Why does my AI walk through wall corners diagonally?

mp_grid_path with allowdiag = true considers diagonal cells walkable as long as the diagonal cell itself is open. It doesn't check that both adjacent cardinal cells are also open, so the path squeezes between walls.

How do I prevent corner cutting?

Either set allowdiag = false (only cardinal moves), or pad walls so the diagonal cell is blocked too. The simplest pad is to grow each wall by half a cell on every side using mp_grid_add_rectangle.

Should I use mp_grid or roll my own A*?

mp_grid is fine for prototypes and small maps. Custom A* gives you control over corner-cutting, weighted terrain, and diagonal post-process smoothing. Switch when mp_grid limitations bite.

Fix: GameMaker mp_grid Pathfinding allowdiag Cuts Corners

Quick answer: Either set allowdiag = false on mp_grid_path to disable diagonals entirely, or grow walls by half a cell so the diagonal cell is blocked as well. mp_grid’s diagonal logic doesn’t check the cardinal neighbors of a diagonal step.

Your enemy AI uses mp_grid_path with allowdiag enabled. It produces a path that diagonally clips a wall corner, sliding through stone. Visually wrong, gameplay-breakable.

The Symptom

AI walks diagonally through corners where the geometry has a wall on the cardinal sides. The diagonal cell itself is open, but the path shouldn’t use it because the entity can’t physically squeeze through.

What Causes This

mp_grid_path with allowdiag = true allows diagonal transitions if the destination cell is open. It doesn’t check that both adjacent cardinal cells are also open — so a diagonal between two walls (NW open, N blocked, W blocked) is considered legal.

The Fix

Option 1: Disable diagonals. Cleanest if your art style permits cardinal-only movement.

path_found = mp_grid_path(grid, path, x, y, target_x, target_y, false);   // allowdiag = false

Option 2: Pad walls. When you build the grid, add wall rectangles slightly larger than the actual wall. The diagonal cell adjacent to a wall corner becomes blocked.

var cell_w = 32;
var cell_h = 32;
grid = mp_grid_create(0, 0, room_width div cell_w, room_height div cell_h, cell_w, cell_h);

with (obj_wall) {
    mp_grid_add_rectangle(other.grid,
        bbox_left  - 8,
        bbox_top   - 8,
        bbox_right + 8,
        bbox_bottom+ 8);
}

The +/-8 padding around each wall blocks the diagonal cell touching the corner. Paths route around. Tune the pad size to match your sprite size; too big and you lose passable corridors.

Custom A* Alternative

If you need fine control, write A* with explicit corner-cut prevention:

// Allow NW move only if N or W is also open
if (dir_is_diagonal) {
    if (!cell_open(x, y - 1) || !cell_open(x - 1, y))
        continue;   // no corner cut
}

Verifying

Spawn an enemy near a wall corner with a target across the wall. The path debug overlay should route around the corner, not through it. mp_grid_draw shows the grid; verify diagonal cells next to walls are blocked when padded.

“Disable diagonals or pad the walls. mp_grid stops cheating.”

Related Issues

For surface lost on resize, see surface lost. For instance deactivate, see deactivate region.

Pad or disallow. Paths respect walls.