diff --git a/src/stealth.lua b/src/stealth.lua
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+-- Monster alertness state machine layered on top of ai.lua's chase/pack/etc
+-- behaviours. A wild monster the builder places starts dormant (asleep) and
+-- takes no action at all until something wakes it: on each of its own turns
+-- a still-dormant monster rolls a wake chance driven by distance, line of
+-- sight and the player's detection-penalty skills (see skills.lua's
+-- camouflage), guaranteed once the player stands right next to it. Landing
+-- any hit on it - melee, spell or ranged - wakes it outright regardless of
+-- the roll.
+--
+-- Once alert a monster hunts exactly as ai.lua always has; losing line of
+-- sight for several turns in a row drops it to searching, where it makes
+-- for wherever it last saw the player instead of running its usual
+-- behaviour. Giving up the search with nothing found puts it back to sleep.
+--
+-- A blow struck against a monster that is still dormant is a sneak attack:
+-- it never saw the swing coming, so it lands for several times the normal
+-- damage (see Stealth.sneakMultiplier, read by Game:playerAttack).
+--
+-- Bosses, the shopkeeper and anything summoned or split mid-fight (see
+-- game.lua's spawnMinion and bosses.lua's create) start "hunting" outright
+-- rather than dormant, so none of this changes behaviour that was already
+-- always-alert. A monster built directly without ever passing through
+-- builder.lua's placement loop - as most of tests/run.lua's fixtures do -
+-- also defaults to "hunting" whenever its awareness field is simply absent,
+-- which keeps every existing hand-built scenario acting exactly as it did
+-- before this module existed.
+
+local Los = require("los")
+local Skills = require("skills")
+
+local Stealth = {}
+
+Stealth.DORMANT = "dormant"
+Stealth.HUNTING = "hunting"
+Stealth.SEARCHING = "searching"
+
+Stealth.WAKE_RADIUS = 6
+Stealth.TRACK_RADIUS = 20
+Stealth.LOST_TURNS_LIMIT = 4
+Stealth.SEARCH_TURNS_LIMIT = 4
+Stealth.SNEAK_MULTIPLIER = 3
+
+function Stealth.isUnaware(monster)
+  return monster.awareness == Stealth.DORMANT
+end
+
+-- Read by Game:playerAttack before Stealth.alert (below) fires on the same
+-- swing, so the bonus is computed against the state the monster was
+-- actually in the instant the blow landed.
+function Stealth.sneakMultiplier(monster)
+  if Stealth.isUnaware(monster) then return Stealth.SNEAK_MULTIPLIER end
+  return 1
+end
+
+-- Called the instant a monster is provoked by any offensive player action -
+-- whatever it was doing before, it now knows exactly where the player is.
+function Stealth.alert(monster, px, py)
+  monster.awareness = Stealth.HUNTING
+  monster.lastKnownX, monster.lastKnownY = px, py
+  monster.lostTurns = 0
+end
+
+-- Called when noise.lua's flood reaches a still-dormant monster (see
+-- game.lua's Game:emitNoise). Unlike alert() above - a direct provocation,
+-- which snaps a monster straight to hunting with the player's exact
+-- position - a monster that only heard something does not know it was the
+-- player specifically, or where they actually are now: it starts searching
+-- the noise's own origin instead, exactly like a hunter that just lost
+-- track (see Stealth.tick's searching branch below), so it may well arrive
+-- to find nothing there and give up. A monster that is not currently
+-- dormant is already tracking something better than a noise and ignores
+-- this outright.
+function Stealth.hear(monster, x, y)
+  if monster.awareness ~= Stealth.DORMANT then return end
+  monster.awareness = Stealth.SEARCHING
+  monster.lastKnownX, monster.lastKnownY = x, y
+  monster.searchTurns = 0
+end
+
+local function chebyshev(ax, ay, bx, by)
+  return math.max(math.abs(ax - bx), math.abs(ay - by))
+end
+
+local function canSpot(game, monster, radius)
+  local penalty = Skills.detectionPenalty(game.player, game.level.dark)
+  local effective = radius - penalty
+  if effective < 1 then effective = 1 end
+  if chebyshev(monster.x, monster.y, game.player.x, game.player.y) > effective then return false end
+  return Los.clear(game.level.map, monster.x, monster.y, game.player.x, game.player.y)
+end
+
+-- Whether a dormant monster notices the player this turn: guaranteed once
+-- adjacent, otherwise a chance that improves the closer the player stands,
+-- scaled against the monster's own (penalty-adjusted) waking radius.
+local function rollWake(game, monster)
+  if not canSpot(game, monster, Stealth.WAKE_RADIUS) then return false end
+  local dist = chebyshev(monster.x, monster.y, game.player.x, game.player.y)
+  if dist <= 1 then return true end
+  local penalty = Skills.detectionPenalty(game.player, game.level.dark)
+  local radius = math.max(Stealth.WAKE_RADIUS - penalty, 1)
+  local roll = game.rng:range(1, radius)
+  return roll <= (radius - dist + 1)
+end
+
+-- Advances one monster's alertness ahead of its upcoming turn and reports
+-- whether game.lua should still run its normal ability/ai dispatch this
+-- turn. A monster that stays dormant, or one that spends the turn marching
+-- back toward wherever it last saw the player, returns false - stealth.lua
+-- has already fully resolved its turn (for dormant, that means doing
+-- nothing at all).
+function Stealth.tick(game, monster)
+  local awareness = monster.awareness or Stealth.HUNTING
+  if awareness == Stealth.DORMANT then
+    if rollWake(game, monster) then
+      Stealth.alert(monster, game.player.x, game.player.y)
+      game.log:add(string.format("%s stirs awake", monster.name))
+      return true
+    end
+    return false
+  end
+  if awareness == Stealth.SEARCHING then
+    if canSpot(game, monster, Stealth.TRACK_RADIUS) then
+      Stealth.alert(monster, game.player.x, game.player.y)
+      return true
+    end
+    monster.searchTurns = (monster.searchTurns or 0) + 1
+    if monster.lastKnownX and (monster.x ~= monster.lastKnownX or monster.y ~= monster.lastKnownY) then
+      game:stepMonsterToward(monster, monster.lastKnownX, monster.lastKnownY)
+    end
+    if monster.searchTurns >= Stealth.SEARCH_TURNS_LIMIT then
+      monster.awareness = Stealth.DORMANT
+      monster.lastKnownX, monster.lastKnownY = nil, nil
+      monster.lostTurns, monster.searchTurns = nil, nil
+      game.log:add(string.format("%s gives up the search and settles back down", monster.name))
+    end
+    return false
+  end
+  -- hunting
+  if canSpot(game, monster, Stealth.TRACK_RADIUS) then
+    monster.lastKnownX, monster.lastKnownY = game.player.x, game.player.y
+    monster.lostTurns = 0
+    return true
+  end
+  monster.lostTurns = (monster.lostTurns or 0) + 1
+  if monster.lostTurns >= Stealth.LOST_TURNS_LIMIT and monster.lastKnownX then
+    monster.awareness = Stealth.SEARCHING
+    monster.searchTurns = 0
+    game.log:add(string.format("%s loses track of you", monster.name))
+    return Stealth.tick(game, monster)
+  end
+  return true
+end
+
+return Stealth