What is a JSONL file?
JSONL — JSON Lines
Use the format JSONL
A concrete example
Prepare a CSV from a JSONL collection with homogeneous fields.
Useful checks
Objects may have different keys. Check for missing columns and how lists are represented.
Choose a conversion
Keep the original and open the result in the software or on the device that will need to use it.
Your questions about this format
Why does my JSON change structure when converted to CSV?
CSV represents a table of rows and columns. JSON can contain objects and nested lists that do not transpose directly. Convert a copy, check how the fields are represented, and keep the JSON if your usage requires retaining its entire structure.
How can one avoid losing the leading zeros in identifiers?
When opening the CSV in a spreadsheet, import the identifier columns as text. Automatic opening can turn 00123 into 123. Also check the dates and decimal separators before re-saving.
Can I convert a file containing secrets?
Remove passwords, API keys, and other secrets before sending a sample to the service or support. File processing is done online. For data that must not leave your device, use a suitable local tool.
Why aren't all outputs offered?
Formats are not interchangeable in all cases. ConvertLab displays the conversions from its catalog; renaming an extension does not change the content. Specialized outputs like certain XML exports or spreadsheets are identified as NoLimit.
What is it?
JSONL refers to exactly the same thing as NDJSON: one JSON object per line. Two names, two communities, a single format — worth knowing before you waste time hunting for a difference that isn't there.
Usage has settled the matter in practice: people tend to say JSONL in machine learning circles, and NDJSON in the world of application logs and data streams.
History & origin
The .jsonl extension spread alongside the explosion of datasets for artificial intelligence, where each line represents a single training example.
It's now the format expected by most model fine-tuning tools: one line, one sample.
Why it is useful
It lets you process datasets several gigabytes in size without ever loading the whole thing into memory.
Adding data simply means appending lines to the end of the file — no rewriting required.
Limits & drawbacks
It can't be read by tools that expect standard JSON.
The lack of an enclosing structure makes it impossible to validate the file as a whole.
Practical tips
JSONL is the format to use for a training dataset or any stream-based processing.
Convert it to CSV to inspect it in a spreadsheet, or to JSON if a tool requires a single array — but watch out for memory usage if the file is large.
Convert your JSONL in seconds, on our secure servers in France.
Convert a JSONL