JSON Lint — Validate JSON & Find Errors

Free, no signup, runs entirely in your browser — your data never leaves your device.

Usage — catch a trailing comma with its exact position

curl "https://json-lint-formatho.filesformatho.workers.dev/api?json=%7B%22a%22%3A%201%2C%7D"

Returns valid: false with {"line":1,"column":9,"message":"Trailing comma before '}'"} — plus the offending context.

Usage — duplicate key detection

curl -X POST "https://json-lint-formatho.filesformatho.workers.dev/api" -d '{"id": 1, "name": "a", "name": "b"}'

Strict JSON.parse accepts this (last key silently wins) — the linter flags $.name as a duplicate with both line numbers.

What it checks

• Strict JSON syntax with line/column positions and context snippets
• Trailing commas, // and /* */ comments, BOM — the three classic paste errors
• Duplicate keys per object (with full paths like $.users[0].name)
• Stats: bytes, lines, max nesting depth, object/array/key counts

Full parameter reference: /api endpoint.

Full browser tool

Lint JSON entirely client-side: JSON Lint on formatho.com.

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