Leading zero in a JSON number
JSON's number grammar forbids a leading zero on a multi-digit integer, to avoid any suggestion of octal. Zip codes, phone numbers and zero-padded IDs hit this constantly.
The input
{"a": 013}{"a": "013"}How to fix it
Quote it. `"013"` is a string and keeps its padding; `13` is a number and does not.
If a value has a leading zero that matters, it is an identifier, not a quantity — it belongs in a string.
What each parser reports
Every message below was captured by running this exact input through the parser named. 15 of 21 parsers rejected it; 6 accepted it.
| Parser | Exact message |
|---|---|
| CPython json.loads CPython 3.12.3 | Expecting ',' delimiter: line 1 column 8 (char 7)what this message means → |
| encoding/json Unmarshal Go go1.22.2 | invalid character '1' after object key:value pairwhat this message means → |
| Gson Java 21.0.12 | accepted — no error |
| Gson JsonParser (lenient off) Java 21.0.12 | accepted — no error |
| hjson Node.js 22.22.2 | End of input while parsing an object (missing '}') at line 1,10 >>>"a": 013} ...what this message means → |
| Jackson ObjectMapper Java 21.0.12 | Invalid numeric value: Leading zeroes not allowedwhat this message means → |
| JavaScriptCore JSON.parse JavaScriptCore (Bun 1.4.1) | JSON Parse error: Expected '}'what this message means → |
| json-bigint Node.js 22.22.2 | accepted — no error |
| JSON5 Node.js 22.22.2 | JSON5: invalid character '1' at 1:8what this message means → |
| JSON::PP Perl 5.038002 | malformed number (leading zero must not be followed by another digit), at character offset 7 (before "13}")what this message means → |
| json_decode PHP 8.3.6 | Syntax error |
| Newtonsoft.Json JsonConvert Newtonsoft.Json 13.0.3 on .NET 8.0.30 | accepted — no error |
| Newtonsoft.Json JsonTextReader Newtonsoft.Json 13.0.3 on .NET 8.0.30 | accepted — no error |
| orjson CPython 3.12.3 | number with leading zero is not allowed: line 1 column 7 (char 6)what this message means → |
| Ruby JSON.parse Ruby 3.2.3 | unexpected token at '{"a": 013}' |
| serde_json::from_str Rust (serde_json 1.x) | invalid number at line 1 column 8what this message means → |
| SpiderMonkey JSON.parse SpiderMonkey 115 | JSON.parse: expected ',' or '}' after property value in object at line 1 column 8 of the JSON datawhat this message means → |
| System.Text.Json Deserialize .NET 8.0.30 | Invalid leading zero before '1'. Path: $ | LineNumber: 0 | BytePositionInLine: 7.what this message means → |
| System.Text.Json JsonDocument .NET 8.0.30 | Invalid leading zero before '1'. LineNumber: 0 | BytePositionInLine: 7.what this message means → |
| ujson CPython 3.12.3 | accepted — no error |
| V8 JSON.parse Node.js 22.22.2 | Unexpected number in JSON at position 7 (line 1 column 8)what this message means → |
What the specification says
The exact sections that govern this error. Descriptions are our paraphrase; follow the links for the normative text.
| Body | Section |
|---|---|
| IETF | A number is base 10, optionally signed with a minus, with an optional fraction and exponent. The section states that leading zeros are not allowed, and that values which cannot be represented in its grammar — Infinity and NaN among them — are not permitted. |
| Ecma International | A number is decimal digits with no superfluous leading zero. Values such as Infinity and NaN are not permitted. |
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