Unexpected character encountered while parsing 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. Then the same message for plain text and for the word undefined, a missing comma, and a string that stops early. A comma after the last member, single quotes and an empty body throw nothing in Newtonsoft, so they have no line here.

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 '' is the top of the document, and line 0, position 0 is what Newtonsoft says when it fails on the first character. Log the body and the status code.

2. 2

   e or u: plain text, or undefined

   A text error such as error: upstream timeout gives while parsing value: e. The word undefined gives while parsing value: u, with Path 'status' naming the field that held it. A client that builds JSON by joining strings writes undefined when a variable has no value. See undefined is not valid JSON for the JavaScript side.

3. 3

   Reading Path, line and position

   Path is the property being read. Path '' is the top, and Path 'status' is the status property. Past the first character the line count starts at 1. In the undefined example the u is the 21st character on line 1, and the message says position 21. In the missing comma example the quote is the third character on its line, and the message says position 2. So the position is close, not exact. Read the Path and the character it names as well.

4. 4

   After parsing a value an unexpected character was encountered: a comma is missing

   The example has no comma after "id": 7. Newtonsoft read the 7, then met the quote of "status" on the next line. Path 'id' is the last property it read, and line 3 is the line of the quote. Add the comma at the end of the line above.

5. 5

   Unterminated string. Expected delimiter: the text stops early

   A string with no closing quote at the end of the text gives Unterminated string. Expected delimiter. The Path names the property that was open, Path 'status' here. Look for a connection that dropped or a buffer that was cut.

6. 6

   Newtonsoft accepts what System.Text.Json rejects

   A comma after the last member and single quotes both read with no error. So does an empty body: DeserializeObject<Order> throws nothing, so there is no error at the read and the problem shows up later, where your code uses the result. System.Text.Json rejects all three, so a payload that worked here can fail after you switch. See the System.Text.Json page.

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