Surface Equivalency

A project that exposes functionality to humans and agents grows surfaces: a REST API, a CLI, an MCP server. Built independently, they drift — operations exist on one surface but not another, parameter models diverge, error contracts fork, and the docs match only one of them. Drift is not a hygiene problem; it makes agents fail in ways users can't debug ("works in the CLI but not over MCP").

The fix is structural, not disciplinary: one core, N projections. Every surface is a thin client of the same core; equivalence is enforced by a test gate, not by review vigilance.

Step 1: Triage — where is this project today?

Always start here. The path differs by starting point: