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

What the error means

SyntaxError: Unexpected token ï 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

• The payload starts with a UTF-8 BOM (byte order mark) or an invisible leading character before '{' or '['.
• The file/export was saved as UTF-8 with BOM and your pipeline doesn't strip it.
• You decoded bytes with the wrong encoding (BOM bytes can appear as the visible prefix '').
• You are parsing a non-UTF-8 text file (UTF-16/Windows-1251/etc.) as UTF-8.

Fast debugging steps

• Print the first characters (and their char codes) to confirm BOM/leading junk.
• Strip BOM and retry parsing (U+FEFF or the literal '' prefix).
• If you read from file/bytes, decode using the correct encoding (Python: utf-8-sig can strip BOM).
• Validate the cleaned JSON locally before any further processing (no upload).

Code example (node)

// Strip UTF-8 BOM (U+FEFF) and the common '' prefix before JSON.parse
const stripBom = (s) => s.replace(/^\uFEFF/, '').replace(/^\u00EF\u00BB\u00BF/, '');

const res = await fetch(url);
const text = await res.text();
const cleaned = stripBom(text);

const data = JSON.parse(cleaned);

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.