backend-rules

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• SKILL.md
• skill.yaml

SKILL.md (first lines)

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# Backend Rules

Every API route must pass this checklist before it ships. These rules are drawn from post-incident reviews on production systems.

---

## The Non-Negotiable Order

Every route handler must execute in this order, without exception:

1. **Rate limit check** — before any auth or business logic
2. **Auth verification** — `getUser()`, never `getSession()` alone
3. **Input validation** — Zod schema before touching the database
4. **Authorization** — check ownership/permission, not just authentication
5. **Business logic** — only if all above pass
6. **Response** — structured, never leaking internals

(BLOCKING — any deviation requires explicit justification in PR)

---

## Zod Input Validation in API Routes

```typescript
// app/api/orders/route.ts
import { z } from 'zod'
import { createServerClient } from '@supabase/ssr'
import { NextRequest } from 'next/server'

const createOrderSchema = z.object({
  branchId: z.string().uuid(),
  items: z.array(z.object({
    productId: z.string().uuid(),
    quantity: z.number().int().positive().max(100),
  })).min(1),
  notes: z.string().max(500).optional(),
})

export async function POST(request: NextRequest) {
  // 1. Rate limit (see section below)
  const ip = request.headers.get('x-forwarded-for') ?? 'unknown'
  const { success: rateLimitOk } = await orderRatelimit.limit(ip)
  if (!rateLimitOk) {
    return Response.json({ error: 'Too many requests' }, { status: 429 })
  }

  // 2. Auth
  const supabase = createServerClient(/* cookies */)
  const { data: { user }, error: authError } = await supabase.auth.getUser()
  if (!user || authError) {
    return Response.json({ error: 'Unauthorized' }, { status: 401 })
  }

  // 3. Input validation
  let body: unknown
  try {
    body = await request.json()
  } catch {
    return Response.json({ error: 'Invalid JSON' }, { status: 400 })
  }

  const result = createOrderSchema.safeParse(body)
  if (!result.success) {
    return Response.json(
      { error: 'Validation failed', details: result.error.flatten().fieldErrors },
      { status: 400 }
    )
  }
  const input = result.data

  // 4. Authorization — verify user has access to this branch
  const { data: membership } = await supabase
    .from('branch_members')
    .select('role')
    .eq('branch_id', input.branchId)
    .eq('user_id', user.id)
    .single()

  if (!membership) {
    return Response.json({ error: 'Not found' }, { status: 404 })
    // Return 404, not 403 — don't reveal resource existence
  }

  // 5. Business logic
  try {
    const order = await createOrder(input, user.id)
    return Response.json(order, { status: 201 })
  } catch (err) {
    // 6. Error handling — log full error server-side, return generic to client
    console.error('[POST /api/orders]', err)
    return Response.json({ error: 'Internal server error' }, { status: 500 })
  }
}
```

---

## Supabase auth.getUser() Pattern

```typescript
// lib/auth.ts — shared helper for all route handlers
import { cookies } from 'next/headers'
import { createServerClient } from '@supabase/ssr'

export async function getAuthenticatedUser() {
  const cookieStore = await cookies()

  const supabase = createServerClient(
    process.env.SUPABASE_URL!,
    process.env.SUPABASE_ANON_KEY!,
    {
      cookies: {
        getAll: () => cookieStore.getAll(),
        setAll: (cookiesToSet) => {
          cookiesToSet.forEach(({ name, value, options }) =>
            cookieStore.set(name, value, options)
          )

From Nadav011/apex-skills, MIT.

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