JSONL Viewer Online
Drop a .jsonl, .ndjson or .jsonl.gz file to see it as a table with one column per key. Sort, find text, list the bad lines, see the values of any key and read chat records as a chat. No account and no upload.
JSONL Viewer, Large JSONL Table & NDJSON Viewer for VS Code and Cursor, free
- Click a .jsonl, .ndjson or .jsonl.gz file in VS Code: files up to 4 GB stream in, and the first page shows in about 20 ms.
- Filters such as level == error and ctx.ms > 500, a Columns menu, Copy JSONPath, and live tail that follows a growing log.
- Chat view for OpenAI, ShareGPT and Gemini training files, and export of the rows you see to CSV, JSON or JSON Lines.
Optional Zovo Pro, USD 19 once: check a fine-tuning dataset, and split a file into train and validation sets. 3 free tries for each.
Drop a .jsonl or .ndjson file here
or
JSON Lines and NDJSON (.jsonl, .ndjson, .jsonlines), packed .jsonl.gz, and logs saved as .json, .log or .txt with one JSON value per line.
| Key | Types | Records | Distinct |
|---|
Click a key to see its values
Click a column title to sort (up, down, file order). Click a row to open the record.
Open this in VS Code or Cursor
The free JSONL Viewer extension opens a .jsonl file with a click inside VS Code and Cursor. It streams files up to 4 GB, so a 40 MB file shows its first page in about 20 ms, and it adds filters such as level == error and ctx.ms > 500, a Columns menu, export of the rows you see to CSV, JSON or JSON Lines, and live tail of a growing log.
Free, MIT license. About JSONL Viewer for VS Code.
Zovo Pro
Check a fine-tuning dataset and split it into train and validation. Zovo Pro, USD 19 once.
In the JSONL Viewer extension, Zovo Pro checks every line of a fine-tuning file the way the training job reads it (roles, empty content, missing answers, duplicates, token limit) with a report that links each problem line, and splits a file into train and validation sets or parts with a fixed seed. Each Pro tool works 3 times for free. One payment covers the Pro tools in 22 Zovo extensions. Everything that is free stays free.
After a card payment you get a key on the page. With GitHub Sponsors you sign in from the extension. Details on the Zovo Pro page.
Related tools
- XLSX, Excel, CSV & Parquet Viewer on the VS Code Marketplace (also on Open VSX). Opens xlsx, xls, csv and parquet files as a sortable table with SQL filters.
- Large JSON Viewer & JSON Tree. Shows large JSON files as a searchable tree in VS Code.
- Agent Session Viewer. Opens Claude Code and Codex session files as a readable transcript.
- Excel, CSV and Parquet Viewer. Opens xlsx, csv and parquet files as a sortable table in VS Code.
- HAR Viewer Online, free in your browser.
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Questions
Is my file uploaded anywhere?
No. The page reads the file inside this tab. Its Content Security Policy sets connect-src to none, so the page cannot send your data anywhere, and it loads no library. Training data and logs often hold private text, so you can check it: open the Network tab of your browser tools and you will see no request when a file opens.
What is a JSONL file?
JSON Lines, also called NDJSON: one JSON value per line, usually an object. Logs, dataset exports, eval results and fine-tuning files for OpenAI, Gemini and other models use it. Each line stands alone, so one bad line does not break the rest.
How are the columns made?
One column per top-level key, from the first 1,000 records. Each title shows the type (string, int, float, bool, object, array or mixed) and how many of those records fill that key. Nested values show as JSON in the cell. Click a row to see the whole record.
What about lines that are not valid JSON?
They stay in the table in red with their line number and the parser message, and the rest of the file opens. The facts at the top count them, and Only bad lines shows just those.
Can it read chat and fine-tuning files?
Yes. A record with messages (OpenAI), conversations (ShareGPT), contents (Gemini) or prompt and completion opens as a chat above its JSON, with tool calls shown in their own box.
How big a file can it open?
The page reads the whole file into the tab, so files of a few hundred MB work on a normal computer. A .jsonl.gz file is unpacked up to 512 MB. For files of many GB, the free VS Code extension streams them and keeps only the lines on screen in memory.
What is free and what is Zovo Pro?
Everything on this page is free. In the JSONL Viewer extension the table, record and chat views, find, filters, sort, export and live tail are free too. Checking a fine-tuning dataset and splitting a file into train and validation sets are Zovo Pro, with 3 free tries each. One USD 19 payment covers the Pro tools in every Zovo extension.