Expecting ',' delimiter

The parser finished one value and found another one starting immediately, with no comma between them. The trap is that this is reported both when a comma is genuinely missing and when a string ended earlier than you intended — and the second case is far more common.

How the message appears

CPython's json module reports line, column and character offset. Treat the position as an upper bound: an unterminated string moves it well past the real mistake.

json.decoder.JSONDecodeError: Expecting ',' delimiter: line 2 column 22 (char 23)
json.decoder.JSONDecodeError: Expecting ':' delimiter: line 1 column 10 (char 9)

What causes it

An unescaped double quote inside a string

This is the one worth checking first. "she said "hi" to me" is not one string containing quotes — it is the string "she said ", then the bare word hi, then more text. The parser sees two values in a row and reports a missing comma, pointing at innocent-looking text several characters after the real mistake.

A genuinely missing comma

Two members or two array elements with nothing between them, usually after hand-editing or after a merge that dropped a line.

A colon typed as a comma

Produces the sibling message Expecting ':' delimiter instead: a key was read, and the thing after it was not a colon.

The fix

Escape every double quote that belongs inside a string as \", and add any comma that is genuinely missing between members. When the reported column points at text that looks fine, read leftwards for a quote that closed a string too early.

Fails to parse

{
  "quote": "she said "hi" to me",
  "id": 2
}

Parses

{
  "quote": "she said \"hi\" to me",
  "id": 2
}

The example travels in the URL fragment, which browsers never send to a server.

Worth knowing

Do not interpolate text into a JSON template with an f-string or %. The moment a value contains a quote, an apostrophe-heavy sentence or a Windows path, you get this error. json.dumps escapes correctly and is not slower in any way that matters.
The sibling message Expecting ':' delimiter has the same shape of cause: something appeared where the parser knew a specific single character had to be.