UTF-8 BOM before a JSON document

A byte order mark (EF BB BF) at the start of the file. Windows editors, Excel exports and PowerShell's `Out-File` add one by default. RFC 8259 says a JSON text must not begin with a BOM.

Reproduce it

The input

Fails to parse

{"a": 1}

Parses correctly

{"a": 1}

Open this broken input in the formatter →

The fix

How to fix it

Save the file as UTF-8 without BOM. In PowerShell use `Set-Content -Encoding utf8NoBOM`; in Notepad choose 'UTF-8' rather than 'UTF-8 with BOM'.

The tell is an error at position 0 on a file that looks perfectly fine in your editor — the BOM is invisible. Check with `head -c 3 file.json | xxd`.

Cross-runtime

What each parser reports

Every message below was captured by running this exact input through the parser named. 17 of 21 parsers rejected it; 4 accepted it.

Parser	Exact message
CPython json.loads CPython 3.12.3	`Unexpected UTF-8 BOM (decode using utf-8-sig): line 1 column 1 (char 0)` what this message means →
encoding/json Unmarshal Go go1.22.2	`invalid character 'ï' looking for beginning of value` what 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	accepted — no error
Jackson ObjectMapper Java 21.0.12	`Unexpected character ('?' (code 65279 / 0xfeff)): expected a valid value (JSON String, Number, Array, Object or token 'null', 'true' or 'false')` what this message means →
JavaScriptCore JSON.parse JavaScriptCore (Bun 1.4.1)	`JSON Parse error: Unrecognized token ''` what this message means →
json-bigint Node.js 22.22.2	`Unexpected ''`
JSON5 Node.js 22.22.2	accepted — no error
JSON::PP Perl 5.038002	`malformed JSON string, neither array, object, number, string or atom, at character offset 0 (before "\x{ef}\x{bb}\x{bf}{"...")` 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	`Unexpected character encountered while parsing value: . Path '', line 0, position 0.` what this message means →
Newtonsoft.Json JsonTextReader Newtonsoft.Json 13.0.3 on .NET 8.0.30	`Unexpected character encountered while parsing value: . Path '', line 0, position 0.` what this message means →
orjson CPython 3.12.3	`UTF-8 byte order mark (BOM) is not supported: line 1 column 1 (char 0)` what this message means →
Ruby JSON.parse Ruby 3.2.3	`unexpected token at '{"a": 1}'`
serde_json::from_str Rust (serde_json 1.x)	`expected value at line 1 column 1` what this message means →
SpiderMonkey JSON.parse SpiderMonkey 115	`JSON.parse: unexpected character at line 1 column 1 of the JSON data` what this message means →
System.Text.Json Deserialize .NET 8.0.30	`'0xEF' is an invalid start of a value. Path: $ \| LineNumber: 0 \| BytePositionInLine: 0.` what this message means →
System.Text.Json JsonDocument .NET 8.0.30	`'0xEF' is an invalid start of a value. LineNumber: 0 \| BytePositionInLine: 0.` what this message means →
ujson CPython 3.12.3	`Expected object or value`
V8 JSON.parse Node.js 22.22.2	`Unexpected token '', "{"a": 1}" is not valid JSON` 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) §8.1 — Character Encoding JSON exchanged outside a closed ecosystem must be encoded in UTF-8. The section also states that implementations must not add a byte order mark, while parsers may choose to ignore one.
IETF	RFC 3629 (STD 63) §6 — Byte order mark The encoding RFC 8259 §8.1 requires for JSON exchanged between systems.
Unicode Consortium	The Unicode Standard Normatively referenced by RFC 8259 for the definition of a character.

Curly (smart) quotes in JSON

Typographic quotes U+201C and U+201D instead of the ASCII double quote. Produced by word processors, chat clients, Google Docs and…

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