External API calls inside loops use Promise.all batching

Sequential await fetch( inside a loop over a non-trivial collection is one of the most common AI-generated code patterns that degrades gracefully in testing and catastrophically in production. Each awaited call holds a thread-of-execution and an HTTP connection open until it resolves; a list of 200 items becomes 200 serialized round-trips, inflating p99 latency from 200ms to 40 seconds. The pattern also prevents proper error isolation — a single slow upstream response blocks everything behind it. On metered hosting (Vercel function duration, Lambda invocation time), sequential loops inflate compute costs linearly with list size.

Replace the sequential loop with Promise.all or pMap for bounded parallelism. In src/lib/sync.ts or whichever file contains the offending pattern:

// Bad: 200 items × 200ms = 40 seconds
for (const user of users) {
  await fetch(`/api/sync/${user.id}`)
}

// Good: 200 items at concurrency 10 ≈ 4 seconds
import pMap from 'p-map'
await pMap(
  users,
  async (user) => fetch(`/api/sync/${user.id}`, { signal: AbortSignal.timeout(5000) }),
  { concurrency: 10 },
)

Always pair with input length validation so the array itself is bounded before any parallelism runs.

• ID: external-api-loops-batched

• Severity: high

• What to look for: Walk source files for await fetch(, await axios., await ky., await got( calls inside for/while/forEach/.map(/.filter(/.reduce( loops. For each match, count all loop-bound external API call sites and verify the call is inside Promise.all([...]) or Promise.allSettled([...]) OR is intentionally rate-paced. EXCLUDE calls with a setTimeout / sleep / delay between them.

• Pass criteria: 100% of loop-bound external API calls use Promise.all/Promise.allSettled OR have intentional pacing. Report: "X loop-bound API call sites, Y batched, 0 sequential."

• Fail criteria: At least 1 sequential await fetch( in a loop with no batching or pacing.

• Skip (N/A) when: No external API call patterns found.

• Detail on fail: "1 sequential loop fetch: src/lib/sync.ts loops 'for (const u of users) { await fetch(\\/api/sync/${u.id}\) }' — N+1 latency"

• Remediation: Sequential awaits in loops are slow AND wasteful. Batch with bounded concurrency:

  // Bad: serial
  for (const user of users) {
    await fetch(`/api/sync/${user.id}`)
  }
  
  // Good: parallel with concurrency cap
  import pMap from 'p-map'
  await pMap(users, async (u) => fetch(`/api/sync/${u.id}`), { concurrency: 10 })