ai-regression-testing

# AI Regression Testing Testing patterns specifically designed for AI-assisted development, where the same model writes code and reviews it — creating systematic blind spots that only automated tests can catch. ## When to Activate - AI agent (Claude Code, Cursor, Codex) has modified API routes or backend logic - A bug was found and fixed — need to prevent re-introduction - Project has a sandbox/mock mode that can be leveraged for DB-free testing - Running `/bug-check` or similar review commands after code changes - Multiple code paths exist (sandbox vs production, feature flags, etc.) ## The Core Problem When an AI writes code and then reviews its own work, it carries the same assumptions into both steps. This creates a predictable failure pattern: ``` AI writes fix → AI reviews fix → AI says "looks correct" → Bug still exists ``` **Real-world example** (observed in production): ``` Fix 1: Added notification_settings to API response → Forgot to add it to the SELECT query → AI reviewed and missed it (same blind spot) Fix 2: Added it to SELECT query → TypeScript build error (column not in generated types) → AI reviewed Fix 1 but didn't catch the SELECT issue Fix 3: Changed to SELECT * → Fixed production path, forgot sandbox path → AI reviewed and missed it AGAIN (4th occurrence) Fix 4: Test caught it instantly on first run PASS: ``` The pattern: **sandbox/production path inconsistency** is the #1 AI-introduced regression. ## Sandbox-Mode API Testing Most projects with AI-friendly architecture have a sandbox/mock mode. This is the key to fast, DB-free API testing. ### Setup (Vitest + Next.js App Router) ```typescript // vitest.config.ts import { defineConfig } from "vitest/config"; import path from "path"; export default defineConfig({ test: { environment: "node", globals: true, include: ["__tests__/**/*.test.ts"], setupFiles: ["__tests__/setup.ts"], }, resolve: { alias: { "@": path.resolve(__dirname, "."), }, }, }); ``` ```typescript // __tests__/setup.ts // Force sandbox mode — no database needed process.env.SANDBOX_MODE = "true"; process.env.NEXT_PUBLIC_SUPABASE_URL = ""; process.env.NEXT_PUBLIC_SUPABASE_ANON_KEY = ""; ``` ### Test Helper for Next.js API Routes ```typescript // __tests__/helpers.ts import { NextRequest } from "next/server"; export function createTestRequest( url: string, options?: { method?: string; body?: Record; headers?: Record; sandboxUserId?: string; }, ): NextRequest { const { method = "GET", body, headers = {}, sandboxUserId } = options || {}; const fullUrl = url.startsWith("http") ? url : `http://localhost:3000${url}`; const reqHeaders: Record = { ...headers }; if (sandboxUserId) { reqHeaders["x-sandbox-user-id"] = sandboxUserId; } const init: { method: string; headers: Record; body?: string } = { method, headers: reqHeaders, }; if (bod