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jsonbeautifiers
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V8 Verified by running the parser on Node v24.15.0, V8 13.6.233.17.

Bad control character in string literal in JSON at position N (line L column C)

A string in your document contains a literal control character: most often a real newline, sometimes a tab. JSON strings cannot contain characters below U+0020 unescaped. A newline has to be written as \n, a tab as \t. The position points at the offending character.

Paste your JSON and see exactly where it breaks

Input

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What actually causes it

Ranked by how often each one turns out to be the answer.

  1. 01 Multi-line text pasted directly into a string

    SQL, HTML, a log excerpt or an error message with real line breaks in it. The line break ends the string as far as the parser is concerned.

    Breaks

    {
  "query": "SELECT *
FROM users"
}

    Works

    {
  "query": "SELECT *\nFROM users"
}

  2. 02 JSON built by string concatenation

    This is the root cause behind almost every instance. Hand-assembled JSON does not escape the values you interpolate into it. Serialise the object instead and the problem cannot occur.

    Breaks

    const body = '{"note": "' + note + '"}';

    Works

    const body = JSON.stringify({ note });

  3. 03 A tab character from a copy and paste

    Tabs are invisible and land in strings easily when copying from a spreadsheet or a terminal. Escape as \t, or strip them before building the document.

  4. 04 Windows line endings inside a value

    A carriage return, U+000D, is also a control character. Text read from a Windows file and embedded in a string carries \r as well as \n.

The same mistake in other runtimes

The underlying problem is identical; only the wording differs. If a colleague reports one of these, they are looking at the same thing you are.

Python Invalid control character at: line 1 column 8 (char 7)
Java (Jackson) Illegal unquoted character ((CTRL-CHAR, code 10)): has to be escaped using backslash to be included in string value

Questions

Which characters have to be escaped?
Everything from U+0000 to U+001F, plus the double quote and the backslash. JSON defines short escapes for backspace, form feed, newline, carriage return and tab; anything else in that range needs the \u form, for example \u0000.
Can I make the parser accept them?
Jackson has ALLOW_UNESCAPED_CONTROL_CHARS and some parsers have similar switches, but that hides the problem rather than fixing it, and the document stays invalid for every other consumer. Escape at the point where the JSON is produced.

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