What is a JSON parse error? (decoder cheatsheet)

Getting Unexpected token or failed to parse JSON? Match the exact error text below, apply the fix, then re-validate your payload.

Common JSON.parse errors decoded

Error message	Usually means	Quick fix
Unexpected token < in JSON at position 0	You parsed HTML (often an error page) instead of JSON.	Inspect response body/status. Verify endpoint and Content-Type before JSON.parse.
Unexpected token ' in JSON at position ...	Single quotes were used in keys/strings.	Use double quotes for keys and string values.
Unexpected token } / ] in JSON at position ...	Trailing comma or malformed structure.	Remove trailing commas and re-check object/array nesting.
Unexpected end of JSON input	Truncated payload or empty response.	Check server output length, buffering, and whether body is empty.
Unexpected number in JSON at position ...	Missing comma between fields/array values.	Add commas between adjacent values.