This helps stop C#'s HttpClient from puking on Icecast and Shoutcat servers that send a ICY header instead of an HTTP header.

IceCastICYHttpClientSupport.cs

// Main()
{
    var socketHandler = new SocketsHttpHandler {
        PlaintextStreamFilter = (filterContext, ct) => new ValueTask<Stream>(new HttpFixerDelegatingStream(filterContext.PlaintextStream))
    };

    var httpClient = new HttpClient(socketHandler);

    var httpGetter = await httpClient.GetAsync("http://icyserver:8000/my_awesome_stream.mp3", HttpCompletionOption.ResponseHeadersRead);
}

internal class HttpFixerDelegatingStream : DelegatingStream
{
    byte[] icyHeaderSignature = Encoding.ASCII.GetBytes("ICY");

    internal HttpFixerDelegatingStream(Stream innerStream) : base(innerStream) { }

    public override async ValueTask<int> ReadAsync(Memory<byte> buffer, CancellationToken cancellationToken = default)
    {
        var read = await base.ReadAsync(buffer, cancellationToken);        

        if (buffer.Span[0] == icyHeaderSignature[0] && buffer.Span[1] == icyHeaderSignature[1] && buffer.Span[2] == icyHeaderSignature[2])
        {
            var incomingHeader = Encoding.ASCII.GetString(buffer.ToArray(), 0, read);

            var replacedHeader = incomingHeader.Replace("ICY 200 OK", "HTTP/1.0 200 OK");

            var replacedHeaderBytes = Encoding.ASCII.GetBytes(replacedHeader);

            for (var i = 0; i < replacedHeaderBytes.Length; i++)
            {
                buffer.Span[i] = replacedHeaderBytes[i];
            }

            read = replacedHeaderBytes.Length;
        }

        return read;
    }
}

internal class DelegatingStream : Stream
{
    private readonly Stream _innerStream;

    #region Properties

    public override bool CanRead
    {
        get { return _innerStream.CanRead; }
    }

    public override bool CanSeek
    {
        get { return _innerStream.CanSeek; }
    }

    public override bool CanWrite
    {
        get { return _innerStream.CanWrite; }
    }

    public override long Length
    {
        get { return _innerStream.Length; }
    }

    public override long Position
    {
        get { return _innerStream.Position; }
        set { _innerStream.Position = value; }
    }

    public override int ReadTimeout
    {
        get { return _innerStream.ReadTimeout; }
        set { _innerStream.ReadTimeout = value; }
    }

    public override bool CanTimeout
    {
        get { return _innerStream.CanTimeout; }
    }

    public override int WriteTimeout
    {
        get { return _innerStream.WriteTimeout; }
        set { _innerStream.WriteTimeout = value; }
    }

    #endregion Properties

    protected DelegatingStream(Stream innerStream)
    {
        Debug.Assert(innerStream != null);

        _innerStream = innerStream;
    }

    protected override void Dispose(bool disposing)
    {
        if (disposing)
        {
            _innerStream.Dispose();
        }

        base.Dispose(disposing);
    }

    public override ValueTask DisposeAsync()
    {
        return _innerStream.DisposeAsync();
    }

    #region Read

    public override long Seek(long offset, SeekOrigin origin)
    {
        return _innerStream.Seek(offset, origin);
    }

    public override int Read(byte[] buffer, int offset, int count)
    {
        return _innerStream.Read(buffer, offset, count);
    }

    public override int Read(Span<byte> buffer)
    {
        return _innerStream.Read(buffer);
    }

    public override int ReadByte()
    {
        return _innerStream.ReadByte();
    }

    public override Task<int> ReadAsync(byte[] buffer, int offset, int count, CancellationToken cancellationToken)
    {
        return _innerStream.ReadAsync(buffer, offset, count, cancellationToken);
    }

    public override ValueTask<int> ReadAsync(Memory<byte> buffer, CancellationToken cancellationToken = default)
    {
        return _innerStream.ReadAsync(buffer, cancellationToken);
    }

    public override IAsyncResult BeginRead(byte[] buffer, int offset, int count, AsyncCallback? callback, object? state)
    {
        return _innerStream.BeginRead(buffer, offset, count, callback, state);
    }

    public override int EndRead(IAsyncResult asyncResult)
    {
        return _innerStream.EndRead(asyncResult);
    }

    public override void CopyTo(Stream destination, int bufferSize)
    {
        _innerStream.CopyTo(destination, bufferSize);
    }

    public override Task CopyToAsync(Stream destination, int bufferSize, CancellationToken cancellationToken)
    {
        return _innerStream.CopyToAsync(destination, bufferSize, cancellationToken);
    }

    #endregion Read

    #region Write

    public override void Flush()
    {
        _innerStream.Flush();
    }

    public override Task FlushAsync(CancellationToken cancellationToken)
    {
        return _innerStream.FlushAsync(cancellationToken);
    }

    public override void SetLength(long value)
    {
        _innerStream.SetLength(value);
    }

    public override void Write(byte[] buffer, int offset, int count)
    {
        _innerStream.Write(buffer, offset, count);
    }

    public override void Write(ReadOnlySpan<byte> buffer)
    {
        _innerStream.Write(buffer);
    }

    public override void WriteByte(byte value)
    {
        _innerStream.WriteByte(value);
    }

    public override Task WriteAsync(byte[] buffer, int offset, int count, CancellationToken cancellationToken)
    {
        return _innerStream.WriteAsync(buffer, offset, count, cancellationToken);
    }

    public override ValueTask WriteAsync(ReadOnlyMemory<byte> buffer, CancellationToken cancellationToken = default)
    {
        return _innerStream.WriteAsync(buffer, cancellationToken);
    }

    public override IAsyncResult BeginWrite(byte[] buffer, int offset, int count, AsyncCallback? callback, object? state)
    {
        return _innerStream.BeginWrite(buffer, offset, count, callback, state);
    }

    public override void EndWrite(IAsyncResult asyncResult)
    {
        _innerStream.EndWrite(asyncResult);
    }
    #endregion Write
}