Slot / ItemStack

A slot is the wire encoding of a single ItemStack plus its overlay of data components. The canonical codec is ItemStack.OPTIONAL_STREAM_CODEC (which the non-optional STREAM_CODEC delegates to and additionally rejects empty stacks).

Wire layout

+-------------------+----------------------+--------------------------+
| Item Count        | Item ID  (only if    | Components Patch         |
| (VarInt)          | Count > 0; VarInt)   | (only if Count > 0)      |
+-------------------+----------------------+--------------------------+

Item Count — VarInt. 0 (or any negative value, when accepted) means the slot is empty (ItemStack.EMPTY); the rest of the slot is then absent. Otherwise the value is the stack size; the encoder rejects sizes greater than the item's max_stack_size component.
Item ID — VarInt id within the minecraft:item registry, written via Item.STREAM_CODEC. Also accepts a Holder<Item> direct value (id 0 followed by the item payload), but vanilla servers and clients only send registry references in this position.
Components Patch — see Data Component patch.

The non-optional STREAM_CODEC is identical but throws on an empty stack (used in places where an empty value would be invalid, e.g. inside crafting recipes).

The "untrusted" variant OPTIONAL_UNTRUSTED_STREAM_CODEC wraps each individual component payload in a length prefix, so that a malformed component type cannot desync the rest of the buffer; it is used for stacks coming from the client.

Data Component patch

A patch is a partial mutation against an item type's prototype component map: it can add or replace entries, and it can mark prototype entries for removal.

+------------------------+------------------------+
| Added Count   (VarInt) | Removed Count (VarInt) |
+------------------------+------------------------+
| repeated Added Count times:                     |
|   Component Type ID (VarInt, see below)         |
|   Component Payload (codec depends on type)     |
+-------------------------------------------------+
| repeated Removed Count times:                   |
|   Component Type ID (VarInt)                    |
+-------------------------------------------------+

If both counts are zero the patch is empty and the stack carries the item type's prototype components unmodified.

Component Type ID is a VarInt id within the minecraft:data_component_type registry. The id space is fixed by the protocol version; a client and server on the same version share the table. The payload's encoding is determined entirely by the type — there is no length prefix in the trusted form.

In the untrusted variant (OPTIONAL_UNTRUSTED_STREAM_CODEC), each component payload is preceded by a VarInt byte length and decoded against a sliced buffer.

Removed entries

Each entry in the Removed section names a component the receiver should delete from its prototype map for this stack. It is an error to mark a component as both added and removed in the same patch.

Common component types

The full list lives in net.minecraft.core.component.DataComponents. The following are the most frequently observed on the wire. Each row gives the registry id (the minecraft: namespace is implied) and the payload encoding.

Identifier	Payload
`custom_data`	NBT Compound. Free-form server/data-pack data; not interpreted.
`max_stack_size`	VarInt (1–99).
`max_damage`	VarInt (≥ 1).
`damage`	VarInt (≥ 0).
`unbreakable`	Empty (`Unit`).
`custom_name`	Text Component.
`item_name`	Text Component (overrides default name).
`item_model`	Identifier.
`lore`	Prefixed array of Text Component.
`rarity`	VarInt enum (0=common, 1=uncommon, 2=rare, 3=epic).
`enchantments`	Map of (enchantment Holder → VarInt level) + Boolean `show_in_tooltip` (legacy).
`can_place_on`	Adventure-mode block predicate list.
`can_break`	Adventure-mode block predicate list.
`attribute_modifiers`	Prefixed array of attribute-modifier entries + display flag.
`custom_model_data`	Prefixed array of floats / flags / strings / colours.
`tooltip_display`	Boolean `hide_tooltip` + BitSet of hidden component types.
`repair_cost`	VarInt.
`creative_slot_lock`	Empty.
`enchantment_glint_override`	Boolean.
`food`	VarInt nutrition + Float saturation + Boolean `can_always_eat`.
`consumable`	Float consume seconds + animation enum + sound + particles + effects.
`use_remainder`	A nested ItemStack.
`use_cooldown`	Float seconds + Optional Identifier group.
`damage_resistant`	Identifier (damage-type tag).
`tool`	Rules list + default mining speed + damage per block.
`weapon`	Item damage per attack + disable-blocking-for ticks.
`enchantable`	VarInt enchantment value.
`equippable`	Equipment slot + sound + asset id + camera overlay + allowed entities.
`repairable`	HolderSet of allowed repair items.
`glider`	Empty.
`tooltip_style`	Identifier.
`death_protection`	Effects to apply on near-death save.
`dyed_color`	Int (RGB).
`map_id`	VarInt.
`map_decorations`	NBT Compound describing markers.
`charged_projectiles`	Prefixed array of ItemStack.
`bundle_contents`	Prefixed array of ItemStack.
`potion_contents`	Optional potion Holder + Optional Int colour + custom effects.
`container`	Prefixed array of slot/ItemStack pairs.
`stored_enchantments`	Same shape as `enchantments`; stored in books.
`block_state`	Map of String → String (block-state property overrides).
`bees`	Prefixed array of bee entries (NBT + ticks-in-hive + min ticks).
`lock`	NBT predicate.
`note_block_sound`	Identifier.
`instrument`	Holder of `instrument`.
`recipes`	Prefixed array of Identifier (unlocked recipes for knowledge book).
`firework_explosion`	Shape enum + colours + fade colours + trail + flicker.
`fireworks`	VarInt flight duration + array of explosions.
`profile`	Resolved game profile (for player heads).
`banner_patterns`	Array of (pattern Holder + DyeColor enum).
`base_color`	DyeColor enum.
`pot_decorations`	Four item-Holder ids (sherds).
`writable_book_content`	Array of String pages (raw author text).
`written_book_content`	Array of `Filterable<Text Component>` pages + title + author + generation.
`trim`	Material Holder + pattern Holder.
`debug_stick_state`	Map of block Holder → String (last selected property per block).
`bucket_entity_data`	NBT Compound.
`block_entity_data`	NBT Compound (synced into the bound block entity on placement).

For the exact codec of each component see the corresponding class under net.minecraft.world.item.component.* or the inline registration in DataComponents.java. Components flagged transient() in DataComponents are server-only and never appear in a patch on the wire.

NOTE: The data_component_type registry order is part of the protocol contract. Servers and clients of the same protocol version (26.1.2) share a fixed numeric id for each component identifier; ids are not negotiated.

Empty slot

An empty slot is encoded as a single VarInt 0 and nothing else. Item ID and the component patch are both omitted.

ON THIS PAGE

Slot / ItemStack#

Wire layout#

Data Component patch#

Removed entries#