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JSONL Validator for AI Training Data
Validate JSONL files line by line: find JSON syntax errors, missing messages/roles, empty content and duplicates before fine-tuning.
🔒 Processed locally in your browser — we don’t upload your input.
How to use the JSONL Validator
A single malformed line can make a fine-tuning job fail after you have already uploaded your file. This validator checks JSON Lines data in your browser before you submit it. Paste the contents or open a .jsonl file and every line is parsed individually, so errors are reported with their exact line number rather than as one confusing message.
Beyond basic JSON syntax, the validator understands common training schemas. For chat-format data it checks that each line has a messages array, that every message has a valid role (system, user, assistant or tool) and non-empty content, and that each example ends with an assistant reply the model can learn from. For prompt/completion data it checks both fields exist. It also flags exact duplicate lines and reports dataset statistics: number of examples, average estimated tokens per example and the total, which helps you estimate training cost.
Fix the reported lines, re-run the check until it is clean, then split your data with the Dataset Splitter. Files are read locally using the browser’s File API, so even large private datasets are never uploaded to EasyTool.
Frequently asked questions
What does it check?
JSON syntax per line, messages/role/content structure, prompt/completion fields, empty values and duplicate lines.
Is there a file size limit?
It depends on your browser memory; files of tens of megabytes usually work fine.
Does it fix errors automatically?
No, it reports each problem with its line number so you can fix it accurately.
Is my file uploaded?
No, it is read and checked locally.
About this tool
Validate JSONL files line by line: find JSON syntax errors, missing messages/roles, empty content and duplicates before fine-tuning. People also look for: jsonl validator, validate jsonl, fine tuning data checker, jsonl lint.
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