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JSONL Validator & Training-File Checker

Validate JSON Lines line by line, check chat-format structure, and estimate what training on the file would cost.

This validator checks a JSON Lines file line by line, reporting the exact line number and cause of every parse failure, then applies the structural rules for chat training formats: role alternation, required first and last turns, and empty content. It also counts tokens per row, flags rows that exceed the training context, and reports which top-level keys are missing from which rows.

Runs in your browser
Privacy
Runs entirely in your browser — nothing is uploaded
Cost
Free, unlimited, no sign-up

Frequently asked questions

Why does my file fail with "Unexpected end of JSON input"?

Almost always because the JSON was pretty-printed. JSONL requires each record to be one complete JSON value on one line — a multi-line object is valid JSON and invalid JSONL. Minify each record before writing it.

Does a valid file mean training will work?

It means the file will be accepted. Whether the content teaches anything useful is a different question, and no validator can answer it. Consistent formatting, enough examples, and assistant turns that actually demonstrate the behaviour you want are what decide that.

How to use the jsonl validator

  1. 1Enter or paste your jsonl.
  2. 2Choose the check as.
  3. 3Set the warn over.
  4. 4Choose the price against.
  5. 5Press Run, then download the result when it is ready.

Sources & specifications

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Put the working jsonl validator on your own site. It runs in your visitors' browsers exactly as it does here — free, no account, nothing uploaded.

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