Error guide
Unexpected token '<' / DOCTYPE is not valid JSON
Fix JSON parsing failures caused by HTML responses. Inspect the response status, content type and body before parsing in JavaScript, Python or Go.
JSON troubleshooting
JSON errors: find the cause and fix the input
Start with the message you saw. Each guide explains the input that triggers it, with reproduced JavaScript and Python errors, cited diagnostics from other runtimes and code to fix the cause.
Error guide
Unexpected end of JSON input: empty or truncated data
Find and fix empty responses and truncated JSON behind Unexpected end of JSON input, Python Expecting value at char 0 and Go EOF errors.
Error guide
"undefined" or "[object Object]" is not valid JSON
Fix JSON.parse calls that receive undefined or an already-parsed object. Learn when to parse, when to stringify and how to guard missing storage values.
Error guide
Expecting property name enclosed in double quotes
Fix single-quoted strings and unquoted keys in JSON. Compare exact Python and JavaScript errors and serialize objects without unsafe quote replacement.
Error guide
JSON trailing comma errors in objects and arrays
Remove trailing commas that strict JSON parsers reject. See version-specific JavaScript, Python and .NET messages and generate valid JSON with serializers.
Error guide
JSONDecodeError: Extra data and multiple JSON values
Fix Python JSONDecodeError: Extra data and JavaScript errors after a JSON value. Distinguish one JSON document from NDJSON and parse each format correctly.
Error guide
Bad control character in string literal in JSON
Fix unescaped newlines, tabs and other control characters inside JSON strings. See exact JavaScript and Python errors and use serializers to escape values.
Error guide
Go JSON: invalid character looking for beginning of value
Diagnose unexpected prefixes, plain-text errors and byte-order marks in Go JSON input. Check HTTP bodies and use SyntaxError.Offset to locate the bad byte.
Check the input before changing the parser
For API failures, read the actual response body and check status and Content-Type. For file failures, inspect the saved text. A message at position zero often means the wrong data reached the parser. A message later in the document can point to a missing separator or an unescaped character.
Paste the original text into the JSON validator to inspect its line and column. Read the syntax reference when you need to check a rule. Tool Kumar keeps parsing on your device.
Frequently asked questions
How do I identify the cause of a JSON parse error?
Inspect the exact text passed to the parser, then read the reported line, column or offset. Check the HTTP response status and Content-Type when the text came from an API. The same message can have more than one cause.
Why do runtimes show different JSON error messages?
Each parser defines its own diagnostic wording, and messages change across versions. JSON standards define valid syntax, not a required error message. Match the input and cause as well as the wording.
Can I validate JSON without uploading it?
Yes. Tool Kumar parses the document in your browser and reports syntax issues with line and column information. Its validator and repair tools keep the input on your device.