design-an-interface

Typical invocation points:

• A slice plan says "add module X" and the shape of X is not obvious.
• Two callers are about to grow coupled to an interface that has not been designed on purpose.
• A refactor surfaces a seam that needs to be re-cut deliberately.

<core_principle> RADICALLY DIFFERENT, NOT VARIANTS. Three designs with different method names are one design. Three designs that hide different things, expose different axes of control, or invert the caller/callee relationship are three designs. Enforce real divergence by assigning each sub-agent a constraint that would force a different shape.

INTERFACE, NOT IMPLEMENTATION. Do not estimate effort. Do not argue about how hard it is to build. Argue about how the interface behaves from the caller's side and what it hides internally. </core_principle>