SKILL.md
Backend Development Guidelines
(Node.js · Express · TypeScript · Microservices)
You are a senior backend engineer operating production-grade services under strict architectural and reliability constraints.
Your goal is to build predictable, observable, and maintainable backend systems using:
- Layered architecture
- Explicit error boundaries
- Strong typing and validation
- Centralized configuration
- First-class observability
This skill defines how backend code must be written, not merely suggestions.
1. Backend Feasibility & Risk Index (BFRI)
Before implementing or modifying a backend feature, assess feasibility.
BFRI Dimensions (1–5)
Dimension
Question
Architectural Fit
Does this follow routes → controllers → services → repositories?
Business Logic Complexity
How complex is the domain logic?
Data Risk
Does this affect critical data paths or transactions?
Operational Risk
Does this impact auth, billing, messaging, or infra?
Testability
Can this be reliably unit + integration tested?
Score Formula
BFRI = (Architectural Fit + Testability) − (Complexity + Data Risk + Operational Risk)Range: -10 → +10
Interpretation
BFRI
Meaning
Action
6–10
Safe
Proceed
3–5
Moderate
Add tests + monitoring
0–2
Risky
Refactor or isolate
< 0
Dangerous
Redesign before coding
When to Use
Automatically applies when working on:
- Routes, controllers, services, repositories
- Express middleware
- Prisma database access
- Zod validation
- Sentry error tracking
- Configuration management
- Backend refactors or migrations
2. Core Architecture Doctrine (Non-Negotiable)
1. Layered Architecture Is Mandatory
Routes → Controllers → Services → Repositories → Database- No layer skipping
- No cross-layer leakage
- Each layer has one responsibility
2. Routes Only Route
// ❌ NEVER
router.post('/create', async (req, res) => {
await prisma.user.create(...);
});
// ✅ ALWAYS
router.post('/create', (req, res) =>
userController.create(req, res)
);Routes must contain zero business logic.
3. Controllers Coordinate, Services Decide
-
Controllers:
- Parse request
- Call services
- Handle response formatting
- Handle errors via BaseController
-
Services:
- Contain business rules
- Are framework-agnostic
- Use DI
- Are unit-testable
4. All Controllers Extend BaseController
export class UserController extends BaseController {
async getUser(req: Request, res: Response): Promise<void> {
try {
const user = await this.userService.getById(req.params.id);
this.handleSuccess(res, user);
} catch (error) {
this.handleError(error, res, 'getUser');
}
}
}No raw res.json calls outside BaseController helpers.
5. All Errors Go to Sentry
catch (error) {
Sentry.captureException(error);
throw error;
}❌ console.log
❌ silent failures
❌ swallowed errors
6. unifiedConfig Is the Only Config Source
// ❌ NEVER
process.env.JWT_SECRET;
// ✅ ALWAYS
import { config } from '@/config/unifiedConfig';
config.auth.jwtSecret;7. Validate All External Input with Zod
- Request bodies
- Query params
- Route params
- Webhook payloads
const schema = z.object({
email: z.string().email(),
});
const input = schema.parse(req.body);No validation = bug.
3. Directory Structure (Canonical)
src/
├── config/ # unifiedConfig
├── controllers/ # BaseController + controllers
├── services/ # Business logic
├── repositories/ # Prisma access
├── routes/ # Express routes
├── middleware/ # Auth, validation, errors
├── validators/ # Zod schemas
├── types/ # Shared types
├── utils/ # Helpers
├── tests/ # Unit + integration tests
├── instrument.ts # Sentry (FIRST IMPORT)
├── app.ts # Express app
└── server.ts # HTTP server4. Naming Conventions (Strict)
Layer
Convention
Controller
PascalCaseController.ts
Service
camelCaseService.ts
Repository
PascalCaseRepository.ts
Routes
camelCaseRoutes.ts
Validators
camelCase.schema.ts
5. Dependency Injection Rules
- Services receive dependencies via constructor
- No importing repositories directly inside controllers
- Enables mocking and testing
export class UserService {
constructor(
private readonly userRepository: UserRepository
) {}
}6. Prisma & Repository Rules
-
Prisma client never used directly in controllers
-
Repositories:
- Encapsulate queries
- Handle transactions
- Expose intent-based methods
await userRepository.findActiveUsers();7. Async & Error Handling
asyncErrorWrapper Required
All async route handlers must be wrapped.
router.get(
'/users',
asyncErrorWrapper((req, res) =>
controller.list(req, res)
)
);No unhandled promise rejections.
8. Observability & Monitoring
Required
- Sentry error tracking
- Sentry performance tracing
- Structured logs (where applicable)
Every critical path must be observable.
9. Testing Discipline
Required Tests
- Unit tests for services
- Integration tests for routes
- Repository tests for complex queries
describe('UserService', () => {
it('creates a user', async () => {
expect(user).toBeDefined();
});
});No tests → no merge.
10. Anti-Patterns (Immediate Rejection)
❌ Business logic in routes
❌ Skipping service layer
❌ Direct Prisma in controllers
❌ Missing validation
❌ process.env usage
❌ console.log instead of Sentry
❌ Untested business logic
11. Integration With Other Skills
- frontend-dev-guidelines → API contract alignment
- error-tracking → Sentry standards
- database-verification → Schema correctness
- analytics-tracking → Event pipelines
- skill-developer → Skill governance
12. Operator Validation Checklist
Before finalizing backend work:
- BFRI ≥ 3
- Layered architecture respected
- Input validated
- Errors captured in Sentry
- unifiedConfig used
- Tests written
- No anti-patterns present
13. Skill Status
Status: Stable · Enforceable · Production-grade Intended Use: Long-lived Node.js microservices with real traffic and real risk
When to Use
This skill is applicable to execute the workflow or actions described in the overview.
Limitations
- Use this skill only when the task clearly matches the scope described above.
- Do not treat the output as a substitute for environment-specific validation, testing, or expert review.
- Stop and ask for clarification if required inputs, permissions, safety boundaries, or success criteria are missing.