// how-to guide

How to Validate JSON

Check if your JSON is valid and get precise error messages with line numbers for any syntax issues.

1

Paste your JSON

Paste the JSON you want to validate into the input editor.
2

Check the result

The validator instantly checks your JSON syntax. Valid JSON gets a green confirmation. Invalid JSON shows the exact error with line and column numbers.
3

Fix errors

Use the error details to locate and fix syntax issues — missing commas, unclosed brackets, trailing commas, or invalid values.
4

Format if needed

Once valid, you can format the JSON for readability or switch to the JSON Formatter tool.

JSON syntax is strict — a single misplaced comma or missing quote will cause your parser to reject the entire document. When you are staring at a 500-line config file and the error message just says “Unexpected token,” the JSON Validator pinpoints exactly where the problem is.

Understanding JSON validation

JSON validation checks whether a string conforms to the JSON specification defined in RFC 8259. Unlike more forgiving formats, JSON has no optional syntax or alternative styles. Every rule is mandatory: double quotes for strings, no trailing commas, no comments, no single quotes, no undefined values.

This strictness is by design. JSON was created to be a minimal, unambiguous data interchange format that any parser in any language can handle identically. But it also means that formats that look like JSON — JavaScript object literals, Python dictionaries, JSON5, JSONC — will fail strict validation even though they are “close.”

If you are working with a format that allows comments or trailing commas (like tsconfig.json or VS Code settings), you are actually working with JSONC or JSON5, not standard JSON. These are supersets that need their own parsers. A standard JSON validator will rightfully reject them.

Tips and best practices

Validate before deploying. A broken JSON config file can bring down an application at startup. Run validation as part of your CI pipeline, especially for i18n files, feature flags, and environment configs.
Use the error line number. When the validator reports an error at line 47, the actual mistake is often on line 46 — a missing comma at the end of the previous line is the most frequent culprit.
Validate API responses during development. If your API returns invalid JSON, clients will crash. Add validation to your integration tests to catch serialization bugs early.
Be careful with numbers. JSON supports numbers but has no distinction between integers and floats. Extremely large integers (beyond 2^53) lose precision in JavaScript parsers. If you need exact large numbers, send them as strings.
Check encoding. JSON must be UTF-8 encoded. If your file contains characters from a different encoding (like Latin-1), the parser may choke on byte sequences that are not valid UTF-8.

Troubleshooting

“Unexpected token at position 0”: This usually means the input is not JSON at all — it might be HTML (from a 404 page), XML, or plain text. Check what your API is actually returning.
“Unexpected end of input”: You have an unclosed bracket, brace, or string somewhere. The parser reached the end of the document while still expecting a closing delimiter. Use the JSON Formatter to help locate the mismatch.
Valid JSON but wrong structure: Validation only checks syntax, not semantics. {"name": 42} is valid JSON even if your application expects name to be a string. For structural validation, you need JSON Schema.