JSONL (JSON Lines) Validator & Converter

What is JSONL?

JSON Lines (also called NDJSON) is a format where each line is a complete JSON value, usually an object. It's the standard for streaming logs, machine-learning training data and LLM conversation dumps — because you can append a line without rewriting the file, and process records one at a time.

Why line numbers matter

A 100,000-line dataset with one malformed line is a debugging nightmare if all you get is "unexpected token at position 3123912". This tool reports the exact line number of the error plus the parser's message, so you can jump straight to the bad record.

Common uses

• LLM fine-tuning: conversation datasets (ChatML/ShareGPT style) are JSONL with messages arrays.
• Log pipelines: app logs emitted as one JSON object per line for easy grep and replay.
• Migration: convert a JSONL export into a JSON array to load into tools that expect an array (or the reverse).
• Schema checks: the inspect mode shows each field's type across your rows, catching mixed types (a user_id that's sometimes a string, sometimes a number) before they bite.

Notes

Empty lines are ignored. If a line parses as a non-object (e.g. a bare number), conversion to array still works — but the schema view focuses on object rows. Everything runs locally; nothing is uploaded.