JSONL validator
Validates JSON Lines (JSONL/NDJSON) line by line in your browser. Shows errors, statistics and exports cleaned files.
One JSON object per line, for example {"name":"Alice","score":95}. You can also open a .jsonl, .ndjson or .txt file via "Load file"; checking runs only in your browser.
How this tool works
The JSONL validator checks JSON Lines (JSONL/NDJSON) line by line directly in the browser. Each line is validated against JSON syntax individually — invalid lines are flagged with position and error message. The tool shows statistics (valid/invalid lines, found keys) and offers export of a cleaned file without invalid lines. JSONL is the standard format for LLM fine-tuning (OpenAI, Anthropic, Google), monitoring exports and streaming APIs (OpenAPI 3.2).
Typical use cases
- Validate fine-tuning datasets for OpenAI/Claude/Gemini before upload
- Check monitoring exports (uptime, analytics, alerts) for completeness
- Scan NDJSON streaming responses from an API for broken lines
- Remove invalid lines from large JSONL files and export cleaned version
- Analyze key structure of a JSONL file without fully parsing it
Frequently asked questions
What is the difference between JSONL and NDJSON?
No functional difference — JSONL (JSON Lines) and NDJSON (Newline Delimited JSON) describe the same format: one JSON object per line, separated by newline. The file extensions .jsonl and .ndjson are interchangeable.
Is my data sent to a server?
No. Validation runs entirely in the browser. Your data never leaves your computer — not even for sensitive fine-tuning data or API exports.
How large can the JSONL file be?
In the browser, files up to about 10 MB work fine. For larger files, we recommend jq (command line) or a stream parser in Python/Node.js.
Can I export only valid lines?
Yes. When invalid lines are found, two buttons appear: "Download cleaned .jsonl" saves only valid lines as a file, "Copy valid lines" puts them in your clipboard.
Does the validator detect empty lines?
Empty lines are skipped — that is JSONL-compliant. Only non-empty lines with invalid JSON are flagged as errors.
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