dotnet/aspnetcore · error · RuntimeException

Extension types are not supported

Error message

Extension types are not supported

What it means

The MessagePack reader explicitly refuses EXTENSION value types — only nil/boolean/integer/float/string/binary/array/map are handled. Any extension-type byte encountered in a value triggers this throw. MessagePack extension types are used for things like timestamps and custom user types.

Source

Thrown at src/SignalR/clients/java/signalr/messagepack/src/main/java/com/microsoft/signalr/messagepack/MessagePackHubProtocol.java:633

                    if (itemType == null) {
                        return null;
                    }
                    return objectMapper.readValue(payloadBytes, payload.position() + (int) readBytesStart, (int) (unpacker.getTotalReadBytes() - readBytesStart),
                            typeFactory.constructType(itemType));
                } else {
                    // This is an inner call to readValue - we just need to read the right number of bytes
                    // We can return null, and the outermost call will know how many bytes to give to objectMapper.
                    return null;
                }
            case EXTENSION:
                /*
                ExtensionTypeHeader extension = unpacker.unpackExtensionTypeHeader();
                byte[] extensionValue = new byte[extension.getLength()];
                unpacker.readPayload(extensionValue);
                //Convert this to an object?
                item = extensionValue;
                */
                throw new RuntimeException("Extension types are not supported");
            default:
                return null;
        }
        // If itemType was null, we were just in this method to advance the buffer. return null.
        if (itemType == null) {
            return null;
        }
        // If we get here, the item isn't a map or a collection/array, so we use the Class to cast it
        if (itemClass.isPrimitive()) {
            return Utils.toPrimitive(itemClass, item);
        }
        return itemClass.cast(item);
    }

    private void writeValue(Object o, MessagePacker packer) throws IOException {
        packer.addPayload(objectMapper.writeValueAsBytes(o));
    }
}

View on GitHub (pinned to 294cab2f9b)

Solutions

1. Encode timestamps/dates as ISO-8601 strings or epoch numbers instead of MessagePack ext.
2. Configure the server's MessagePack options to not use extension types for dates.
3. Avoid sending custom extension-typed values across the hub boundary.

Example fix

// before: server emits DateTime as msgpack timestamp ext

// after: server sends DateTime as ISO-8601 string
public record Dto(string timestampUtc);

// C# server - serialize dates as strings, not msgpack ext
public record Dto([property: JsonExtensionData] ... string timestampUtc);

// Java
hubConnection.start().subscribe(ok -> {}, err -> {
    if (err.getMessage() != null && err.getMessage().contains("Extension types")) {
        // server is emitting msgpack ext (likely timestamps); switch to string dates
    }
});

When it happens

Trigger: A peer sends a MessagePack extension type (e.g. a timestamp ext) in a hub argument or value; a custom encoder uses extension types; jackson-datatype-jsr310 producing ext-encoded timestamps.

Common situations: Server serializing DateTime/OffsetDateTime as a MessagePack timestamp extension; custom MessagePack encoder on the server; sending types whose serializer emits ext codes.

Related errors

• Cannot read message size.
• Messages bigger than 2GB are not supported.
• Incomplete message.
• MessagePack message was length %d but claimed to be length %
• Error reading length header.

AI-assisted analysis of dotnet/aspnetcore@294cab2f9b (2026-08-06). Data as JSON: /api/errors/94b47bd522963f98. Report an issue: GitHub.