---
name: principle-boundary-discipline
description: "Concentrates validation and error handling at system boundaries, trusts internal types, and keeps business logic pure."
disable-model-invocation: true
---

# Boundary Discipline

Place validation, type narrowing, and error handling at system boundaries. Trust internal code unconditionally. Business logic lives in pure functions. The shell is thin and mechanical.

**Why:** Scattered validation is noisy, redundant, and gives a false sense of safety. Keep logic out of framework wiring so it can be tested without the framework.

**The pattern:**

- **At boundaries** (CLI args, config files, external APIs, network protocols): validate, return errors, handle defensively.
- **Inside the system:** typed data, error propagation, no re-validation. Trust the types.
- **Across the boundary.** Expose domain concepts, not the boundary's private representation. Keep general-purpose mechanism inside and special-purpose policy at the edge.
- **Read the platform's contract.** Before changing the order or timing of calls into a framework, such as lifecycle steps, delivery, or acknowledgment, read its documented contract and cite it when you offer options. Your memory of an API is a proxy for it.

**Applications:**

Validation and error handling:

- Validate config at parse time (the boundary), not inside business logic
- Parse raw data into domain types at the boundary
- Do not re-export transport, storage, framework, or wire types through the public surface
- No redundant nil checks deep in call chains if the boundary already validated
- When translating an error, keep the original cause

Code organization:

- Business logic in pure functions with no framework dependencies
- Keep construction, dependency wiring, persistence, and vendor APIs out of business logic. Wrap a vendor API only where it would leak; an adapter that only forwards calls is a one-caller wrapper
- Isolate security mechanisms; keep permission rules with the behavior they protect
- Parse functions: pure transforms from raw bytes to typed state
- Prompt construction: structured state in, string out
- Scoring and assessment: pure transforms from state to results

**The tests:**

- "Is this data crossing a system boundary right now?" If not, validation is redundant.
- "Can this be a pure function that the shell just calls?" If yes, extract it.