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Vibe codingHardvc-g191
Subject Ai code reviewLevel Senior–Staff~22 minCommon in Networking & APIs interviewsIndustries Software development

Question

An AI wrote this Express payment endpoint and added an idempotency check so retries don't double-charge. Review whether the dedup actually holds under concurrent retries.

typescript
app.post('/charge', async (req, res) => {  const key = req.header('Idempotency-Key');  const existing = await db.charges.findByKey(key);  if (existing) return res.json(existing);   const charge = await processor.charge(req.body.amount);  await db.charges.insert({ key, id: charge.id, amount: req.body.amount });  res.json(charge);});

A flaky client retries fast. Why can it still double-charge, and how do you make the key truly idempotent?

What a strong answer looks like

Treat the AI’s output as a draft to verify, not an answer to trust. Name the specific flaw and the input that triggers it, say how you’d catch it — tests, edge cases, reading critically — and how you’d re-prompt or decompose to get it right.

Learn the concepts
Describe your solution

Vibe coding: describe the solution in plain language (or narrate it) and the coach grades your approach. Generating runnable code from your description is coming next.

Run or narrate your approach, then ask the coach.