# Principle Boundary Discipline

> 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.

- Skill: `cursor/principle-boundary-discipline` (Agent Skill)
- Install (CLI): `npx skillmds@latest add cursor/principle-boundary-discipline`
- Raw SKILL.md: https://api.skillmd.com/api/skills/cursor/principle-boundary-discipline/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: cursor (https://skillmd.com/u/cursor)
- Updated: 2026-09-09
- Page: https://skillmd.com/skills/cursor/principle-boundary-discipline

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# 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.