Parse at the Boundary

Every piece of data entering your code from outside your control must be parsed through a schema exactly once, at the boundary. After parsing, downstream code receives typed data and never re-validates.

If parsing fails, fail loudly. Never silently coerce.

Principle #8 (Trust the boundary, not the middle) — this skill is the canonical operationalization. The trust gradient is outside (paranoid) → boundary (parse) → inside (trust); defensive code in the middle of your system is a sign that the boundary is leaking. See /principles.

"We require Codex to parse data shapes at the boundary, but are not prescriptive on how that happens." — OpenAI Harness Engineering

The Rule