feat(rules): Stealth + opposed Perception - opt-in hide move action (Stealth d20 + trained class + ACP + size mod vs each enemy Perception with -1/10ft distance; observed creatures cannot be hidden from), per-observer hidden_from state treated as total concealment + unseen-attacker +2/deny-Dex for failed observers, attacking or running/charging breaks stealth, move/5-foot-step/withdraw re-check the opposed roll, observation exposes hidden_from (SRD Stealth/Perception/Skill Checks), 13 tests

src/pf1e_simulator/combat.py

@@ -272,6 +272,17 @@ _TUMBLE_SPACE_ENTRY_DC = 5
 _TUMBLE_EXTRA_OPPONENT_DC = 2
 # SRD "Acrobatics" - Tumble: "You move at half speed when you tumble".
 _TUMBLE_SPEED_FACTOR = 0.5
+# PF1e (SRD "Stealth" - Modifiers): size bonus/penalty on Stealth checks.
+_STEALTH_SIZE_MOD: dict[str, int] = {
+    "fine": 16, "diminutive": 12, "tiny": 8, "small": 4, "medium": 0,
+    "large": -4, "huge": -8, "gargantuan": -12, "colossal": -16,
+}
+# PF1e (SRD "Perception"): notice checks take -1 per 10 feet of distance.
+_STEALTH_DISTANCE_FT = 10
+# SRD "Stealth": moving faster than half speed takes -5 on the Stealth check
+# (engine: a move/withdraw action's budget is the full speed, so its re-check
+# carries the -5; see _stealth_after_move).
+_STEALTH_MOVE_PENALTY = -5
 _HELPLESS_MELEE_BONUS = 4  # PF1e: +4 melee attack rolls (and crit confirms) vs helpless
 # PF1e: a helpless creature (paralyzed, held, bound, sleeping, unconscious —
 # stunned is NOT helpless) has an effective Dexterity score of 0, i.e. a -5
@@ -294,6 +305,9 @@ _DISABLED_MOVE_KINDS: frozenset[str] = frozenset(
         "wait",
         "retrieve_weapon",
         "repair_weapon",
+        # CRB "Disabled" lists "moving 5 feet or more" as strenuous; hiding
+        # in place is a move action that does not move - not strenuous.
+        "hide",
     }
 )
 # PF1e (CRB, Combat Maneuver): attempting a combat maneuver provokes an
@@ -1029,6 +1043,16 @@ class CombatantState:
     # combatant while the target lives and stands in reach, dissipated when
     # this combatant casts another spell. None when no charge is held.
     held_charge: HeldCharge | None = None
+    # PF1e Stealth (SRD "Stealth": "Creatures that fail to beat your Stealth
+    # check are not aware of you and treat you as if you had total
+    # concealment", KB rules/ultimate-intrigue/perception-and-stealth.md):
+    # the ids of observers that currently cannot perceive this combatant -
+    # per-observer invisibility, which the global condition machinery cannot
+    # express (a creature can be hidden from one foe and seen by another).
+    # Set by the `hide` action's opposed check, re-rolled when the creature
+    # hides or moves again, and cleared the moment the creature makes an
+    # attack roll (SRD "Breaking Stealth").
+    hidden_from: set[str] = field(default_factory=set)
 
     def usable_attacks(self) -> list[AttackSpec]:
         # PF1e (CRB "Breaking Items" / "Conditions - Broken"): a broken
@@ -1359,7 +1383,10 @@ class Action:
     Combat Casting) or the spell fizzles. ``move`` accepts ``tumble``: the
     move happens at half speed and each opponent crossed is passed with one
     Acrobatics check against its CMD (SRD "Acrobatics" - Tumble; see the
-    ``tumble`` field comment).
+    ``tumble`` field comment). ``hide`` is a move action: a Stealth check
+    opposed by each enemy observer's Perception check (distance -1 per 10
+    ft); observers that fail cannot perceive the creature until it attacks
+    or moves again (SRD "Stealth" - see ``_execute_hide``).
     """
 
     kind: Literal[
@@ -1369,7 +1396,7 @@ class Action:
         "inspire_courage", "retrieve_weapon", "repair_weapon",
         "activate_power_attack", "delay", "ready",
         "total_defense", "fight_defensively", "stand_up", "drop_prone",
-        "aid_another", "run",
+        "aid_another", "run", "hide",
     ]
     target_id: str | None = None
     spell_name: str | None = None
@@ -1852,6 +1879,10 @@ class CombatEngine:
             "weapon_dropped": tuple(sorted(state.weapon_dropped)),
             "carried_items": tuple(state.carried_items),
             "grappled_with": state.grappled_with,
+            # SRD "Stealth": ids of observers that currently cannot perceive
+            # this combatant (they failed the opposed Perception check); the
+            # policy sees who is hidden from whom (per-observer invisibility).
+            "hidden_from": tuple(sorted(state.hidden_from)),
             "moved_this_turn": state.moved_this_turn,
             "power_attack": state.power_attack,
             "defensive_ac_bonus": state.defensive_ac_bonus,
@@ -3407,8 +3438,16 @@ class CombatEngine:
         if self._has_condition_named(attacker, "prone"):
             atk_mods.append(StatModifier(target="attack", value=-4))
         # PF1e: an invisible attacker strikes from an unknown direction (+2 on
-        # attack and crit confirmation, target loses its Dex bonus to AC).
+        # attack and crit confirmation, target loses its Dex bonus to AC). A
-        if self._has_condition_named(attacker, "invisible"):
+        # creature hidden from its target by a successful Stealth check strikes
+        # as an unseen attacker exactly the same way (SRD "Stealth": observers
+        # that fail the check "are not aware of you and treat you as if you
+        # had total concealment" - CRB "Unseen Attackers and Targets" gives
+        # the unseen attacker +2 and denies the defender its Dex bonus).
+        attacker_unseen = self._has_condition_named(
+            attacker, "invisible"
+        ) or self._is_hidden_from(attacker, target)
+        if attacker_unseen:
             atk_mods.append(StatModifier(target="attack", value=_INVISIBLE_ATTACK_BONUS))
         # PF1e (CRB): melee attacks (melee touch included) against a helpless
         # target gain +4; the helpless state includes paralyzed, asleep and
@@ -3434,11 +3473,12 @@ class CombatEngine:
         # loses its DEX bonus too (U5e), reusing the same AC path — unless it
         # has the Run feat (SRD Run: "You lose any Dexterity bonus to AC
         # unless you have the Run feat"). An invisible attacker also denies
-        # the target's DEX bonus to AC.
+        # the target's DEX bonus to AC, and so does one hidden from the
+        # target by Stealth (attacker_unseen above).
         denies_dex = (
             self.has_condition_flag(target, "denies_dex_to_ac")
             or self._running_denies_dex(target)
-            or self._has_condition_named(attacker, "invisible")
+            or attacker_unseen
         )
         ac = self._attack_ac(attacker, target, weapon, dist_ft, denies_dex=denies_dex)
         hit = roll == _NATURAL_TWENTY or (roll != _NATURAL_ONE and total >= ac)
@@ -3495,6 +3535,13 @@ class CombatEngine:
             else:
                 splash_total = self._splash_scatter(attacker, target, weapon)
         attacker.aid_attack_bonus = 0
+        # SRD "Stealth" - Breaking Stealth: "Your Stealth immediately ends
+        # after you make an attack roll, whether or not the attack is
+        # successful" (KB rules/ultimate-intrigue/perception-and-stealth.md
+        # awareness states: attacking makes the observers aware of you) -
+        # the roll above already benefited from the unseen-attacker treatment
+        # and the target's failed check, so the hidden state is spent now.
+        attacker.hidden_from.clear()
         return AttackResult(
             hit=hit, crit=crit, damage=damage, roll=roll, total=total, ac=ac,
             penalty=penalty, flank_bonus=flank, high_ground_bonus=high_ground,
@@ -3603,7 +3650,7 @@ class CombatEngine:
             crit = confirm == _NATURAL_TWENTY or (
                 confirm != _NATURAL_ONE and confirm_total >= ac
             )
-        concealment = self._concealment_chance(target)
+        concealment = self._concealment_chance_vs(attacker, target)
         # PF1e: a blinded character's opponents count as having total
         # concealment against it — the blinded attacker itself misses 50%.
         attacker_blinded = self._has_condition_named(attacker, "blinded")
@@ -3819,6 +3866,28 @@ class CombatEngine:
         chances += [cond.miss_chance for cond in target.conditions]
         return max((c for c in chances if c > 0), default=0)
 
+    def _is_hidden_from(self, subject: CombatantState, observer: CombatantState) -> bool:
+        """True when ``subject`` is hidden from ``observer`` (failed Stealth)."""
+        return observer.combatant.id in subject.hidden_from
+
+    def _concealment_chance_vs(
+        self, observer: CombatantState, target: CombatantState
+    ) -> int:
+        """Concealment the ``target`` has against attacks from ``observer``.
+
+        PF1e (SRD "Stealth", KB rules/ultimate-intrigue/perception-and-stealth.md
+        quoting the CRB: creatures that fail the opposed Perception check "are
+        not aware of the sneaking character"): a hidden creature is treated as
+        if it had TOTAL concealment (50% miss chance, CRB "Concealment") -
+        but only relative to the observers that failed the check, which the
+        global condition machinery cannot express, hence the per-observer
+        ``hidden_from`` set folded in here.
+        """
+        chance = self._concealment_chance(target)
+        if self._is_hidden_from(target, observer):
+            return max(chance, _TOTAL_CONCEALMENT_PCT)
+        return chance
+
     def _sneak_attack_dice(
         self,
         attacker: CombatantState,
@@ -4398,7 +4467,7 @@ class CombatEngine:
         suppressed too. Melee line: terrain cover only — creatures grant no
         melee cover.
         """
-        concealment = self._concealment_chance(target)
+        concealment = self._concealment_chance_vs(attacker, target)
         if self._has_condition_named(attacker, "blinded"):
             concealment = max(concealment, _TOTAL_CONCEALMENT_PCT)
         if concealment >= _TOTAL_CONCEALMENT_PCT:
@@ -4900,6 +4969,9 @@ class CombatEngine:
         """
         if attacker.moved_this_turn:
             return
+        # SRD "Stealth": "It's impossible to use Stealth while attacking,
+        # running, or charging" - charging forfeits the hidden state outright.
+        self._break_stealth(attacker, "charging")
         inner_cells, reach_cells = _melee_reach_zone_cells(attacker, weapon)
         speed_cells = _speed_ft(attacker) // _SQUARE_FT
         max_cells = self._charge_speed_mult(attacker) * speed_cells
@@ -4985,7 +5057,7 @@ class CombatEngine:
             return self._execute_delay(state, target, action)
         if kind == "ready":
             return self._execute_ready(state, action)
-        if kind in ("total_defense", "fight_defensively", "stand_up", "drop_prone"):
+        if kind in ("total_defense", "fight_defensively", "stand_up", "drop_prone", "hide"):
             return self._execute_self_action(state, kind)
         if kind == "aid_another":
             return self._execute_aid_another(state, target, action)
@@ -5090,6 +5162,8 @@ class CombatEngine:
             return self._execute_fight_defensively(state)
         if kind == "stand_up":
             return self._execute_stand_up(state)
+        if kind == "hide":
+            return self._execute_hide(state)
         return self._execute_drop_prone(state)
 
     def _execute_total_defense(self, state: CombatantState) -> ActionResult:
@@ -5308,6 +5382,9 @@ class CombatEngine:
         ``missing_target``.
         """
         kind = "run"
+        # SRD "Stealth": "It's impossible to use Stealth while attacking,
+        # running, or charging" - running forfeits the hidden state outright.
+        self._break_stealth(state, "running")
         if state.moved_this_turn:
             # Defense in depth, mirroring the _move/_5foot_step no-op-ok
             # convention. Unreachable through the filtered path: run is a
@@ -5576,6 +5653,178 @@ class CombatEngine:
             f"Power Attack activated"
         )
 
+    def _stealth_size_mod(self, state: CombatantState) -> int:
+        """The size bonus/penalty the state's Stealth checks carry.
+
+        PF1e (SRD "Stealth" - Modifiers, d20pfsrd Stealth (CRB): "Creatures
+        gain a bonus or penalty on Stealth checks based on their size: Fine
+        +16, Diminutive +12, Tiny +8, Small +4, Medium +0, Large -4, Huge -8,
+        Gargantuan -12, Colossal -16"). The data model bakes size into AC and
+        attack JSON values but not into skill ranks, so the table rides the
+        check here.
+        """
+        return _STEALTH_SIZE_MOD.get(state.combatant.size.lower(), 0)
+
+    def _stealth_check(
+        self, state: CombatantState, *, penalty: int = 0
+    ) -> SkillCheckResult:
+        check = self.skill_check(state, "stealth")
+        return SkillCheckResult(
+            roll=check.roll, total=check.total + self._stealth_size_mod(state) + penalty
+        )
+
+    def _notice_check(
+        self, observer: CombatantState, target: CombatantState
+    ) -> SkillCheckResult:
+        """One observer's Perception check to notice ``target``.
+
+        PF1e (SRD "Perception", KB rules/pathfinder-unchained/perception-wis.md
+        notice-modifier table: "Distance to the source, object, or creature:
+        +1 per 10 feet" DC modifier = -1 per 10 feet on the check). The
+        table's "distracted +5" modifier is NOT applied: the SRD grants it to
+        a specifically distracted creature and defines no automatic
+        in-combat = distracted rule, and the model has no distracted state
+        (documented out of scope).
+        """
+        dist_ft = self.creature_distance(observer, target) * _SQUARE_FT
+        penalty = -(dist_ft // _STEALTH_DISTANCE_FT)
+        mods: tuple[StatModifier, ...] = ()
+        if penalty:
+            mods = (StatModifier(target="perception", value=penalty),)
+        return self.skill_check(observer, "perception", extra_mods=mods)
+
+    def _directly_observed(
+        self, hider: CombatantState, observer: CombatantState
+    ) -> bool:
+        """True when the observer directly sees the hider (Stealth impossible).
+
+        PF1e (SRD "Stealth" - Being Observed, KB rules/ultimate-intrigue/
+        perception-and-stealth.md: "characters can't use Stealth while being
+        observed... finding cover or concealment allows you to use Stealth"):
+        line of effect, no terrain cover, no concealment on the hider and a
+        sighted observer means the hider IS observed.
+        """
+        if self._has_condition_named(observer, "blinded"):
+            return False
+        if not has_line_of_effect_cells(
+            self._grid, observer.occupied_cells(), hider.occupied_cells()
+        ):
+            return False
+        if self._concealment_chance(hider) > 0:
+            return False
+        return (
+            cover_bonus_cells(
+                self._grid,
+                observer.occupied_cells(),
+                hider.occupied_cells(),
+                ranged=True,
+                occupied=frozenset(),
+            )
+            == 0
+        )
+
+    def _execute_hide(self, state: CombatantState) -> ActionResult:
+        """Execute the hide action (PF1e, SRD "Stealth"): a move action.
+
+        One Stealth check (d20 + ranks + Dex + trained class skill + armor
+        check penalty + size) opposed by every enemy observer's Perception
+        check (distance -1 per 10 ft). Observers that fail to BEAT the
+        Stealth total cannot perceive the creature until it attacks or moves
+        (SRD "Stealth": "Creatures that fail to beat your Stealth check are
+        not aware of you and treat you as if you had total concealment";
+        opposed-check resolution per CRB "Skill Checks"). An observer that
+        directly sees the hider is never beaten - Stealth against it is
+        impossible (SRD "Being Observed").
+        """
+        kind = "hide"
+        stealth: SkillCheckResult | None = None
+        hidden: set[str] = set()
+        for observer in self._states:
+            if (
+                observer is state
+                or not observer.active
+                or observer.side == state.side
+            ):
+                continue
+            if self._directly_observed(state, observer):
+                self._log(
+                    f"round {self._current_round} {state.combatant.id}: "
+                    f"hide vs {observer.combatant.id} -> observed "
+                    f"(Stealth impossible)"
+                )
+                continue
+            if stealth is None:
+                stealth = self._stealth_check(state)
+            perc = self._notice_check(observer, state)
+            if stealth.total > perc.total:
+                hidden.add(observer.combatant.id)
+                verdict = "hidden"
+            else:
+                verdict = "noticed"
+            self._log(
+                f"round {self._current_round} {state.combatant.id}: "
+                f"hide stealth d20={stealth.roll}+{stealth.total - stealth.roll}"
+                f"={stealth.total} vs {observer.combatant.id} perception "
+                f"d20={perc.roll}+{perc.total - perc.roll}={perc.total} "
+                f"-> {verdict}"
+            )
+        state.hidden_from = hidden
+        return ActionResult(kind=kind, ok=True, reason=_REASON_OK)
+
+    def _stealth_after_move(self, state: CombatantState, *, penalty: int) -> None:
+        """Re-run the opposed check when a hidden creature moves.
+
+        PF1e (SRD "Stealth" - Breaking Stealth, d20pfsrd Stealth (CRB): "you
+        can leave cover or concealment and remain unobserved as long as you
+        succeed at a Stealth check and end your turn in cover or
+        concealment"; "You can move up to half your normal speed and use
+        Stealth at no penalty. When moving at a speed greater than half...
+        you take a -5 penalty"). Engine simplification (documented): the
+        half-speed option is not separately expressible for a move action
+        (its budget is the full speed), so a move/withdraw re-check carries
+        the -5; a 5-foot step (always within half speed) carries 0. An
+        observer that ends the move directly observing the creature is
+        aware of it with no roll.
+        """
+        if not state.hidden_from:
+            return
+        stealth: SkillCheckResult | None = None
+        for observer_id in sorted(state.hidden_from):
+            observer = next(
+                s for s in self._states if s.combatant.id == observer_id
+            )
+            if not observer.active:
+                continue
+            if self._directly_observed(state, observer):
+                state.hidden_from.discard(observer_id)
+                self._log(
+                    f"round {self._current_round} {state.combatant.id}: "
+                    f"no longer hidden from {observer_id} (observed)"
+                )
+                continue
+            if stealth is None:
+                stealth = self._stealth_check(state, penalty=penalty)
+            perc = self._notice_check(observer, state)
+            self._log(
+                f"round {self._current_round} {state.combatant.id}: "
+                f"hide stealth d20={stealth.roll}+{stealth.total - stealth.roll}"
+                f"={stealth.total} vs {observer_id} perception "
+                f"d20={perc.roll}+{perc.total - perc.roll}={perc.total} "
+                f"-> {'hidden' if stealth.total > perc.total else 'noticed'}"
+            )
+            if perc.total >= stealth.total:
+                state.hidden_from.discard(observer_id)
+
+    def _break_stealth(self, state: CombatantState, reason: str) -> None:
+        """End a hidden state explicitly (SRD "Stealth": impossible while
+        running or charging - the action itself forfeits the check)."""
+        if state.hidden_from:
+            state.hidden_from.clear()
+            self._log(
+                f"round {self._current_round} {state.combatant.id}: "
+                f"stealth broken ({reason})"
+            )
+
     def _check_reactions(self, event: ReactionEvent) -> None:
         """Fire readied actions first, then offer the event to the reaction policy.
 
@@ -7521,6 +7770,7 @@ class CombatEngine:
             self.has_condition_flag(target, "denies_dex_to_ac")
             or self._running_denies_dex(target)
             or self._has_condition_named(caster, "invisible")
+            or self._is_hidden_from(caster, target)
         )
         if denies_dex:
             ac = target.combatant.ac.touch - max(0, target.combatant.abilities.dex_mod)
@@ -7563,6 +7813,7 @@ class CombatEngine:
             self.has_condition_flag(target, "denies_dex_to_ac")
             or self._running_denies_dex(target)
             or self._has_condition_named(caster, "invisible")
+            or self._is_hidden_from(caster, target)
         )
         if denies_dex:
             ac = target.combatant.ac.touch - max(0, target.combatant.abilities.dex_mod)
@@ -7588,7 +7839,11 @@ class CombatEngine:
         """Finish a touch-attack roll: hit check, crit threat/confirm (only
         when the spell deals damage, CRB "Cast a Spell — Touch Attacks"), then
         the concealment miss chance (a blinded caster faces total concealment,
-        same max(target, 50) convention as _resolve_hit_damage)."""
+        same max(target, 50) convention as _resolve_hit_damage). A hidden
+        caster delivers the touch as an unseen attacker (Dex denial in the
+        caller) and this attack roll ends its Stealth (SRD "Breaking
+        Stealth")."""
+        caster.hidden_from.clear()
         hit = roll == _NATURAL_TWENTY or (roll != _NATURAL_ONE and roll + bonus >= ac)
         self._log(
             f"round {self._current_round} {caster.combatant.id}: {label} vs "
@@ -7608,7 +7863,7 @@ class CombatEngine:
                 f"crit confirm d20={confirm}+{bonus}={confirm + bonus} AC {ac} "
                 f"-> {'CRIT' if crit else 'no crit'}"
             )
-        concealment = self._concealment_chance(target)
+        concealment = self._concealment_chance_vs(caster, target)
         blinded = self._has_condition_named(caster, "blinded")
         miss_chance = max(concealment, _TOTAL_CONCEALMENT_PCT) if blinded else concealment
         if miss_chance > 0:
@@ -9108,6 +9363,8 @@ class CombatEngine:
             f"round {self._current_round} {state.combatant.id}: "
             f"5ft step ({start[0]},{start[1]})->({path[0][0]},{path[0][1]})"
         )
+        # A 5-foot step is always within half speed: Stealth re-check at 0.
+        self._stealth_after_move(state, penalty=0)
 
     def _step5_to(self, state: CombatantState, destination: Pos) -> ActionResult:
         """5-foot step to an explicit adjacent cell — never provokes (U5c)."""
@@ -9145,6 +9402,8 @@ class CombatEngine:
             f"round {self._current_round} {state.combatant.id}: "
             f"5ft step ({start[0]},{start[1]})->({destination[0]},{destination[1]})"
         )
+        # A 5-foot step is always within half speed: Stealth re-check at 0.
+        self._stealth_after_move(state, penalty=0)
         return ActionResult(kind=kind, ok=True, reason=_REASON_OK)
 
     def _move(
@@ -9166,6 +9425,7 @@ class CombatEngine:
             state.moved_this_turn = True
             coords = "->".join(f"({p[0]},{p[1]})" for p in (start, *actual_path))
             self._log(f"round {self._current_round} {state.combatant.id}: move {coords}")
+        self._stealth_after_move(state, penalty=_STEALTH_MOVE_PENALTY)
 
     def _move_to(
         self, state: CombatantState, destination: Pos, *, tumble: bool = False
@@ -9204,6 +9464,7 @@ class CombatEngine:
             f"round {self._current_round} {state.combatant.id}: "
             f"move -> ({destination[0]},{destination[1]})"
         )
+        self._stealth_after_move(state, penalty=_STEALTH_MOVE_PENALTY)
         return ActionResult(kind=kind, ok=True, reason=_REASON_OK)
 
     def _destination_reachable(
@@ -9658,6 +9919,8 @@ class CombatEngine:
             self._log(
                 f"round {self._current_round} {state.combatant.id}: withdraw {coords}"
             )
+        # Withdraw moves at double speed: the Stealth re-check carries -5.
+        self._stealth_after_move(state, penalty=_STEALTH_MOVE_PENALTY)
 
     def _log(self, line: str) -> None:
         self._transcript.append(line)
@@ -10094,7 +10357,7 @@ _ACTION_ECONOMY_STANDARD_KINDS: frozenset[str] = frozenset({
     "delay", "ready", "total_defense", "aid_another",
 })
 _ACTION_ECONOMY_MOVE_KINDS: frozenset[str] = frozenset({
-    "move", "stand_up", "retrieve_weapon", "repair_weapon",
+    "move", "stand_up", "retrieve_weapon", "repair_weapon", "hide",
 })
 _ACTION_ECONOMY_SWIFT_KINDS: frozenset[str] = frozenset(
     {"swift", "deed", "inspire_courage"}

tests/test_conformance_action_economy.py

@@ -72,6 +72,7 @@ _ALL_KINDS: tuple[_Kind, ...] = (
     "inspire_courage", "retrieve_weapon", "repair_weapon",
     "activate_power_attack", "delay", "ready", "total_defense",
     "fight_defensively", "stand_up", "drop_prone", "aid_another", "run",
+    "hide",
 )
 
 
@@ -99,7 +100,7 @@ def test_slot_per_kind_matches_pf1e_action_type() -> None:
         "aid_another": "standard",
         # move actions
         "move": "move", "stand_up": "move", "retrieve_weapon": "move",
-        "repair_weapon": "move",
+        "repair_weapon": "move", "hide": "move",
         # run: PF1e full-round action (4x speed); consumes standard + move.
         "run": "full_round",
         # swift actions

tests/test_stealth.py

@@ -0,0 +1,324 @@
+"""PF1e Stealth + opposed Perception (SRD "Stealth" / "Perception").
+
+Verified sources:
+- KB rules/ultimate-intrigue/perception-and-stealth.md: "a creature using
+  Stealth (which calls for an opposed Perception check)"; CRB quoted there:
+  perceiving creatures that "fail to beat a sneaking character's Stealth check
+  result are not aware of the sneaking character"; "characters can't use
+  Stealth while being observed... finding cover or concealment allows you to
+  use Stealth".
+- KB rules/pathfinder-unchained/perception-wis.md (notice table): "Notice a
+  creature using Stealth: Opposed by Stealth"; "Distance to the source,
+  object, or creature: +1 per 10 feet" (DC modifier = -1/10 ft on the
+  observer's check). The table's "distracted +5" is NOT applied: no SRD rule
+  makes in-combat observers automatically distracted and the model has no
+  distracted state (documented out of scope).
+- KB rules/prpg-core-rulebook/skill-checks.md: opposed checks - "the attempt
+  is successful if your check result exceeds the result of the target"; no
+  natural 1/20 auto rules on skill checks.
+- SRD Stealth (d20pfsrd, CRB text): hidden creatures are treated as having
+  TOTAL CONCEALMENT vs unaware observers; the unseen attacker gets +2 and
+  denies the defender its Dex bonus; "Your Stealth immediately ends after
+  you make an attack roll"; "impossible to use Stealth while attacking,
+  running, or charging"; size modifiers (Small +4 ... Large -4); moving
+  faster than half speed takes -5.
+
+Engine: `Action(kind="hide")` (move action) runs the opposed check; the
+per-observer hidden state lives in `CombatantState.hidden_from`.
+"""
+
+from __future__ import annotations
+
+from pf1e_simulator.combat import Action, CombatantState, CombatEngine
+from pf1e_simulator.conditions import CONDITIONS_BY_NAME
+from pf1e_simulator.dice import parse_dice
+from pf1e_simulator.effects import ActiveCondition
+from pf1e_simulator.grid import Grid
+from pf1e_simulator.map import MapSpec, TerrainType
+from pf1e_simulator.models import (
+    AbilityScores,
+    ACProfile,
+    AttackSpec,
+    Combatant,
+    DamageComponent,
+    Saves,
+    SkillRank,
+)
+from pf1e_simulator.rng import ScriptedRng
+
+
+def _sword(cid: str, bonus: int = 3) -> AttackSpec:
+    return AttackSpec(
+        id=f"{cid}-sword",
+        name="short sword",
+        kind="melee",
+        attack_bonus=bonus,
+        damage=[DamageComponent(formula=parse_dice("1d6"), types=["slashing"])],
+    )
+
+
+def _combatant(
+    cid: str,
+    *,
+    size: str = "Medium",
+    dex: int = 10,
+    wis: int = 10,
+    stealth: SkillRank | None = None,
+    perception: SkillRank | None = None,
+    ac: int = 15,
+    touch: int = 10,
+    flat: int = 10,
+) -> Combatant:
+    skills: dict[str, SkillRank] = {}
+    if stealth is not None:
+        skills["stealth"] = stealth
+    if perception is not None:
+        skills["perception"] = perception
+    return Combatant(
+        id=cid,
+        name=cid,
+        level=1,
+        size=size,
+        abilities=AbilityScores(
+            str_score=10, dex_score=dex, con_score=10,
+            int_score=10, wis_score=wis, cha_score=10,
+        ),
+        hp_max=30,
+        ac=ACProfile(total=ac, touch=touch, flat_footed=flat),
+        bab=1,
+        initiative_mod=0,
+        speed_land_ft=30,
+        saves=Saves(fort=0, ref=0, will=0),
+        attacks=[_sword(cid)],
+        skills=skills,
+    )
+
+
+def _engine(
+    rng: list[int],
+    placements: list[tuple[Combatant, tuple[int, int], str]],
+) -> CombatEngine:
+    states = [
+        CombatantState(combatant=c, side=side, pos=pos, hp=c.hp_max)
+        for c, pos, side in placements
+    ]
+    legend = {".": TerrainType(type="floor", move_cost=1)}
+    spec = MapSpec(name="test", terrain=tuple(["." * 8] * 8), legend=legend)
+    return CombatEngine(ScriptedRng(rng), Grid.from_spec(spec), states)
+
+
+def _state(eng: CombatEngine, cid: str) -> CombatantState:
+    return next(s for s in eng._states if s.combatant.id == cid)
+
+
+def _blind(state: CombatantState) -> None:
+    state.active_conditions.append(
+        ActiveCondition(
+            condition=CONDITIONS_BY_NAME["blinded"], rounds_left=5, source="test"
+        )
+    )
+
+
+_ROGUE_SKILL = SkillRank(rank=4, class_skill=True)
+
+
+# ── the opposed check ─────────────────────────────────────────────────────────
+
+
+def test_hide_opposed_check_hides_from_observer_that_fails_to_beat_it() -> None:
+    rogue = _combatant("rogue", dex=18, stealth=_ROGUE_SKILL)
+    goblin = _combatant("goblin", wis=8, perception=SkillRank(rank=1, class_skill=True))
+    # Blinded observer cannot see it: the opposed (hearing) check applies.
+    eng = _engine([10, 5], [(rogue, (0, 0), "players"), (goblin, (0, 2), "monsters")])
+    _blind(eng._states[1])
+    s = _state(eng, "rogue")
+    eng._current_round = 1
+    result = eng._execute(s, Action(kind="hide"))
+    assert result.ok
+    assert result.reason == "ok"
+    # Stealth: 10 + 4 ranks + 4 Dex + 3 class = 21. Perception: 5 + 1 + 3
+    # class - 1 Wis - 1 (10 ft distance) = 7. 21 beats 7 -> hidden.
+    assert s.hidden_from == {"goblin"}
+    assert any(
+        "hide stealth d20=10+11=21 vs goblin perception d20=5+2=7 -> hidden"
+        in ln
+        for ln in eng._transcript
+    )
+
+
+def test_hide_fails_when_observer_beats_stealth() -> None:
+    rogue = _combatant("rogue", dex=18, stealth=_ROGUE_SKILL)
+    goblin = _combatant("goblin", wis=14, perception=SkillRank(rank=5, class_skill=True))
+    eng = _engine([1, 20], [(rogue, (0, 0), "players"), (goblin, (0, 2), "monsters")])
+    _blind(eng._states[1])
+    s = _state(eng, "rogue")
+    eng._current_round = 1
+    eng._execute(s, Action(kind="hide"))
+    # Stealth 1+11=12 vs Perception 20+5+3+2-1=29: noticed (no auto-20 on a
+    # skill check is irrelevant here - the total wins outright).
+    assert s.hidden_from == set()
+    assert any("-> noticed" in ln for ln in eng._transcript)
+
+
+def test_no_auto_success_or_fail_on_the_skill_check() -> None:
+    # CRB "Skill Checks": the opposed attempt succeeds only if it EXCEEDS the
+    # target's result - a natural 20 on Stealth that ties the Perception
+    # total is not an auto-success, and a natural 1 with a big bonus is not
+    # an auto-fail.
+    rogue = _combatant("rogue", dex=18, stealth=_ROGUE_SKILL)
+    goblin = _combatant("goblin", wis=18, perception=SkillRank(rank=5, class_skill=True))
+    eng = _engine([20, 20], [(rogue, (0, 0), "players"), (goblin, (0, 2), "monsters")])
+    _blind(eng._states[1])
+    s = _state(eng, "rogue")
+    eng._current_round = 1
+    eng._execute(s, Action(kind="hide"))
+    # 20+11=31 vs 20+5+3+4-1=31: the attempt did not exceed -> noticed.
+    assert s.hidden_from == set()
+
+
+def test_observed_creature_cannot_hide() -> None:
+    rogue = _combatant("rogue", dex=18, stealth=_ROGUE_SKILL)
+    goblin = _combatant("goblin")
+    # Open arena, line of effect, no cover, sighted observer: the hider is
+    # directly observed -> Stealth is impossible against it (no roll needed).
+    eng = _engine([10], [(rogue, (0, 0), "players"), (goblin, (0, 2), "monsters")])
+    s = _state(eng, "rogue")
+    eng._current_round = 1
+    eng._execute(s, Action(kind="hide"))
+    assert s.hidden_from == set()
+    assert any(
+        "hide vs goblin -> observed (Stealth impossible)" in ln
+        for ln in eng._transcript
+    )
+
+
+def test_stealth_carries_the_size_modifier() -> None:
+    rogue = _combatant(
+        "rogue", size="Small", dex=18, stealth=_ROGUE_SKILL
+    )
+    goblin = _combatant("goblin", wis=8)
+    eng = _engine([10, 1], [(rogue, (0, 0), "players"), (goblin, (0, 2), "monsters")])
+    _blind(eng._states[1])
+    s = _state(eng, "rogue")
+    eng._current_round = 1
+    eng._execute(s, Action(kind="hide"))
+    # Small: +4 size bonus on top of 10 + 4 + 4 + 3 = 21 -> 25.
+    assert any("hide stealth d20=10+15=25" in ln for ln in eng._transcript)
+
+
+# ── the hidden creature is invisible to the failed observers ─────────────────
+
+
+def test_hidden_attacker_gets_unseen_bonus_and_denies_target_dex() -> None:
+    rogue = _combatant("rogue", dex=18)
+    goblin = _combatant("goblin", ac=20, touch=10, flat=10)
+    # Attack roll d20=10, then the 1d6 damage die.
+    eng = _engine([10, 3], [(rogue, (0, 0), "players"), (goblin, (0, 1), "monsters")])
+    s = _state(eng, "rogue")
+    s.hidden_from = {"goblin"}
+    weapon = rogue.attacks[0]
+    result = eng.resolve_attack(s, eng._states[1], weapon)
+    # Unseen attacker: +2 rides the roll; the target is denied its Dex bonus
+    # so the flat-footed AC 10 applies (not 20).
+    assert result.total == 10 + 3 + 2
+    assert result.ac == 10
+    assert result.hit
+
+
+def test_attacking_breaks_stealth() -> None:
+    rogue = _combatant("rogue", dex=18)
+    goblin = _combatant("goblin")
+    eng = _engine([10, 1], [(rogue, (0, 0), "players"), (goblin, (0, 1), "monsters")])
+    s = _state(eng, "rogue")
+    s.hidden_from = {"goblin"}
+    eng.resolve_attack(s, eng._states[1], rogue.attacks[0])
+    assert s.hidden_from == set()
+
+
+def test_unaware_attacker_faces_total_concealment_vs_hidden_creature() -> None:
+    rogue = _combatant("rogue", dex=18, ac=10, touch=10, flat=10)
+    goblin = _combatant("goblin", ac=10)
+    # Attack roll hits (d20=10+3 >= 10), then the d100 concealment roll: 30
+    # <= 50 -> the unaware attacker misses the hidden creature entirely.
+    eng = _engine([10, 30], [(rogue, (0, 0), "players"), (goblin, (0, 1), "monsters")])
+    s = _state(eng, "rogue")
+    s.hidden_from = {"goblin"}
+    result = eng.resolve_attack(eng._states[1], s, goblin.attacks[0])
+    assert result.hit
+    assert result.damage == 0
+    assert s.hp == s.combatant.hp_max
+    assert any("concealment vs rogue" in ln for ln in eng._transcript)
+
+
+def test_unaware_enemy_gets_no_aoop_against_a_hidden_mover() -> None:
+    rogue = _combatant("rogue", dex=18)
+    goblin = _combatant("goblin")
+    # AoO attack roll would consume a die if taken; none is available.
+    eng = _engine([], [(rogue, (0, 0), "players"), (goblin, (0, 1), "monsters")])
+    s = _state(eng, "rogue")
+    s.hidden_from = {"goblin"}
+    eng._current_round = 1
+    eng._execute(s, Action(kind="move", destination=(0, 3)))
+    assert any(
+        "AoO vs rogue suppressed (total concealment)" in ln
+        for ln in eng._transcript
+    )
+
+
+# ── stealth lifecycle ────────────────────────────────────────────────────────
+
+
+def test_moving_while_hidden_rechecks_and_open_ground_reveals() -> None:
+    rogue = _combatant("rogue", dex=18)
+    goblin = _combatant("goblin")
+    eng = _engine([], [(rogue, (0, 0), "players"), (goblin, (0, 4), "monsters")])
+    s = _state(eng, "rogue")
+    s.hidden_from = {"goblin"}
+    eng._current_round = 1
+    eng._execute(s, Action(kind="move", destination=(0, 2)))
+    # The goblin directly observes the new position -> aware, no roll consumed.
+    assert s.hidden_from == set()
+    assert any(
+        "no longer hidden from goblin (observed)" in ln for ln in eng._transcript
+    )
+
+
+def test_run_breaks_stealth_without_a_check() -> None:
+    rogue = _combatant("rogue", dex=18)
+    goblin = _combatant("goblin")
+    eng = _engine([], [(rogue, (0, 0), "players"), (goblin, (0, 5), "monsters")])
+    s = _state(eng, "rogue")
+    s.hidden_from = {"goblin"}
+    eng._current_round = 1
+    eng._execute(s, Action(kind="run", destination=(0, 3)))
+    assert s.hidden_from == set()
+    assert any("stealth broken (running)" in ln for ln in eng._transcript)
+
+
+def test_charge_breaks_stealth_without_a_check() -> None:
+    rogue = _combatant("rogue", dex=18)
+    goblin = _combatant("goblin")
+    eng = _engine([10, 2], [(rogue, (0, 0), "players"), (goblin, (0, 3), "monsters")])
+    s = _state(eng, "rogue")
+    s.hidden_from = {"goblin"}
+    eng._current_round = 1
+    eng._execute(s, Action(kind="charge", target_id="goblin"))
+    assert any("stealth broken (charging)" in ln for ln in eng._transcript)
+
+
+# ── observation contract ─────────────────────────────────────────────────────
+
+
+def test_observation_exposes_hidden_from() -> None:
+    rogue = _combatant("rogue", dex=18)
+    goblin = _combatant("goblin")
+    eng = _engine([10, 5], [(rogue, (0, 0), "players"), (goblin, (0, 2), "monsters")])
+    _blind(eng._states[1])
+    s = _state(eng, "rogue")
+    eng._current_round = 1
+    eng._execute(s, Action(kind="hide"))
+    obs = eng._build_observation(s)
+    me = next(c for c in obs.combatants if c["id"] == "rogue")
+    assert me["hidden_from"] == ("goblin",)
+    foe = next(c for c in obs.combatants if c["id"] == "goblin")
+    assert foe["hidden_from"] == ()