Unexpected token < in JSON at position 0

Quick Answer

Unexpected token < in JSON at position 0 usually means the response starts with <, so the parser is seeing HTML instead of JSON. Common causes include a 404 page, a login page, a server error page, or a development proxy returning the front-end app. Debug the HTTP response before changing JSON syntax.

Why This Matters

This error is one of the most common API debugging traps. The JSON parser is not telling you that < belongs in your JSON. It is telling you that the first character of the response is not JSON at all.

Common Causes

Practical Workflow

1. Open the failed request in the browser Network tab.
2. Check status code and Request URL.
3. Check the Content-Type header.
4. Copy the response body and inspect the first 200 characters.
5. Use JSON Error Doctor only after confirming the body is meant to be JSON.

API Response Checklist

Example

const response = await fetch("/api/profile");
const text = await response.text();
console.log(response.status, response.headers.get("content-type"), text.slice(0, 200));

If text starts with HTML, fix routing, authentication, or server error handling. Do not try to "repair" the HTML as JSON.

Common Mistakes

• Calling response.json() without checking status code.
• Missing a base URL difference between local and production.
• Treating a login redirect as a successful API response.
• Assuming HTTP 200 always means JSON.

Practical FAQ

Is Unexpected token < a JSON syntax problem?

Usually no. The < character normally points to HTML, so the first task is to find why the API returned a page instead of JSON.

Why does this happen only in production?

Production may use a different base URL, rewrite rule, authentication cookie, CORS path, or proxy target. Compare the full request URL and response headers between local and production.

Should I call response.text() or response.json() while debugging?

Use response.text() temporarily when the body is suspicious. It lets you inspect the raw response before deciding whether JSON parsing is appropriate.

Can HTTP 200 still return HTML?

Yes. Some login pages, app shells, proxy fallbacks, and CDN error pages can return a 200 status with HTML. Check Content-Type and the first characters, not status alone.

What should the server return for API errors?

For JSON APIs, return a clear status code and a JSON error body. Avoid silently redirecting API clients to HTML login pages when they expect machine-readable JSON.

Related Tool

• JSON Error Doctor

Related Article

• JSON Unexpected token Error
• JSON formatter vs validator

Updated

2026-06-25