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JSONL Validator

Validate a JSONL file line by line, and check fine-tuning datasets before you upload.

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Or drop a .jsonl file — it is read in your browser, never uploaded

Quick Tool Overview

The JSONL Validator parses every line of a JSON Lines file on its own, reports each broken line by number, checks rows against OpenAI chat, ShareGPT, Alpaca, completion or Gemini fine-tuning formats, and offers a cleaned file with only the valid rows.

What it does
Validates JSONL files and fine-tuning datasets
You give it
A .jsonl file or pasted JSON Lines
You get back
Errors by line number, statistics and a cleaned file
Best for
Checking training data before an upload fails
AI Tools Free, unlimited use No sign-up Runs in your browser

About the JSONL Validator

A JSON Lines file holds one JSON object per line, and a single broken line is enough for a fine-tuning upload to fail with a message that rarely says which one. An ordinary JSON validator cannot help, because the file as a whole is not JSON.

Paste the file or drop it in, and every line is parsed on its own. Errors are listed by line number with the offending text, whether they are broken JSON — a trailing comma, an unescaped quote — or rows that parse but do not match the training format. The format is detected from the first row, or you can choose it: OpenAI chat messages, ShareGPT conversations, Alpaca instruction and output, prompt and completion, or Gemini contents. Unknown roles, missing content and examples with no assistant turn are all caught.

The report also flags exact duplicate rows, rows whose keys differ from the rest, blank lines and a whole-file JSON array saved with a .jsonl name. Token estimates show the size of one training epoch, and a cleaned file with only the valid rows can be downloaded in one click.

Key features of the JSONL Validator

Exact line numbers

Every invalid line is listed with its number, the parser message and the start of the line.

Training format rules

Roles, content, assistant turns and weights are checked for five common trainer formats.

Dataset hygiene checks

Duplicates, mismatched keys, blank lines and JSON arrays saved as JSONL are all caught.

Token estimates

Average and largest example, and the tokens in one training epoch.

Cleaned download

One click gives a file containing only the rows that passed.

How to use the JSONL Validator

  1. 1

    Add your file

    Paste JSONL into the box or drop a .jsonl file. Large files are read without being uploaded.

  2. 2

    Choose the format

    Leave it on detect, or pick the training format you are about to upload to.

  3. 3

    Fix the listed lines

    Each problem shows its line number, the reason and the start of the line.

  4. 4

    Download the clean file

    Get a file with only the rows that passed, one object per line.

Frequently asked questions

JSON Lines: a text file with one complete JSON object on each line and no surrounding brackets or commas. It is the standard format for fine-tuning datasets and batch API requests.

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