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jsonbeautifiers
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Python Verified by running the parser on CPython 3.14.3.

Expecting ',' delimiter: line L column C (char N)

The parser expected the current container to continue with a comma. The confusing part is that the position often points somewhere that looks perfectly fine, because the real mistake is slightly earlier: usually a quote that ended a string sooner than you intended.

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Input

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What actually causes it

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

  1. 01 An unescaped double quote inside a string

    The inner quote closes the string early, so the following text is read as a new token where a comma was expected. The top Stack Overflow question for this error is literally titled "Valid JSON giving JSONDecodeError", which is the whole experience in one line.

    Breaks

    {"quote": "he said "yes" loudly"}

    Works

    {"quote": "he said \"yes\" loudly"}

  2. 02 A genuinely missing comma

    Between two members of an object or two elements of an array.

  3. 03 A single backslash before the closing quote

    A Windows path ending in a backslash escapes the quote, so the string runs on and the next comma never arrives where the parser expects it.

  4. 04 The file is truncated

    If the position is the end of the input, the document was cut short rather than misplaced.

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.

JavaScript (V8) Expected ',' or '}' after property value in JSON at position N

Questions

The JSON looks correct to me. Why does it fail?
Look at the character immediately before the reported position rather than at the position itself. An early-terminated string is invisible until you notice the quote count. Pasting the document into a validator that colours strings makes it obvious in a second.
How do I stop this happening?
Never build JSON by concatenating strings. Build a dict and call json.dumps, which escapes every value correctly by construction.

Fix it now

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