diff --git a/BauSystem/BauSystem_Main/build.gradle.kts b/BauSystem/BauSystem_Main/build.gradle.kts
index f51b33cd..6d594c79 100644
--- a/BauSystem/BauSystem_Main/build.gradle.kts
+++ b/BauSystem/BauSystem_Main/build.gradle.kts
@@ -34,6 +34,7 @@ dependencies {
     compileOnly(libs.classindex)
     annotationProcessor(libs.classindex)
     compileOnly(project(":SpigotCore", "default"))
+    compileOnly(project(":KotlinCore", "default"))
 
     compileOnly(libs.axiom)
     compileOnly(libs.authlib)
diff --git a/BauSystem/BauSystem_Main/src/de/steamwar/bausystem/features/TestCommand.java b/BauSystem/BauSystem_Main/src/de/steamwar/bausystem/features/TestCommand.java
new file mode 100644
index 00000000..2aa35725
--- /dev/null
+++ b/BauSystem/BauSystem_Main/src/de/steamwar/bausystem/features/TestCommand.java
@@ -0,0 +1,38 @@
+/*
+ *   This file is a part of the SteamWar software.
+ *
+ *   Copyright (C) 2026 SteamWar.de-Serverteam
+ *
+ *   This program is free software: you can redistribute it and/or modify
+ *   it under the terms of the GNU Affero General Public License as published by
+ *   the Free Software Foundation, either version 3 of the License, or
+ *   (at your option) any later version.
+ *
+ *   This program is distributed in the hope that it will be useful,
+ *   but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ *   GNU Affero General Public License for more details.
+ *
+ *   You should have received a copy of the GNU Affero General Public License
+ *   along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+package de.steamwar.bausystem.features;
+
+import de.steamwar.command.SWCommand;
+import de.steamwar.kotlin.ui.TestInv;
+import de.steamwar.linkage.Linked;
+import org.bukkit.entity.Player;
+
+@Linked
+public class TestCommand extends SWCommand {
+    public TestCommand() {
+        super("supertestcommand");
+    }
+
+    @Register
+    public void test(Player player) {
+        TestInv inv = new TestInv(player);
+        inv.open();
+    }
+}
diff --git a/BauSystem/build.gradle.kts b/BauSystem/build.gradle.kts
index de9fe1b8..f2843bcf 100644
--- a/BauSystem/build.gradle.kts
+++ b/BauSystem/build.gradle.kts
@@ -37,5 +37,7 @@ tasks.register<DevServer>("DevBau21") {
     dependsOn(":SpigotCore:shadowJar")
     dependsOn(":BauSystem:shadowJar")
     dependsOn(":SchematicSystem:shadowJar")
+    dependsOn(":KotlinCore:shadowJar")
     template = "Bau21"
+    worldName = "73"
 }
diff --git a/KotlinCore/build.gradle.kts b/KotlinCore/build.gradle.kts
index 6c56593d..8467d3ac 100644
--- a/KotlinCore/build.gradle.kts
+++ b/KotlinCore/build.gradle.kts
@@ -1,3 +1,5 @@
+import org.jetbrains.kotlin.gradle.tasks.KotlinCompile
+
 /*
  *   This file is a part of the SteamWar software.
  *
@@ -41,3 +43,8 @@ dependencies {
     implementation(libs.mysql)
     implementation("org.slf4j:slf4j-simple:2.0.17")
 }
+val compileKotlin: KotlinCompile by tasks
+
+compileKotlin.compilerOptions {
+    freeCompilerArgs.set(listOf("-XXLanguage:+ContextParameters"))
+}
\ No newline at end of file
diff --git a/KotlinCore/src/de/steamwar/kotlin/ui/Observer.kt b/KotlinCore/src/de/steamwar/kotlin/ui/Observer.kt
new file mode 100644
index 00000000..569aad9b
--- /dev/null
+++ b/KotlinCore/src/de/steamwar/kotlin/ui/Observer.kt
@@ -0,0 +1,63 @@
+/*
+ *   This file is a part of the SteamWar software.
+ *
+ *   Copyright (C) 2026 SteamWar.de-Serverteam
+ *
+ *   This program is free software: you can redistribute it and/or modify
+ *   it under the terms of the GNU Affero General Public License as published by
+ *   the Free Software Foundation, either version 3 of the License, or
+ *   (at your option) any later version.
+ *
+ *   This program is distributed in the hope that it will be useful,
+ *   but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ *   GNU Affero General Public License for more details.
+ *
+ *   You should have received a copy of the GNU Affero General Public License
+ *   along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+package de.steamwar.kotlin.ui
+
+import kotlin.properties.ReadWriteProperty
+import kotlin.reflect.KProperty
+
+class Observer<T>(private var value: T): ReadWriteProperty<Any?, T> {
+    private val listeners = mutableSetOf<RenderBoundary>()
+
+    fun removeListener(render: RenderBoundary) {
+        listeners.remove(render)
+    }
+
+    override fun getValue(thisRef: Any?, property: KProperty<*>): T {
+        return value
+    }
+
+    override fun setValue(thisRef: Any?, property: KProperty<*>, value: T) {
+        this.value = value
+        notifyListeners()
+    }
+
+    fun set(value: T) {
+        this.value = value
+        notifyListeners()
+    }
+
+    fun get() = value
+
+    fun update(update: (T) -> T) {
+        this.value = update(value)
+        notifyListeners()
+    }
+
+    private fun notifyListeners() {
+        listeners.forEach { it.update() }
+    }
+
+    context(render: RenderBoundary)
+    fun listen(): Observer<T> {
+        listeners.add(render)
+        render.observers.add(this)
+        return this
+    }
+}
diff --git a/KotlinCore/src/de/steamwar/kotlin/ui/RenderBoundary.kt b/KotlinCore/src/de/steamwar/kotlin/ui/RenderBoundary.kt
new file mode 100644
index 00000000..5074dad5
--- /dev/null
+++ b/KotlinCore/src/de/steamwar/kotlin/ui/RenderBoundary.kt
@@ -0,0 +1,40 @@
+/*
+ *   This file is a part of the SteamWar software.
+ *
+ *   Copyright (C) 2026 SteamWar.de-Serverteam
+ *
+ *   This program is free software: you can redistribute it and/or modify
+ *   it under the terms of the GNU Affero General Public License as published by
+ *   the Free Software Foundation, either version 3 of the License, or
+ *   (at your option) any later version.
+ *
+ *   This program is distributed in the hope that it will be useful,
+ *   but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ *   GNU Affero General Public License for more details.
+ *
+ *   You should have received a copy of the GNU Affero General Public License
+ *   along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+package de.steamwar.kotlin.ui
+
+abstract class RenderBoundary(private val parent: RenderBoundary?) {
+    val observers = mutableSetOf<Observer<*>>()
+
+    fun update() {
+        render()
+        updateInv()
+    }
+
+    protected open fun updateInv() {
+        parent?.updateInv()
+    }
+
+    abstract fun render()
+
+    open fun destroy() {
+        observers.forEach { it.removeListener(this) }
+        observers.clear()
+    }
+}
\ No newline at end of file
diff --git a/KotlinCore/src/de/steamwar/kotlin/ui/RenderMarker.kt b/KotlinCore/src/de/steamwar/kotlin/ui/RenderMarker.kt
new file mode 100644
index 00000000..be7aaaed
--- /dev/null
+++ b/KotlinCore/src/de/steamwar/kotlin/ui/RenderMarker.kt
@@ -0,0 +1,23 @@
+/*
+ *   This file is a part of the SteamWar software.
+ *
+ *   Copyright (C) 2026 SteamWar.de-Serverteam
+ *
+ *   This program is free software: you can redistribute it and/or modify
+ *   it under the terms of the GNU Affero General Public License as published by
+ *   the Free Software Foundation, either version 3 of the License, or
+ *   (at your option) any later version.
+ *
+ *   This program is distributed in the hope that it will be useful,
+ *   but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ *   GNU Affero General Public License for more details.
+ *
+ *   You should have received a copy of the GNU Affero General Public License
+ *   along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+package de.steamwar.kotlin.ui
+
+@DslMarker
+annotation class RenderMarker()
diff --git a/KotlinCore/src/de/steamwar/kotlin/ui/TestInv.kt b/KotlinCore/src/de/steamwar/kotlin/ui/TestInv.kt
new file mode 100644
index 00000000..14a0c959
--- /dev/null
+++ b/KotlinCore/src/de/steamwar/kotlin/ui/TestInv.kt
@@ -0,0 +1,49 @@
+/*
+ *   This file is a part of the SteamWar software.
+ *
+ *   Copyright (C) 2026 SteamWar.de-Serverteam
+ *
+ *   This program is free software: you can redistribute it and/or modify
+ *   it under the terms of the GNU Affero General Public License as published by
+ *   the Free Software Foundation, either version 3 of the License, or
+ *   (at your option) any later version.
+ *
+ *   This program is distributed in the hope that it will be useful,
+ *   but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ *   GNU Affero General Public License for more details.
+ *
+ *   You should have received a copy of the GNU Affero General Public License
+ *   along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+package de.steamwar.kotlin.ui
+
+import de.steamwar.kotlin.ui.components.item
+import org.bukkit.Material
+import org.bukkit.entity.Player
+import org.bukkit.inventory.ItemStack
+
+class TestInv(player: Player): UIInventory(player) {
+
+    override fun render() {
+        inventory(3, title = "TEST_TITLE") {
+            group {
+                item {
+                    var test by Test.test.listen()
+                    x = 1
+                    y = 1
+                    item = ItemStack.of(Material.STONE, test)
+
+                    onClick {
+                        test++
+                    }
+                }
+            }
+        }
+    }
+}
+
+object Test {
+    val test = Observer(1)
+}
diff --git a/KotlinCore/src/de/steamwar/kotlin/ui/UIInventory.kt b/KotlinCore/src/de/steamwar/kotlin/ui/UIInventory.kt
new file mode 100644
index 00000000..a8fbc09b
--- /dev/null
+++ b/KotlinCore/src/de/steamwar/kotlin/ui/UIInventory.kt
@@ -0,0 +1,53 @@
+/*
+ *   This file is a part of the SteamWar software.
+ *
+ *   Copyright (C) 2026 SteamWar.de-Serverteam
+ *
+ *   This program is free software: you can redistribute it and/or modify
+ *   it under the terms of the GNU Affero General Public License as published by
+ *   the Free Software Foundation, either version 3 of the License, or
+ *   (at your option) any later version.
+ *
+ *   This program is distributed in the hope that it will be useful,
+ *   but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ *   GNU Affero General Public License for more details.
+ *
+ *   You should have received a copy of the GNU Affero General Public License
+ *   along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+package de.steamwar.kotlin.ui
+
+import de.steamwar.kotlin.KotlinCore
+import de.steamwar.kotlin.ui.context.WindowContext
+import net.kyori.adventure.text.event.ClickEvent
+import org.bukkit.Bukkit
+import org.bukkit.entity.Player
+import org.bukkit.event.EventHandler
+import org.bukkit.event.Listener
+import org.bukkit.event.inventory.InventoryClickEvent
+import org.bukkit.event.inventory.InventoryCloseEvent
+import org.bukkit.event.inventory.InventoryType
+
+abstract class UIInventory(val player: Player) {
+    lateinit var window: UIWindow
+
+    abstract fun render()
+
+    fun open() {
+        if (!::window.isInitialized) {
+            render()
+        }
+
+        window.open()
+    }
+
+    protected fun inventory(size: Int, title: String, init: WindowContext.() -> Unit) {
+        window = UIWindow(size, title, player, init)
+    }
+
+    protected fun inventory(type: InventoryType, title: String, init: WindowContext.() -> Unit) {
+        window = UIWindow(type, title, player, init)
+    }
+}
\ No newline at end of file
diff --git a/KotlinCore/src/de/steamwar/kotlin/ui/UIWindow.kt b/KotlinCore/src/de/steamwar/kotlin/ui/UIWindow.kt
new file mode 100644
index 00000000..cf28f4cb
--- /dev/null
+++ b/KotlinCore/src/de/steamwar/kotlin/ui/UIWindow.kt
@@ -0,0 +1,83 @@
+/*
+ *   This file is a part of the SteamWar software.
+ *
+ *   Copyright (C) 2026 SteamWar.de-Serverteam
+ *
+ *   This program is free software: you can redistribute it and/or modify
+ *   it under the terms of the GNU Affero General Public License as published by
+ *   the Free Software Foundation, either version 3 of the License, or
+ *   (at your option) any later version.
+ *
+ *   This program is distributed in the hope that it will be useful,
+ *   but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ *   GNU Affero General Public License for more details.
+ *
+ *   You should have received a copy of the GNU Affero General Public License
+ *   along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+package de.steamwar.kotlin.ui
+
+import de.steamwar.kotlin.KotlinCore
+import de.steamwar.kotlin.ui.context.WindowContext
+import net.kyori.adventure.text.Component
+import org.bukkit.Bukkit
+import org.bukkit.entity.Player
+import org.bukkit.event.EventHandler
+import org.bukkit.event.Listener
+import org.bukkit.event.inventory.InventoryClickEvent
+import org.bukkit.event.inventory.InventoryCloseEvent
+import org.bukkit.event.inventory.InventoryType
+import org.bukkit.inventory.Inventory
+
+class UIWindow(val bukkitInv: Inventory, val player: Player, val render: WindowContext.() -> Unit): Listener {
+    val onClicks = mutableMapOf<Int, (event: InventoryClickEvent) -> Unit>()
+    private var updating = false
+
+    constructor(size: Int, title: String, player: Player, render: WindowContext.() -> Unit): this(
+        Bukkit.createInventory(null, size * 9, Component.translatable(title)), player, render
+    )
+
+    constructor(type: InventoryType, title: String, player: Player, render: WindowContext.() -> Unit): this(
+        Bukkit.createInventory(null, type, Component.translatable(title)), player, render
+    )
+
+    fun open() {
+        render(getContext())
+        openInv()
+    }
+
+    fun openInv() {
+        updating = true
+        player.openInventory(bukkitInv)
+        Bukkit.getPluginManager().registerEvents(this, KotlinCore.plugin)
+        updating = false
+    }
+
+    @EventHandler
+    fun onInventoryClick(event: InventoryClickEvent) {
+        if (event.whoClicked != player) return
+
+        event.isCancelled = true
+        onClicks[event.slot]?.invoke(event)
+    }
+
+    @EventHandler
+    fun onInventoryClose(event: InventoryCloseEvent) {
+        if (event.player != player) return
+
+        onClose()
+
+        InventoryClickEvent.getHandlerList().unregister(this)
+        InventoryCloseEvent.getHandlerList().unregister(this)
+    }
+
+    fun getContext() = WindowContext(this)
+
+    fun onClose() {
+        if (updating) return
+
+        onClicks.clear()
+    }
+}
\ No newline at end of file
diff --git a/KotlinCore/src/de/steamwar/kotlin/ui/components/ItemContext.kt b/KotlinCore/src/de/steamwar/kotlin/ui/components/ItemContext.kt
new file mode 100644
index 00000000..3a0407a1
--- /dev/null
+++ b/KotlinCore/src/de/steamwar/kotlin/ui/components/ItemContext.kt
@@ -0,0 +1,50 @@
+/*
+ *   This file is a part of the SteamWar software.
+ *
+ *   Copyright (C) 2026 SteamWar.de-Serverteam
+ *
+ *   This program is free software: you can redistribute it and/or modify
+ *   it under the terms of the GNU Affero General Public License as published by
+ *   the Free Software Foundation, either version 3 of the License, or
+ *   (at your option) any later version.
+ *
+ *   This program is distributed in the hope that it will be useful,
+ *   but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ *   GNU Affero General Public License for more details.
+ *
+ *   You should have received a copy of the GNU Affero General Public License
+ *   along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+package de.steamwar.kotlin.ui.components
+
+import de.steamwar.kotlin.ui.RenderBoundary
+import de.steamwar.kotlin.ui.RenderMarker
+import de.steamwar.kotlin.ui.context.GroupContext
+import org.bukkit.Material
+import org.bukkit.event.inventory.InventoryClickEvent
+import org.bukkit.inventory.ItemStack
+
+@RenderMarker
+class ItemContext(val parent: GroupContext, val renderFunc: ItemContext.() -> Unit): RenderBoundary(parent) {
+    override fun render() {
+        renderFunc(this)
+        parent.renderItem(x, y, item, onClick)
+    }
+
+    init {
+        render()
+    }
+
+    var item: ItemStack = ItemStack.of(Material.AIR)
+    var onClick: (event: InventoryClickEvent) -> Unit = {}
+    var x: Int = 0
+    var y: Int = 0
+
+    fun onClick(func: (event: InventoryClickEvent) -> Unit) {
+        onClick = func
+    }
+}
+
+fun GroupContext.item(init: ItemContext.() -> Unit) = children.add(ItemContext(this, init))
diff --git a/KotlinCore/src/de/steamwar/kotlin/ui/context/GroupContext.kt b/KotlinCore/src/de/steamwar/kotlin/ui/context/GroupContext.kt
new file mode 100644
index 00000000..a923a520
--- /dev/null
+++ b/KotlinCore/src/de/steamwar/kotlin/ui/context/GroupContext.kt
@@ -0,0 +1,53 @@
+/*
+ *   This file is a part of the SteamWar software.
+ *
+ *   Copyright (C) 2026 SteamWar.de-Serverteam
+ *
+ *   This program is free software: you can redistribute it and/or modify
+ *   it under the terms of the GNU Affero General Public License as published by
+ *   the Free Software Foundation, either version 3 of the License, or
+ *   (at your option) any later version.
+ *
+ *   This program is distributed in the hope that it will be useful,
+ *   but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ *   GNU Affero General Public License for more details.
+ *
+ *   You should have received a copy of the GNU Affero General Public License
+ *   along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+package de.steamwar.kotlin.ui.context
+
+import de.steamwar.kotlin.ui.RenderBoundary
+import de.steamwar.kotlin.ui.RenderMarker
+import org.bukkit.event.inventory.InventoryClickEvent
+import org.bukkit.inventory.ItemStack
+
+@RenderMarker
+open class GroupContext(val parent: GroupContext?, val init: GroupContext.() -> Unit): RenderBoundary(parent) {
+    val children = mutableListOf<RenderBoundary>()
+
+    override fun destroy() {
+        children.forEach { it.destroy() }
+        super.destroy()
+    }
+
+    init {
+        init(this)
+    }
+
+    open fun renderItem(x: Int, y: Int, item: ItemStack, onClick: (event: InventoryClickEvent) -> Unit) {
+        parent?.renderItem(x, y, item, onClick)
+    }
+
+    fun group(init: GroupContext.() -> Unit) {
+        children.add(GroupContext(this, init))
+    }
+
+    override fun render() {
+        children.forEach { it.destroy() }
+        children.clear()
+        init(this)
+    }
+}
\ No newline at end of file
diff --git a/KotlinCore/src/de/steamwar/kotlin/ui/context/WindowContext.kt b/KotlinCore/src/de/steamwar/kotlin/ui/context/WindowContext.kt
new file mode 100644
index 00000000..d77b1ef2
--- /dev/null
+++ b/KotlinCore/src/de/steamwar/kotlin/ui/context/WindowContext.kt
@@ -0,0 +1,39 @@
+/*
+ *   This file is a part of the SteamWar software.
+ *
+ *   Copyright (C) 2026 SteamWar.de-Serverteam
+ *
+ *   This program is free software: you can redistribute it and/or modify
+ *   it under the terms of the GNU Affero General Public License as published by
+ *   the Free Software Foundation, either version 3 of the License, or
+ *   (at your option) any later version.
+ *
+ *   This program is distributed in the hope that it will be useful,
+ *   but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ *   GNU Affero General Public License for more details.
+ *
+ *   You should have received a copy of the GNU Affero General Public License
+ *   along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+package de.steamwar.kotlin.ui.context
+
+import de.steamwar.kotlin.ui.RenderMarker
+import de.steamwar.kotlin.ui.UIWindow
+import org.bukkit.event.inventory.InventoryClickEvent
+import org.bukkit.inventory.ItemStack
+
+@RenderMarker
+class WindowContext(val window: UIWindow): GroupContext(null, {}) {
+    override fun renderItem(x: Int, y: Int, item: ItemStack, onClick: (event: InventoryClickEvent) -> Unit) {
+        window.bukkitInv.setItem(x + y * 9, item)
+        window.onClicks[x + y * 9] = onClick
+    }
+
+    override fun updateInv() = window.openInv()
+
+    fun outsideClick(click: (event: InventoryClickEvent) -> Unit) {
+        window.onClicks[-999] = click
+    }
+}
\ No newline at end of file
diff --git a/buildSrc/src/steamwar.devserver.gradle b/buildSrc/src/steamwar.devserver.gradle
index eea3fdca..8de7abf0 100644
--- a/buildSrc/src/steamwar.devserver.gradle
+++ b/buildSrc/src/steamwar.devserver.gradle
@@ -27,7 +27,6 @@ plugins {
 class DevServer extends DefaultTask {
 
     @Input
-    @Optional
     boolean debug = false
 
     @Input