diff --git a/core/include/vf/Behavior.h b/core/include/vf/Behavior.h
new file mode 100644
index 0000000..f846de8
--- /dev/null
+++ b/core/include/vf/Behavior.h
@@ -0,0 +1,156 @@
+#pragma once
+
+#include <cstdint>
+#include <functional>
+#include <map>
+#include <memory>
+#include <string>
+#include <vector>
+
+#include "Item.h"
+#include "Math.h"
+#include "Mob.h"
+#include "Pathfind.h"
+#include "World.h"
+
+namespace vf {
+
+// Behaviour trees layered on top of the reactive creature AI in Mob.h. A node
+// answers with one of three verdicts every tick and the tree remembers which
+// node was left Running, so the next tick resumes there instead of restarting
+// the whole traversal.
+enum class BehaviorStatus : uint8_t {
+    Failure = 0,
+    Success = 1,
+    Running = 2,
+};
+
+const char* behaviorStatusName(BehaviorStatus status);
+
+// Traversal is recursive, so a malformed tree would blow the stack. Trees are
+// measured when the root is installed and refuse to run past this.
+constexpr int maxBehaviorDepth = 48;
+
+// Deterministic xorshift shared by every module built on this one. No clock,
+// no global state: the caller owns the stream.
+uint32_t behaviorNextRandom(uint32_t& state);
+float behaviorUnitFloat(uint32_t& state);
+int behaviorRandomRange(uint32_t& state, int minInclusive, int maxInclusive);
+
+Vec3i behaviorCellOf(const Vec3& position);
+Vec3 behaviorCellCenter(const Vec3i& cell);
+float behaviorHorizontalDistance(const Vec3& a, const Vec3& b);
+
+// Typed shared storage between nodes. One map per value type rather than a
+// variant, so a key written as a route can never be read back as a number.
+// Ordered maps keep iteration stable for anyone dumping the contents.
+class Blackboard {
+public:
+    void setNumber(const std::string& key, float value);
+    void setCount(const std::string& key, int value);
+    void setFlag(const std::string& key, bool value);
+    void setPoint(const std::string& key, const Vec3& value);
+    void setCell(const std::string& key, const Vec3i& value);
+    void setRoute(const std::string& key, std::vector<Vec3i> cells);
+
+    bool hasNumber(const std::string& key) const;
+    bool hasCount(const std::string& key) const;
+    bool hasFlag(const std::string& key) const;
+    bool hasPoint(const std::string& key) const;
+    bool hasCell(const std::string& key) const;
+    bool hasRoute(const std::string& key) const;
+
+    float number(const std::string& key, float fallback = 0.0f) const;
+    int count(const std::string& key, int fallback = 0) const;
+    bool flag(const std::string& key, bool fallback = false) const;
+    Vec3 point(const std::string& key, const Vec3& fallback = {}) const;
+    Vec3i cell(const std::string& key, const Vec3i& fallback = {}) const;
+    const std::vector<Vec3i>* route(const std::string& key) const;
+
+    void eraseNumber(const std::string& key);
+    void eraseRoute(const std::string& key);
+    void clear();
+    std::size_t entryCount() const;
+
+private:
+    std::map<std::string, float> numbers;
+    std::map<std::string, int> counts;
+    std::map<std::string, bool> flags;
+    std::map<std::string, Vec3> points;
+    std::map<std::string, Vec3i> cells;
+    std::map<std::string, std::vector<Vec3i>> routes;
+};
+
+// Tunables the stock trees read. Kept out of the node code so a caller can
+// retune a behaviour without rebuilding the tree.
+struct BehaviorTuning {
+    float arriveEpsilon = 0.35f;
+    float fleeRadius = 8.0f;
+    float followFoodRadius = 8.0f;
+    float breedRadius = 3.0f;
+    float homeReturnRadius = 1.2f;
+    int breedCooldownTicks = 120;
+    int grazeDurationTicks = 40;
+    int attackCooldownTicks = 20;
+    int repathIntervalTicks = 10;
+    int shadeSearchRadius = 6;
+    int shadeRunTicks = 120;
+    int sunLightThreshold = 12;
+    int shelterHoldTicks = 60;
+};
+
+// The mob the stock trees steer. Movement is expressed in blocks per tick so
+// the whole simulation stays integer-tick driven and reproducible.
+struct BehaviorAgent {
+    Species species = Species::Zombie;
+    Vec3 position;
+    Vec3 homePosition;
+    Vec3 heading{1.0f, 0.0f, 0.0f};
+    float health = 20.0f;
+    float maxHealth = 20.0f;
+    float moveStep = 0.25f;
+    int grazeTicks = 0;
+    int attacksLanded = 0;
+    int breedCount = 0;
+    int shelterTicks = 0;
+    bool sheltered = false;
+    std::vector<Vec3i> route;
+    int routeCursor = 0;
+    long long routePlannedTick = -4096;
+    Vec3i routeGoal;
+    uint32_t rng = 0x9e3779b9u;
+
+    Vec3i cell() const { return behaviorCellOf(position); }
+    bool routeFinished() const { return routeCursor >= static_cast<int>(route.size()); }
+};
+
+// Everything a node may read or write. `agent` is null for trees that drive
+// something other than a mob (the boss fights capture their own state in the
+// node lambdas instead).
+struct BehaviorContext {
+    const World* world = nullptr;
+    BehaviorAgent* agent = nullptr;
+    Blackboard blackboard;
+    BehaviorTuning tuning;
+    PathConfig path;
+
+    long long tick = 0;
+    uint32_t rng = 0x2545f491u;
+
+    Vec3 playerPosition;
+    bool playerPresent = false;
+    ItemId playerHeldItem = ItemId::None;
+
+    Vec3 matePosition;
+    bool matePresent = false;
+
+    int skyLight = 15;
+    bool daylight = true;
+
+    bool traceEnabled = false;
+    std::vector<std::string> trace;
+};
+
+using BehaviorPredicate = std::function<bool(BehaviorContext&)>;
+using BehaviorAction = std::function<BehaviorStatus(BehaviorContext&)>;
+