diff --git a/src/world/OverworldController.lua b/src/world/OverworldController.lua
index bb0a7f3a..0dbc0770 100644
--- a/src/world/OverworldController.lua
+++ b/src/world/OverworldController.lua
@@ -329,6 +329,10 @@ function OverworldState:setMap(mapId, x, y, facing, opts)
     MapLoader.invalidateAll()
   end
   self.map = MapLoader.load(Game.data, mapId)
+  -- EnterMap -> ClearVariablesOnEnterMap zeroes wStepCounter
+  -- (engine/overworld/clear_variables.asm:7), but a connection crossing
+  -- never reaches EnterMap (home/overworld.asm:675 .loadNewMap)
+  if not (opts and opts.seamless) then Game.save.poisonSteps = 0 end
   -- STRENGTH deactivates on every real map load (home/overworld.asm
   -- EnterMap -> ResetUsingStrengthOutOfBattleBit clears BIT_STRENGTH_ACTIVE
   -- of wStatusFlags1).  setMap is the single choke point for every map-id
@@ -4296,6 +4300,9 @@ end
 -- battle is optional; when given, Oak's Lab OPP_RIVAL1 losses skip the
 -- blackout (pret HandlePlayerBlackOut) so the map script can HealParty.
 function OverworldState:afterBattle(result, battle)
+  -- .battleOccurred tail-jumps to EnterMap (home/overworld.asm:353), so
+  -- ClearVariablesOnEnterMap zeroes wStepCounter on every battle return
+  Game.save.poisonSteps = 0
   if battle and battle.kind == "wild" then
     self.wildEncounterGraceSteps = WILD_ENCOUNTER_GRACE_STEPS
   end
diff --git a/tests/parity_field_poison_phase.lua b/tests/parity_field_poison_phase.lua
new file mode 100644
index 00000000..c10241e5
--- /dev/null
+++ b/tests/parity_field_poison_phase.lua
@@ -0,0 +1,87 @@
+-- Parity test (#1519): wStepCounter is zeroed by ClearVariablesOnEnterMap
+-- (engine/overworld/clear_variables.asm:7), reached from EnterMap on a warp
+-- and from the post-battle tail jump (home/overworld.asm:353), but never from
+-- CheckMapConnections (home/overworld.asm:675), so a seam crossing keeps phase.
+-- Self-contained; run via `luajit tests/parity_field_poison_phase.lua`.
+package.path = "./?.lua;./?/init.lua;" .. package.path
+if not _G.love then _G.love = require("tests.love_stub") end
+local Data = require("src.core.Data")
+if not (Data.maps and Data.maps.PALLET_TOWN) then Data:load() end
+local S = require("tests.harness").suite("parity field poison phase")
+local check, eq = S.check, S.eq
+
+require("src.render.Font").load(Data)
+local Game       = require("src.core.Game")
+local Input      = require("src.core.Input")
+local StateStack = require("src.core.StateStack")
+local Renderer   = require("src.render.Renderer")
+local SaveData   = require("src.core.SaveData")
+local Pokemon    = require("src.pokemon.Pokemon")
+local OW         = require("src.world.OverworldController")
+
+Game.data = Data
+Game.input = Input; Input:init()
+Game.renderer = Renderer; Renderer:init()
+Game.stack = StateStack; StateStack:init()
+Game.save = SaveData.newGame()
+Game.save.party = { Pokemon.new(Data, "SQUIRTLE", 12) }
+
+Game.stack:push(OW, "PALLET_TOWN", 5, 6, "down")
+local ow = Game.stack:top()
+Game.overworld = ow
+
+local mon = Game.save.party[1]
+mon.status = "PSN"
+
+-- Walk until the poison bites, and report which step it was.
+local function stepsToTick(limit)
+  for i = 1, limit or 12 do
+    local before = mon.hp
+    ow:applyFieldPoison()
+    if mon.hp < before then return i end
+  end
+  return nil
+end
+
+local function reheal()
+  mon.hp = mon.stats.hp
+  mon.status = "PSN"
+end
+
+-- ---- the free-running four-step cycle ------------------------------------
+Game.save.poisonSteps = 0
+eq(stepsToTick(), 4, "a cleared counter bites on the fourth step")
+reheal()
+eq(stepsToTick(), 4, "and every fourth step after that")
+
+reheal()
+Game.save.poisonSteps = 2
+eq(stepsToTick(), 2, "a counter two steps into the cycle bites two steps later")
+
+-- ---- a real map load re-phases it ----------------------------------------
+reheal()
+Game.save.poisonSteps = 2
+ow:setMap("PALLET_TOWN", 5, 6, "down", { via = "warp" })
+eq(Game.save.poisonSteps, 0, "setMap clears the counter")
+eq(stepsToTick(), 4, "so the first tick after a warp is four steps out")
+
+-- ---- a connection crossing does not --------------------------------------
+reheal()
+Game.save.poisonSteps = 2
+ow:setMap("ROUTE_1", 9, 33, "up", { seamless = true, keepMusic = true })
+eq(Game.save.poisonSteps, 2, "a seamless crossing leaves the counter alone")
+eq(stepsToTick(), 2, "so the seam does not delay the tick")
+
+-- ---- a battle return re-phases it ----------------------------------------
+reheal()
+Game.save.poisonSteps = 2
+ow:afterBattle("win", { kind = "wild" })
+eq(Game.save.poisonSteps, 0, "afterBattle clears the counter")
+eq(stepsToTick(), 4, "so poison always bites four steps after a battle")
+
+reheal()
+Game.save.poisonSteps = 3
+ow:afterBattle("win", { kind = "trainer", oppClass = "OPP_YOUNGSTER" })
+eq(stepsToTick(), 4, "trainer battles re-phase it the same way")
+
+S.finish()