Viper Anti-Dupe

Viper Anti-Dupe is a server-side Fabric mod that blocks item and block duplication exploits on Minecraft servers. It works only on the server: players do not need to install anything. Protection is applied through server-side guards on block interactions, entity behavior, hoppers, pistons, containers and incoming player packets, so common duplication methods are stopped before extra items can be created.

Supported versions

Minecraft	Java	Fabric Loader	Notes
1.21 - 1.21.11	21	0.19.3+	Full support, 25 blocked methods
26.1 - 26.1.2	25	0.19.3+	Full support, 25 blocked methods
26.2	25	0.19.3+	Full support, 25 blocked methods
26.3	25	0.19.5+	Full support, 26 blocked methods (adds shelf protection)

Download the file whose supported Minecraft versions match your server. Fabric API is required on all versions.

How it works

• Every known duplication method has its own on/off switch in the config file. Each switch defaults to blocking.
• When a blocked duplication attempt is detected, the action is cancelled and the attempt is written to the audit log with a timestamp, the player name and the dupe type.
• Suspicious inventory packets (desynced container clicks, invalid stack splits, invalid creative-mode stacks) are cancelled and the player's container is re-synced.
• A configurable list of known dupable items lets you flag risky items. The mod ships with a default list covering rails, carpets, concrete powder, shulker boxes, gravity blocks, TNT, books, tridents, bee nests and related items.
• Configuration can be reloaded in game without restarting the server.

Config semantics

In config/viper-anti-dupe.json, each dupe method key follows one rule:

• false = the method is BLOCKED (the mod prevents it). This is the default.
• true = the method is ALLOWED (the mod does not interfere with it).

Setting a method to true does not create an exploit; it only turns off this mod's protection for that method.

Blocked methods

Method	Config key	What it stops
Book dupe	BookDupe	Oversized or invalid book edits used to duplicate written books
Shulker box nesting	ShulkerBoxDupe	Shulker boxes hidden inside other containers beyond the nesting limit
Rail dupe	RailDupe	Piston-driven rail duplication
Carpet dupe	CarpetDupe	Piston-driven carpet duplication
Horse quick-move	HorseQuickMoveDupe	Shift-click transfers in horse inventories that duplicate items
Stack split	StackSplitDupe	Invalid slot-click splits that create extra items
Hopper dupe	HopperDupe	Suspicious hopper transfers
Piston dupe	PistonDupe	TNT and other blocks duplicated by piston movement
Entity dimension dupe	EntityDimensionDupe	Duplication through vehicle detach and dimension changes
Trident dupe	TridentDupe	Thrown-trident collision duplication
Bee dupe	BeeDupe	Breaking bee nests and beehives for extra drops
Tripwire dupe	TripwireDupe	Tripwire update and break duplication
String dupe	StringDupe	Tripwire string duplication, with optional legacy restoration
Lectern dupe	LecternDupe	Invalid lectern book operations and takes
Tripwire hook dupe	TripwireHookDupe	Tripwire hook duplication, with optional legacy restoration
Gravity block dupe	GravityBlockDupe	Falling sand, gravel, anvil and similar block duplication
NBT list dupe	NBTListDupe	Oversized or malformed NBT data beyond MaximumItemNbtBytes
Item frame dupe	ItemFrameDupe	Rapid item-frame breaks that duplicate the held item
Update suppression	UpdateSuppressionDupe	Update-suppression based duplication setups
Container desync	ContainerDesyncDupe	Desynced container transactions, re-syncs the player container
Death dupe	DeathDupe	Rapid repeated deaths used to duplicate drops
Chunk rollback	ChunkRollbackDupe	Chunk save-state rollback duplication
Portal dupe	PortalDupe	Portal travel duplication
Bundle dupe	BundleDupe	Invalid bundle content removal
Crafter dupe	CrafterDupe	Rapid crafter firing used to duplicate output
Shelf dupe	ShelfDupe	Rapid shelf swap and break duplication with the 26.3 shelf blocks (26.3 file only)

General config options

Option	Default	Description
ConfigVersion	1	Config format version, managed by the mod
ModEnabled	true	Master switch for all protection
LoggingEnabled	true	Write blocked attempts to the audit log
DetailedAuditLogging	false	Log allowed actions in addition to blocked ones
NotifyOperators	true	Notify online operators when something is blocked
CancelSuspiciousTransactions	true	Cancel suspicious inventory transactions instead of only logging them
RestoreLegacyStringDupeWhenAllowed	true	Restore legacy string behavior when the string method is allowed
RestoreLegacyTripwireHookDupeWhenAllowed	true	Restore legacy tripwire hook behavior when that method is allowed
MaximumBookPages	100	Maximum pages accepted in an edited book
MaximumBookPageLength	1024	Maximum characters accepted per book page
MaximumItemNbtBytes	2097152	Maximum NBT payload size accepted per item
MaximumContainerNestingDepth	8	Maximum container-in-container nesting depth
TransactionHistoryTicks	200	How long inventory transactions are remembered for checks
DuplicateEntityDetectionTicks	100	Time window used for duplicate entity detection
OperatorPermissionLevel	4	Reserved operator permission level setting
KnownDupableItems	(built-in list)	Item IDs flagged as known dupable, manageable with the item commands

New options are added to an existing config file automatically with their defaults; your other settings are kept.

Commands

All commands start with /viperantidupe and must be run by a player in game.

Command	Description
/viperantidupe reload	Reload the config file without restarting
/viperantidupe status	Show mod version, enabled state, and blocked/allowed method counts
/viperantidupe methods	List every dupe method with its current BLOCKED or ALLOWED state
/viperantidupe get <method>	Show whether one method is ALLOWED or BLOCKED
/viperantidupe set <method> <true/false>	Allow (true) or block (false) one method, saved to the config
/viperantidupe item get <id>	Check whether an item ID is flagged as known dupable
/viperantidupe item set <id> <true/false>	Flag or unflag an item ID as known dupable
/viperantidupe dumpitems	Write every registered item ID to config/viper-anti-dupe-items.json
/viperantidupe dumpknown	Write the known-dupable item list to config/viper-anti-dupe-known-items.json
/viperantidupe audit	Show the 50 most recent audit log entries in chat

Installation

1. Install Fabric Loader on your server (0.19.3 or newer for 1.21.x, 26.1.x and 26.2; 0.19.5 or newer for 26.3).
2. Install Fabric API for your Minecraft version.
3. Put the matching Viper Anti-Dupe file into the server mods folder.
4. Start the server once so config/viper-anti-dupe.json is created, then adjust the toggles to taste.
5. Run /viperantidupe reload in game after editing the config, or restart the server.

Audit entries are also appended to the log file configured by the logging options.