eva: Skill for Claude Code

Service Layer Architecture

Overview

Two-layer separation: Actions orchestrate domain rules (the "why/when"), while a service layer centralizes reusable operational mechanics (the "how").

This prevents duplicated code, inconsistent behavior, and bugs fixed in one path but not others.

When to Use

• Multiple callers need the same low-level operation (sandbox creation, email sending, payment processing)
• You're copy-pasting operational logic between action files
• A bug fix in one workflow doesn't propagate to others doing the same thing
• Adding a new feature that shares mechanics with existing flows

Don't use when: Logic is truly domain-specific and used by only one caller.

Core Pattern

Orchestration Layer (Actions)          Service Layer (Shared Mechanics)
├── owns business rules                ├── owns reusable operations
├── owns state transitions             ├── owns provider/SDK interactions
├── owns auth/ownership checks         ├── owns command execution details
├── owns failure classification        ├── owns health checks / readiness
├── owns retries / user-facing errors  └── returns structured results
└── calls service functions

Rule of thumb:

• "What this product flow means" → keep in actions
• "How to do this operation reliably" → move to service layer

Quick Reference

Design Principle	Do	Don't
API shape	Composable capability blocks	One giant "do everything" method
Inputs/outputs	Explicit params, structured returns	Hidden global state, reaching into DB
Migration	Extract one block, replace one caller, verify, then migrate rest	Refactor everything at once
Domain logic	Keep auth, policy, error classification in actions	Let service mutate domain state directly
Extraction trigger	Logic repeated across 2+ callers	Logic used once (over-abstraction)

Designing Service Functions

Design as capability blocks, not monoliths:

// Good: composable, each caller chooses what to use
createManagedSandbox(...)
prepareRepo(...)
detectPackageManager(...)
installDependencies(...)
runBuildCommand(...)
startSandboxRuntime(...)