---
name: michael-denyer-principle-boundary-discipline
description: "Apply when wiring validation, error handling, or framework adapters. Concentrate guards at system boundaries (CLI, config, network, external APIs); trust internal types and keep business logic in pure functions."
license: "MIT"
metadata:
  decimalai-skill-id: "1d5e4630-2e09-4bcc-9efc-91a5d0a34557"
  decimalai-version-id: "dbb003c0-fd80-499f-a5c0-963cbd1f4592"
  decimalai-version: "1"
  decimalai-body-sha256: "45df4e48c913c74c9efc114bb8031f4ae7cc97aa04badc29953aedb1a3f809ec"
  decimalai-name: "michael-denyer/principle-boundary-discipline"
  decimalai-format: "1"
  decimalai-invocation: "model"
  decimalai-category: "coding"
  decimalai-source: "https://app.decimal.ai/s/michael-denyer-principle-boundary-discipline@1/SKILL.md"
---
# 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.

**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

Code organization:
- Business logic in pure functions with no framework dependencies
- 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.