JSON Validator Online 2026 - Real-time JSON Syntax Checker
Validate JSON data against standards. Get exact line numbers and detailed error messages for syntax violations in your JSON files. 100% Private & Free.
Key Features
- ✅ Real-time validation
- ✅ Error line highlighting
- ✅ Detailed error reports
- ✅ Schema support
- ✅ Auto-fix common errors
How to Use
- Load your JSON via paste or upload
- The validator runs automatically on every change
- Look for the red error indicators in the gutter
- Hover over errors to see the specific violation message
- Fix the highlighted issues until the "Valid" indicator appears
Expert FAQ
- Why does my JSON fail here but work fine in JavaScript?
JavaScript object literals and JSON are not the same grammar. JS allows unquoted keys, single-quoted strings, trailing commas, and comments — none of which are legal in strict JSON (RFC 8259). This validator enforces the strict spec, so code that works as a JS literal (or with a lenient parser like JSON5) will correctly fail here if it relies on any of those JS-only conveniences. That's the point: catching exactly the cases that will break `JSON.parse()` or a strict backend parser in production. - My JSON has a number like 9223372036854775807 and it "loses precision" after I round-trip it — is that a validator bug?
No — this is inherent to JSON itself. The spec doesn't define an integer type with arbitrary precision; most parsers (including JavaScript's) decode JSON numbers as 64-bit floats, which only safely represent integers up to 2^53. Values larger than that (common with 64-bit database IDs, Snowflake/Twitter-style IDs) silently lose precision on decode. The fix is upstream: serialize large IDs as JSON strings, not numbers, if exact round-tripping matters. - Does it catch duplicate keys in the same object?
Yes, with a warning — duplicate keys are technically not forbidden by the JSON grammar itself (the spec is silent on it), but every real-world parser resolves them differently: some keep the first occurrence, most (including JavaScript) keep the last. Since this is a frequent source of "works on my machine" bugs when JSON is generated by string concatenation, the validator flags duplicate keys even though strict grammar validation alone wouldn't reject them. - It says my JSON is valid, but my API still rejects it — why?
Syntax validity and schema validity are different checks. This tool confirms your JSON parses correctly per the JSON grammar; it does not check whether the right fields are present, whether types match what an API expects, or whether values satisfy business rules (e.g. an enum field, a required nested object). For that, you need schema validation (JSON Schema) against your specific API contract — syntactic validity is a necessary but not sufficient condition.
Technical Details
JSON validation means checking input against the JSON grammar defined in RFC 8259 — correctly matched braces/brackets, properly quoted (double-quote only) string keys and values, no trailing commas, no unquoted keys, and numbers that match JSON's specific numeric grammar (no leading zeros, no hex literals, no NaN/Infinity, unlike JavaScript number literals). The validator reports the exact line and column where the parser's expectations diverge from the input, since "unexpected token" with no location is the single most useless error message in JSON tooling. A few edge cases are worth understanding rather than just fixing blindly: large integers beyond 2^53 silently lose precision in any standard JSON decoder because JSON doesn't mandate arbitrary-precision integers, so 64-bit IDs should be transmitted as strings if exact values matter downstream. Duplicate object keys aren't actually forbidden by the grammar — different parsers resolve collisions differently (first-wins vs last-wins) — so this validator flags them as a likely-unintentional pattern even though they don't make the document strictly invalid. Validity is also a narrower guarantee than correctness: a syntactically valid document can still violate an API's expected shape (wrong field types, missing required keys). For that level of checking, JSON Schema validation is the right tool — this validator answers "will JSON.parse() throw," not "does this match my contract." Once your JSON parses cleanly, use the JSON Formatter to make structural review easier, or JSON Compare to diff it against a known-good reference.