dotnet/aspnetcore · error · RuntimeException

Error reading MessagePack data.

Error message

Error reading MessagePack data.

What it means

The outer try in parseMessages catches MessagePackException (from the msgpack-core library) and IOException raised while unpacking the array header, message type, and message body. Any such failure - bad varint, unexpected token, truncated array - is wrapped as 'Error reading MessagePack data.' This is the general parse-failure sink for MessagePack message bodies.

Solutions

1. Align server and client SignalR versions so the MessagePack message contracts match.
2. Ensure all types used as hub method arguments are serializable by the MessagePack protocol on both sides (register custom formatters identically).
3. Capture the failing payload (log the ByteBuffer before parse) and validate it with the MessagePack library standalone to localize the bad field.
4. Temporarily switch the hub protocol to JSON to confirm the issue is MessagePack-specific, then fix the contract.

Example fix

// before - argument type not MessagePack-serializable on client
hub.invoke(MyCustomType.class, "Send", new MyCustomType());

// after - register symmetric formatters, or use JSON
// server: services.AddSignalR().AddMessagePackProtocol();
// client: ensure MyCustomType is MessagePack-compatible,
// or
HubConnectionBuilder.create(url)
    .build(); // default JSON protocol on both ends

hub.onClosed(ex -> {
  if (ex != null && ex.getMessage().contains("Error reading MessagePack data")) {
    // body parse failure; capture the failing payload, align contracts
    log.error("MessagePack parse failure", ex);
  }
});

When it happens

Trigger: The MessagePack payload is structurally invalid: wrong array arity, unknown message-type int, missing fields for the declared message type, or a stream that ended mid-message after the length header passed.

Common situations: Protocol version mismatch (server sends a newer message shape the client cannot unpack); MessagePack library version skew; payload corrupted in transit but the length header happened to be valid; a server hub argument type that has no symmetric MessagePack representation on the client.

Related errors

• Unexpected message type
• Error reading JSON.
• Error reading length header.
• Error writing MessagePack data.
• Invalid invocation result kind.

AI-assisted analysis of dotnet/aspnetcore@3600ca084e (2026-08-11). Data as JSON: /api/errors/fe35e90eda2cc287. Report an issue: GitHub.

Appendix: source

Thrown at src/SignalR/clients/java/signalr/messagepack/src/main/java/com/microsoft/signalr/messagepack/MessagePackHubProtocol.java:137

                // Make sure that we actually read the right number of bytes
                int readBytes = (int) unpacker.getTotalReadBytes();
                if (readBytes != length) {
                    // Check what the last message was
                    // If it was an invocation binding failure, we have to correct the position of the buffer
                    if (hubMessages.get(hubMessages.size() - 1).getMessageType() == HubMessageType.INVOCATION_BINDING_FAILURE) {
                        // Cast to a Buffer to avoid the Java 9+ behavior where ByteBuffer.position(int) overrides Buffer.position(int),
                        // Returning a ByteBuffer rather than a Buffer. This causes issues on Android - see https://github.com/dotnet/aspnetcore/pull/26614
                        ((Buffer) payload).position(payload.position() + (length - readBytes));
                    } else {
                        throw new RuntimeException(String.format("MessagePack message was length %d but claimed to be length %d.", readBytes, length));
                    }
                }
                unpacker.close();
                // Cast to a Buffer to avoid the Java 9+ behavior where ByteBuffer.position(int) overrides Buffer.position(int),
                // Returning a ByteBuffer rather than a Buffer. This causes issues on Android - see https://github.com/dotnet/aspnetcore/pull/26614
                ((Buffer) payload).position(payload.position() + readBytes);
            } catch (MessagePackException | IOException ex) {
                throw new RuntimeException("Error reading MessagePack data.", ex);
            }
        }
        return hubMessages;
    }

    @Override
    public ByteBuffer writeMessage(HubMessage hubMessage) {
        HubMessageType messageType = hubMessage.getMessageType();

        try {
            byte[] message;
            switch (messageType) {
                case INVOCATION:
                    message = writeInvocationMessage((InvocationMessage) hubMessage);
                    break;
                case STREAM_ITEM:
                    message = writeStreamItemMessage((StreamItem) hubMessage);
                    break;

View on GitHub (pinned to 3600ca084e)