diff --git a/src/common/rpg/components/inventory.ts b/src/common/rpg/components/inventory.ts
index fc3bdf7..b955c03 100644
--- a/src/common/rpg/components/inventory.ts
+++ b/src/common/rpg/components/inventory.ts
@@ -17,6 +17,8 @@ interface InventoryState {
 
 @component
 export class Inventory extends Component<InventoryState> {
+    #cachedVars: RPGVariables | null = null;
+
     constructor(slotDefs: Array<InventorySlotInput>) {
         super({
             slots: slotDefs.map(def => {
@@ -45,6 +47,7 @@ export class Inventory extends Component<InventoryState> {
 
     @action
     add({ itemId, amount, slotId }: { itemId: string; amount: number; slotId?: SlotId }): boolean {
+        this.#cachedVars = null;
         if (amount < 0) return false;
         if (amount === 0) return true;
 
@@ -105,6 +108,7 @@ export class Inventory extends Component<InventoryState> {
 
     @action
     remove({ itemId, amount, slotId }: { itemId: string; amount: number; slotId?: SlotId }): boolean {
+        this.#cachedVars = null;
         if (amount < 0) return false;
         if (amount === 0) return true;
 
@@ -193,6 +197,7 @@ export class Inventory extends Component<InventoryState> {
     }
 
     override getVariables(): RPGVariables {
+        if (this.#cachedVars) return this.#cachedVars;
         const result: RPGVariables = {};
         for (const [itemId, amount] of this.getItems()) {
             result[itemId] = amount;
@@ -203,6 +208,7 @@ export class Inventory extends Component<InventoryState> {
                 }
             }
         }
+        this.#cachedVars = result;
         return result;
     }
 }
diff --git a/src/common/rpg/components/questLog.ts b/src/common/rpg/components/questLog.ts
index 66383d4..ef5f0ef 100644
--- a/src/common/rpg/components/questLog.ts
+++ b/src/common/rpg/components/questLog.ts
@@ -22,6 +22,8 @@ interface QuestLogState {
 
 @component
 export class QuestLog extends Component<QuestLogState> {
+    #cachedVars: RPGVariables | null = null;
+
     constructor(quests: Quest[] = []) {
         const questsRecord: Record<string, Quest> = {};
         const runtimeStates: Record<string, QuestRuntimeState> = {};
@@ -41,6 +43,8 @@ export class QuestLog extends Component<QuestLogState> {
         this.state.runtimeStates[quest.id] = { status: 'inactive', stageIndex: 0 };
     }
 
+    #invalidate() { this.#cachedVars = null; }
+
     #transition(op: string, questId: string, from: QuestStatus, to: QuestStatus, event: string): boolean {
         const runtimeState = this.state.runtimeStates[questId];
         if (!runtimeState) {
@@ -53,6 +57,7 @@ export class QuestLog extends Component<QuestLogState> {
         }
         runtimeState.status = to;
         if (to === 'active' || to === 'inactive') runtimeState.stageIndex = 0;
+        this.#invalidate();
         this.emit(event, { questId });
         return true;
     }
@@ -105,7 +110,7 @@ export class QuestLog extends Component<QuestLogState> {
     }
 
     /** @internal used by QuestSystem */
-    *entries(): Generator<[string, QuestEntry]> {
+    *entries(): Generator<[string, QuestEntry], void, unknown> {
         for (const [id, quest] of Object.entries(this.state.quests)) {
             yield [id, { quest, state: this.state.runtimeStates[id]! }];
         }
@@ -116,6 +121,7 @@ export class QuestLog extends Component<QuestLogState> {
         const quest = this.state.quests[questId];
         const runtimeState = this.state.runtimeStates[questId];
         if (!quest || !runtimeState) return;
+        this.#invalidate();
         if (runtimeState.stageIndex + 1 < quest.stages.length) {
             runtimeState.stageIndex++;
             this.emit('stage', { questId, index: runtimeState.stageIndex, stage: quest.stages[runtimeState.stageIndex] });
@@ -140,11 +146,13 @@ export class QuestLog extends Component<QuestLogState> {
     }
 
     override getVariables(): RPGVariables {
+        if (this.#cachedVars) return this.#cachedVars;
         const result: RPGVariables = {};
         for (const [questId, runtimeState] of Object.entries(this.state.runtimeStates)) {
             result[`${questId}.status`] = runtimeState.status;
             result[`${questId}.stage`] = runtimeState.stageIndex;
         }
+        this.#cachedVars = result;
         return result;
     }
 }
diff --git a/src/common/rpg/components/variables.ts b/src/common/rpg/components/variables.ts
index 532915f..0043399 100644
--- a/src/common/rpg/components/variables.ts
+++ b/src/common/rpg/components/variables.ts
@@ -12,6 +12,13 @@ interface VariablesState {
     vars: RPGVariables;
 }
 
+/**
+ * Generic runtime key-value store set by dialog actions, quest scripts, and game events —
+ * values whose keys are only known at runtime or come from data files.
+ *
+ * Prefer a typed `Component<TState>` when the shape is fixed at compile time
+ * (e.g. health, stats, slot definitions).
+ */
 @component
 export class Variables extends Component<VariablesState> {
     constructor() {
diff --git a/src/common/rpg/core/registry.ts b/src/common/rpg/core/registry.ts
index 6c47b96..3d9f945 100644
--- a/src/common/rpg/core/registry.ts
+++ b/src/common/rpg/core/registry.ts
@@ -1,17 +1,29 @@
 import type { Component } from './world';
 
 type ComponentConstructor = abstract new (...args: any[]) => Component<any>;
-type ComponentDecorator = (target: ComponentConstructor, ctx: ClassDecoratorContext) => void;
+type MigrationFn = (state: Record<string, unknown>) => Record<string, unknown>;
 
-const registry = new Map<string, ComponentConstructor>();
+interface ComponentMeta {
+    ctor: ComponentConstructor;
+    version: number;
+}
+
+interface MigrationEntry {
+    toVersion: number;
+    fn: MigrationFn;
+}
+
+const registry = new Map<string, ComponentMeta>();
 const reverseRegistry = new Map<ComponentConstructor, string>();
+/** migrations[name][fromVersion] → { toVersion, fn } */
+const migrations = new Map<string, Map<number, MigrationEntry>>();
 
-function register(name: string, ctor: ComponentConstructor): void {
-    registry.set(name, ctor);
+function register(name: string, ctor: ComponentConstructor, version: number): void {
+    registry.set(name, { ctor, version });
     reverseRegistry.set(ctor, name);
 }
 
-export function getComponentClass(name: string): ComponentConstructor | undefined {
+export function getComponentMeta(name: string): ComponentMeta | undefined {
     return registry.get(name);
 }
 
@@ -19,15 +31,70 @@ export function getComponentName(ctor: Function): string | undefined {
     return reverseRegistry.get(ctor as ComponentConstructor);
 }
 
+/**
+ * Register a migration that upgrades component state from `fromVersion` to `toVersion`.
+ * Migrations are chained automatically: registering 0→1 and 1→2 handles saves at version 0.
+ */
+export function registerMigration(
+    name: string,
+    fromVersion: number,
+    toVersion: number,
+    fn: MigrationFn,
+): void {
+    if (!migrations.has(name)) migrations.set(name, new Map());
+    migrations.get(name)!.set(fromVersion, { toVersion, fn });
+}
+
+/**
+ * Apply all registered migrations to bring `state` from `fromVersion` up to the current
+ * registered version. Returns the migrated state (may be the same object if no migrations ran).
+ */
+export function migrateState(
+    name: string,
+    state: Record<string, unknown>,
+    fromVersion: number,
+): Record<string, unknown> {
+    const meta = registry.get(name);
+    if (!meta) return state;
+    const chain = migrations.get(name);
+    if (!chain) return state;
+    let current = fromVersion;
+    let s = state;
+    while (current < meta.version) {
+        const entry = chain.get(current);
+        if (!entry) throw new Error(
+            `[registry] No migration for '${name}' from version ${current} to ${meta.version}. ` +
+            `Register one with registerMigration('${name}', ${current}, ...).`
+        );
+        s = entry.fn(s);
+        current = entry.toVersion;
+    }
+    return s;
+}
+
+interface ComponentOptions {
+    name?: string;
+    version?: number;
+}
+
+type ComponentDecorator = (target: ComponentConstructor, ctx: ClassDecoratorContext) => void;
+
 export function component(target: ComponentConstructor, ctx: ClassDecoratorContext): void;
 export function component(name: string): ComponentDecorator;
+export function component(options: ComponentOptions): ComponentDecorator;
 export function component(
-    nameOrTarget: string | ComponentConstructor,
+    nameOrTargetOrOptions: string | ComponentConstructor | ComponentOptions,
     ctx?: ClassDecoratorContext,
 ): void | ComponentDecorator {
-    if (typeof nameOrTarget === 'string') {
-        const name = nameOrTarget;
-        return (target: ComponentConstructor) => register(name, target);
+    if (typeof nameOrTargetOrOptions === 'string') {
+        const name = nameOrTargetOrOptions;
+        return (target: ComponentConstructor) => register(name, target, 0);
     }
-    register(String(ctx!.name), nameOrTarget);
+    if (typeof nameOrTargetOrOptions === 'object') {
+        const { name, version = 0 } = nameOrTargetOrOptions;
+        return (target: ComponentConstructor, ctx: ClassDecoratorContext) =>
+            register(name ?? String(ctx.name), target, version);
+    }
+    // Used as bare @component
+    register(String(ctx!.name), nameOrTargetOrOptions, 0);
 }
diff --git a/src/common/rpg/core/serialization.ts b/src/common/rpg/core/serialization.ts
index 7c4e2e7..654c573 100644
--- a/src/common/rpg/core/serialization.ts
+++ b/src/common/rpg/core/serialization.ts
@@ -1,10 +1,14 @@
 import { World, Entity, Component, COMPONENT_STATE, WORLD_ENTITY_COUNTER } from './world';
-import { getComponentClass, getComponentName } from './registry';
+import { getComponentMeta, getComponentName, migrateState } from './registry';
+
+/** Increment this when the WorldData/EntityData structure itself changes incompatibly. */
+const SCHEMA_VERSION = 1;
 
 interface ComponentData {
     type: 'component';
     name: string;
     key: string;
+    version: number;
     state: unknown;
 }
 
@@ -16,6 +20,7 @@ interface EntityData {
 
 interface WorldData {
     type: 'world';
+    schemaVersion: number;
     globals: Record<string, string | number | boolean | undefined>;
     entityCounter: number;
     entities: EntityData[];
@@ -31,7 +36,8 @@ function serializeComponent(component: Component<any>): ComponentData {
             `Add @component to the class declaration.`
         );
     }
-    return { type: 'component', name, key: component.key, state: component[COMPONENT_STATE]() };
+    const meta = getComponentMeta(name)!;
+    return { type: 'component', name, key: component.key, version: meta.version, state: component[COMPONENT_STATE]() };
 }
 
 function serializeEntity(entity: Entity): EntityData {
@@ -49,6 +55,7 @@ function serializeWorld(world: World): WorldData {
     }
     return {
         type: 'world',
+        schemaVersion: SCHEMA_VERSION,
         globals: { ...world.globals },
         entityCounter: world[WORLD_ENTITY_COUNTER],
         entities,
@@ -56,15 +63,21 @@ function serializeWorld(world: World): WorldData {
 }
 
 function deserializeComponent(data: ComponentData): Component<any> {
-    const ComponentClass = getComponentClass(data.name);
-    if (!ComponentClass) {
+    const meta = getComponentMeta(data.name);
+    if (!meta) {
         throw new Error(`Unknown component '${data.name}'. Ensure it is imported so @component runs.`);
     }
+
+    const savedVersion = data.version ?? 0;
+    const state = savedVersion < meta.version
+        ? migrateState(data.name, data.state as Record<string, unknown>, savedVersion)
+        : data.state;
+
     // Bypass constructor: create a bare instance and restore state directly.
-    // This is safe because constructors must only call super(state) — all
-    // initialization logic goes in onAdd(), which entity.add() calls after this.
-    const instance = Object.create(ComponentClass.prototype) as Component<any>;
-    (instance as unknown as { state: unknown }).state = data.state;
+    // Safe because constructors must only call super(state) — all initialization
+    // logic goes in onAdd(), which entity.add() calls after this.
+    const instance = Object.create(meta.ctor.prototype) as Component<any>;
+    (instance as unknown as { state: unknown }).state = state;
     return instance;
 }
 
@@ -77,6 +90,12 @@ function deserializeEntity(data: EntityData, world: World): Entity {
 }
 
 function deserializeWorld(data: WorldData): World {
+    if (data.schemaVersion !== SCHEMA_VERSION) {
+        throw new Error(
+            `Unsupported save format: schema version ${data.schemaVersion}, ` +
+            `expected ${SCHEMA_VERSION}. The save file is incompatible with this version of the engine.`
+        );
+    }
     const world = new World();
     Object.assign(world.globals, data.globals);
     world[WORLD_ENTITY_COUNTER] = data.entityCounter;
@@ -98,7 +117,6 @@ export namespace Serialization {
         switch (data.type) {
             case 'world': return deserializeWorld(data);
             case 'entity': {
-                // A standalone entity needs a world to live in.
                 const world = new World();
                 return deserializeEntity(data, world);
             }
diff --git a/src/common/rpg/core/world.ts b/src/common/rpg/core/world.ts
index 7193133..1052959 100644
--- a/src/common/rpg/core/world.ts
+++ b/src/common/rpg/core/world.ts
@@ -199,6 +199,7 @@ export class World {
     readonly #entities = new Map<string, Entity>();
     readonly #handlers = new Map<string, Set<EntityEventHandler>>();
     readonly #globalHandlers = new Map<string, Set<WorldEventHandler>>();
+    readonly #onceWrappers = new Map<Function, Function>();
     readonly #systems: System[] = [];
     #entityCounter = 0;
 
@@ -243,13 +244,39 @@ export class World {
         }
     }
 
-    *query<T extends Component<any>>(ctor: Class<T>): Generator<[Entity, string, T]> {
-        for (const entity of this.#entities.values()) {
-            for (const [key, component] of entity) {
-                if (component instanceof ctor) {
-                    yield [entity, key, component as T]
+    /**
+     * Create a new entity whose components are deep copies of `source`'s components.
+     * The clone is immediately live in the world and `onAdd()` fires for each component.
+     * @param source - Entity to clone.
+     * @param newId - ID for the clone; follows the same rules as {@link createEntity}.
+     */
+    cloneEntity(source: Entity, newId?: string): Entity {
+        const target = this.createEntity(newId);
+        for (const [key, component] of source) {
+            const clone = Object.create(component.constructor.prototype) as Component<any>;
+            (clone as unknown as { state: unknown }).state = structuredClone(component[COMPONENT_STATE]());
+            target.add(key, clone);
+        }
+        return target;
+    }
+
+    query<T extends Component<any>>(ctor: Class<T>): Generator<[Entity, string, T]>;
+    query<A extends Component<any>, B extends Component<any>>(ctorA: Class<A>, ctorB: Class<B>): Generator<[Entity, A, B]>;
+    query<A extends Component<any>, B extends Component<any>, C extends Component<any>>(ctorA: Class<A>, ctorB: Class<B>, ctorC: Class<C>): Generator<[Entity, A, B, C]>;
+    *query(...ctors: Class<Component<any>>[]): Generator<unknown[]> {
+        const entities = this.#entities;
+        if (ctors.length === 1) {
+            const ctor = ctors[0];
+            for (const entity of entities.values()) {
+                for (const [key, component] of entity) {
+                    if (component instanceof ctor) yield [entity, key, component];
                 }
             }
+        } else {
+            for (const entity of entities.values()) {
+                const components = ctors.map(ctor => entity.get(ctor));
+                if (components.every(c => c !== undefined)) yield [entity, ...components];
+            }
         }
     }
 
@@ -293,9 +320,13 @@ export class World {
     off(entityId: string, event: string, handler: EntityEventHandler): void;
     off(arg1: string, arg2: WorldEventHandler | string, arg3?: EntityEventHandler): void {
         if (typeof arg2 === 'string') {
-            this.#handlers.get(`${arg1}\0${arg2}`)?.delete(arg3!);
+            const handler = (this.#onceWrappers.get(arg3!) ?? arg3!) as EntityEventHandler;
+            this.#handlers.get(`${arg1}\0${arg2}`)?.delete(handler);
+            this.#onceWrappers.delete(arg3!);
         } else {
-            this.#globalHandlers.get(arg1)?.delete(arg2);
+            const handler = (this.#onceWrappers.get(arg2) ?? arg2) as WorldEventHandler;
+            this.#globalHandlers.get(arg1)?.delete(handler);
+            this.#onceWrappers.delete(arg2);
         }
     }
 
@@ -303,14 +334,17 @@ export class World {
     once(entityId: string, event: string, handler: EntityEventHandler): () => void;
     once(arg1: string, arg2: WorldEventHandler | string, arg3?: EntityEventHandler): () => void {
         if (typeof arg2 === 'string') {
-            const wrapped: EntityEventHandler = data => { unsub(); arg3!(data); };
+            const original = arg3!;
+            const wrapped: EntityEventHandler = data => { this.#onceWrappers.delete(original); unsub(); original(data); };
+            this.#onceWrappers.set(original, wrapped);
             const unsub = this.on(arg1, arg2, wrapped);
-            return unsub;
+            return () => { this.#onceWrappers.delete(original); unsub(); };
         }
-        const h = arg2;
-        const wrapped: WorldEventHandler = data => { unsub(); h(data); };
+        const original = arg2;
+        const wrapped: WorldEventHandler = data => { this.#onceWrappers.delete(original); unsub(); original(data); };
+        this.#onceWrappers.set(original, wrapped);
         const unsub = this.on(arg1, wrapped);
-        return unsub;
+        return () => { this.#onceWrappers.delete(original); unsub(); };
     }
 
     #addHandler<T extends WorldEventHandler | EntityEventHandler>(
@@ -336,3 +370,8 @@ export function isEvalContext(v: unknown): v is EvalContext {
         )
         && (v as EvalContext).world instanceof World;
 }
+
+/** Narrows an {@link EvalContext} to one where `self` is an `Entity`, not a `World`. */
+export function isEntityContext(ctx: EvalContext): ctx is { self: Entity; world: World } {
+    return ctx.self instanceof Entity;
+}
diff --git a/src/common/rpg/utils/conditions.ts b/src/common/rpg/utils/conditions.ts
index 43ebcc6..c8aa9a0 100644
--- a/src/common/rpg/utils/conditions.ts
+++ b/src/common/rpg/utils/conditions.ts
@@ -12,33 +12,47 @@ export interface ParsedCondition {
     value?: ConditionValue;
 }
 
+const parseCache = new Map<string, ParsedCondition>();
+
 export function parseCondition(s: RPGCondition): ParsedCondition {
+    const cached = parseCache.get(s);
+    if (cached) return cached;
+    let result: ParsedCondition;
+
     // ~variable — falsy check, nothing else allowed
-    if (s.startsWith('~') && !s.includes(' '))
-        return { variable: s.slice(1), negate: true };
+    if (s.startsWith('~') && !s.includes(' ')) {
+        result = { variable: s.slice(1), negate: true };
+    } else {
+        const spaceIdx = s.indexOf(' ');
+        if (spaceIdx === -1) {
+            result = { variable: s, negate: false };
+        } else {
+            const variable = s.slice(0, spaceIdx);
+            const rest = s.slice(spaceIdx + 1).trim();
 
-    const spaceIdx = s.indexOf(' ');
-    if (spaceIdx === -1)
-        return { variable: s, negate: false };
+            if (rest === 'null') {
+                result = { variable, negate: false, operator: 'null' };
+            } else if (rest === '~null') {
+                result = { variable, negate: false, operator: '~null' };
+            } else {
+                const opMatch = rest.match(/^(==|!=|>=|<=|>|<)\s*(.+)$/);
+                if (!opMatch) throw new Error(`Invalid condition: "${s}"`);
 
-    const variable = s.slice(0, spaceIdx);
-    const rest = s.slice(spaceIdx + 1).trim();
+                const [, operator, rawValue] = opMatch;
+                let value: ConditionValue;
+                if (rawValue === 'null') value = null;
+                else if (rawValue === 'true') value = true;
+                else if (rawValue === 'false') value = false;
+                else if (!isNaN(Number(rawValue))) value = Number(rawValue);
+                else value = rawValue.replace(/^['"]|['"]$/g, '');
 
-    if (rest === 'null') return { variable, negate: false, operator: 'null' };
-    if (rest === '~null') return { variable, negate: false, operator: '~null' };
+                result = { variable, negate: false, operator: operator as ConditionOperator, value };
+            }
+        }
+    }
 
-    const opMatch = rest.match(/^(==|!=|>=|<=|>|<)\s*(.+)$/);
-    if (!opMatch) throw new Error(`Invalid condition: "${s}"`);
-
-    const [, operator, rawValue] = opMatch;
-    let value: ConditionValue;
-    if (rawValue === 'null') value = null;
-    else if (rawValue === 'true') value = true;
-    else if (rawValue === 'false') value = false;
-    else if (!isNaN(Number(rawValue))) value = Number(rawValue);
-    else value = rawValue.replace(/^['"]|['"]$/g, '');
-
-    return { variable, negate: false, operator: operator as ConditionOperator, value };
+    parseCache.set(s, result);
+    return result;
 }
 
 function evalParsed({ negate, operator, value }: ParsedCondition, val: RPGVariables[string]): boolean {
diff --git a/src/common/rpg/utils/variables.ts b/src/common/rpg/utils/variables.ts
index 3ee1fd6..e4c52ee 100644
--- a/src/common/rpg/utils/variables.ts
+++ b/src/common/rpg/utils/variables.ts
@@ -77,8 +77,7 @@ export async function executeAction(action: RPGAction, ctx: EvalContext | Contex
         ctx = ctx.context;
     }
     if (!action.type) {
-        console.warn(`[executeAction] action missing 'type' property`);
-        return;
+        throw new TypeError(`[executeAction] action object is missing a 'type' property`);
     }
     let entity = ctx.self;
     let actionType = action.type;
@@ -87,12 +86,12 @@ export async function executeAction(action: RPGAction, ctx: EvalContext | Contex
     if (action.type.startsWith('@')) {
         const dotIdx = action.type.indexOf('.', 1);
         if (dotIdx === -1) {
-            console.warn(`[executeAction] malformed cross-entity action '${action.type}': missing '.' after entity id`);
-            return;
+            throw new Error(`[executeAction] malformed cross-entity action '${action.type}': missing '.' after entity id`);
         }
         const entityId = action.type.slice(1, dotIdx);
         const found = ctx.world.getEntity(entityId);
         if (!found) {
+            // Entity may have been destroyed legitimately — warn and skip rather than throw.
             console.warn(`[executeAction] entity '${entityId}' not found (action '${action.type}')`);
             return;
         }
@@ -102,8 +101,7 @@ export async function executeAction(action: RPGAction, ctx: EvalContext | Contex
 
     const actions = resolveActions(entity);
     if (!(actionType in actions)) {
-        console.warn(`[executeAction] action '${actionType}' not found on entity '${entity.id}'`);
-        return;
+        throw new Error(`[executeAction] action '${actionType}' not found on entity '${entity instanceof World ? 'world' : entity.id}'`);
     }
     return actions[actionType](action.arg, ctx);
 }