Zoom in on the seam
Seamless has an exact meaning: the pixels on one tile's edge continue pixel-for-pixel into the next. Here is the join between two tiles, magnified - ours against a popular diffusion tool's exported tileset patch, repeated the way an engine would repeat it.
SPRITEFORGE · 8x ZOOM
TILE 1TILE 2
The same tile, repeated - find the join
RETRO DIFFUSION · SAME ZOOM
COPY 1COPY 2
Their patch repeated - its own join is a wall
HOW THESE WERE MADE · prompt "make me a cobblestone road" · both 16px · SpriteForge low quality · Retro Diffusion dedicated Tileset option · each image shows the tool's REPEATING UNIT drawn twice, meeting at the centre - SpriteForge's unit is one 16px tile; the diffusion export has no internal tile grid, so its unit is the whole 96px patch
01Diffusion tools paint a big image and shrink it. The downscale averages pixels, so tile edges land wherever they land - almost matching is not tiling.
02The common workaround is tiles that fade to a neutral border so they never have to match a neighbour - which is why those maps read as one stamp, printed forever.
03SpriteForge places pixels on the tile grid directly and derives the 16-tile autotile family so every edge relationship holds by construction. The design deliberately reaches across tiles.
Full mechanics, Wang sets included: why AI tilesets don't tile.