API Response JSON Validator — Check REST API JSON

REST API JSON Response Validator

Validate REST API JSON responses against RFC 8259 online. Check for syntax errors, unexpected tokens, and malformed payloads before you ship.

Paste any REST API response to validate it against RFC 8259 before your application processes it. Catches unexpected token errors, malformed payloads, and encoding issues with exact line-number reporting.

Why Validate API Responses Before Parsing?

Even well-known APIs occasionally return malformed JSON due to server errors, encoding issues, or content negotiation failures. Validating before calling JSON.parse() prevents runtime crashes in production.

Common API JSON Issues Detected

HTML error pages returned as JSON (Content-Type mismatch)
Trailing commas from server-side template rendering
Non-UTF-8 encoding or BOM characters at start of response
Partial payloads from network timeouts or truncated responses
NaN or Infinity values from numeric computation bugs

Who this guide is for

Validate API responses are strict RFC 8259 JSON before parsing
Catch malformed responses that would crash JSON.parse() in production
Verify third-party API responses during integration testing
Check webhooks payloads before writing handler logic

Frequently Asked Questions

Why would an API return invalid JSON?

APIs return invalid JSON when: a server error returns an HTML page, templating engines insert JavaScript-style comments, encoding issues corrupt the payload, or network timeouts truncate the response.

Should I validate JSON in my application code or just catch JSON.parse errors?

For production code, always wrap JSON.parse() in try/catch. For debugging and integration testing, use this validator to get human-readable error messages with line numbers.