How MCPG is built — a thin protocol-authority host, an operator-defined backend layer, and a plugin system that loads every cross-cutting concern across a stable binary boundary.

Every tool call flows through one lifecycle — who is calling (access), are they allowed (policy), does a human need to sign off (approvals), and what gets recorded (audit). One umbrella, fail-closed by default.

MCPG is a thin host; everything with a policy, an upstream, or a side-effect is a plugin loaded across a stable binary boundary. The entity model, the three runtimes, capabilities, signing, and trust — explained.

MCPG negotiates two MCP revisions on one listener — the stateful 2025-11-25 release and the stateless DRAFT-2026-v1 draft. How version selection works, what changed between them, and why a single deployment serves both.