Technē: A Deterministic Craft Layer for Model Context Protocol Servers, Grounded in LLM-Authored X3D

When a language model builds 3D content by calling tools through the Model Context Protocol (MCP), the JSON Schema ensures each call is well-formed—but not that the result will render correctly. A texture placed in the wrong slot vanishes. A humanoid skeleton routed to the wrong parent field is invisible. The schema passes; the pixels fail. We present Technē, a deterministic MCP proxy for the Web3D x3d_mcp server that catches these failures. Sitting between the model and the real server, Technē intercepts each tool call and (1) repairs sloppy arguments using Postel-style coercion, (2) checks a catalog of documented X3D silent-failure modes, returning prescriptive corrections that name the exact fix, and (3) renders the scene and checks for blank frames—a test no schema validator can perform. An opt-in provenance gate extends the same approach to content honesty. On correct input Technē blocks nothing and adds no model round-trips. The system runs no LLM in the enforcement loop, carries 234 passing unit tests, and is measured against the bare server in a controlled comparison. None of this is specific to X3D: wherever a model authors through a tool surface, the craft that surface assumes can be compiled out of documentation and into the transport, where it holds whether or not the model understood it.