JSONL Validator
Validate a JSONL (JSON Lines) file line by line.
Processed locally in your browser. Nothing you paste is sent anywhere.
- Pinpoints the exact line number and parse error, not just "invalid file".
- Optional schema mode catches the field-level mistakes that actually cause most fine-tuning upload rejections — a missing `messages` array, an invalid `role`, or non-string `content` — not just JSON syntax errors.
- Checks each line independently, so one bad row doesn't hide the rest.
- Runs in your browser — training data isn't uploaded.
Why this is private
- Processed locally in your browser. Nothing you paste is sent anywhere.
- Open your browser's DevTools → Network tab while using this tool — you won't see a request carrying your input, because it never leaves your device.
- Nothing you enter is uploaded to a server or shared with us — the processing happens entirely on your device.
Overview
Validates a JSONL (JSON Lines) file — one JSON value per line — commonly used for fine-tuning datasets and log files. Reports exactly which lines fail to parse, with an optional mode that also checks each line against OpenAI's chat fine-tuning schema.
How to use it
- 1Paste JSONL content, one JSON object per line.
- 2Pick "Syntax only" or "OpenAI chat fine-tuning schema" mode.
- 3Invalid lines are listed with their line number and error.
Frequently asked questions
What people use it for
Debug a rejected fine-tuning uploadSwitch to schema mode to catch a missing messages array or bad role value, not just malformed JSON.
Check an exported datasetConfirm every record is valid JSON before starting a training run.
Validate a JSONL log streamSpot lines that were truncated or double-written.
Review a hand-edited datasetCatch a trailing comma or unquoted key introduced during edits.
Related tools
All ai & llm toolsJSONL FormatterFormat or minify a JSONL file, with malformed lines isolated instead of failing the whole file.JSON ValidatorValidate JSON syntax and pinpoint errors instantly.Token CounterExact token counts for OpenAI models, plus calibrated per-model estimates for Claude, Llama, Gemini and Mistral.Context Window CalculatorSee how much of a model's context window your text uses.
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JSONL Validator