'<' is an invalid start of a value

The fix

Log the raw body and the status code before you deserialize. If the body is an HTML page, fix the request or the proxy, not the class. The two samples here are the same failed call: what arrived, and what the API should have sent.

<!DOCTYPE html>
<html><body>502 Bad Gateway</body></html>

{
  "error": "bad gateway",
  "status": 502
}

How the message appears

The first line is the example on this page. The rest are the same message for other bodies: plain text, the word undefined, and the faults System.Text.Json rejects that other parsers let through, which are a comma after the last member, single quotes, an empty body, a missing comma and a string that stops early.

What causes it

1. 1

   '<': the body is an HTML page

   A 502 from a proxy, a 404 page, a login redirect or a WAF challenge usually comes back as HTML. Your code asked for JSON and read the page. Path: $ and LineNumber: 0 | BytePositionInLine: 0 say it failed on the very first character, before it had read anything. Nothing in your class can fix that. Log the body and the status code.

2. 2

   'e' or another letter: the body is plain text

   A text error such as error: upstream timeout starts with e, so the message says 'e' is an invalid start of a value. The letter in quotes is the first letter of the text. Log the body and read it.

3. 3

   'u': undefined from JavaScript

   A client that builds JSON by joining strings writes undefined when a variable has no value. The message says 'u', and Path names the field, $.status in this example. BytePositionInLine: 20 is the 21st character on the line, because the count starts at 0. See undefined is not valid JSON for the JavaScript side.

4. 4

   Reading Path, LineNumber and BytePositionInLine

   Path says where in the document it stopped. $ is the top, and $.status is the status field. LineNumber and BytePositionInLine both start at 0, so LineNumber: 0 is the first line. Take a document with no comma after "id": 7. The message says LineNumber: 2 | BytePositionInLine: 2, which is the quote that starts "status" on the third line. The comma belongs at the end of the line above it.

5. 5

   It is strict: a trailing comma, single quotes, an empty body

   A comma after the last member gives The JSON object contains a trailing comma at the end which is not supported in this mode. Change the reader options. Single quotes give ''' is an invalid start of a property name. Expected a '"'. An empty body gives The input does not contain any JSON tokens. Newtonsoft reads the first two with no error, so a payload that worked before you switched libraries can fail now. See the Newtonsoft page. The message points at the reader options. JsonSerializerOptions.AllowTrailingCommas is the one for a trailing comma, and ReadCommentHandling is the one for comments. Not tested here. The better fix is to get the sender to write valid JSON.

6. 6

   The text stops early

   A body cut off inside a string says Expected end of string, but instead reached end of data. Path names the field that was open, $.status here. Look for a connection that dropped or a buffer that was cut.

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