Hello, similar error, downloaded the weights (for Minimax), pointed Sprite Sheep to the ComfyUI folder, but I still have "2 files not visible to ComfyUI yet"
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Re-check never linked anything. The weights only got linked into ComfyUI's folder at the end of a download, or via "Select folder…". If you downloaded them before pointing Sprite Sheep at ComfyUI, neither happened — and Re-check only looked, while the text under it promised it would link. So it said "not visible to ComfyUI yet" forever.
That's also the 400. It is local, and ComfyUI did answer: it just rejected the graph, because the weights weren't in its models/ folder, so the filename wasn't among the values its loader nodes accept. Sprite Sheep was throwing away the response body, where ComfyUI explains exactly that. Now it shows the reason and names the file.
Fixed in 0.0.3, just uploaded. Nothing to re-download: install it, open Settings, press Re-check once and the weights get linked in place.
(Also gone: the <null> under every file in the Models panel.)