Malformed \u escape in a JSON string

A `\u` not followed by exactly four hexadecimal digits.

Reproduce it

The input

Fails to parse

{"a": "\uZZZZ"}

Parses correctly

{"a": "\u0041"}

Open this broken input in the formatter →

The fix

How to fix it

Supply four hex digits, or escape the backslash if you meant a literal `\u`.

A common origin is double-escaping: text that already contained `\u0041` gets escaped again and loses its digits.

Cross-runtime

What each parser reports

Every message below was captured by running this exact input through the parser named. 20 of 21 parsers rejected it; 1 accepted it.

Parser	Exact message
CPython json.loads CPython 3.12.3	`Invalid \uXXXX escape: line 1 column 9 (char 8)` what this message means →
encoding/json Unmarshal Go go1.22.2	`invalid character 'Z' in \u hexadecimal character escape` what this message means →
Gson Java 21.0.12	`\uZZZZ`
Gson JsonParser (lenient off) Java 21.0.12	`java.lang.NumberFormatException: \uZZZZ` what this message means →
hjson Node.js 22.22.2	`Bad \u char Z at line 1,9 >>>"a": "\uZZZZ"} ...` what this message means →
Jackson ObjectMapper Java 21.0.12	`Unexpected character ('Z' (code 90)): expected a hex-digit for character escape sequence` what this message means →
JavaScriptCore JSON.parse JavaScriptCore (Bun 1.4.1)	`JSON Parse error: "\uZZZZ" is not a valid unicode escape` what this message means →
json-bigint Node.js 22.22.2	accepted — no error
JSON5 Node.js 22.22.2	`JSON5: invalid character 'Z' at 1:10` what this message means →
JSON::PP Perl 5.038002	`unexpected end of string while parsing JSON string, at character offset 10 (before "ZZZ"}")` 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	`Invalid Unicode escape sequence: \uZZZZ. Path 'a', line 1, position 9.` what this message means →
Newtonsoft.Json JsonTextReader Newtonsoft.Json 13.0.3 on .NET 8.0.30	`Invalid Unicode escape sequence: \uZZZZ. Path 'a', line 1, position 9.` what this message means →
orjson CPython 3.12.3	`invalid escaped sequence in string: line 1 column 8 (char 7)` what this message means →
Ruby JSON.parse Ruby 3.2.3	`unexpected token at '{"a": "\uZZZZ"}'`
serde_json::from_str Rust (serde_json 1.x)	`invalid escape at line 1 column 13` what this message means →
SpiderMonkey JSON.parse SpiderMonkey 115	`JSON.parse: bad Unicode escape at line 1 column 10 of the JSON data` what this message means →
System.Text.Json Deserialize .NET 8.0.30	`'Z' is not a hex digit following '\u' within a JSON string. The string should be correctly escaped. Path: $ \| LineNumber: 0 \| BytePositionInLine: 9.` what this message means →
System.Text.Json JsonDocument .NET 8.0.30	`'Z' is not a hex digit following '\u' within a JSON string. The string should be correctly escaped. LineNumber: 0 \| BytePositionInLine: 9.` what this message means →
ujson CPython 3.12.3	`Unexpected character in unicode escape sequence when decoding 'string'` what this message means →
V8 JSON.parse Node.js 22.22.2	`Bad Unicode escape in JSON at position 9 (line 1 column 10)` what this message means →

Standards

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	RFC 8259 (STD 90) §7 — Strings A string begins and ends with a quotation mark. Any Unicode character may appear between them except the quotation mark, the reverse solidus, and the control characters U+0000 through U+001F, which the section requires to be escaped. It also fixes the escape set and the `\uXXXX` form.
Ecma International	ECMA-404, 2nd edition §9 Strings A string is code points wrapped in quotation marks; the quotation mark, reverse solidus and U+0000–U+001F must be escaped. Table 3 fixes the escape sequences.
Unicode Consortium	The Unicode Standard Normatively referenced by RFC 8259 for the definition of a character.

Literal newline inside a JSON string

A real line break between the quotes. JSON forbids unescaped control characters (U+0000 to U+001F) inside strings, and a newline i…

Literal tab inside a JSON string

A raw tab character inside string content. Like a newline, it is a control character and must be escaped.…

Invalid escape sequence in a JSON string

A backslash followed by a character JSON does not define. The complete set is `\" \\ \/ \b \f \n \r \t` and `\uXXXX`. Windows path…

Truncated \u escape in a JSON string

A `\u` escape with fewer than four hex digits, typically where a string was cut mid-escape by a length limit.…

Lone surrogate in a JSON string

A high surrogate (`\uD800`–`\uDBFF`) with no following low surrogate. Characters outside the Basic Multilingual Plane — emoji, mos…

Raw NUL byte inside a JSON string

A U+0000 byte in string content, usually from a C string boundary, a fixed-width database column, or binary data that was treated …

← All JSON parser errors