fix(rules): stop a run from crossing difficult terrain

PF1e CRB 'Movement, Position and Distance - Difficult Terrain': 'You
can't run or charge across difficult terrain.' The run action reused the
move machinery (_descend_path), so it crossed rubble at 2x cost within
the 4x budget. The descent now takes an avoid_difficult option that
refuses every step whose footprint would touch a cell with move cost > 1
(the same rejection charge applies in _charge_path_clear); the run
moves up to the obstacle and stops there (deterministic, documented).
Move and charge behavior are unchanged.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

README.md

@@ -693,10 +693,17 @@ Règles modélisées :
   s'arrête adjacent à la cible) ou vers une `destination` explicite (U5c), en
   réutilisant exactement la mécanique de `move` (descente du champ de coût
   Dijkstra, pas à pas avec provocation d'AoO — courir ne protège pas des
-  AoO). À l'achèvement, le combattant est `running` jusqu'au début de son
-  prochain tour : il **perd son bonus de Dextérité à la CA** (résolu par le
-  même chemin flat-footed/`denies_dex_to_ac` que les conditions). Une
-  `destination` hors budget 4× (ou invalide) est rejetée
+  AoO), **sauf que la course ne traverse pas le terrain difficile**
+  (conformance R2 — CRB « Movement, Position and Distance — Difficult
+  Terrain » : « You can't run or charge across difficult terrain ») : la
+  descente refuse tout pas dont le footprint toucherait une case de coût > 1
+  (mécanisme `avoid_difficult` de `_descend_path`, symétrique du refus de la
+  charge dans `_charge_path_clear`) ; le combattant **avance jusqu'à
+  l'obstacle et s'y arrête** (comportement déterministe, retenu plutôt que
+  le rejet de l'action). À l'achèvement, le combattant est `running` jusqu'au
+  début de son prochain tour : il **perd son bonus de Dextérité à la CA**
+  (résolu par le même chemin flat-footed/`denies_dex_to_ac` que les
+  conditions). Une `destination` hors budget 4× (ou invalide) est rejetée
   (`unreachable_destination`) ; sans cible ni destination, rejet
   (`missing_target`). `default_policy` ne choisit pas la course (disponible
   via une politique personnalisée).

src/pf1e_simulator/combat.py

@@ -3473,10 +3473,16 @@ class CombatEngine:
 
         Reuses the move machinery: the Dijkstra descent toward an explicit
         destination (U5c) or the resolved target, walked step-by-step with
-        the usual AoO provocation, under a 4x speed budget. On completion the
-        combatant is ``running`` until the start of its next turn, which
-        denies it its Dexterity bonus to AC. An explicit destination that
-        cannot be honoured within the 4x budget is rejected with
+        the usual AoO provocation, under a 4x speed budget. PF1e forbids
+        running across difficult terrain (CRB "Movement, Position and
+        Distance — Difficult Terrain": "You can't run or charge across
+        difficult terrain"), so the descent refuses every step into a
+        difficult cell: the run moves up to the obstacle and stops there
+        (deterministic; the same stop-short tolerance the greedy descent
+        already applies to any other blockage). On completion the combatant
+        is ``running`` until the start of its next turn, which denies it its
+        Dexterity bonus to AC. An explicit destination that cannot be
+        honoured within the 4x budget is rejected with
         ``unreachable_destination``; with neither target nor destination the
         U1 gate rejects the action with ``missing_target``.
         """
@@ -3503,13 +3509,16 @@ class CombatEngine:
                     kind=kind, ok=False, reason=_REASON_UNREACHABLE_DESTINATION
                 )
             path = self._descend_path(
-                state, action.destination, speed_factor=_RUN_SPEED_FACTOR
+                state,
+                action.destination,
+                speed_factor=_RUN_SPEED_FACTOR,
+                avoid_difficult=True,
             )
             self._walk_steps(state, path)
             state.moved_this_turn = True
         elif target is not None:
             path = self._descend_path(
-                state, target.pos, speed_factor=_RUN_SPEED_FACTOR
+                state, target.pos, speed_factor=_RUN_SPEED_FACTOR, avoid_difficult=True
             )
             if self._walk_steps(state, path):
                 state.moved_this_turn = True
@@ -5720,11 +5729,19 @@ class CombatEngine:
         return self._descend_path(state, target.pos)
 
     def _descend_path(
-        self, state: CombatantState, anchor: Pos, *, speed_factor: int = 1
+        self,
+        state: CombatantState,
+        anchor: Pos,
+        *,
+        speed_factor: int = 1,
+        avoid_difficult: bool = False,
     ) -> list[Pos]:
         """Greedy walk of the anchor over the cost field from ``anchor``.
 
         ``speed_factor`` widens the movement budget (run: 4x speed, U5e).
+        ``avoid_difficult`` excludes difficult-terrain cells from the walk
+        (PF1e: a run cannot cross difficult terrain), so the walk stops in
+        front of the obstacle.
         """
         speed_mult = self._get_speed_mult(state)
         speed_cells = int(
@@ -5743,7 +5760,9 @@ class CombatEngine:
         current = state.pos
         budget = speed_cells
         while budget > 0:
-            best = self._best_move_step(state, current, to_target, budget)
+            best = self._best_move_step(
+                state, current, to_target, budget, avoid_difficult=avoid_difficult
+            )
             if best is None:
                 break
             _, step_cost, nxt = best
@@ -5758,6 +5777,8 @@ class CombatEngine:
         current: Pos,
         to_target: dict[Pos, int],
         budget: int,
+        *,
+        avoid_difficult: bool = False,
     ) -> tuple[int, int, Pos] | None:
         """Pick the adjacent anchor that best descends the cost field toward the target."""
         current_cost = self._footprint_cost(self._footprint_at(current, state), to_target)
@@ -5770,6 +5791,8 @@ class CombatEngine:
             footprint = self._footprint_at(nxt, state)
             if not self._cells_free(footprint, state):
                 continue
+            if avoid_difficult and self._footprint_touches_difficult(footprint):
+                continue
             nxt_cost = self._footprint_cost(footprint, to_target)
             if nxt_cost is None or nxt_cost >= current_cost:
                 continue
@@ -5781,6 +5804,17 @@ class CombatEngine:
                 best = (score, step, nxt)
         return best
 
+    def _footprint_touches_difficult(self, footprint: frozenset[Pos]) -> bool:
+        """True if any footprint cell is difficult terrain (move cost > 1)."""
+        return any(self._is_difficult_cell(cell) for cell in footprint)
+
+    def _is_difficult_cell(self, pos: Pos) -> bool:
+        """True for a passable cell whose terrain costs more than one square."""
+        if not self._grid.in_bounds(pos):
+            return False
+        terrain = self._grid.terrain_at(pos)
+        return terrain.move_cost is not None and terrain.move_cost > 1
+
     def _flee_path(self, state: CombatantState, threat: CombatantState) -> list[Pos]:
         """Movement path away from ``threat``, up to double speed.
 

tests/test_aid_and_run.py

@@ -117,6 +117,16 @@ def _open_grid(size: int = 16) -> Grid:
     return Grid.from_spec(spec)
 
 
+def _rubble_row_grid(size: int = 16, row: int = 5) -> Grid:
+    """Open grid whose single ``row`` is rubble (difficult terrain, cost 2)."""
+    terrain = tuple("." * size if r != row else "~" * size for r in range(size))
+    legend = {
+        ".": TerrainType(type="floor", move_cost=1),
+        "~": TerrainType(type="rubble", move_cost=2),
+    }
+    return Grid.from_spec(MapSpec(name="rubble", terrain=terrain, legend=legend))
+
+
 # ── (a) aid (attack): +2 on the ally's next attack, consumed one-shot ────────
 
 
@@ -334,9 +344,72 @@ def test_run_toward_target_outreaches_normal_move() -> None:
     assert grid.distance(run_hero.pos, foe.pos) < grid.distance(move_hero.pos, foe2.pos)
 
 
+# ── (d2) R2: a run cannot cross difficult terrain ────────────────────────────
+
+
+def test_run_stops_before_difficult_terrain_toward_target() -> None:
+    """Given: hero at (0,0), a rubble row across row 3, foe beyond it at (6,0)
+    When: hero runs (4x speed = 24 cells, far beyond the 3-cell obstacle)
+    Then: hero stops in front of the rubble and never enters it (CRB: "You
+    can't run or charge across difficult terrain")."""
+
+    def policy(_engine: CombatEngine, state: CombatantState) -> tuple[Action, ...]:
+        if state.combatant.id == "hero":
+            return (Action(kind="run", target_id="foe"),)
+        return _WAIT
+
+    grid = _rubble_row_grid(size=8, row=3)
+    hero = make_state(_combatant("hero", initiative_mod=2), "players", (0, 0))
+    foe = make_state(_combatant("foe"), "monsters", (6, 0))
+    eng = _duo(policy, [10, 5], round_cap=1, hero=hero, foe=foe, grid=grid)
+    eng.run()
+    # The footprint never touches the rubble row, and the run stopped at it.
+    assert all(cell[0] != 3 for cell in eng._states[0].occupied_cells())
+    assert eng._states[0].pos == (2, 0)
+
+
+def test_run_stops_before_difficult_terrain_to_destination() -> None:
+    """Given: the same rubble row
+    When: hero runs to an explicit destination beyond it
+    Then: the descent stops in front of the rubble (the destination is
+    reachable in budget, but the run cannot cross — it moves up to the
+    obstacle and stops)."""
+
+    def policy(_engine: CombatEngine, state: CombatantState) -> tuple[Action, ...]:
+        if state.combatant.id == "hero":
+            return (Action(kind="run", destination=(6, 0)),)
+        return _WAIT
+
+    grid = _rubble_row_grid(size=8, row=3)
+    hero = make_state(_combatant("hero", initiative_mod=2), "players", (0, 0))
+    # The default foe sits at (0, 1), adjacent to the start: the run walks
+    # out of its threatened disk and provokes one (missed) AoO.
+    eng = _duo(policy, [10, 5, 1, 1], round_cap=1, hero=hero, grid=grid)
+    result = eng.run()
+    assert eng._states[0].pos == (2, 0)
+    assert "round 1 hero: run" in result.transcript
+
+
+def test_move_can_enter_difficult_terrain_but_run_cannot() -> None:
+    """Contrast: the same 3-cell geometry with `move` (6-cell budget) pays the
+    2x cost and DOES enter the rubble row — only the run refuses it."""
+
+    def policy(_engine: CombatEngine, state: CombatantState) -> tuple[Action, ...]:
+        if state.combatant.id == "hero":
+            return (Action(kind="move", target_id="foe"),)
+        return _WAIT
+
+    grid = _rubble_row_grid(size=8, row=3)
+    hero = make_state(_combatant("hero", initiative_mod=2), "players", (0, 0))
+    foe = make_state(_combatant("foe"), "monsters", (6, 0))
+    eng = _duo(policy, [10, 5], round_cap=1, hero=hero, foe=foe, grid=grid)
+    eng.run()
+    # Move: (0,0)->(1,0)->(2,0) then the rubble step costs 2 of the 6-cell
+    # budget, so it crosses: the hero ends beyond row 3.
+    assert eng._states[0].pos[0] > 3
+
+
 # ── (e) a running combatant loses its Dex bonus to AC ────────────────────────
-
-
 def test_running_combatant_is_denied_dex_to_ac() -> None:
     def policy(_engine: CombatEngine, state: CombatantState) -> tuple[Action, ...]:
         if state.combatant.id == "hero":