JSON Validator
Check strict JSON syntax, locate errors and identify ambiguous duplicate keys without rewriting your data.
Input
Validation answers one question.
Is this strict JSON? Paste a document and choose Validate JSON. Unlike formatting, validation does not rewrite whitespace or produce a replacement document. Invalid input receives the parser’s explanation and a location when available; Go to error moves the editor cursor there.
Valid syntax can still be ambiguous.
Duplicate keys produce a warning, not a syntax error. Earlier values may be discarded by other parsers. Large numeric tokens are checked without rounding, including integers beyond JavaScript precision. A valid number does not guarantee that your consuming application can represent it exactly.
Strict, bounded and local.
Comments, trailing commas, single-quoted strings and a leading BOM are rejected consistently with JSON Formatter. Raw unpaired Unicode surrogates are rejected; literal Unicode escapes are preserved. Positions count UTF-16 code units, with CRLF as one line break.
Limits are 128 KiB UTF-8, 64 nesting levels and 20,000 values to keep interactive validation bounded. Input and diagnostics stay in browser memory. Reset or navigation clears them; nothing is uploaded, logged or saved. This checks syntax, not conformance to a JSON Schema.