FormatException: Unexpected character Flutter / Dart Origin: On your device OSI layer 7 · Application Last updated 4 Nov 2026

Dart: FormatException: Unexpected character (at character 1) <!DOCTYPE html> — jsonDecode of an HTML page

jsonDecode received something that is not JSON, commonly an HTML error page, an empty string, or a plain-text message.

Exact message you may see FormatException: Unexpected character (at character 1)FormatException: Unexpected end of input (at character 1)FormatException: Unexpected character <!DOCTYPE html>FormatException: Invalid numberUnexpected character (at line 1, character 1)
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Seen on: Flutter

Meaning

The server returned HTML (404/500 page, login redirect, proxy error) but the app tried to parse it as JSON. Check status code and Content-Type, log the body, and confirm the URL. Trailing commas or single quotes also cause Unexpected character.

Common causes

  • Server returned HTML for an error or redirect
  • Wrong URL or missing API path
  • Empty body
  • Invalid JSON (single quotes, trailing comma)
  • Gzip/encoding problem

⚡ Quick fix

  1. Print response.statusCode and response.body
  2. Verify the URL in a browser or curl
  3. Only decode when status is 200 and Content-Type is JSON

Detailed fix by platform

Dart

  1. if (res.statusCode != 200) throw Exception('HTTP ${res.statusCode}: ${res.body}');

How to diagnose

  1. Body — HTML or JSON?
  2. Status — 200?
  3. URL — Correct?

🔧 Still not fixed?

Many errors look alike. If the steps above didn’t solve it, one of these is probably what you’re facing:

Where this error sits

Origin: On your device. Errors that are often confused with or caused by this one (click to open):

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