C# JSON: '<' is an invalid start of a value / Unexpected character

What the error means

The .NET JSON reader found a character that cannot start a JSON value. System.Text.Json quotes the character, or its byte in hexadecimal such as 0xEF, and adds the JSON path, a zero-based line number and a byte position. Json.NET (Newtonsoft) names the character, the path, the line and the position. A < almost always means HTML: a wrong URL, a login redirect, an error page or, in Blazor WebAssembly, the app's index.html returned by the fallback route for an unknown path. Other characters point to plain-text errors or form data.

Exact messages by runtime

Messages depend on the input and runtime version. Node and Python rows were reproduced locally. Other rows cite official documentation or source. Rows marked as fragments or templates preserve the source wording and are not complete exception transcripts. Browsers and libraries may append location details.

Causes you can reproduce

// "api/itemz" is not an API route, so the fallback returns index.html
var items = await Http.GetFromJsonAsync<List<Item>>("api/itemz");

using var response = await http.GetAsync("api/items");
var items = await response.Content.ReadFromJsonAsync<List<Item>>(); // the body is an HTML error page

byte[] bytes = File.ReadAllBytes("data.json"); // file saved with a BOM
var item = JsonSerializer.Deserialize<Item>(bytes); // '0xEF' is an invalid start of a value

How to fix it

1. Log the status code, Content-Type and the first 200 characters of the body while debugging. ReadAsStringAsync shows exactly what the deserializer received.
2. In Blazor, open the browser Network panel. A 200 response with text/html for an API path means the request reached the fallback route; fix the path or the server routing.
3. Check IsSuccessStatusCode and the media type before reading JSON, and handle error pages separately.
4. For files, read through a stream or File.ReadAllText, which handle a BOM, or strip the three BOM bytes before deserializing a byte array.

using var response = await http.GetAsync("api/items");
var mediaType = response.Content.Headers.ContentType?.MediaType;
if (!response.IsSuccessStatusCode || mediaType != "application/json")
{
    var body = await response.Content.ReadAsStringAsync();
    throw new HttpRequestException(
        $"Expected JSON, got {(int)response.StatusCode} {mediaType}: {body[..Math.Min(body.Length, 200)]}");
}
var items = await response.Content.ReadFromJsonAsync<List<Item>>();

ReadOnlySpan<byte> json = File.ReadAllBytes("data.json");
if (json.StartsWith("\uFEFF"u8))
{
    json = json[3..];
}
var item = JsonSerializer.Deserialize<Item>(json);

Find the exact position with Tool Kumar

1. Copy the exact input text, including its original whitespace. For HTTP failures, copy the raw response body.
2. Open the Tool Kumar JSON validator and paste the text. The document stays in your browser.
3. Read the line and column in the error or fixable-issue banner. For an invalid document, select Go to error to move the caret to that location.
4. Inspect the preceding token as well as the marked character. A parser marks where it discovered the problem; the missing quote, separator or value can start earlier.

Python character offsets are zero-based, while its displayed lines and columns are one-based. V8 string positions count UTF-16 code units; Go offsets count bytes read. Unicode text can make these offsets differ. Use the original text and its line/column context rather than copying an offset between runtimes.

Tool Kumar can classify some input as fixable loose JSON. Review a proposed change before using the JSON repair tool, then validate the result against your API contract. For empty input, inspect the producer or response because there is no character to select.

Frequently asked questions

Related errors and tools

• Unexpected token '<' / DOCTYPE is not valid JSON
• JSON.parse: unexpected character at line 1 column 1 of the JSON data
• Jackson: Unrecognized token / Unexpected character ('<' (code 60))
• Go JSON: invalid character looking for beginning of value
• All JSON error guides
• JSON syntax reference
• Generate C# classes from valid JSON

Standards and diagnostic sources

RFC 8259 and ECMA-404 define JSON syntax. Diagnostic wording belongs to the parser. The table links each cited message to its source.

• System.Text.Json source: stream reading skips a UTF-8 BOM