Unity-Technologies/UnityCsReference · error · JSONParseException

Cannot parse json value starting with '{}' at {}

Error message

Cannot parse json value starting with '{}' at {}

What it means

ParseValue dispatches on the current character: '{' for object, '[' for array, '"' for string, '-' or digit for number, 't'/'f'/'n' for true/false/null. The default branch throws JSONParseException ("Cannot parse json value starting with '<first 5 chars>' at <pos>") for any other leading character. The placeholder shows the actual offending prefix so the bad byte is visible.

Source

Thrown at Editor/Mono/AssetStore/Json.cs:463

                    return ParseString();
                case '-':
                case '0':
                case '1':
                case '2':
                case '3':
                case '4':
                case '5':
                case '6':
                case '7':
                case '8':
                case '9':
                    return ParseNumber();
                case 't':
                case 'f':
                case 'n':
                    return ParseConstant();
                default:
                    throw new JSONParseException("Cannot parse json value starting with '" + json.Substring(idx, 5) + "' at " + PosMsg());
            }
        }

        private JSONValue ParseArray()
        {
            Next();
            SkipWs();
            List<JSONValue> arr = new List<JSONValue>();
            while (cur != ']')
            {
                arr.Add(ParseValue());
                SkipWs();
                if (cur == ',')
                {
                    Next();
                    SkipWs();
                }
            }

View on GitHub (pinned to 225b0fbdb5)

Solutions

1. Inspect the first characters of the input (the message already shows the first 5) to identify the stray byte/markup.
2. Strip a leading BOM and surrounding whitespace before parsing.
3. Verify the response Content-Type is JSON and that any compression was decoded.
4. Catch JSONParseException and surface the endpoint/length so misrouted responses are obvious.

Example fix

// before
var v = JSON.Parse(responseBody); // body starts with '<' (HTML)

// after
string body = responseBody.TrimStart('\uFEFF').Trim();
if (body.StartsWith("<"))
    throw new InvalidOperationException("Expected JSON, got markup: " + body.Substring(0, Math.Min(80, body.Length)));
var v = JSON.Parse(body);

string CleanJson(string raw) {
    var s = raw.TrimStart('\uFEFF').Trim();
    if (s.Length == 0 || (s[0] != '{' && s[0] != '[' && s[0] != '"' && s[0] != '-' && !char.IsDigit(s[0]) && s[0] != 't' && s[0] != 'f' && s[0] != 'n'))
        throw new InvalidOperationException("Input does not start with a JSON value: " + s.Substring(0, System.Math.Min(40, s.Length)));
    return s;
}

try { v = JSON.Parse(json); }
catch (JSONParseException ex) {
    // ex.Message shows the first 5 chars; log endpoint + content-type to spot HTML/BOM
    throw;
}

When it happens

Trigger: Leading byte-order mark (BOM), HTML/markup returned instead of JSON (e.g. '<' from an error page), unstripped leading whitespace that was not consumed, control characters, or an encoding mismatch producing unexpected bytes.

Common situations: Server returns an HTML error page with 200 status; UTF-8 BOM not stripped; gzip body not decompressed; binary garbage from a wrong endpoint.

Related errors

• Key not string type at {}
• Missing dict entry delimiter ':' at {}
• End of json while parsing at {}
• missing '"' to end string at {}
• End of json while parsing while parsing string at {}

AI-assisted analysis of Unity-Technologies/UnityCsReference@225b0fbdb5 (2026-08-13). Data as JSON: /api/errors/dc9c03e36b81c86b. Report an issue: GitHub.