What a local readiness check can and cannot tell you
Before a model reaches a slicer, it helps to know whether the surface mesh itself is sound. A watertight, "manifold" mesh, where every edge borders exactly two triangles and the surface forms one or more fully closed shells, is what most slicers expect. A model exported from CAD with a missing face, an accidental gap, or overlapping geometry can still open in a slicer, but the result is unpredictable: some slicers silently patch small holes, others generate a broken or missing wall exactly where the defect sits. This check reads that structure directly from the file's triangles, in your browser, before you commit to a slice.
How the check works
The file is parsed as binary STL and every triangle's edges are compared against its neighbors. An edge shared by exactly two triangles is healthy. An edge belonging to only one triangle marks a boundary, an opening in the surface. An edge shared by three or more triangles marks non-manifold geometry, usually from overlapping or self-intersecting surfaces. The tool also counts separate connected shells and looks for zero-area (degenerate) triangles, which are usually harmless export artifacts but worth knowing about. None of this leaves your device; the parsing and analysis both run locally.
What the two outcomes look like
A closed, single-shell mesh with no boundary or non-manifold edges returns a "proceed to slicing" result: this is a readiness signal, not a slicer or print guarantee. A mesh with even one boundary edge, commonly from a missing face or a modeling gap, is flagged as an open-boundary concern and routed toward repair in your CAD tool or a dedicated mesh-repair tool rather than treated as auto-fixable.
Limitations
This check reads triangle and edge structure only. It has no way to know whether your geometry is dimensionally correct, whether walls are thick enough to print, or whether the part will actually fit or function as intended; those questions belong to the other planning tools on this site and, ultimately, to a real test print. A mesh that passes this check can still fail to print for reasons entirely outside what a local structural scan can see.