I rebuilt my tiny homage to Warhammer 40k this week. The original, Hive City Rampage, was a Python/Pygame (monstrocity), top-down grimdark shooter that ran but was a prototype. Now we have a sequel, Ashgate Siege, is Godot 4: gothic isometric, two missions, built for Mac and Android. I have a confession though, the 95k lines of GDScript, 8 commits, roughly five hours was written by an AI agent under my direction. The sprites are generated too. I'm disclaiming that up front because the interesting part is what came out of it. The easy part was the code This is the simple finding. An engine port is exactly the shape of coding my agents were very good at: the target is well documented, the semantics known, and correctness is checkable by running the thing. Godot 4 plus GDScript is heavily represented in training data for any model. Zero agent struggle. The hard part was four pictures of the same orc Here's the hill I'm whiling to die on: for AI-assisted games, 2D sprite games are harder than 3D. That sounds backwards, so: in 3D, the engine guarantees coherence. One mesh, one material, one light rig, and every frame of animation is consistent because it's the same object being transformed. The renderer is doing the work. In sprite based games there is no shared object. Each sprite is an independent generated map of pixels. So: Frame 2 of a walk cycle can be a different character than frame 1. The light source can move between sprites in the same scene. Proportions drift. Your massive size boss shrinks. Limbs get cropped at cell edges. None of that is caught by anything automatically. It just ships, and the game looks odd, like a badly executed collage. Prompts as interface specs The fix was to stop treating generation as commissioning art and start treating it as calling an API with a strict schema. Every prompt pins the geometry: Production game sprite sheet: exactly 4 columns x 3 rows, 1536x1024 canvas, equal cells, solid pure magenta #FF00FF background for color-key import. NO text, shadows, smoke, or border. Every figure entirely within its own cell with ample margin; feet centered at same baseline in each row. All figures face screen RIGHT in three-quarter isometric view [...] Four columns are four coherent walking-cycle poses: left foot forward, passing, right foot forward, passing. Each sentence has to do some work: Magenta #FF00FF, not transparency. Alpha comes back unreliable. A color key is deterministic to strip. Explicit grid and canvas. The importer crops on fixed coordinates. If the grid drifts, every sprite is off. "feet centered at same baseline in each row." Without it the character bobs while walking. Named poses. "Walking animation" returns four unrelated drawings. Naming the four phases is what makes them a cycle. Negative constraints. "No text, shadows, smoke, or border" because models add decoration that breaks the silhouette. The importer is a compiler, and tests are CI If the prompt is a spec, something has to enforce it. Generated atlases go into art/, an importer crops and scales them into runtime assets, and a native test suite checks the result: 2,972 reference and combat assertions, 17,112 silhouette combinations, plus a 600-frame combat simulation. make test runs it. Silhouette checks are the super important. A regenerated atlas that's subtly wrong (a pose 8px taller, a gun scaled differently between frames) passes every functional test and looks broken in motion. Comparing silhouettes against frozen baselines should catch most of it. The way this can generalize: if AI generation is nondeterministic, everything downstream has to be strict. The prompt is a spec that gets compiled by the importer, and CI runs with silhouette tests. Without that you don't have a pipeline, at bess it is a slot machine you need to keep pulling until the art looks okay. What I'd tell you to steal from my repo Color-key over alpha. Deterministic beats convenient. Pin the grid numerically in the prompt and crop on those exact numbers. Name every animation phase. Never say "walk cycle" and hope. Baseline your silhouettes and diff them. This is the whole safety net. Keep source atlases and prompts in the repo. Regeneration is a build step, so its inputs are source code. Try it Mac and Android builds are on GitHub Releases. They're previews: macOS is ad-hoc signed and not notarized, Android uses a debug key, so both will warn you. Source builds with make run if you have Godot 4.3. https://github.com/nonatofabio/hive-city-rampage-ii