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jsonbeautifiers
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C# Message text transcribed from reported occurrences; not executed here.

Unexpected character encountered while parsing value: <. Path '', line 0, position 0.

JsonConvert.DeserializeObject was handed something that does not start with a JSON value. Line 0 position 0 means it failed on the very first character. The character named in the message tells you what you actually have.

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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 You passed a file path instead of file content

    DeserializeObject takes a JSON string, not a filename. Passing a path means the parser reads C:\... as the document and fails on the letter C or on a slash. This is a very common mistake and the message does not hint at it.

    Breaks

    var data = JsonConvert.DeserializeObject<Model>(@"C:\data\input.json");

    Works

    var json = File.ReadAllText(@"C:\data\input.json");
var data = JsonConvert.DeserializeObject<Model>(json);

  2. 02 The response is HTML

    An error page, a login redirect or an IIS error. The character in the message will be <. Read the body as a string and inspect it before deserialising.

    Breaks

    var data = JsonConvert.DeserializeObject<Model>(await res.Content.ReadAsStringAsync());

    Works

    var body = await res.Content.ReadAsStringAsync();
if (!res.IsSuccessStatusCode || !body.TrimStart().StartsWith("{"))
    throw new InvalidOperationException($"{(int)res.StatusCode}: {body[..Math.Min(300, body.Length)]}");
var data = JsonConvert.DeserializeObject<Model>(body);

  3. 03 A byte order mark at the start of the string

    Reading a file without letting the encoding detector strip the BOM leaves U+FEFF in front of the JSON.

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.

C# (System.Text.Json) '<' is an invalid start of a value. Path: $ | LineNumber: 0 | BytePositionInLine: 0.
JavaScript (V8) Unexpected token '<', "<!DOCTYPE "... is not valid JSON

Questions

Should I migrate to System.Text.Json?
It is the built-in option from .NET Core 3.0 onwards and is faster, but it is also stricter: no single-quoted strings, no trailing commas, no comments unless you enable them explicitly. Code that relied on Newtonsoft being forgiving will need the producer fixed.

Fix it now

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