jsonl.co

JSONL puts one JSON object on each line, which is why it shows up in log exports, dataset dumps and model fine-tuning files. It's also why ordinary JSON viewers choke on it, and why a multi-gigabyte log is awkward to open in a text editor. jsonl.co is built for exactly that file. Drop in a .jsonl, .ndjson, .json or .txt file, or paste some lines, and it streams the content with syntax highlighting and validates each line as it goes.

The headline numbers are files up to 1 GB, no sign-up, and no upload. The site says your data never leaves your device, which matters if the file is a production log or a training set you can't share.

The viewer is the main tool, but the site has a full set of small helpers next to it. You can turn JSONL into JSON, CSV, YAML or Excel and back, format or validate a file line by line, flatten nested JSON, and split one large file into several or merge several into one. There's a sample data button if you just want to see how the viewer behaves, and the FAQ covers common cases like checking an OpenAI fine-tuning file.

If you'd rather not leave your editor, jsonl.co also offers a VS Code extension, linked from the top of the site. The guides section explains JSON versus JSONL for anyone meeting the format for the first time. For a quick look inside a large line-delimited file, or a format conversion you don't want to script, it covers the job without an account.