diff --git a/core/include/vf/SlotAction.h b/core/include/vf/SlotAction.h
new file mode 100644
index 0000000..7d59a54
--- /dev/null
+++ b/core/include/vf/SlotAction.h
@@ -0,0 +1,97 @@
+#pragma once
+
+#include <cstdint>
+#include <vector>
+
+#include "ContainerMenu.h"
+
+namespace vf {
+
+// Click semantics for an open ContainerMenu. This is where inventories usually
+// break, so the whole vocabulary lives in one place and every action reports
+// exactly what it moved.
+
+enum class ClickKind : uint8_t {
+    Left,            // take all / put all / swap
+    Right,           // take half / put one
+    ShiftLeft,       // move to the other section, merging before filling
+    ShiftRight,      // same rule; kept distinct so callers can bind both
+    HotbarSwap,      // number key: exchange the slot with a hotbar slot
+    Middle,          // creative clone, otherwise nothing happens
+    Double,          // gather every mergeable stack onto the cursor
+    DragLeftStart,
+    DragLeftAdd,
+    DragLeftEnd,     // split the held stack evenly, remainder stays held
+    DragRightStart,
+    DragRightAdd,
+    DragRightEnd,    // one item per dragged slot
+    DropOne,         // Q
+    DropStack,       // Ctrl+Q
+    Count
+};
+
+const char* clickKindName(ClickKind kind);
+
+// The stack the mouse carries between slots. It is deliberately its own type:
+// the cursor is not a menu slot and must never be reachable by index.
+struct CursorStack {
+    MetaStack held;
+
+    bool empty() const { return held.empty(); }
+    void clear() { held.clear(); }
+};
+
+// A drag paints a set of slots while the button is down. The items stay on the
+// cursor for the whole gesture and are only distributed on release, so an
+// abandoned drag cannot lose anything.
+struct DragState {
+    bool active = false;
+    bool rightButton = false;
+    std::vector<int> paintedSlots;
+    // Cursor contents when the drag began, needed to compute an even split.
+    MetaStack payload;
+
+    void reset();
+    bool alreadyPainted(int index) const;
+};
+
+struct SlotAction {
+    ClickKind kind = ClickKind::Left;
+    int slotIndex = -1;
+    // Which hotbar slot a HotbarSwap targets, 0..8.
+    int hotbarButton = -1;
+};
+
+SlotAction clickAt(ClickKind kind, int slotIndex);
+SlotAction hotbarSwapAt(int slotIndex, int hotbarButton);
+
+struct SlotActionResult {
+    bool applied = false;
+    // Items that changed slot, or moved between a slot and the cursor.
+    int moved = 0;
+    // Items that left the menu entirely. They are also listed in droppedStacks
+    // so a caller can spawn the right entities.
+    int dropped = 0;
+    std::vector<MetaStack> droppedStacks;
+    // Only a creative middle click can be non-zero here. Every other action is
+    // conservative, which is what the invariant test pins down.
+    int created = 0;
+    bool cursorChanged = false;
+    bool craftedResult = false;
+    int craftedCount = 0;
+
+    void noteDrop(const MetaStack& stack);
+};
+
+// Applies one click. `cursor` and `drag` are the player's own state and are
+// carried between calls; the menu is the screen being clicked on.
+SlotActionResult applySlotAction(ContainerMenu& menu, CursorStack& cursor, DragState& drag,
+                                 const SlotAction& action);
+
+// Total items of one kind across everything the action model can touch: the
+// menu, the cursor, and whatever a drag is holding. The invariant is that this
+// number only ever changes by what an action reports as dropped or created.
+int totalAcrossMenuAndCursor(const ContainerMenu& menu, const CursorStack& cursor, ItemId item);
+int totalItemsAcrossMenuAndCursor(const ContainerMenu& menu, const CursorStack& cursor);
+
+} // namespace vf