feat(mods): add battle menu auxiliary action

docs/modding.md

@@ -272,6 +272,16 @@ data.
 
 See RFC 0003, RFC 0004, and RFC 0005 for exact contracts and error codes.
 
+At that same settled ordinary wild/trainer decision boundary, a tool may claim
+START through `battle.menu_auxiliary`. It receives `(next, game, context)`, where
+`context` is the data-only `{ kind = "wild" }` or `{ kind = "trainer" }`; it
+never receives the live battle controller. Return `true` to consume START after
+opening source-owned UI, or call `next(game, context)` to allow lower-priority
+handlers. With no handler, START remains inert. The hook is never reached for
+link/Safari/ghost/demo/scripted battles, action queues, animation/messages,
+forced choices, or any phase that cannot safely be checkpointed. Exceptions are
+contained by normal hook isolation and fall through without advancing a turn.
+
 ## Developer console
 
 Boot with developer mode on to unlock the in-game console and hot-reload

docs/rfcs/0007-battle-menu-auxiliary-actions.md

@@ -0,0 +1,46 @@
+# RFC 0007: Battle menu auxiliary actions
+
+## Status
+
+Proposed.
+
+## Problem
+
+Tool mods can inspect/capture a persistent checkpoint only at a settled
+ordinary wild/trainer player-decision boundary. Before this proposal, that
+boundary had no public semantic input/action seam: `BattleState` consumed the
+command loop directly. A mod could reach it only through private battle/input
+internals, which would be unsafe and incompatible with controller/touch input.
+
+## Contract
+
+`mod.hooks:wrap("battle.menu_auxiliary", callback)` is called only when START
+is pressed at the existing checkpoint-safe player-decision boundary. The
+callback signature is:
+
+```lua
+function callback(next, game, context)
+  -- context is { kind = "wild" } or { kind = "trainer" }
+  -- return true after claiming START, otherwise return next(game, context)
+end
+```
+
+The context is data-only. No live battle controller, input object, serializer,
+or restoration primitive is exposed. A `true` result consumes START for that
+fixed step without selecting a battle command. With no installed handler,
+START is inert exactly as before. Hook priorities and error isolation are the
+existing generic wrapper semantics: a throwing handler is skipped and cannot
+advance battle state.
+
+The engine reuses the same internal safety predicate as battle checkpoint
+capture. Link, Safari, ghost/demo, unsupported origins, scripts, queues,
+animations, messages, forced replacement/locked actions, and unsettled HP or
+status presentation never invoke the hook.
+
+## Compatibility and verification
+
+The call is additive and no-op with no handler. ROM-free engine tests prove
+wild/trainer delivery, cursor/turn preservation, and unsafe-phase refusal;
+the mod-SDK fixture proves a loaded mod can consume the semantic action using
+only its public hook facade. `gate_hooks` automatically includes the new call
+site in no-mod parity coverage.

src/battle/BattleSafety.lua

@@ -0,0 +1,86 @@
+-- Shared settled ordinary-player-decision predicate.  Checkpoint capture and
+-- the public auxiliary action deliberately use this one engine-owned rule so
+-- a tool cannot open at a phase that it could not subsequently checkpoint.
+-- It exposes no controller; callers receive only the result/reason.
+
+local BattleSafety = {}
+
+local BATTLE_BUSY_FIELDS = {
+  "current", "afterQueue", "nextInsert", "pendingHit", "waitingUI",
+  "waitingSound", "waitFrames", "draining", "animPlaying", "growIn",
+  "introSlide", "ghostReveal", "mimicCtx", "mimicMoves", "result",
+}
+
+local function nonempty(value)
+  return type(value) == "table" and next(value) ~= nil
+end
+
+local function running(runner)
+  return runner and runner.isRunning and runner:isRunning()
+end
+
+local function scriptsBusy(overworld)
+  return running(overworld and overworld.runner)
+    or nonempty(overworld and overworld.parallelRunners)
+    or nonempty(overworld and overworld.pendingScripts)
+    or nonempty(overworld and overworld.parallelQueue)
+    or nonempty(overworld and overworld.scriptMoves)
+end
+
+function BattleSafety.inspect(game, battle)
+  if type(game) ~= "table" or type(game.save) ~= "table"
+      or type(game.save.version) ~= "string" then
+    return nil, "not_in_playthrough", "A checkpoint requires an identified active playthrough."
+  end
+  if type(battle) ~= "table" then
+    return nil, "not_battle", "No battle is active."
+  end
+  if battle.kind == "link" then
+    return nil, "link_battle_unsupported", "Network battles cannot be checkpointed."
+  end
+  if battle.safari or battle.ghost or battle.scopeReveal or battle.demo or battle.noCatch then
+    return nil, "battle_variant_unsupported",
+      "This battle variant does not have a checkpoint contract."
+  end
+  if battle.kind ~= "wild" and battle.kind ~= "trainer" then
+    return nil, "battle_variant_unsupported",
+      "This battle kind does not have a checkpoint contract."
+  end
+  local expectedOrigin = battle.kind == "wild" and "wild_encounter"
+    or "trainer_encounter"
+  if type(battle.checkpointOrigin) ~= "table"
+      or battle.checkpointOrigin.kind ~= expectedOrigin then
+    return nil, "battle_origin_unsupported",
+      "The battle completion path cannot be reconstructed safely."
+  end
+  if scriptsBusy(game.overworld) then
+    return nil, "script_busy", "A suspended or queued script cannot be checkpointed."
+  end
+  if battle.phase ~= "menu" or nonempty(battle.queue) then
+    return nil, "battle_phase_busy",
+      "Wait for the player command menu before creating a checkpoint."
+  end
+  for _, field in ipairs(BATTLE_BUSY_FIELDS) do
+    if battle[field] ~= nil and battle[field] ~= false then
+      return nil, "battle_phase_busy", "Wait for the current battle action to finish."
+    end
+  end
+  if not battle.player or not battle.enemy or not battle.player.mon
+      or battle.player.mon.hp <= 0
+      or (battle.menuLockedAction and battle:menuLockedAction(battle.player)) then
+    return nil, "battle_phase_busy",
+      "Wait for an ordinary player decision before creating a checkpoint."
+  end
+  for _, battler in ipairs({ battle.player, battle.enemy }) do
+    if not battler.mon or battler.shownHP ~= battler.mon.hp
+        or battler.shownStatus ~= battler.mon.status
+        or battler.drainFloor ~= nil or battler.drainHold ~= nil
+        or battler.faintQueued then
+      return nil, "battle_phase_busy",
+        "Wait for battle status and HP presentation to settle."
+    end
+  end
+  return true
+end
+
+return BattleSafety

src/battle/BattleState.lua

@@ -21,6 +21,7 @@ local MoveEffects = require("src.battle.MoveEffects")
 local Party = require("src.pokemon.Party")
 local Pokemon = require("src.pokemon.Pokemon")
 local Runtime = require("src.mods.Runtime")
+local BattleSafety = require("src.battle.BattleSafety")
 local Screens = require("src.ui.Screens")
 local Status = require("src.battle.Status")
 local Timing = require("src.core.Timing")
@@ -1942,6 +1943,17 @@ function BattleState:update(dt)
       self:resolveTurn(locked)
       return
     end
+    -- START has no vanilla action at the settled ordinary player-decision
+    -- boundary.  A tool mod may claim this semantic auxiliary action through
+    -- the public hook, receiving only game plus a data-only kind.  The shared
+    -- safety predicate keeps every unsupported/forced/animated phase inert.
+    if input:wasPressed("start") and Runtime.wantsHook("battle.menu_auxiliary") then
+      local safe = BattleSafety.inspect(self.game, self)
+      if safe and Runtime.call("battle.menu_auxiliary", function() return false end,
+          self.game, { kind = self.kind }) == true then
+        return
+      end
+    end
     local col = (self.menuIndex - 1) % 2
     local row = math.floor((self.menuIndex - 1) / 2)
     if input:wasPressed("left") then

src/core/Checkpoint.lua

@@ -7,6 +7,7 @@ local Version = require("src.core.Version")
 local BattleState = require("src.battle.BattleState")
 local BattleCheckpoint = require("src.core.BattleCheckpoint")
 local ModRuntime = require("src.mods.Runtime")
+local BattleSafety = require("src.battle.BattleSafety")
 
 local Checkpoint = {}
 
@@ -36,61 +37,9 @@ local function scriptsBusy(ow)
     or nonempty(ow.scriptMoves)
 end
 
-local BATTLE_BUSY_FIELDS = {
-  "current", "afterQueue", "nextInsert", "pendingHit", "waitingUI",
-  "waitingSound", "waitFrames", "draining", "animPlaying", "growIn",
-  "introSlide", "ghostReveal", "mimicCtx", "mimicMoves", "result",
-}
-
-local function inspectBattle(ow, battle)
-  if battle.kind == "link" then
-    return refusal("battle", "link_battle_unsupported",
-      "Network battles cannot be checkpointed.")
-  end
-  if battle.safari or battle.ghost or battle.scopeReveal or battle.demo
-      or battle.noCatch then
-    return refusal("battle", "battle_variant_unsupported",
-      "This battle variant does not have a checkpoint contract.")
-  end
-  if battle.kind ~= "wild" and battle.kind ~= "trainer" then
-    return refusal("battle", "battle_variant_unsupported",
-      "This battle kind does not have a checkpoint contract.")
-  end
-  local origin = battle.checkpointOrigin
-  local expectedOrigin = battle.kind == "wild" and "wild_encounter"
-    or "trainer_encounter"
-  if type(origin) ~= "table" or origin.kind ~= expectedOrigin then
-    return refusal("battle", "battle_origin_unsupported",
-      "The battle completion path cannot be reconstructed safely.")
-  end
-  if scriptsBusy(ow) then
-    return refusal("battle", "script_busy",
-      "A suspended or queued script cannot be checkpointed.")
-  end
-  if battle.phase ~= "menu" or nonempty(battle.queue) then
-    return refusal("battle", "battle_phase_busy",
-      "Wait for the player command menu before creating a checkpoint.")
-  end
-  for _, field in ipairs(BATTLE_BUSY_FIELDS) do
-    if battle[field] ~= nil and battle[field] ~= false then
-      return refusal("battle", "battle_phase_busy",
-        "Wait for the current battle action to finish.")
-    end
-  end
-  if not battle.player or not battle.enemy or battle.player.mon.hp <= 0
-      or (battle.menuLockedAction and battle:menuLockedAction(battle.player)) then
-    return refusal("battle", "battle_phase_busy",
-      "Wait for an ordinary player decision before creating a checkpoint.")
-  end
-  for _, battler in ipairs({ battle.player, battle.enemy }) do
-    if battler.shownHP ~= battler.mon.hp
-        or battler.shownStatus ~= battler.mon.status
-        or battler.drainFloor ~= nil or battler.drainHold ~= nil
-        or battler.faintQueued then
-      return refusal("battle", "battle_phase_busy",
-        "Wait for battle status and HP presentation to settle.")
-    end
-  end
+local function inspectBattle(game, battle)
+  local allowed, reason, message = BattleSafety.inspect(game, battle)
+  if not allowed then return refusal("battle", reason, message) end
   return { canCapture = true, canRestore = true, kind = "battle" }
 end
 
@@ -109,7 +58,7 @@ function Checkpoint.inspect(game)
   end
   local top = game.stack and game.stack.top and game.stack:top()
   if getmetatable(top) == BattleState then
-    return inspectBattle(ow, top)
+    return inspectBattle(game, top)
   end
   if top ~= ow then
     return refusal("overworld", "screen_busy",

tests/engine/battle_menu_auxiliary.lua

@@ -0,0 +1,91 @@
+-- Public battle auxiliary actions are a narrow semantic entry point for tool
+-- mods.  They run only at the same settled ordinary decision boundary as a
+-- battle checkpoint, consume no FIGHT/PKMN/ITEM/RUN action, and receive no
+-- live BattleState object.
+
+package.path = "./?.lua;./?/init.lua;" .. package.path
+love = love or require("tests.love_stub")
+
+local T = require("tests.harness").suite("battle menu auxiliary action")
+local Fixtures = require("tests.modkit").fixtures
+local BattleState = require("src.battle.BattleState")
+local Hooks = require("src.mods.Hooks")
+local Runtime = require("src.mods.Runtime")
+local Pokemon = require("src.pokemon.Pokemon")
+local SaveData = require("src.core.SaveData")
+local StateStack = require("src.core.StateStack")
+
+local Data = Fixtures.fresh()
+
+local function makeGame(kind)
+  local save = SaveData.newGame()
+  save.meta.playthroughId = "battle-menu-playthrough"
+  save.party = { Pokemon.new(Data, "FIXMON_A", 20) }
+  local stack = setmetatable({ states = {} }, { __index = StateStack })
+  local overworld = {
+    map = { id = save.player.map },
+    player = { cellX = save.player.x, cellY = save.player.y, facing = save.player.facing },
+    runner = { isRunning = function() return false end },
+    parallelRunners = {}, pendingScripts = {}, parallelQueue = {}, scriptMoves = {},
+  }
+  local game = { data = Data, save = save, stack = stack }
+  game.input = { wasPressed = function(_, button) return button == "start" end }
+  game.overworld = overworld
+  stack.states[1] = overworld
+  local battle = kind == "trainer"
+    and BattleState.newTrainer(game, "OPP_FIX_YOUNGSTER", 1)
+    or BattleState.newWild(game, "FIXMON_B", 12)
+  battle.phase, battle.queue = "menu", {}
+  battle.checkpointOrigin = kind == "trainer"
+    and { kind = "trainer_encounter", map = save.player.map, npcId = "TRAINER_1",
+      trainerClass = "OPP_FIX_YOUNGSTER", partyIndex = 1, event = "EVENT_BEAT_TRAINER_1" }
+    or { kind = "wild_encounter", map = save.player.map }
+  battle.onFinish = function() end
+  stack.states[2] = battle
+  return game, battle
+end
+
+local oldHooks = Runtime.hooks
+local hooks = Hooks.new()
+Runtime.hooks = hooks
+
+local game, battle = makeGame("wild")
+local calls = 0
+hooks:wrap("battle.menu_auxiliary", function(nextFn, liveGame, context)
+  calls = calls + 1
+  T.check(liveGame == game, "auxiliary action receives the live game")
+  T.same(context, { kind = "wild" }, "auxiliary action receives only data-only battle context")
+  return true
+end, 0, "tool_fixture")
+
+local originalIndex = battle.menuIndex
+battle:update(1 / 60)
+T.eq(calls, 1, "START reaches the public auxiliary action at a wild decision")
+T.eq(battle.phase, "menu", "handled auxiliary action does not advance the battle")
+T.eq(battle.menuIndex, originalIndex, "handled auxiliary action preserves cursor")
+T.eq(#battle.queue, 0, "handled auxiliary action does not enqueue a turn")
+
+hooks:removeOwner("tool_fixture")
+local trainerGame, trainer = makeGame("trainer")
+local trainerCalls = 0
+hooks:wrap("battle.menu_auxiliary", function(_, liveGame, context)
+  trainerCalls = trainerCalls + 1
+  T.check(liveGame == trainerGame, "trainer action receives its live game")
+  T.same(context, { kind = "trainer" }, "trainer context remains data-only")
+  return true
+end, 0, "trainer_fixture")
+trainer:update(1 / 60)
+T.eq(trainerCalls, 1, "START reaches the public auxiliary action at a trainer decision")
+hooks:removeOwner("trainer_fixture")
+
+local unsafeGame, unsafe = makeGame("wild")
+unsafe.phase = "messages"
+local unsafeCalls = 0
+hooks:wrap("battle.menu_auxiliary", function() unsafeCalls = unsafeCalls + 1 return true end,
+  0, "unsafe_fixture")
+unsafe:update(1 / 60)
+T.eq(unsafeCalls, 0, "messages never expose the auxiliary action")
+hooks:removeOwner("unsafe_fixture")
+
+Runtime.hooks = oldHooks
+T.finish()

tests/modkit/cases/checkpoints.lua

@@ -140,7 +140,10 @@ local files = {
     '{"id":"probe","name":"probe","version":"1.0.0",'
       .. '"entry":"main.lua","api":2,"profile":"content"}',
   ["mods/probe/main.lua"] = [[
-return function(mod) _G.MOD_CHECKPOINTS = mod.checkpoints end
+return function(mod)
+  _G.MOD_CHECKPOINTS = mod.checkpoints
+  _G.MOD_HOOKS = mod.hooks
+end
 ]],
 }
 local game, ow = makeGame()
@@ -433,9 +436,28 @@ if battleSnapshot then
     "public battle capture/restore/capture is a normalized differential roundtrip")
 end
 
+-- The mod receives the normal public hook facade, never BattleState. START
+-- at the restored safe decision reaches its semantic auxiliary action without
+-- selecting a native command.
+local auxiliaryCalls = 0
+_G.MOD_HOOKS:wrap("battle.menu_auxiliary", function(nextFn, liveGame, context)
+  auxiliaryCalls = auxiliaryCalls + 1
+  T.check(liveGame == battleGame, "public battle auxiliary action receives the game")
+  T.same(context, { kind = "wild" }, "public auxiliary context is data-only")
+  return true
+end)
+battleGame.input = { wasPressed = function(_, button) return button == "start" end }
+local boundary = battleGame.stack:top()
+local originalMenuIndex = boundary.menuIndex
+boundary:update(1 / 60)
+T.eq(auxiliaryCalls, 1, "public mod hook receives START at the checkpoint boundary")
+T.eq(boundary.phase, "menu", "public auxiliary hook does not advance the turn")
+T.eq(boundary.menuIndex, originalMenuIndex, "public auxiliary hook preserves cursor")
+
 Runtime.events, Runtime.hooks = savedEvents, savedHooks
 Runtime.currentMod = nil
 _G.MOD_CHECKPOINTS = nil
+_G.MOD_HOOKS = nil
 love.math.getRandomState = oldGetRandomState
 love.math.setRandomState = oldSetRandomState