D029 - Backends are abstracted at guest semantics

Status: decided Date: 2026-08-19

A backend contract describes what the guest asks for, not what one host API provides: render commands, samples at a rate, pad state, guest path semantics. Any string crossing a boundary is a named constant declared once.

Why: a contract designed from one host API carries that API's model, and a second backend then fits badly. A seam is structural when it pays for itself in testability; the render contract lets command-stream translation be tested with no device through a recording backend.

Rejected:

• A contract shaped by the first host API: a second backend fits badly.
• Seams that pay off only hypothetically: speculation.