diff --git a/src/pf1e_simulator/combat.py b/src/pf1e_simulator/combat.py
index d59c495..6c71621 100644
--- a/src/pf1e_simulator/combat.py
+++ b/src/pf1e_simulator/combat.py
@@ -55,8 +55,13 @@ Phase 0 documented deviations from PF1e (conventions):
 - Movement: a move action travels up to the combatant's speed in cells along
   the true shortest path (Dijkstra cost field from the target, 5-10-5
   diagonals), stopping adjacent to the nearest enemy; ties keep delta order.
-  The speed budget is the best of land and fly speed (V4); flying over
-  obstacles or difficult terrain is NOT modeled (grid costs still apply).
+  The speed budget is the best of land and fly speed (V4). A creature whose
+  budget comes from its fly speed (fly >= land) is AIRBORNE: it ignores
+  difficult-terrain costs and crosses ground cells impassable to walkers
+  (pits, rubble, water), except cells that block line of effect (solid
+  barriers, assumed full height) — SRD "Movement, Position, and Distance":
+  "Flying and incorporeal creatures are not hampered by difficult terrain" /
+  "are able to avoid most obstacles". No fly maneuverability, no 3D flight.
 - Line of effect gates all attacks: a target fully behind blocking terrain
   cannot be attacked, and the policy moves to gain sight instead.
 - Cover (corner rule) grants +4 AC on hit and crit-confirm rolls; melee and
@@ -774,15 +779,35 @@ def _melee_reach_zone_cells(state: CombatantState, weapon: AttackSpec) -> tuple[
 def _speed_ft(state: CombatantState) -> int:
     """Movement budget in feet: the best of the combatant's land and fly speed.
 
-    Simplification (V4): a creature that has a fly speed is assumed to use it;
-    flying OVER obstacles or difficult terrain is NOT modeled — the grid's
-    passability and terrain costs still apply to the whole budget. No
-    fly maneuverability, no 3D flight. Swim/climb/burrow speeds do not exist
-    in the model (fly only).
+    Simplification (V4): a creature that has a fly speed is assumed to use it
+    when its fly speed is at least its land speed (see ``_is_airborne``). The
+    budget itself is land-or-fly; an airborne creature additionally moves over
+    ground terrain at cost 1 (``Grid.airborne_view``). No fly maneuverability,
+    no 3D flight. Swim/climb/burrow speeds do not exist in the model (fly only).
     """
     return max(state.combatant.speed_land_ft, state.combatant.speed_fly_ft or 0)
 
 
+def _is_airborne(state: CombatantState) -> bool:
+    """True when the creature's movement budget comes from its fly speed.
+
+    The engine's V4 assumption is that a creature with a fly speed uses it
+    (budget = max(land, fly)); the creature is AIRBORNE — flying over the
+    ground, so difficult terrain does not hamper it and it avoids the
+    obstacles that merely block walkers (SRD "Movement, Position, and
+    Distance": "Flying and incorporeal creatures are not hampered by difficult
+    terrain"; "Flying and incorporeal creatures are able to avoid most
+    obstacles") — exactly when the fly speed is the source of that budget,
+    i.e. ``speed_fly_ft >= speed_land_ft``. A creature whose fly speed is
+    slower than its land speed walks (the budget is the land speed) and pays
+    ground terrain costs normally. There is no explicit altitude flag: the
+    model has no 3D position, so "airborne" is derived from the speeds alone
+    (documented decision).
+    """
+    fly = state.combatant.speed_fly_ft
+    return fly is not None and fly >= state.combatant.speed_land_ft
+
+
 def _bresenham_line(start: Pos, end: Pos) -> list[Pos]:
     """Integer Bresenham line from start to end (exclusive of start, inclusive of end)."""
     r0, c0 = start
@@ -1598,6 +1623,7 @@ class CombatEngine:
             raise ValueError(msg)
         self._rng = rng
         self._grid = grid
+        self._airborne_grid = grid.airborne_view()
         self._states = states
         self._round_cap = round_cap
         if llm_policy is not None:
@@ -3022,11 +3048,25 @@ class CombatEngine:
         """Height of the creature's anchor cell (0 when no elevation layer)."""
         return self._grid.elevation_at(state.pos)
 
+    def _movement_grid(self, state: CombatantState) -> Grid:
+        """The terrain grid the mover walks on: the air view when airborne.
+
+        An airborne creature prices and gates every movement action (move,
+        run, charge, withdraw, 5-foot step) on ``Grid.airborne_view``: ground
+        difficult terrain costs 1 and impassable non-barrier cells (pits,
+        water, rubble) are crossable; ``blocks_los`` cells (solid barriers,
+        assumed full height) stay impassable. Line of effect, cover and
+        high ground always use the real ``_grid`` — the air view preserves
+        their terrain flags, so combat geometry is unaffected by flight.
+        """
+        return self._airborne_grid if _is_airborne(state) else self._grid
+
     def _cells_free(self, cells: Iterable[Pos], mover: CombatantState) -> bool:
         """True if every cell is in bounds, passable, and unoccupied by others."""
         blocked = self._blocked(mover)
+        grid = self._movement_grid(mover)
         return all(
-            self._grid.passable(cell) and cell not in blocked for cell in cells
+            grid.passable(cell) and cell not in blocked for cell in cells
         )
 
     def _cells_walkable(self, cells: Iterable[Pos], mover: CombatantState) -> bool:
@@ -3037,8 +3077,9 @@ class CombatEngine:
         though the movement may not END there (``_cells_free`` gates that).
         """
         hostile = self._blocked_hostile(mover)
+        grid = self._movement_grid(mover)
         return all(
-            self._grid.passable(cell) and cell not in hostile for cell in cells
+            grid.passable(cell) and cell not in hostile for cell in cells
         )
 
     def _footprint_at(self, anchor: Pos, state: CombatantState) -> frozenset[Pos]:
@@ -4420,16 +4461,18 @@ class CombatEngine:
 
         (API publique, lecture pour politique LLM.) Same cost model as
         ``Grid.reachable``: 5-10-5 diagonals, terrain, elevation, blocked
-        creatures excluded. ``budget`` is in feet; None = full field.
+        creatures excluded. An airborne mover prices the field on the air
+        view (``_movement_grid``). ``budget`` is in feet; None = full field.
         """
         blocked = self._blocked(state)
+        grid = self._movement_grid(state)
         if budget is None:
-            field = self._grid.reachable(state.pos, None, blocked)
+            field = grid.reachable(state.pos, None, blocked)
         else:
             budget_cells = budget // _SQUARE_FT
             if budget_cells < 1:
                 return {}
-            field = self._grid.reachable(state.pos, budget_cells, blocked)
+            field = grid.reachable(state.pos, budget_cells, blocked)
         return {pos: cost * _SQUARE_FT for pos, cost in field.items()}
 
     def movement_cost_ft(self, state: CombatantState, target: CombatantState) -> int | None:
@@ -4440,10 +4483,11 @@ class CombatEngine:
         ``_move_path``: a full Dijkstra field from the target's anchor (other
         active states' cells blocked), then the cheapest field value over
         ``state``'s footprint, compared against the combatant's effective
-        (land-or-fly) speed. Read-only: never mutates state nor consumes RNG.
+        (land-or-fly) speed. An airborne ``state`` prices the field on the air
+        view. Read-only: never mutates state nor consumes RNG.
         """
         blocked = self._blocked(state, target)
-        field = self._grid.reachable(target.pos, None, blocked)
+        field = self._movement_grid(state).reachable(target.pos, None, blocked)
         cost_cells = self._footprint_cost(state.occupied_cells(), field)
         if cost_cells is None:
             return None
@@ -4840,12 +4884,12 @@ class CombatEngine:
                 return ChargeResult(weapon=weapon, distance_ft=distance_ft, reason=None)
         return ChargeResult(weapon=None, distance_ft=0, reason="no_charge_path")
 
-    def _path_cost_5105(self, path: list[Pos]) -> int:
-        """Total 5-10-5 movement cost of a path (diagonal parity threaded)."""
+    def _path_cost_5105(self, path: list[Pos], grid: Grid) -> int:
+        """Total 5-10-5 movement cost of a path on ``grid`` (parity threaded)."""
         total = 0
         diagonals_used = 0
         for frm, to in pairwise(path):
-            total += self._grid.step_cost(frm, to, diagonals_used)
+            total += grid.step_cost(frm, to, diagonals_used)
             if frm[0] != to[0] and frm[1] != to[1]:
                 diagonals_used += 1
         return total
@@ -4869,8 +4913,11 @@ class CombatEngine:
         Ties break toward lower distance from state.
         """
         target_cells = target.occupied_cells()
+        grid = self._movement_grid(state)
         candidates: list[tuple[int, Pos]] = []
-        for end in self._charge_candidates(target, reach_cells, blocked, state.pos):
+        for end in self._charge_candidates(
+            target, reach_cells, blocked, state.pos, grid
+        ):
             footprint = self._footprint_at(end, state)
             if not self._cells_free(footprint, state):
                 continue
@@ -4888,7 +4935,7 @@ class CombatEngine:
                 continue
             # The charge budget is a movement budget: count it in 5-10-5
             # squares (diagonal parity threaded), not raw cells.
-            if self._path_cost_5105(path) > max_cells:
+            if self._path_cost_5105(path, grid) > max_cells:
                 continue
             candidates.append((len(path), end))
         if not candidates:
@@ -4902,8 +4949,13 @@ class CombatEngine:
         reach_cells: int,
         blocked: frozenset[Pos],
         start: Pos,
+        grid: Grid,
     ) -> list[Pos]:
-        """Anchors within reach of some target cell, passable, unoccupied, not the start."""
+        """Anchors within reach of some target cell, passable, unoccupied, not the start.
+
+        ``grid`` is the charger's movement grid (the air view when airborne:
+        an airborne charger ends its charge over cells walkers cannot enter).
+        """
         result: set[Pos] = set()
         for tr, tc in target.occupied_cells():
             for dr in range(-reach_cells, reach_cells + 1):
@@ -4915,7 +4967,7 @@ class CombatEngine:
                     end = (tr + dr, tc + dc)
                     if end == start:
                         continue
-                    if not self._grid.in_bounds(end) or not self._grid.passable(end):
+                    if not grid.in_bounds(end) or not grid.passable(end):
                         continue
                     if end in blocked:
                         continue
@@ -4939,19 +4991,24 @@ class CombatEngine:
         below. Difficult terrain is tested over the WHOLE footprint, not
         just the anchor cell (SRD "Movement, Position and Distance": "You
         can't run or charge across difficult terrain" - a 2x2 creature with
-        one trailing square in rubble cannot charge).
+        one trailing square in rubble cannot charge). An AIRBORNE charger is
+        not hampered by difficult terrain and avoids ground obstacles, so it
+        walks these gates on the air view (SRD "Movement, Position, and
+        Distance": "Flying and incorporeal creatures are not hampered by
+        difficult terrain"); solid barriers still stop the charge.
         """
+        grid = self._movement_grid(state)
         prev = state.pos
         for pos in path:
             footprint = self._footprint_at(pos, state)
             if not self._cells_free(footprint, state):
                 return False
-            if self._footprint_touches_difficult(footprint):
+            if self._footprint_touches_difficult(state, footprint):
                 return False
             if (
                 prev[0] != pos[0]
                 and prev[1] != pos[1]
-                and not self._grid.diagonal_allowed(prev, pos)
+                and not grid.diagonal_allowed(prev, pos)
             ):
                 return False
             prev = pos
@@ -9349,12 +9406,15 @@ class CombatEngine:
         # SRD "Take 5-Foot Step": "You can only take a 5-foot-step if your
         # movement isn't hampered by difficult terrain or darkness." — a step
         # out of OR into a difficult cell is blocked (darkness not modeled).
-        if self._footprint_touches_difficult(self._footprint_at(state.pos, state)):
+        # An airborne creature's movement is not hampered by difficult
+        # terrain (SRD "Movement, Position, and Distance"), so the gate
+        # passes for it.
+        if self._footprint_touches_difficult(state, self._footprint_at(state.pos, state)):
             self._log(f"round {self._current_round} {state.combatant.id}: wait (cannot step)")
             return
         path = self._move_path(state, target)
         start = state.pos
-        if self._footprint_touches_difficult(self._footprint_at(path[0], state)):
+        if self._footprint_touches_difficult(state, self._footprint_at(path[0], state)):
             self._log(f"round {self._current_round} {state.combatant.id}: wait (cannot step)")
             return
         state.pos = path[0]
@@ -9376,16 +9436,19 @@ class CombatEngine:
             return ActionResult(kind=kind, ok=True, reason=_REASON_OK)
         adjacent = self._grid.distance(state.pos, destination) == 1
         free = self._cells_free(self._footprint_at(destination, state), state)
+        grid = self._movement_grid(state)
         # PF1e (SRD Measuring Distance): "You can't move diagonally past a
         # corner (even by taking a 5-foot step)."
         diagonal = state.pos[0] != destination[0] and state.pos[1] != destination[1]
-        corner_clear = not diagonal or self._grid.diagonal_allowed(state.pos, destination)
+        corner_clear = not diagonal or grid.diagonal_allowed(state.pos, destination)
         # SRD "Take 5-Foot Step": "You can only take a 5-foot-step if your
         # movement isn't hampered by difficult terrain or darkness." — a step
-        # out of OR into a difficult cell is blocked (darkness not modeled).
+        # out of OR into a difficult cell is blocked (darkness not modeled);
+        # flight is not hampered by ground terrain, so the gate passes for an
+        # airborne creature.
         step_clear = not (
-            self._footprint_touches_difficult(self._footprint_at(state.pos, state))
-            or self._footprint_touches_difficult(self._footprint_at(destination, state))
+            self._footprint_touches_difficult(state, self._footprint_at(state.pos, state))
+            or self._footprint_touches_difficult(state, self._footprint_at(destination, state))
         )
         if not adjacent or not free or not corner_clear or not step_clear:
             self._log(
@@ -9497,22 +9560,23 @@ class CombatEngine:
         if not self._cells_free(self._footprint_at(destination, state), state):
             return False
         if avoid_difficult and self._footprint_touches_difficult(
-            self._footprint_at(destination, state)
+            state, self._footprint_at(destination, state)
         ):
             return False
         blocked = self._blocked_hostile(state) if friendly_passable else self._blocked(state)
+        grid = self._movement_grid(state)
         if avoid_difficult:
-            blocked = blocked | self._difficult_cells()
-        field = self._grid.reachable(state.pos, speed_cells, blocked)
+            blocked = blocked | self._difficult_cells(grid)
+        field = grid.reachable(state.pos, speed_cells, blocked)
         return destination in field
 
-    def _difficult_cells(self) -> frozenset[Pos]:
-        """Every passable cell on the grid whose terrain costs more than one."""
+    def _difficult_cells(self, grid: Grid) -> frozenset[Pos]:
+        """Every passable cell of ``grid`` whose terrain costs more than one."""
         return frozenset(
             (row, col)
-            for row in range(self._grid.height)
-            for col in range(self._grid.width)
-            if self._is_difficult_cell((row, col))
+            for row in range(grid.height)
+            for col in range(grid.width)
+            if self._is_difficult_cell(grid, (row, col))
         )
 
     def _walk_steps(
@@ -9595,7 +9659,7 @@ class CombatEngine:
         if speed_cells <= 0:
             return []
         blocked = self._blocked_hostile(state) if friendly_passable else self._blocked(state)
-        to_target = self._grid.reachable(anchor, None, blocked)
+        to_target = self._movement_grid(state).reachable(anchor, None, blocked)
         if self._footprint_cost(state.occupied_cells(), to_target) is None:
             return []
         path: list[Pos] = []
@@ -9640,9 +9704,13 @@ class CombatEngine:
          so an auto-directed run ends adjacent to the target — except one the
          mover may pass through (three size categories apart, SRD "Moving
          Through a Square"), which is crossed with an entry AoO and never
-         ended on. Friendly squares
-        are still crossed (SRD Movement, conformance L5) but never ended on —
-        the tail is trimmed like ``_descend_path``.
+          ended on. Friendly squares
+         are still crossed (SRD Movement, conformance L5) but never ended on —
+         the tail is trimmed like ``_descend_path``. An AIRBORNE runner prices
+         the line on the air view: ground difficult terrain and impassable
+         non-barrier cells do not stop it (SRD "Movement, Position, and
+         Distance": "Flying and incorporeal creatures are not hampered by
+         difficult terrain").
         """
         speed_mult = self._get_speed_mult(state)
         budget = int(
@@ -9650,6 +9718,7 @@ class CombatEngine:
         )
         if budget <= 0:
             return []
+        grid = self._movement_grid(state)
         path: list[Pos] = []
         current = state.pos
         diagonals_used = 0
@@ -9661,15 +9730,15 @@ class CombatEngine:
             if (
                 current[0] != nxt[0]
                 and current[1] != nxt[1]
-                and not self._grid.diagonal_allowed(current, nxt)
+                and not grid.diagonal_allowed(current, nxt)
             ):
                 break
             footprint = self._footprint_at(nxt, state)
-            if self._footprint_touches_difficult(footprint):
+            if self._footprint_touches_difficult(state, footprint):
                 break
             if not self._cells_walkable(footprint, state):
                 break
-            step = self._grid.step_cost(current, nxt, diagonals_used)
+            step = grid.step_cost(current, nxt, diagonals_used)
             if step > budget:
                 break
             path.append(nxt)
@@ -9698,6 +9767,7 @@ class CombatEngine:
             return None
         best: tuple[int, int, Pos] | None = None
         row, col = current
+        grid = self._movement_grid(state)
         fits = self._cells_walkable if friendly_passable else self._cells_free
         for d_row, d_col in _STEP_DELTAS:
             nxt = (row + d_row, col + d_col)
@@ -9705,12 +9775,12 @@ class CombatEngine:
             # a corner (even by taking a 5-foot step)" — same corner rule the
             # explicit step (``_step5_to``) and flee (``_best_flee_step``)
             # paths already enforce.
-            if not self._grid.diagonal_allowed(current, nxt):
+            if not grid.diagonal_allowed(current, nxt):
                 continue
             footprint = self._footprint_at(nxt, state)
             if not fits(footprint, state):
                 continue
-            if avoid_difficult and self._footprint_touches_difficult(footprint):
+            if avoid_difficult and self._footprint_touches_difficult(state, footprint):
                 continue
             nxt_cost = self._footprint_cost(footprint, to_target)
             if nxt_cost is None or nxt_cost >= current_cost:
@@ -9766,9 +9836,17 @@ class CombatEngine:
                     frontier.append((nxt, cost))
         return False
 
-    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 _footprint_touches_difficult(
+        self, state: CombatantState, footprint: frozenset[Pos]
+    ) -> bool:
+        """True if any footprint cell is difficult terrain (move cost > 1).
+
+        Priced on the mover's movement grid: an airborne creature is not
+        hampered by difficult terrain (SRD "Movement, Position, and
+        Distance"), so no cell is difficult for it.
+        """
+        grid = self._movement_grid(state)
+        return any(self._is_difficult_cell(grid, cell) for cell in footprint)
 
     def _footprint_step_cost(
         self, state: CombatantState, frm: Pos, to: Pos, diagonals_used: int
@@ -9783,22 +9861,26 @@ class CombatEngine:
         at ``to`` (the 5-10-5 + elevation formula stays in ``grid.py``). For a
         1x1 creature the footprint is the anchor cell, so this is exactly
         ``grid.step_cost`` - bit-identical to the old anchor-only pricing.
+        The pricing grid is the mover's movement grid: every cell costs 1 to
+        an airborne creature (SRD "Movement, Position, and Distance": flying
+        creatures are not hampered by difficult terrain).
         """
+        grid = self._movement_grid(state)
         footprint = self._footprint_at(to, state)
         hardest = 1
         for cell in footprint:
-            if not self._grid.in_bounds(cell):
+            if not grid.in_bounds(cell):
                 continue
-            cost = self._grid.terrain_at(cell).move_cost
+            cost = grid.terrain_at(cell).move_cost
             if cost is not None and cost > hardest:
                 hardest = cost
-        return self._grid.step_cost_with_terrain(frm, to, diagonals_used, hardest)
+        return grid.step_cost_with_terrain(frm, to, diagonals_used, hardest)
 
-    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):
+    def _is_difficult_cell(self, grid: Grid, pos: Pos) -> bool:
+        """True for a passable cell of ``grid`` whose terrain costs more than one."""
+        if not grid.in_bounds(pos):
             return False
-        terrain = self._grid.terrain_at(pos)
+        terrain = 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]:
@@ -9819,7 +9901,7 @@ class CombatEngine:
         if max_cells <= 0:
             return []
         blocked = self._blocked(state)
-        from_threat = self._grid.reachable(threat.pos, None, blocked)
+        from_threat = self._movement_grid(state).reachable(threat.pos, None, blocked)
         if self._footprint_cost(state.occupied_cells(), from_threat) is None:
             return []
         path: list[Pos] = []
@@ -9854,12 +9936,13 @@ class CombatEngine:
             return None
         best: tuple[int, int, Pos] | None = None
         row, col = current
+        grid = self._movement_grid(state)
         for d_row, d_col in _STEP_DELTAS:
             nxt = (row + d_row, col + d_col)
             footprint = self._footprint_at(nxt, state)
             if not self._cells_free(footprint, state):
                 continue
-            if not self._grid.diagonal_allowed(current, nxt):
+            if not grid.diagonal_allowed(current, nxt):
                 continue
             nxt_cost = self._footprint_cost(footprint, from_threat)
             if nxt_cost is None or nxt_cost <= current_cost:
diff --git a/src/pf1e_simulator/grid.py b/src/pf1e_simulator/grid.py
index 7cfb055..a01a31a 100644
--- a/src/pf1e_simulator/grid.py
+++ b/src/pf1e_simulator/grid.py
@@ -18,6 +18,7 @@ if TYPE_CHECKING:
     from pf1e_simulator.map import MapSpec, Pos, TerrainType
 
 _DIFFICULT_TERRAIN_COST = 2  # rubble, water, etc. count as 2 squares (PF1e)
+_AIRBORNE_TERRAIN_COST = 1  # flying is not hampered by ground terrain (PF1e)
 
 _DIRECTIONS: tuple[Pos, ...] = (
     (-1, -1),
@@ -48,6 +49,41 @@ class Grid:
         self._width = len(cells[0]) if cells else 0
         self._elevation = elevation
 
+    @staticmethod
+    def _airborne_terrain(terrain: TerrainType) -> TerrainType:
+        """The same square seen from the air (PF1e flight over ground terrain).
+
+        SRD "Movement, Position, and Distance" (Terrain and Obstacles):
+        "Flying and incorporeal creatures are not hampered by difficult
+        terrain" and "Flying and incorporeal creatures are able to avoid most
+        obstacles" — so a cell that only slows walking (move_cost > 1) or
+        stops it (pits, rubble, water: move_cost None) becomes a plain
+        cost-1 cell in the air view. The exception is a SOLID BARRIER
+        (`blocks_los`, the engine's encoding of CRB "Aiming a Spell — Line of
+        Effect": "A line of effect is canceled by a solid barrier"), which is
+        assumed full height and still stops flight. Cover flags and the
+        terrain name are preserved; only the walking cost is normalized.
+        """
+        if terrain.blocks_los:
+            return terrain
+        cost = terrain.move_cost
+        if cost is None or cost > _AIRBORNE_TERRAIN_COST:
+            return terrain.model_copy(update={"move_cost": _AIRBORNE_TERRAIN_COST})
+        return terrain
+
+    def airborne_view(self) -> Grid:
+        """A terrain view for airborne creatures (see ``_airborne_terrain``).
+
+        Elevation is preserved: the layer stores STEP heights of the ground,
+        not wall or cliff heights, so the climbing cost still applies to
+        flight (documented simplification — true 3D flight is not modeled).
+        """
+        cells = tuple(
+            tuple(self._airborne_terrain(terrain) for terrain in row)
+            for row in self._cells
+        )
+        return Grid(cells, elevation=self._elevation)
+
     @classmethod
     def from_spec(cls, spec: MapSpec) -> Grid:
         cells = tuple(tuple(spec.legend[ch] for ch in row) for row in spec.terrain)
diff --git a/tests/test_fly_speed.py b/tests/test_fly_speed.py
index 9f48ba5..eba614d 100644
--- a/tests/test_fly_speed.py
+++ b/tests/test_fly_speed.py
@@ -1,14 +1,18 @@
-"""V4: movement uses the best of land and fly speed.
+"""V4: movement uses the best of land and fly speed + 3D flight over terrain.
 
 The engine budgets every movement action (move, 5-foot step toward a target,
 charge at 2x, withdraw at 2x, run at 4x) from ``_speed_ft(state)`` — the
 maximum of the creature's ``speed_land_ft`` and ``speed_fly_ft`` (a missing
-fly speed counts as 0, so a land-only creature is unchanged). Simplification:
-a creature that has a fly speed is assumed to use it, but flying OVER
-obstacles or difficult terrain is NOT modeled — the grid's passability and
-terrain costs still apply to the whole budget. The observation snapshot adds
-an additive ``speed_effective_ft`` key; ``speed_land_ft``/``speed_fly_ft`` are
-kept as-is.
+fly speed counts as 0, so a land-only creature is unchanged). A creature
+whose budget comes from its fly speed (fly >= land) is AIRBORNE
+(``_is_airborne``): it moves on ``Grid.airborne_view``, where ground difficult
+terrain costs 1 and cells impassable to walkers (pits, water, rubble) are
+crossable — SRD "Movement, Position, and Distance": "Flying and incorporeal
+creatures are not hampered by difficult terrain"; "Flying and incorporeal
+creatures are able to avoid most obstacles". Cells that block line of effect
+(``blocks_los`` — solid barriers, assumed full height) still stop flight and
+still block attacks. The observation snapshot adds an additive
+``speed_effective_ft`` key; ``speed_land_ft``/``speed_fly_ft`` are kept as-is.
 """
 from __future__ import annotations
 
@@ -77,6 +81,45 @@ def _corridor(cols: int) -> Grid:
     return Grid.from_spec(spec)
 
 
+_LEGEND = {
+    ".": TerrainType(type="floor", move_cost=1),
+    "T": TerrainType(type="rubble", move_cost=2),
+    "P": TerrainType(type="pit", move_cost=None),
+    "#": TerrainType(type="wall", move_cost=None, blocks_los=True),
+}
+
+
+def _terrain_grid(rows: tuple[str, ...]) -> Grid:
+    """A grid from ASCII rows over the shared ``_LEGEND`` terrain types."""
+    spec = MapSpec(name="terrain", terrain=rows, legend=_LEGEND)
+    return Grid.from_spec(spec)
+
+
+def _run_move_on(
+    *, land: int, fly: int | None, destination: tuple[int, int], grid: Grid,
+) -> CombatantState:
+    """Hero acts first and takes one ``move`` to ``destination`` on ``grid``.
+
+    The foe idles far to the right (never threatens). Returns the hero state.
+    """
+    cols = grid.width
+    hero = make_state(
+        _combatant("hero", land=land, fly=fly, initiative_mod=2), "players", (0, 0),
+    )
+    foe = make_state(_combatant("foe", land=30), "monsters", (0, cols - 1))
+
+    def policy(_engine: CombatEngine, state: CombatantState) -> tuple[Action, ...]:
+        if state.combatant.id == "hero":
+            return (Action(kind="move", destination=destination),)
+        return _WAIT
+
+    eng = CombatEngine(
+        ScriptedRng([10, 5]), grid, [hero, foe], policy=policy, round_cap=1,
+    )
+    eng.run()
+    return eng._states[0]
+
+
 def _run_move(
     *, land: int, fly: int | None, destination: tuple[int, int], cols: int,
 ) -> CombatantState:
@@ -220,3 +263,236 @@ def test_fly_lower_than_land_uses_land() -> None:
     assert mover.pos == (0, 6)
     mover = _run_move(land=30, fly=10, destination=(0, 7), cols=12)
     assert mover.pos == (0, 0)
+
+
+# ── 3D flight over ground terrain (airborne creatures) ────────────────────────
+#
+# SRD "Movement, Position, and Distance" (Terrain and Obstacles): "Flying and
+# incorporeal creatures are not hampered by difficult terrain" and "Flying and
+# incorporeal creatures are able to avoid most obstacles." An airborne creature
+# (fly >= land, ``_is_airborne``) moves on ``Grid.airborne_view``: ground
+# difficult terrain costs 1 and cells impassable to walkers (pits) are
+# crossable, EXCEPT cells that block line of effect (solid barriers, assumed
+# full height) which still stop flight. Walkers are bit-identical. The foe sits
+# two rows below the hero's lane so it never threatens the row being crossed.
+
+_LEGEND = {
+    ".": TerrainType(type="floor", move_cost=1),
+    "T": TerrainType(type="rubble", move_cost=2),
+    "P": TerrainType(type="pit", move_cost=None),
+    "#": TerrainType(type="wall", move_cost=None, blocks_los=True),
+}
+
+
+def _lane_grid(lane: str) -> Grid:
+    """A 3-row grid: the terrain ``lane`` on row 0, floor rows below."""
+    width = len(lane)
+    rows = (lane, "." * width, "." * width)
+    spec = MapSpec(name="lane", terrain=rows, legend=_LEGEND)
+    return Grid.from_spec(spec)
+
+
+def _move_on_lane(*, land: int, fly: int | None, lane: str,
+                  destination: tuple[int, int]) -> tuple[int, int]:
+    """Hero (row 0, col 0) acts first and takes one ``move`` to ``destination``.
+
+    The foe idles at (2, 0), two rows below and never threatening. Returns the
+    hero's final position.
+    """
+    hero = make_state(
+        _combatant("hero", land=land, fly=fly, initiative_mod=2), "players", (0, 0),
+    )
+    foe = make_state(_combatant("foe", land=30), "monsters", (2, 0))
+
+    def policy(_engine: CombatEngine, state: CombatantState) -> tuple[Action, ...]:
+        if state.combatant.id == "hero":
+            return (Action(kind="move", destination=destination),)
+        return _WAIT
+
+    eng = CombatEngine(
+        ScriptedRng([10, 5]), _lane_grid(lane), [hero, foe],
+        policy=policy, round_cap=1,
+    )
+    eng.run()
+    return eng._states[0].pos
+
+
+def test_flyer_crosses_rubble_at_normal_cost() -> None:
+    # Rubble at cols 4-5: the flyer prices every cell at 1, so (0, 9) costs
+    # 9 cells and is within the 16-cell (fly 80) budget.
+    assert _move_on_lane(land=30, fly=80, lane="....TT....", destination=(0, 9)) == (0, 9)
+
+
+def test_walker_still_pays_for_rubble() -> None:
+    # Same lane, same 10-cell span: a land-30 walker pays 3 + 2*2 + 4 = 11
+    # cells for (0, 9), beyond its 6-cell budget, so it is rejected in place.
+    assert _move_on_lane(land=30, fly=None, lane="....TT....", destination=(0, 9)) == (0, 0)
+
+
+def test_flyer_crosses_pit_impassable_to_walkers() -> None:
+    # A pit (move_cost None, no blocks_los) at col 3: the flyer avoids the
+    # obstacle and reaches (0, 7); the walker cannot enter the pit cell.
+    assert _move_on_lane(land=30, fly=80, lane="...P....", destination=(0, 7)) == (0, 7)
+    assert _move_on_lane(land=30, fly=None, lane="...P....", destination=(0, 7)) == (0, 0)
+
+
+def test_flyer_still_stopped_by_wall() -> None:
+    # A wall column (blocks_los) is a solid barrier assumed full height: it
+    # stops flight just like walking — no crossing and no detour.
+    hero = make_state(
+        _combatant("hero", land=30, fly=80, initiative_mod=2), "players", (0, 0),
+    )
+    foe = make_state(_combatant("foe", land=30), "monsters", (2, 7))
+
+    def policy(_engine: CombatEngine, state: CombatantState) -> tuple[Action, ...]:
+        if state.combatant.id == "hero":
+            return (Action(kind="move", destination=(0, 7)),)
+        return _WAIT
+
+    spec = MapSpec(
+        name="wall", terrain=("...#....", "...#....", "...#...."), legend=_LEGEND,
+    )
+    eng = CombatEngine(
+        ScriptedRng([10, 5]), Grid.from_spec(spec), [hero, foe],
+        policy=policy, round_cap=1,
+    )
+    eng.run()
+    assert eng._states[0].pos == (0, 0)
+
+
+def test_flyer_runs_over_rubble_walker_run_stops() -> None:
+    # Run is a straight line: an airborne runner is not hampered by the
+    # rubble; a walker's run stops in front of it (CRB "Difficult Terrain":
+    # "You can't run or charge across difficult terrain").
+    def _run(land: int, fly: int | None) -> tuple[int, int]:
+        hero = make_state(
+            _combatant("hero", land=land, fly=fly, initiative_mod=2), "players", (0, 0),
+        )
+        foe = make_state(_combatant("foe", land=30), "monsters", (2, 0))
+
+        def policy(_e: CombatEngine, s: CombatantState) -> tuple[Action, ...]:
+            if s.combatant.id == "hero":
+                return (Action(kind="run", destination=(0, 9)),)
+            return _WAIT
+
+        eng = CombatEngine(
+            ScriptedRng([10, 5]), _lane_grid("....TT...."),
+            [hero, foe], policy=policy, round_cap=1,
+        )
+        eng.run()
+        return eng._states[0].pos
+
+    assert _run(30, 80) == (0, 9)
+    # The walker's run destination lies beyond difficult terrain: the
+    # reachability verdict and the walked line agree, so the run is rejected
+    # outright (unreachable_destination) and the walker never starts.
+    assert _run(30, None) == (0, 0)
+
+
+def test_flyer_5foot_step_into_rubble() -> None:
+    # SRD "Take 5-Foot Step": the step is legal only if movement isn't
+    # hampered by difficult terrain — a flyer's isn't, a walker's is.
+    def _step(land: int, fly: int | None) -> tuple[int, int]:
+        hero = make_state(
+            _combatant("hero", land=land, fly=fly, initiative_mod=2), "players", (0, 3),
+        )
+        foe = make_state(_combatant("foe", land=30), "monsters", (2, 0))
+
+        def policy(_e: CombatEngine, s: CombatantState) -> tuple[Action, ...]:
+            if s.combatant.id == "hero":
+                return (Action(kind="5foot_step", destination=(0, 4)),)
+            return _WAIT
+
+        eng = CombatEngine(
+            ScriptedRng([10, 5]), _lane_grid("...T...."),
+            [hero, foe], policy=policy, round_cap=1,
+        )
+        eng.run()
+        return eng._states[0].pos
+
+    assert _step(30, 80) == (0, 4)
+    assert _step(30, None) == (0, 3)
+
+
+def test_movement_cost_ft_uses_the_air_view() -> None:
+    # Cost from the hero's cell to the target's anchor: rubble prices at 1 for
+    # the flyer (8 cells = 40 ft) and at 2 for the walker (13 cells = 65 ft,
+    # beyond its 30 ft speed so None).
+    grid = _lane_grid("..TTTTT..")
+    flyer = make_state(_combatant("flyer", land=30, fly=80), "players", (0, 0))
+    walker = make_state(_combatant("walker", land=30), "players", (0, 0))
+    target_foe = make_state(_combatant("target", land=30), "monsters", (0, 8))
+    eng = CombatEngine(ScriptedRng([]), grid, [flyer, target_foe])
+    assert eng.movement_cost_ft(flyer, target_foe) == 40
+    target_walker = make_state(_combatant("target", land=30), "monsters", (0, 8))
+    eng2 = CombatEngine(ScriptedRng([]), grid, [walker, target_walker])
+    assert eng2.movement_cost_ft(walker, target_walker) is None
+
+
+def test_fly_equal_to_land_is_airborne() -> None:
+    # fly >= land: the budget comes from the fly speed, so the creature flies
+    # (6-cell budget, rubble priced at 1, (0, 6) reachable).
+    assert _move_on_lane(land=30, fly=30, lane="....TT....", destination=(0, 6)) == (0, 6)
+
+
+def test_fly_slower_than_land_walks_and_pays() -> None:
+    # fly < land: the creature walks (budget = land) and pays the rubble cost
+    # like any walker — (0, 9) costs 11 cells > the 6-cell budget.
+    assert _move_on_lane(land=30, fly=10, lane="....TT....", destination=(0, 9)) == (0, 0)
+
+
+def test_flyer_still_pays_elevation_climb() -> None:
+    # The elevation layer stores STEP heights, not wall heights, so the
+    # climbing cost still applies to flight (documented simplification): with
+    # a 2-cell budget (fly 10 = land 10) the step up onto the higher ledge
+    # costs 2 and only the flat neighbour is reachable.
+    legend = {".": TerrainType(type="floor", move_cost=1)}
+    spec = MapSpec(
+        name="ledge", terrain=("....", "...."), legend=legend,
+        elevation=("..1.", "...."),
+    )
+    grid = Grid.from_spec(spec)
+
+    def _go(destination: tuple[int, int]) -> tuple[int, int]:
+        hero = make_state(
+            _combatant("hero", land=10, fly=10, initiative_mod=2), "players", (0, 0),
+        )
+        foe = make_state(_combatant("foe", land=30), "monsters", (1, 3))
+
+        def policy(_e: CombatEngine, s: CombatantState) -> tuple[Action, ...]:
+            if s.combatant.id == "hero":
+                return (Action(kind="move", destination=destination),)
+            return _WAIT
+
+        eng = CombatEngine(
+            ScriptedRng([10, 5]), grid, [hero, foe], policy=policy, round_cap=1,
+        )
+        eng.run()
+        return eng._states[0].pos
+
+    assert _go((0, 2)) == (0, 0)
+    assert _go((0, 1)) == (0, 1)
+
+
+def test_airborne_view_preserves_combat_flags() -> None:
+    # The air view normalizes only the walking cost: solid barriers stay
+    # impassable (so LoE/cover geometry is untouched by flight), cover flags
+    # and the elevation layer are preserved.
+    legend = dict(_LEGEND)
+    legend["C"] = TerrainType(type="pillar", move_cost=None, cover=True)
+    spec = MapSpec(
+        name="flags", terrain=(".TP#C",), legend=legend, elevation=(".1...",),
+    )
+    grid = Grid.from_spec(spec)
+    view = grid.airborne_view()
+    wall = view.terrain_at((0, 3))
+    assert wall.blocks_los
+    assert wall.move_cost is None
+    pit = view.terrain_at((0, 2))
+    assert pit.move_cost == 1
+    assert not pit.blocks_los
+    assert view.terrain_at((0, 1)).move_cost == 1
+    pillar = view.terrain_at((0, 4))
+    assert pillar.move_cost == 1
+    assert pillar.cover
+    assert view.elevation_at((0, 1)) == grid.elevation_at((0, 1)) == 1