JSONL Formatter
Format or minify a JSONL file, with malformed lines isolated instead of failing the whole file.
Processed locally in your browser. Nothing you paste is sent anywhere.
- Pretty-prints each record so you can actually read a JSONL file.
- Minifies back to strict one-object-per-line form that upload APIs expect.
- A malformed line is preserved as-is and reported, not dropped — one bad record in a large dataset never costs you the rest of the file.
- Runs in your browser — no upload.
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
Formats each line of a JSONL file for readability, or minifies it back to compact one-object-per-line form for upload. A malformed line doesn't stop the whole file — it's left untouched in place and reported by line number, so real-world datasets with a few bad records still process cleanly.
How to use it
- 1Paste JSONL content.
- 2Choose Format (readable) or Minify (compact).
- 3Fix any malformed lines listed below the output, if any.
- 4Copy or download the result.
Frequently asked questions
What people use it for
Inspect a fine-tuning datasetExpand each line to check the structure of a few records.
Find bad records in a large exportProcess a file with thousands of lines and get the exact line numbers of any that don't parse.
Prepare a file for uploadMinify a hand-edited, pretty-printed file back to valid JSONL.
Diff two datasets cleanlyFormat both so a line-by-line comparison is readable.
Tidy a generated exportNormalise inconsistent spacing across rows.
Related tools
All ai & llm toolsJSONL ValidatorValidate a JSONL (JSON Lines) file line by line.JSON Viewer & FormatterView, format, validate, edit and beautify JSON with an interactive tree view, plus a shareable link.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 Formatter