Why would an API return invalid JSON?

Common causes: the server returned an HTML error page instead of JSON (check the HTTP status code), a proxy injected content, character encoding issues corrupted the JSON, or a backend exception printed a stack trace before the JSON output.

How do I validate a JSON API response in JavaScript?

In JavaScript, try { JSON.parse(responseText) } catch(e) { console.error(e) } will throw for invalid JSON. For a quick manual check, paste the response into FixTools.

Does FixTools send my API response to a server?

No. Validation happens entirely in your browser. Your API data — which may contain sensitive information — never leaves your machine.

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Validate JSON API Responses

API responses should be valid JSON — but sometimes they are not. Servers can return HTML error pages wrapped in a JSON content-type header, encoding issues can corrupt data, or a backend bug can produce malformed output. Paste your API response into FixTools to confirm it is valid JSON before trusting it in your code.

⚡ Use Tool Now How it works

Cost: Free forever
Sign-up: Not required
Processing: In your browser
Privacy: Files stay local

⚡

Quickly checks API response validity

🔒

Catches malformed or partially-escaped JSON

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Safe for sensitive API data — no server calls

JSON Tool

JSON Validator

All processing happens in your browser — your files are never uploaded to any server.

🚀Open JSON Validator

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How to use this tool

💡

Copy the raw body from your API response and paste it here to validate it.

How It Works

Step-by-step guide to validate json api responses:

1
Capture the API response
Copy the raw response body from curl, Postman, Insomnia, browser DevTools Network tab, or your HTTP client.
2
Paste and validate
Paste into FixTools and click Validate. Any issues are reported immediately.
3
Investigate and fix
If the response is invalid, check whether the server returned an HTML error page, a status message, or has an encoding problem. Format valid responses to inspect their structure.