Ultima Online Packets

Moongate speaks the classic Ultima Online wire protocol. Every message is a packet: a byte stream that starts with a one-byte opcode and is decoded into (or encoded from) a small C# record struct — incoming packets implement IIncomingPacket<T>, outgoing ones IOutgoingPacket. This page maps the protocol surface currently implemented for the ClassicUO 7.x client target.

implemented packets

incoming (client → server)

outgoing (server → client)

7.x

client target

Opcode	Name	Dir	Size	Description
`0x02`	Move Request	C → S	7 bytes (fixed)	One step or turn, with anti-fastwalk key.
`0x06`	Double Click	C → S	5 bytes (fixed)	The client double-clicked an entity, identified by its serial.
`0x09`	Single Click	C → S	5 bytes (fixed)	The client clicked an entity, identified by its serial.
`0x11`	Status Bar Info	S → C	Variable	The player's own status window.
`0x1B`	Login Confirm	S → C	37 bytes (fixed)	The first packet of the enter-world burst.
`0x1D`	Delete Object	S → C	5 bytes (fixed)	The entity is gone — stop drawing it.
`0x20`	Draw Game Player	S → C	19 bytes (fixed)	Positions and renders the player's own mobile.
`0x22`	Movement Ack	S → C	3 bytes (fixed)	Confirms the step with the client's sequence number.
`0x24`	Draw Container	S → C	7 bytes (fixed)	Opens the container's gump on the client.
`0x25`	Add Item To Container	S → C	21 bytes (fixed)	Drops one item into an already-open container gump.
`0x27`	Lift Reject	S → C	2 bytes (fixed)	The lift the client asked for is refused, and why.
`0x2E`	Worn Item	S → C	15 bytes (fixed)	Draws a single item on a mobile that the client already knows about.
`0x3A`	Skill Lock Change	C → S	Variable	The client sets the up/down/lock arrow on one skill.
`0x3A`	Skills	S → C	Variable	Skill list (0x3A), in the absolute-with-caps form (type 0x02): the client's whole skill list in one go.
`0x3C`	Container Content	S → C	Variable	Every item inside a container, in one variable-length packet.
`0x4E`	Personal Light Level	S → C	6 bytes (fixed)	The light radiating around the given mobile.
`0x4F`	Overall Light Level	S → C	2 bytes (fixed)	0 is full daylight, higher is darker.
`0x55`	Login Complete	S → C	1 bytes (fixed)	The "you are now in the world" marker that unblocks the client.
`0x5B`	Game Time	S → C	4 bytes (fixed)	The in-world clock shown to the client.
`0x5D`	Character Select	C → S	73 bytes (fixed)	The client picks an existing character slot to enter the world with.
`0x72`	War Mode	S → C	5 bytes (fixed)	Toggles the client's combat stance.
`0x73`	Ping	C → S	2 bytes (fixed)	The client sends this periodically with a rolling sequence byte and expects the server to echo it straight back, or it eventually drops the connection.
`0x73`	Ping Ack	S → C	2 bytes (fixed)	Echoes the client's keep-alive sequence byte straight back.
`0x78`	Draw Object	S → C	Variable	Draws a mobile and its equipped items on the client.
`0x80`	Account Login Request	C → S	62 bytes (fixed)	Credentials for the login server.
`0x82`	Login Denied	S → C	2 bytes (fixed)	Rejects the login with a protocol reason code.
`0x83`	Delete Character	C → S	39 bytes (fixed)	The client asks to delete the character in the given slot.
`0x85`	Character Delete Result	S → C	2 bytes (fixed)	Why a deletion was refused.
`0x86`	Character List Update	S → C	304 bytes (fixed)	The account's character list after it changed, so the client can redraw the selection screen.
`0x88`	Paperdoll	S → C	66 bytes (fixed)	Tells the client to open the character window for a mobile.
`0x8C`	Connect To Game Server	S → C	11 bytes (fixed)	Redirects the client to the game port with an auth key.
`0x91`	Game Server Login	C → S	65 bytes (fixed)	The auth key from the redirect plus the account credentials.
`0xA0`	Select Server	C → S	3 bytes (fixed)	The shard index the client picked from the server list.
`0xA8`	Server List	S → C	Variable	Advertises the available shards.
`0xA9`	Character List	S → C	Variable	The character slots followed by the starting cities, in the extended 7.0.13+ layout.
`0xB9`	Support Features	S → C	5 bytes (fixed)	Unlocks the client feature set at login, sent right before the character list.
`0xBC`	Season Change	S → C	3 bytes (fixed)	Sets the client's season and optionally plays the season-change sound.
`0xBD`	Client Version	C → S	Variable	The client answers the server's version request with its build string (e.g.
`0xBF`	General Information	C → S	Variable	A multiplexed request whose meaning is chosen by a leading `SubCommand` (ushort).
`0xBF/0x08`	Map Change	S → C	6 bytes (fixed)	Switches the client to the given map.
`0xBF/0x18`	Map Patches	S → C	41 bytes (fixed)	Declares the static/land map-diff block counts for the four classic facets.
`0xBF/0x19`	Stat Lock Info	S → C	12 bytes (fixed)	The up/down/lock state of the three stats, packed two bits each into a single byte.
`0xEF`	Login Seed	C → S	21 bytes (fixed)	Connection seed and client version, sent first by ClassicUO.
`0xF3`	World Item	S → C	24 bytes (fixed)	Draws an item lying in the world.
`0xF8`	Character Creation	C → S	106 bytes (fixed)	The new 106-byte creation packet sent by clients 7.0.16.0 and later.

How the protocol works

Opcodes. The first byte of every packet identifies it. The same opcode can mean different things per direction (for example 0x73 is a ping from the client and a ping acknowledgement from the server).
Fixed vs variable length. Most packets have a fixed size known from the opcode. Variable-length packets carry their total length as a big-endian ushort right after the opcode.
Login vs game phase. The client first talks to the login server (account auth, shard list, server select), receives a redirect (0x8C), then reconnects to the game server and enters the world.
The 0xBF multiplexer. General Information (0xBF) is a container: a ushort sub-command selects the actual meaning (map patches, stat locks, and many more). Moongate models each implemented sub-command as its own outgoing packet class.

How a packet is implemented

Each packet is a self-contained readonly record struct that knows how to read or write itself, and declares its documentation family with the [PacketDocumentation] attribute:

/// <summary>
/// Double click (0x06): the client double-clicked an entity, identified by its serial. 5 bytes fixed.
/// Whether the target is a mobile or an item is decided later from the serial's range.
/// </summary>
[PacketDocumentation(PacketFamilyType.InteractionKeepalive, Length = 5)]
public readonly record struct DoubleClickPacket(Serial Target) : IIncomingPacket<DoubleClickPacket>
{
    public static byte PacketId => 0x06;

    public static DoubleClickPacket Read(ref SpanReader reader)
    {
        reader.ReadByte(); // packet id
        var target = new Serial(reader.ReadUInt32());

        return new(target);
    }
}

The reference pages in this section are generated from those classes by scripts/generate-packet-docs.cs — run dotnet run scripts/generate-packet-docs.cs from the repository root after adding or changing a packet.

Packet families

Every packet belongs to a family — click a card for the family's packet list.

Table of Contents

Ultima Online Packets

How the protocol works

How a packet is implemented

Packet families

Login & shard select

Characters

Enter world

World state

Items & containers

Movement

Status & skills

Interaction & keepalive