Unexpected token in JSON — causes and fixes

"Unexpected token" is the most common JSON parse error. It means the parser encountered a character it did not expect at that position — a mismatch between what the JSON grammar allows and what was in the input.

What the position tells you

Most parsers report a byte offset or line/column alongside the error. Three positions cover the vast majority of cases:

• Position 0 / line 1, col 1 — the very first byte is wrong. Common causes: empty string, an HTML error page returned instead of JSON, a UTF-8 BOM, or the literal string "undefined".
• Somewhere in the middle — the document started as valid JSON but something broke it: a trailing comma, single-quoted string, unquoted key, or an embedded comment.
• "Unexpected end of JSON input" — the parser hit end-of-file before all brackets and quotes were closed. The document is truncated.

Most common causes

1. Trailing comma

{ "a": 1, "b": 2, }   // ✗ trailing comma before }
{ "a": 1, "b": 2 }    // ✓

2. Single-quoted string or key

{ 'name': 'Ada' }    // ✗ single quotes are not JSON
{ "name": "Ada" }    // ✓

3. Unquoted key

{ name: "Ada" }      // ✗ JavaScript object literal syntax
{ "name": "Ada" }    // ✓

4. JavaScript-style comment

{ "a": 1 // note }  // ✗ comments are not valid JSON
{ "a": 1 }           // ✓ — move notes to a sibling key

5. Python-style literals

{ "ok": True, "x": None }  // ✗ Python, not JSON
{ "ok": true, "x": null }  // ✓

6. Truncated response

Receiving only part of an HTTP response produces "Unexpected end of JSON input". Check Content-Length headers, network errors, and streaming-read logic that might stop early.

How to locate and fix the error

Paste the JSON into the validator. The error panel shows the exact line, column, and a hint pointing at the invalid token.