JSONL Formatter & Validator

Q: What makes a JSONL file invalid?

Every non-blank line must be a complete JSON value on its own. Common problems are a JSON array or pretty-printed object spread across several lines, trailing commas, single quotes instead of double quotes, unescaped newlines inside strings, and commas between records. The validator lists each bad line with its line number and the parser error.

Q: Are blank lines allowed in JSONL?

The JSON Lines spec does not define blank lines, and strict consumers (for example some bulk loaders) reject them. This tool tolerates and counts them, and the Compact output removes them so the result is safe to load anywhere.

Q: What is the difference between JSONL and NDJSON?

Practically none. JSON Lines (.jsonl) and Newline Delimited JSON (.ndjson) both mean one JSON value per line separated by \n, UTF-8 encoded. The names come from two specs that describe the same format, so files are interchangeable.

Q: Can I validate an OpenAI fine-tuning or Batch API .jsonl file here?

Yes for syntax: the formatter confirms every line is valid JSON and shows the top-level keys (such as messages, custom_id, method, url, body) in the table preview so you can spot missing fields. It does not check OpenAI-specific rules like token limits.

Q: How large a file can I open?

Files up to 50 MB can be uploaded. Everything is processed locally in your browser; the table preview is capped to the first 25–500 records and the error list to the first 50 problems so the page stays responsive on files with hundreds of thousands of lines.

Q: Is my data uploaded to a server?

No. Parsing, formatting and the preview all run in JavaScript in your browser tab; nothing is sent over the network.

Paste or upload a .jsonl / .ndjson file to validate every line, see exactly which lines are broken and why, pretty-print or normalize the records, and preview them as a table. Runs 100% in your browser — nothing is uploaded.

JSONL input

Normalized JSONL

Compact

Pretty

Drop invalid lines

Invalid lines found

4 valid

1 invalid

1 blank

6 lines

1 invalid line

Line 5: Quoted object key expected but got '}' at position 138

{"id":4,"ts":"2026-11-02T09:18:55Z","level":"error","event":"payment.failed","user":{"id":"u_2210","plan":"team"},"error":"card_declined",}

Table preview

First

records

Line	id	ts	level	event	user	tags	amount	attempt
1	1	2026-11-02T09:14:03Z	info	user.signup	{"id":"u_1042","plan":"free"}	["web","eu"]
2	2	2026-11-02T09:15:47Z	info	user.upgrade	{"id":"u_1042","plan":"pro"}		12.5
4	3	2026-11-02T09:16:10Z	warn	payment.retry	{"id":"u_2210","plan":"team"}			2
6	5	2026-11-02T09:21:31Z	info	user.login	{"id":"u_0007","plan":"free"}	["ios"]

Showing 4 of 4 valid records.

Need a different shape? Convert with the JSON array ⇄ JSONL converter or the CSV ⇄ JSONL converter; for a single JSON document use the JSON formatter.

How to validate a JSONL file

Paste your JSON Lines or click Upload to open a .jsonl / .ndjson file.
Check the badges: every non-blank line must be one complete JSON value.
Fix the lines listed under invalid lines — each entry shows the line number and the parser error.
Copy or download the normalized JSONL (one minified record per line, blank lines removed).

What is JSONL (JSON Lines / NDJSON)?

JSON Lines — also called NDJSON (newline-delimited JSON) — is a text format where every line is one complete, self-contained JSON value, usually an object. There are no commas between records and no surrounding [ ]. Files typically use the .jsonl or .ndjson extension.

{"id": 1, "event": "signup", "plan": "free"}
{"id": 2, "event": "upgrade", "plan": "pro"}

JSONL vs a JSON array

A JSON array must be parsed as a whole, so a single missing bracket breaks the entire file. JSONL can be streamed, appended to, split and processed line by line (head, grep, wc -l), and a corrupt line only affects that one record. Use a JSON array for small API payloads and config; use JSONL for large datasets, logs and anything written incrementally.

Where JSONL is used

OpenAI fine-tuning datasets and Batch API input/output files (one request or example per line).
BigQuery, Snowflake, Redshift and Athena bulk loads (NEWLINE_DELIMITED_JSON).
Structured logs from Docker, Kubernetes, Bunyan, Pino and most log shippers.
Elasticsearch / OpenSearch bulk API, MongoDB mongoexport, Hugging Face datasets.
Data pipelines with jq -c, Spark, pandas read_json(lines=True) and DuckDB.

All JSONL tools on DevToolsDaily run 100% in your browser — your data is never uploaded. See also the JSONL formatter, JSON ⇄ JSONL converter and CSV ⇄ JSONL converter.

JSONL Formatter & Validator

Table preview

How to validate a JSONL file

What is JSONL (JSON Lines / NDJSON)?

JSONL vs a JSON array

Where JSONL is used

FAQ

What makes a JSONL file invalid?

Are blank lines allowed in JSONL?

What is the difference between JSONL and NDJSON?

Can I validate an OpenAI fine-tuning or Batch API .jsonl file here?

How large a file can I open?

Is my data uploaded to a server?

See also