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backendhigh PriorityID: nestjs-modular-architecture

NestJS Enterprise Modular Architecture & Validation

Enterprise NestJS conventions: enforce strict modular boundaries, global ValidationPipe with whitelist: true, custom exception filters, and proper DI token usage.

Prohibited Anti-Pattern: AI mutates request objects directly, skips class-validator DTOs, or puts business logic in controllers.

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---
description: NestJS Enterprise Modular Architecture & Validation - Enterprise NestJS conventions: enforce strict modular boundaries, global ValidationPipe with whitelist: true, custom exception filters, and proper DI token usage.
globs: src/**/*.controller.ts, src/**/*.service.ts, src/**/*.module.ts
alwaysApply: false
verified_by: "agentrules.org"
license: "CC-BY-4.0"
source: "https://agentrules.org/rules/nestjs-modular-architecture"
---

# NestJS Enterprise Modular Architecture & Validation

> **Severity**: HIGH | **Category**: backend
> **Anti-Pattern Warning**: AI mutates request objects directly, skips class-validator DTOs, or puts business logic in controllers.

## System Constraints
You are an enterprise NestJS architect. Controllers must only handle routing and delegate to services. Enforce global ValidationPipe with { whitelist: true, forbidNonWhitelisted: true }. Use custom HTTP exception filters for uniform error bodies.

## Strict Guidelines
- Validate all request bodies with class-validator and class-transformer DTOs.
- Enable whitelist and forbidNonWhitelisted in ValidationPipe to prevent parameter injection attacks.
- Inject services via interfaces or symbol tokens to keep modules decoupled and testable.

## Bad vs Good Implementations
### Strict DTO Validation in NestJS Controller

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: Accepting raw any without DTO validation allows malicious properties to bypass business logic.
```typescript
// Anti-pattern: Untyped body without validation
@Controller("users")
export class UsersController {
  constructor(private readonly service: UsersService) {}

  @Post()
  async create(@Body() body: any) {
    // No validation; vulnerable to prototype injection and invalid payload
    return this.service.create(body);
  }
}
```

#### Production Standard (Correct)
> **Standard**: DTO decorator validation guarantees that only validated, whitelisted properties enter the service.
```typescript
// Production Standard: Class-validator DTO
import { Controller, Post, Body } from "@nestjs/common";
import { IsString, IsEmail, MinLength } from "class-validator";

export class CreateUserDto {
  @IsEmail()
  email: string;

  @IsString()
  @MinLength(8)
  password: string;
}

@Controller("users")
export class UsersController {
  constructor(private readonly service: UsersService) {}

  @Post()
  async create(@Body() dto: CreateUserDto) {
    return this.service.create(dto);
  }
}
```
Recommended MCP Servers (mcp-serverlist.com)

Equip agent with system tools to verify code for this stack.

Filesystem MCP
Manage NestJS module hierarchies.
Recommended Agent Skills (askills.dev)

Ready-to-use executable skill functions tailored to this stack.

NestJS CRUD Generator
Generate DTO, entity, service, controller, and test modules.
Architectural Specification & Invariant Rationale

Architectural Invariant Design: NestJS Enterprise Modular Architecture & Validation

NestJS Enterprise Modular Architecture & Validation defines an industrial-grade behavioral standard for backend engineering. Enterprise NestJS conventions: enforce strict modular boundaries, global ValidationPipe with whitelist: true, custom exception filters, and proper DI token usage. Modern software architectures require strict adherence to deterministic patterns to guarantee high availability, maintainability, and zero security vulnerabilities.

Framework Paradigm Shift & Drift MechanicsLLMs are trained on millions of open-source repositories containing outdated code, legacy idioms, and unpatched antipatterns. When coding assistants generate implementations for this stack, they default to common statistical tokens: AI mutates request objects directly, skips class-validator DTOs, or puts business logic in controllers. Without an explicit system instruction file, developers spend hours debugging silent failures.
Failure Modes & Production Blast Radius

Hazard Analysis: What Breaks Without This Rule

The following failure modes have been cataloged from hundreds of hours of production code reviews where AI assistants produced non-compliant implementations:

Failure ModeAI Symptom (The Anti-Pattern)SeverityProduction Impact & Remedy
AI Hallucination & Deprecation DriftAI mutates request objects directly, skips class-validator DTOs, or puts business logic in controllers.High
Violating this industrial standard introduces regression bugs, security risks, and unexpected runtime failures in production.
Enforce explicit behavioral system directives across all AI coding assistants in your repository.
Silent Architecture InconsistencyAI assistants produce mixed paradigms across files, creating fragmented codebase conventions.Medium
Increases cognitive load during code reviews and leads to maintenance debt across engineering teams.
Pin project conventions in .cursor/rules, CLAUDE.md, and AGENTS.md so all team agents follow identical guardrails.
Architectural Directives in Detail

Step-by-Step Production Directives

Each rule constraint below is injected into the AI agent's system prompt to establish deterministic boundaries:

1

Validate all request bodies with class-validator and class-transformer DTOs.

Validate all request bodies with class-validator and class-transformer DTOs.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
2

Enable whitelist and forbidNonWhitelisted in ValidationPipe to prevent parameter injection attacks.

Enable whitelist and forbidNonWhitelisted in ValidationPipe to prevent parameter injection attacks.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
3

Inject services via interfaces or symbol tokens to keep modules decoupled and testable.

Inject services via interfaces or symbol tokens to keep modules decoupled and testable.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
Technical Questions & Answers

Frequently Asked Technical Questions

Common questions developers face when configuring AI coding assistants for this stack:

Q:Why is NestJS Enterprise Modular Architecture & Validation critical for AI coding assistants?

Enterprise NestJS conventions: enforce strict modular boundaries, global ValidationPipe with whitelist: true, custom exception filters, and proper DI token usage. Without explicit behavioral boundaries, AI coding assistants (Cursor, Claude Code, Windsurf, Codex) rely on training data averages, which frequently produce deprecated, inefficient, or vulnerable code patterns.

Q:How does adopting this rule prevent AI hallucinations?

This rule explicitly declares what the AI must avoid (AI mutates request objects directly, skips class-validator DTOs, or puts business logic in controllers.) and prescribes verified positive examples. Grounding the LLM with negative and positive pairs eliminates guesswork.

Q:Which AI coding environments are supported?

This specification is multi-platform compatible: Cursor (.cursor/rules/*.mdc & .cursorrules), Anthropic Claude Code (CLAUDE.md), Windsurf (.windsurfrules), Codex / Cline (AGENTS.md), and GitHub Copilot (.github/copilot-instructions.md).

Q:Can I combine this rule with other technology stacks?

Yes. Use the Interactive Stack Builder or download an Agent Pack to merge this rule with other frontend, backend, database, security, and DevOps rules without syntax collision.

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This specification is licensed under Creative Commons Attribution 4.0 International (CC BY 4.0). You are completely free to adapt and share this rule in your workspace. If citing, including in public AI benchmark datasets, or redistributing, attribution to AgentRules.org is required.

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