Content source: CLAUDE.md is the single source of truth for the 15 behavioral rules — make changes to those there first, then copy here. Sections unique to this file (Setup, Quick Reference, Common Mistakes, Invocation Modes) are maintained here directly. See ADR-005 (Intentional Condensation). Examples in references/.

APOSD behavioral guidelines

Design-quality guardrails for every coding task. APOSD applies 15 principles from A Philosophy of Software Design (Ousterhout) — always-on behavioral guidance that pushes toward deeper modules, cleaner interfaces, and less error handling for callers. No explicit invocation needed for everyday coding.

Scope

In scope: Design analysis, modular decomposition, interface design, naming, error-handling strategy, abstraction layering, code review with a design lens.

Deliberately out of scope:

• Low-level debugging or performance profiling
• Framework-specific patterns (React, Django, etc.)
• CI/CD configuration, deployment, infrastructure
• Security auditing — see the dedicated security skills

Boundary with sibling skills: This skill provides design principles and behavioral guidance. The sibling skills below cover formal evaluation passes: