FormatException: Unexpected character in JSON

Why This Happens

In Dart, jsonDecode throws a FormatException when the input string is not valid JSON. This commonly occurs when an API returns HTML error pages, empty responses, or malformed JSON. Always validate the response status code before parsing.

The Problem

final response = await http.get(uri);
final data = jsonDecode(response.body); // May not be JSON!

The Fix

final response = await http.get(uri);
if (response.statusCode == 200) {
  final data = jsonDecode(response.body);
} else {
  throw Exception('Server error: ${response.statusCode}');
}

Step-by-Step Fix

1. 1

   Identify the error

   Look at the FormatException about unexpected characters. The input to jsonDecode is not valid JSON.

2. 2

   Find the cause

   Check the HTTP response body — it may be an HTML error page, empty, or truncated. Verify the status code before parsing.

3. 3

   Apply the fix

   Check response.statusCode before calling jsonDecode, and handle non-200 responses appropriately.