AI Regression Testing Agent Skill

AI writes fix → AI reviews fix → AI says "looks correct" → Bug still exists

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 ✅

// 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 = "";

// __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 };
}

// __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);
    const { json } = await parseResponse(res);

    expect("notification_settings" in json.data).toBe(true);
    const ns = json.data.notification_settings;
    expect(ns === null || typeof ns === "object").toBe(true);
  });
});

// Test that sandbox responses match the expected contract
describe("GET /api/user/messages (conversation list)", () => {
  it("includes partner_name in sandbox mode", async () => {
    const req = createTestRequest("/api/user/messages", {
      sandboxUserId: "user-001",
    });
    const res = await GET(req);
    const { json } = await parseResponse(res);

    // This caught a bug where partner_name was added
    // to production path but not sandbox path
    if (json.data.length > 0) {
      for (const conv of json.data) {
        expect("partner_name" in conv).toBe(true);
      }
    }
  });
});

<!-- .claude/commands/bug-check.md -->
# Bug Check

## Step 1: Automated Tests (mandatory, cannot skip)

Run these commands FIRST before any code review:

    npm run test       # Vitest test suite
    npm run build      # TypeScript type check + build

- If tests fail → report as highest priority bug
- If build fails → report type errors as highest priority
- Only proceed to Step 2 if both pass

## Step 2: Code Review (AI review)

1. Sandbox / production path consistency
2. API response shape matches frontend expectations
3. SELECT clause completeness
4. Error handling with rollback
5. Optimistic update race conditions

## Step 3: For each bug fixed, propose a regression test

User: "バグチェックして" (or "/bug-check")
  │
  ├─ Step 1: npm run test
  │   ├─ FAIL → Bug found mechanically (no AI judgment needed)
  │   └─ PASS → Continue
  │
  ├─ Step 2: npm run build
  │   ├─ FAIL → Type error found mechanically
  │   └─ PASS → Continue
  │
  ├─ Step 3: AI code review (with known blind spots in mind)
  │   └─ Findings reported
  │
  └─ Step 4: For each fix, write a regression test
      └─ Next bug-check catches if fix breaks

// ❌ AI adds field to production path only
if (isSandboxMode()) {
  return { data: { id, email, name } };  // Missing new field
}
// Production path
return { data: { id, email, name, notification_settings } };

// ✅ Both paths must return the same shape
if (isSandboxMode()) {
  return { data: { id, email, name, notification_settings: null } };
}
return { data: { id, email, name, notification_settings } };

it("sandbox and production return same fields", async () => {
  // In test env, sandbox mode is forced ON
  const res = await GET(createTestRequest("/api/user/profile"));
  const { json } = await parseResponse(res);

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

// ❌ New column added to response but not to SELECT
const { data } = await supabase
  .from("users")
  .select("id, email, name")  // notification_settings not here
  .single();

return { data: { ...data, notification_settings: data.notification_settings } };
// → notification_settings is always undefined

// ✅ Use SELECT * or explicitly include new columns
const { data } = await supabase
  .from("users")
  .select("*")
  .single();

// ❌ Error state set but old data not cleared
catch (err) {
  setError("Failed to load");
  // reservations still shows data from previous tab!
}

// ✅ Clear related state on error
catch (err) {
  setReservations([]);  // Clear stale data
  setError("Failed to load");
}

// ❌ No rollback on failure
const handleRemove = async (id: string) => {
  setItems(prev => prev.filter(i => i.id !== id));
  await fetch(`/api/items/${id}`, { method: "DELETE" });
  // If API fails, item is gone from UI but still in DB
};

// ✅ Capture previous state and rollback on failure
const handleRemove = async (id: string) => {
  const prevItems = [...items];
  setItems(prev => prev.filter(i => i.id !== id));
  try {
    const res = await fetch(`/api/items/${id}`, { method: "DELETE" });
    if (!res.ok) throw new Error("API error");
  } catch {
    setItems(prevItems);  // Rollback
    alert("削除に失敗しました");
  }
};

Bug found in /api/user/profile     → Write test for profile API
Bug found in /api/user/messages    → Write test for messages API
Bug found in /api/user/favorites   → Write test for favorites API
No bug in /api/user/notifications  → Don't write test (yet)