Defining a Packet

A packet in Architectury is a vanilla CustomPacketPayload: a small object holding the data you want to send, plus a type id and a codec that reads and writes it. Define one as a record.

The payload record

public record OpenGuiPayload(BlockPos pos) implements CustomPacketPayload {
    // A unique id for this packet type.
    public static final Type<OpenGuiPayload> TYPE =
        new Type<>(Identifier.fromNamespaceAndPath(MyMod.MOD_ID, "open_gui"));

    // Reads and writes the payload's fields over the network.
    public static final StreamCodec<RegistryFriendlyByteBuf, OpenGuiPayload> CODEC =
        StreamCodec.composite(
            BlockPos.STREAM_CODEC, OpenGuiPayload::pos,
            OpenGuiPayload::new
        );

    @Override
    public Type<? extends CustomPacketPayload> type() {
        return TYPE;
    }
}

Three pieces make it work:

• TYPE - a CustomPacketPayload.Type wrapping a unique Identifier. This is what you pass when registering and what identifies the packet on the wire.
• CODEC - a StreamCodec that serializes the payload. StreamCodec.composite pairs each field's codec with its accessor, ending with the constructor that rebuilds the record.
• type() - returns TYPE.

Composing the codec

StreamCodec.composite takes each field as a (codec, getter) pair, then your constructor. For example, a payload with two fields:

public record SyncStatsPayload(int level, float xp) implements CustomPacketPayload {
    public static final Type<SyncStatsPayload> TYPE =
        new Type<>(Identifier.fromNamespaceAndPath(MyMod.MOD_ID, "sync_stats"));

    public static final StreamCodec<RegistryFriendlyByteBuf, SyncStatsPayload> CODEC =
        StreamCodec.composite(
            ByteBufCodecs.VAR_INT,   SyncStatsPayload::level,
            ByteBufCodecs.FLOAT,     SyncStatsPayload::xp,
            SyncStatsPayload::new
        );

    @Override
    public Type<? extends CustomPacketPayload> type() { return TYPE; }
}

ByteBufCodecs provides codecs for primitives and common types; many Minecraft types (like BlockPos) expose their own STREAM_CODEC.

Once you've defined a payload, see Sending & Receiving to wire it up.