diff --git a/core/include/vf/Effect.h b/core/include/vf/Effect.h
new file mode 100644
index 0000000..5a4cd55
--- /dev/null
+++ b/core/include/vf/Effect.h
@@ -0,0 +1,106 @@
+#pragma once
+
+#include <cstdint>
+#include <vector>
+
+namespace vf {
+
+constexpr int ticksPerSecond = 20;
+
+enum class EffectKind : uint8_t {
+    Speed, Slowness, Haste, MiningFatigue, Strength, InstantHealth, InstantDamage,
+    JumpBoost, Nausea, Regeneration, Resistance, FireResistance, WaterBreathing,
+    Invisibility, Weakness, Poison, Wither, HealthBoost, Absorption, Saturation,
+    Levitation, NightVision,
+    Count
+};
+
+// Instant effects resolve on the first tick after application and never linger.
+bool isInstantEffect(EffectKind kind);
+bool isHarmfulEffect(EffectKind kind);
+
+// Fermented spider eye corruption. Returns EffectKind::Count for effects that
+// have no counterpart.
+EffectKind invertedEffect(EffectKind kind);
+
+struct EffectInstance {
+    EffectKind kind = EffectKind::Speed;
+    int amplifier = 0;
+    int durationTicks = 0;
+    // Scales instant heal/damage magnitude; splash potions land at less than
+    // one on anything not standing on the impact point.
+    float potency = 1.0f;
+    bool ambient = false;
+
+    int level() const { return amplifier + 1; }
+    bool expired() const { return durationTicks <= 0; }
+};
+
+EffectInstance makeEffect(EffectKind kind, int level, int durationTicks);
+
+// Ticks between two firings of a periodic effect. Higher levels fire faster
+// until the interval bottoms out at a single tick.
+int periodicIntervalTicks(EffectKind kind, int amplifier);
+
+// Attribute deltas contributed by a whole EffectSet, resolved once per tick
+// instead of being queried effect by effect.
+struct EffectModifier {
+    float moveSpeedMultiplier = 1.0f;
+    float miningSpeedMultiplier = 1.0f;
+    float attackDamageBonus = 0.0f;
+    float incomingDamageMultiplier = 1.0f;
+    float jumpVelocityBonus = 0.0f;
+    float maxHealthBonus = 0.0f;
+    float absorptionHearts = 0.0f;
+    float levitationSpeed = 0.0f;
+    float nauseaIntensity = 0.0f;
+    bool fireImmune = false;
+    bool waterBreathing = false;
+    bool invisible = false;
+    bool nightVision = false;
+};
+
+struct EffectSet {
+    // Health and food work produced by one tick, kept separate per source so a
+    // caller can honour the rule that poison stops at half a heart while wither
+    // does not.
+    struct Pending {
+        float regenerationHeal = 0.0f;
+        float instantHeal = 0.0f;
+        float poisonDamage = 0.0f;
+        float witherDamage = 0.0f;
+        float instantDamage = 0.0f;
+        float saturationGain = 0.0f;
+        int expiredCount = 0;
+
+        bool empty() const;
+    };
+
+    std::vector<EffectInstance> active;
+};
+
+const EffectInstance* findEffect(const EffectSet& set, EffectKind kind);
+bool hasEffect(const EffectSet& set, EffectKind kind);
+int effectLevel(const EffectSet& set, EffectKind kind);
+int effectDuration(const EffectSet& set, EffectKind kind);
+
+// Applies an effect using the vanilla precedence: a stronger amplifier always
+// replaces, an equal amplifier replaces only when it lasts longer, a weaker one
+// is dropped. Instant effects stack as separate entries and are consumed by the
+// next tickEffects call. Returns true when the set actually changed.
+bool applyEffect(EffectSet& set, const EffectInstance& effect);
+
+bool removeEffect(EffectSet& set, EffectKind kind);
+void clearEffects(EffectSet& set);
+
+EffectModifier effectModifiers(const EffectSet& set);
+
+// Advances every effect by one tick, dropping expired ones and reporting the
+// periodic and instant work owed to the entity this tick.
+EffectSet::Pending tickEffects(EffectSet& set);
+
+// Applies one tick of pending health work. Poison never takes the last half
+// heart; wither and instant damage can kill. Returns the signed health change.
+float applyEffectTick(const EffectSet::Pending& pending, float& health, float maxHealth);
+
+} // namespace vf