SyntaxError: Unexpected token / in JSON at position 0: what it means and how to fix it

What the error means

SyntaxError: Unexpected token / in JSON at position 0 means the parser expected valid JSON but encountered a character/token that cannot appear there. In practice, it usually means the input is not JSON (or not strict JSON), or it is incomplete.

Most common real-world causes

• Your JSON contains comments (// or /* ... */). Strict JSON does not allow comments.
• You are parsing a path like /api/... (not JSON at all).
• A proxy prepended a banner/comment line before the JSON.
• You are parsing a JavaScript object literal, not strict JSON.

Fast debugging steps

• Check the first non-whitespace character; if it is /, it is not strict JSON.
• Remove comments, then validate again.
• Use a JSON repair step if the source is out of your control.

Code example (node)

// Strict JSON: comments are not allowed
const bad = `{
  // comment
  "ok": true
}`;

// Fix: remove comments (or use a repair step), then parse
const ok = '{"ok": true}';
JSON.parse(ok);

Fix without uploading data

If the JSON contains sensitive data, validate and fix it locally. No Upload Tools runs 100% in your browser.

• JSON Validator to pinpoint the exact syntax error.
• JSON Repair to remove comments/trailing commas when the source is not strict.
• JSON Formatter to pretty-print and inspect structure.
• JSON String Escape if the issue is inside a string (newlines/tabs/quotes).

Workflow: validate -> fix the first error -> validate again -> export/convert.

FAQ

Does the exact token matter? Yes. The token often hints at the root cause: < is usually HTML, u is often undefined, and / often points to comments.

Should I just regex-fix JSON? Avoid blind regex edits. Validate after each change so you know what you fixed and what broke.