private static bool LooksLikeNdjson(string text, int threshold = 2)
{
    int count = 0;
    using var sr = new StringReader(text);
    for (string line = sr.ReadLine(); line != null; line = sr.ReadLine())
    {
        line = line.Trim();
        if (line.Length == 0) continue;
        if (line.StartsWith("{") || line.StartsWith("["))
        {
            if (++count >= threshold) return true;
        }
        else
        {
            // a non-JSON token breaks NDJSON expectation early
            break;
        }
    }
    return false;
}

private static JsonFormatDetector.Format ClassifyJsonHeader(string head)
{
    // fingerprints
    if (head.IndexOf("\"type\":\"Topology\"", StringComparison.OrdinalIgnoreCase) >= 0 &&
        head.IndexOf("\"objects\"", StringComparison.OrdinalIgnoreCase) >= 0)
        return JsonFormatDetector.Format.TopoJson;

    if (head.IndexOf("\"spatialReference\"", StringComparison.OrdinalIgnoreCase) >= 0 ||
        head.IndexOf("\"geometryType\"", StringComparison.OrdinalIgnoreCase) >= 0 ||
        head.IndexOf("\"attributes\"", StringComparison.OrdinalIgnoreCase) >= 0)
        return JsonFormatDetector.Format.EsriJson;

    if (LooksLikeNdjson(head)) return JsonFormatDetector.Format.GeoJsonSeq;

    // GeoJSON: FeatureCollection/Feature or geometry w/ coordinates
    if (head.IndexOf("\"FeatureCollection\"", StringComparison.OrdinalIgnoreCase) >= 0 ||
        head.IndexOf("\"Feature\"", StringComparison.OrdinalIgnoreCase) >= 0 ||
        head.IndexOf("\"coordinates\"", StringComparison.OrdinalIgnoreCase) >= 0)
        return JsonFormatDetector.Format.GeoJson;

    return JsonFormatDetector.Format.Unknown;
}
``

Then replace your current fallback block in Run(...) with:


var head = ReadHeadUtf8(gisInputFilePath, maxBytes: 64 * 1024); // do not read whole file