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)

// 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, "."),
    },
  },
});

// __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

// __tests__/helpers.ts
import { NextRequest } from "next/server";

export function createTestRequest(
  url: string,
  options?: {
    method?: string;
    body?: Record<string, unknown>;
    headers?: Record<string, string>;
    sandboxUserId?: string;
  },
): NextRequest {
  const { method = "GET", body, headers = {}, sandboxUserId } = options || {};
  const fullUrl = url.startsWith("http") ? url : `http://localhost:3000${url}`;
  const reqHeaders: Record<string, string> = { ...headers };

  if (sandboxUserId) {
    reqHeaders["x-sandbox-user-id"] = sandboxUserId;
  }

  const init: { method: string; headers: Record<string, string>; body?: string } = {
    method,
    headers: reqHeaders,
  };

  if (body) {
    init.body = JSON.stringify(body);
    reqHeaders["content-type"] = "application/json";
  }

  return new NextRequest(fullUrl, init);
}

export async function parseResponse(response: Response) {
  const json = await response.json();
  return { status: response.status, json };
}

Writing Regression Tests

The key principle: write tests for bugs that were found, not for code that works.

// __tests__/api/user/profile.test.ts
import { describe, it, expect } from "vitest";
import { createTestRequest, parseResponse } from "../../helpers";
import { GET, PATCH } from "@/app/api/user/profile/route";

// Define the contract — what fields MUST be in the response
const REQUIRED_FIELDS = [
  "id",
  "email",
  "full_name",
  "phone",
  "role",
  "created_at",
  "avatar_url",
  "notification_settings",  // ← Added after bug found it missing
];

describe("GET /api/user/profile", () => {
  it("returns all required fields", async () => {
    const req = createTestRequest("/api/user/profile");
    const res = await GET(req);
    const { status, json } = await parseResponse(res);

    expect(status).toBe(200);
    for (const field of REQUIRED_FIELDS) {
      expect(json.data).toHaveProperty(field);
    }
  });

  // Regression test — this exact bug was introduced by AI 4 times
  it("notification_settings is not undefined (BUG-R1 regression)", async () => {
    const req = createTestRequest("/api/user/profile");
    const res = await GET(req);

…