Stop AI Code Slop: Engineering High-Signal .cursorrules and MDC Directives for Teams

AI-assisted coding with Cursor, Windsurf, and Claude Code has permanently changed engineering velocity. But without disciplined architectural constraints, team codebases rapidly accumulate "AI slop" — subtle code smells, hallucinated APIs, and inconsistent patterns.

The Mechanics of AI Drift in Codebases

Large language models are probabilistic token predictors trained on vast historical snapshots of GitHub repositories. When left unguided:

• They favor obsolete patterns: Prompting for a Next.js component often results in legacy Pages Router code (getStaticProps) rather than React Server Components.
• They take the path of least resistance: When TypeScript complains about an intricate union type, the AI frequently bypasses the problem by injecting as any or // @ts-ignore.
• They litter production code with noise: Unprompted models routinely leave console.log() statements and empty error catch blocks scattered across services.

Legacy .cursorrules vs. Modern .cursor/rules/*.mdc

In early versions of Cursor, developers placed a single monolithic .cursorrules file in the repository root. While helpful, monolithic rules suffer from context saturation: backend instructions waste context tokens when editing CSS files, and frontend styling directives confuse backend database queries.

The modern Cursor 2024+ format introduces MDC (Markdown with Context) rules stored in .cursor/rules/*.mdc. These files feature YAML frontmatter with file glob filters:

---
description: React Server Components and Next.js 15 routing rules
globs: app/**/*.tsx, components/**/*.tsx
alwaysApply: false
---

# Next.js 15 Architecture Directives
- Default to React Server Components (RSC). Only declare 'use client' when state or browser event listeners are strictly required.
- Colocate data queries using async Server Components; use Server Actions for mutations.
- NEVER use 'any' or '@ts-ignore'. Provide strict discriminated union interfaces.

The Four Pillars of High-Signal AI Rules

1. Strict Negative Constraints: Explicitly state what the AI is forbidden from doing. Prohibitions like "NEVER import from 'lodash' when native Array methods exist" or "NEVER catch exceptions without logging structured context" are more effective than open-ended advice.
2. Architectural Scaffolding: Define your layer boundaries (e.g. "Controllers must only call Services; Services must only interact with Repositories; Models are strictly declarative schemas").
3. Banned Dependency Injections: Stop the AI from hallucinating or introducing unapproved npm packages when your monorepo already has preferred utilities.
4. Testing Requirements: Require every new exported function or API route handler to be accompanied by a companion unit test fixture in the same PR.

By treating prompt engineering as an infrastructure discipline, engineering teams maintain clean architecture while capturing the 5x velocity benefits of AI coding assistants.