---
name: ai-regression-testing
description: Prevents AI-introduced regressions with sandbox-mode API testing, automated bug-check workflows, and patterns that catch blind spots where the same model writes and reviews code.
---

# 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<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**.

```typescript
// __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);
  });
});
```

### Testing Sandbox/Production Parity

The most common AI regression: fixing production path but forgetting sandbox path (or vice versa).

```typescript
// 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);
      }
    }
  });
});
```

## Integrating Tests into Bug-Check Workflow

### Custom Command Definition

```markdown
<!-- .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
```

### The Workflow

```
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
```

## Common AI Regression Patterns

### Pattern 1: Sandbox/Production Path Mismatch

**Frequency**: Most common (observed in 3 out of 4 regressions)

```typescript
// FAIL: 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 } };

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

**Test to catch it**:

```typescript
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);
  }
});
```

### Pattern 2: SELECT Clause Omission

**Frequency**: Common with Supabase/Prisma when adding new columns

```typescript
// FAIL: 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

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

### Pattern 3: Error State Leakage

**Frequency**: Moderate — when adding error handling to existing components

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

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

### Pattern 4: Optimistic Update Without Proper Rollback

```typescript
// FAIL: 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
};

// PASS: 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");
  }
};
```

## Strategy: Test Where Bugs Were Found

Don't aim for 100% coverage. Instead:

```
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)
```

**Why this works with AI development:**

1. AI tends to make the **same category of mistake** repeatedly
2. Bugs cluster in complex areas (auth, multi-path logic, state management)
3. Once tested, that exact regression **cannot happen again**
4. Test count grows organically with bug fixes — no wasted effort

## Quick Reference

| AI Regression Pattern | Test Strategy | Priority |
|---|---|---|
| Sandbox/production mismatch | Assert same response shape in sandbox mode |  High |
| SELECT clause omission | Assert all required fields in response |  High |
| Error state leakage | Assert state cleanup on error |  Medium |
| Missing rollback | Assert state restored on API failure |  Medium |
| Type cast masking null | Assert field is not undefined |  Medium |

## DO / DON'T

**DO:**
- Write tests immediately after finding a bug (before fixing it if possible)
- Test the API response shape, not the implementation
- Run tests as the first step of every bug-check
- Keep tests fast (< 1 second total with sandbox mode)
- Name tests after the bug they prevent (e.g., "BUG-R1 regression")

**DON'T:**
- Write tests for code that has never had a bug
- Trust AI self-review as a substitute for automated tests
- Skip sandbox path testing because "it's just mock data"
- Write integration tests when unit tests suffice
- Aim for coverage percentage — aim for regression prevention