.

How it works

The tool parses your CSV with a quote-aware parser, so commas, newlines, and escaped quotes inside fields are handled correctly. You then map each output field to a CSV column. In chat mode it builds a messages array with an optional system message and the user/assistant turns; in prompt/completion mode it emits { "prompt": ..., "completion": ... }. Each CSV row becomes exactly one line of the output file — the structure fine-tuning APIs read one example at a time.

Tips and notes

Keep one training example per row; if a conversation spans multiple turns, model it as separate columns rather than multiple rows. For OpenAI-style chat fine-tuning, include a consistent system message to anchor behaviour. Always spot -check the preview — a wrong column mapping silently produces a syntactically valid but useless dataset. Download is generated in-browser via a Blob, so your data never leaves the page.