MCP Spine: Middleware proxy for LLM tool calls with security and token control

The Model Context Protocol (MCP) has solved the 'connectivity' problem for LLMs, but it created a 'management' problem. As developers connect more servers—GitHub, Slack, local databases—they face bloated context windows filled with tool schemas and a worrying lack of oversight. MCP Spine steps in as a sophisticated middleware proxy, effectively acting as a firewall and optimizer for the tool-calling pipeline. It transforms a chaotic collection of disparate servers into a single, governed entry point. Technically, Spine is impressive for its focus on local-first efficiency. The semantic router uses local embeddings (all-MiniLM-L6-v2) to ensure only relevant tools are exposed to the LLM, preventing the context window from choking on irrelevant definitions. Even more practical is the schema minifier; by stripping non-essential metadata, it claims token savings of up to 61%. For those building production agents, the 'State Guard' is a standout feature, using SHA-256 pins to ensure the LLM isn't editing a version of a file that has changed since it was last read. From a security perspective, Spine is skeptical and thorough. It doesn't just rely on the underlying server's security; it implements its own secret scrubbing, path jails to prevent directory traversal, and a dedicated injection detection engine to scan tool responses before they reach the LLM. The inclusion of Human-in-the-Loop (HITL) confirmation for destructive actions (like writing files) moves this from a hobbyist tool to something viable for professional environments. While the feature set is dense, the primary risk is the added latency of an extra hop in the request chain. However, the inclusion of a built-in LRU cache for read-only tools and concurrent server startup suggests the author is mindful of performance. MCP Spine is essential for any integrator who has moved past the 'hello world' phase of MCP and is now worried about token costs, security leaks, or agent autonomy.