principle
Contract-first interfaces
Treat every handoff (human↔human, human↔system, system↔system) as a contract with explicit schemas, expectations, and validation—so variability doesn’t leak downstream.
Applied in
Single parent
Morning/bedtime “contracts” (mini checklists, ready-by times, who-does-what). Example: a laminated “out-the-door” card kids can self-check.
Engineer
JSON schemas; structured outputs; strict validators; reject+retry on violation. Example: Pydantic guardrail around LLM output.