GUI API — Fabric 1.21.8

A Fabric mod that lets datapacks define and open chest GUIs via JSON files.
No client mod required. No macros. No external dependencies beyond Fabric API.

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Installation

1. Drop guiapi-1.0.4.jar into your mods/ folder.
2. Drop your datapack into world/datapacks/.
3. Run /reload or /guiapi reload.

Optionally install Mod Menu to see loaded GUIs and visually edit them in-game!

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Commands

Command	Description
/guiapi	Show help (same as /guiapi help)
/guiapi open <id>	Open a GUI for yourself
/guiapi open <id> <targets>	Open a GUI for target players
/guiapi list	List all loaded GUI definitions
/guiapi reload	Reload all datapack resources (including GUIs)
/guiapi var get <player> <key>	Get a player's runtime variable
/guiapi var set <player> <key> <value>	Set a player's runtime variable
/guiapi var clear <player>	Clear all runtime variables for a player
/guiapi help	Show command and JSON field reference in-game

Permission level 2 (OP) required by default (configurable in Mod Menu).

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File Location

GUI definition files go in:

data/<namespace>/gui/<name>.json

The GUI ID used in commands is <namespace>:<name> — matching the file path under gui/.

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JSON Schema

Top-level fields

Field	Type	Default	Description
title	string	"GUI"	Inventory title. Supports § color codes and placeholders.
rows	int 1–6	3	Number of rows (9 slots each).
tick_rate	int	0	Auto-refresh interval in ticks (e.g., 20 = 1s). Set 0 to disable.
close_on_move	boolean	false	If true, closes screen if player walks away (> 1.5 blocks).
filler	object	—	Background filler configuration (see below).
on_open	action[]	[]	Actions executed when the GUI is opened.
on_close	action[]	[]	Actions executed when the GUI is closed (any reason).
buttons	button[]	[]	List of button definitions.

Filler fields

Any empty slot in the inventory is automatically populated with this background item.

"filler": {
  "item": "minecraft:gray_stained_glass_pane",
  "name": " ",
  "glint": false,
  "hide_tooltip": true
}

Button fields

Field	Type	Default	Description
slot	int	0	Zero-based slot index (0–rows*9-1).
page	int	0	Which page this button appears on.
item	string	"minecraft:stone"	Item ID.
name	string	""	Display name. Supports color codes and placeholders.
lore	string[]	[]	Lore lines. Supports placeholders.
glint	boolean	false	Apply enchantment glint effect.
amount	string	"1"	Item stack count (supports placeholders like {var:counter}).
hide_tooltip	boolean	false	Hides item name and lore from hover tooltip.
hide_additional_tooltip	boolean	false	Hides attributes, enchantments, and clutter.
custom_model_data	int or object	—	Custom model data component (supports legacy int and 1.21.4+ composite object).
item_model	string	—	Custom item model component ID (1.21.2+).
click_type	string	"any"	Which click triggers actions: any · left · right · shift
condition	object	—	Visibility condition (see below).
actions	action[]	[close]	Actions executed in order on click. Supports "delay": int (ticks).
toggle	object	—	Toggle definition — replaces item/actions (see below).

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Placeholders

Supported in title, button name, lore, message values, and run_command values.

Placeholder	Resolves to
{player}	Player's display name
{gui}	GUI ID (namespace:name)
{page}	Current page index (0-based)
{page1}	Current page index (1-based)
{pages}	Total page count
{score:objective}	Player's score in the given scoreboard objective
{var:key}	Player's runtime variable key (empty string if unset)

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Action types

Any action can be delayed by adding "delay": int (in ticks) to its JSON block.

Type	value format	run_with	Description
run_command	Command string	player · console	Run a command. Default: player. Supports placeholders.
close	—	—	Close the GUI.
refresh	—	—	Refresh the current GUI inventory dynamically (no closing/flicker).
open_gui	namespace:name	—	Close and open another GUI.
message	Text string	—	Send a chat message to the player. Supports placeholders.
action_bar	Text string	—	Send an action bar message directly to the player.
sound	sound.id or sound.id:volume:pitch	—	Play a sound. Volume/pitch default to 1.0. Supports placeholders.
set_score	objective:value	—	Set player's scoreboard objective score directly (supports placeholders).
add_score	objective:value	—	Add score to player's scoreboard objective directly.
sub_score	objective:value	—	Subtract score from player's scoreboard objective directly.
take_item	itemId:amount	—	Deduct a specified amount of an item from the player's inventory.
add_effect	effect_id:duration:amplifier:particles	—	Give player status effect (duration in seconds, particles true/false).
remove_effect	effect_id	—	Remove a specific status effect from the player.
clear_effects	—	—	Clear all status effects from the player.
set_var	New value	—	Set a runtime variable. Requires "var": "key".
add_var	Integer to add	—	Add an integer to a runtime variable. Requires "var": "key".
sub_var	Integer to subtract	—	Subtract an integer from a runtime variable. Requires "var": "key".
reset_var	—	—	Delete a single runtime variable. Requires "var": "key".
clear_vars	—	—	Delete all runtime variables for this player.
next_page	—	—	Go to the next page.
prev_page	—	—	Go to the previous page.
goto_page	Page index (string)	—	Jump to a specific page.

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Condition types

Conditions control button visibility. Hidden buttons cannot be clicked.

Type	value format	Visible when
has_tag	Tag name	Player has the scoreboard tag
not_tag	Tag name	Player does not have the scoreboard tag
score_gt	"objective:threshold"	Player's score > threshold
score_lt	"objective:threshold"	Player's score < threshold
score_eq	"objective:value"	Player's score == value
var_eq	"key:value"	Runtime variable key equals value (string compare)
var_gt	"key:value"	Runtime variable key (int) > value
var_lt	"key:value"	Runtime variable key (int) < value
var_set	key	Runtime variable key is set (any value)
has_item	"itemId:amount"	Player has at least amount of itemId in inventory
not_item	"itemId:amount"	Player has less than amount of itemId in inventory
level_gt	value	Player's XP level > value
level_lt	value	Player's XP level < value
health_gt	value	Player's current health > value
health_lt	value	Player's current health < value
food_gt	value	Player's hunger level > value
food_lt	value	Player's hunger level < value

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Toggle buttons

A toggle button shows different item/name/lore/actions depending on a scoreboard tag on the player. Replace the item and actions fields with a toggle object.

Field	Type	Default	Description
tag	string	""	Scoreboard tag that stores the on/off state.
item_on / item_off	string	lime/gray dye	Item shown in each state.
name_on / name_off	string	§aEnabled / §7Disabled	Display name in each state.
lore_on / lore_off	string[]	[]	Lore in each state.
glint_on / glint_off	boolean	false	Glint in each state.
actions_on	action[]	[tag @s remove <tag>]	Actions executed on click while ON (turning OFF).
actions_off	action[]	[tag @s add <tag>]	Actions executed on click while OFF (turning ON).

Toggle actions also fully support the multi-action engine (separated by ; in the visual editor).

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Client-Side features (Optional)

Installing this mod on the client-side unlocks powerful, highly-polished user experience features:

1. In-Game GUI Editor

• Open the Mod Menu config screen to see a list of loaded GUIs.
• Click any GUI to open the GUI Editor Screen!
• Visually edit GUI title, rows, tick_rate, close_on_move, and background filler.
• Manage buttons list, add new buttons, and edit slot, item, amount, glint, lore (using ; split), multiple actions, and toggle properties.
• Click Apply & Back to open the Gui Save Loading Screen which finds the target datapack folder on the server, safely writes the JSON to disk, and reloads the API definitions. No edits are ever lost on rejoin!

2. Native Keybindings

Integrates natively with Minecraft's official controls menu (Options > Controls > Key Binds > GUI API):

• Accept Rules (Open GUI): Opens the default welcome GUI (Defaults to G).

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Examples

Please refer to the updated example-datapack directory in the repository sources for highly-polished, fully-annotated examples demonstrating auto-refreshing clocks, direct scoreboard trading, status effect controllers, and custom visual models!

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Building

chmod +x gradlew
./gradlew build
# Output: build/libs/guiapi-1.0.4.jar

Requires Java 21.

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License

MIT — see LICENSE.