Coding agents work best when the target is observable. A useful spec states the user outcome, current behavior, constraints, edge cases, and evidence required for completion.
Let Codex inspect the repository before choosing an implementation. Review its plan when the change crosses APIs, schemas, or product decisions. Then let it edit, test, and inspect the diff.
The human still owns intent and acceptance. Codex supplies exploration, implementation, and verification. The output is not generated code; it is a change you understand and can prove works.