backend-dev-guidelines – Coding skill for AI agents

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

Use these patterns when they fit the target repository; its architecture, authorization model and dependency versions take precedence. This documentation does not ship a runnable service.

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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: -13 → +7 (each input is 1–5). This is an informal discussion aid, not a validated risk model or merge gate.

Interpretation

| BFRI | Meaning | Action |

| -------- | --------- | ---------------------- |

| 6–7 | Lower concern | Proceed |

| 3–5 | Moderate | Add tests + monitoring |

| 0–2 | Risky | Refactor or isolate |

| < 0 | Dangerous | Redesign before coding |

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When to Use

Automatically applies when working on:

• Routes, controllers, services, repositories
• Express middleware
• Prisma database access
• Zod validation
• Approved error tracking with redaction and destination/retention controls
• Configuration management
• Backend refactors or migrations

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2. Architecture Pattern (When Adopted by the Repository)

1. Layered Architecture Option

Routes → Controllers → Services → Repositories → Database

• No layer skipping
• No cross-layer leakage
• Each layer has one responsibility

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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. Capture Unexpected Errors at One Approved Boundary

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 server

---

4. 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

For Express 4, route promises need an error-forwarding wrapper. Express 5 forwards rejected returned promises; avoid duplicate handlers.

router.get(
  '/users',
  asyncErrorWrapper((req, res) =>
    controller.list(req, res)
  )
);

Return or await the handler promise; a wrapper does not handle detached background work.

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8. Observability & Monitoring

Required

• Approved error tracking with redaction and destination/retention controls
• Sampled performance tracing appropriate to the installed SDK
• 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. Review Signals

❌ Business logic in routes

❌ Skipping service layer

❌ Direct Prisma in controllers

❌ Missing validation

❌ Unvalidated environment access outside configuration bootstrap

❌ Unredacted or unapproved telemetry

❌ Untested business logic

---

Resource Guide

Read the relevant bundled resources before applying examples. Private project paths and @project-lifecycle-portal/database, SSOMiddleware, BaseController and configuration utilities are illustrative application components, not installed dependencies. Authenticate with access tokens, authorize every object/tenant action, allowlist public DTO fields, project response fields and protect cookie-authenticated mutations against CSRF.

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

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12. Operator Validation Checklist

Before finalizing backend work:

• [ ] Risks discussed with evidence; no numeric score grants approval
• [ ] Layered architecture respected
• [ ] Input validated
• [ ] Errors captured in Sentry
• [ ] unifiedConfig used
• [ ] Tests written
• [ ] No anti-patterns present

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13. Skill Status

Status: Illustrative patterns; target-specific compilation, tests and security review required

Intended Use: Long-lived Node.js microservices with real traffic and real risk

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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.