dotnet/yarp · error · Exception

Expected to receive EndOfMessage = true.

Error message

Expected to receive EndOfMessage = true.

What it means

Asserted in WebSocketsScenario when a received frame's EndOfMessage flag is false. Each test message is small and sent as a single complete frame, so the echo must also arrive as a single frame with EndOfMessage true. A false flag means the frame was fragmented across reads, which the scenario does not reassemble.

Source

Thrown at testassets/TestClient/Scenarios/WebSocketsScenario.cs:48

        stopwatch.Restart();
        for (var i = 0; i < 256; i++)
        {
            var textToSend = $"Hello {i}";
            var numBytes = Encoding.UTF8.GetBytes(textToSend, buffer.AsSpan());
            await client.SendAsync(new ArraySegment<byte>(buffer, 0, numBytes),
                WebSocketMessageType.Text,
                endOfMessage: true,
                cancellation);

            var message = await client.ReceiveAsync(buffer, cancellation);
            if (message.MessageType != WebSocketMessageType.Text)
            {
                throw new Exception($"Expected to receive a text message, got '{message.MessageType}' instead.");
            }

            if (!message.EndOfMessage)
            {
                throw new Exception("Expected to receive EndOfMessage = true.");
            }

            var text = Encoding.UTF8.GetString(buffer.AsSpan(0, message.Count));
            if (text != textToSend)
            {
                throw new Exception($"Expected to receive '{textToSend}', but got '{text}'.");
            }

            Console.Write(".");
        }

        Console.WriteLine();
        Console.WriteLine($"Completed 256 text messages in {stopwatch.ElapsedMilliseconds} ms.");

        Console.WriteLine("Sending binary messages...");
        stopwatch.Restart();
        for (var i = 0; i < 256; i++)
        {

View on GitHub (pinned to bd11867bee)

Solutions

  1. Use the matching test backend that returns single complete frames.
  2. Disable WebSocket extensions (e.g. permessage-deflate) on the client or server if they cause fragmentation.
  3. Remove or reconfigure intermediaries that re-chunk frames.
  4. If you adapt the scenario, loop ReceiveAsync until EndOfMessage to support fragmentation.
Defensive patterns

Strategy: validation

Validate before calling

// If you adapt the scenario, reassemble fragmented messages instead of asserting EndOfMessage.
async Task<(string text, WebSocketMessageType type)> ReceiveFullAsync(WebSocket ws, byte[] buf, CancellationToken ct) {
    using var ms = new MemoryStream();
    WebSocketReceiveResult r;
    do {
        r = await ws.ReceiveAsync(buf, ct);
        ms.Write(buf, 0, r.Count);
        if (r.MessageType == WebSocketMessageType.Close) throw new IOException("peer closed");
    } while (!r.EndOfMessage);
    return (Encoding.UTF8.GetString(ms.ToArray()), r.MessageType);
}

Try / catch

try { await WebSocketsScenario.RunAsync(client, args, cancellation); }
catch (Exception ex) when (ex.Message.Contains("EndOfMessage")) {
    Console.Error.WriteLine("Frames arrived fragmented; disable permessage-deflate or remove re-chunking proxy.");
    return 1;
}

Prevention

When it happens

Trigger: Running WebSocketsScenario against an endpoint or intermediary that fragments a small message into multiple frames, so the first ReceiveAsync sees EndOfMessage false.

Common situations: A proxy that re-chunks WebSocket frames; an echo backend that splits messages; an extension (permessage-deflate) that changes framing; large effective frame size after compression causing fragmentation.

Related errors

AI-assisted analysis of dotnet/yarp@bd11867bee (2026-08-13). Data as JSON: /api/errors/144ef8f66db94298. Report an issue: GitHub.