At first glance, this looked like a simple problem.
Check balance → deduct amount → done.
But I added one small thing:
An async delay.
And everything broke.
🚨 The Problem
The logic seems correct:
- If balance is enough → process order
- Deduct amount
- Return success
But here’s the issue:
Multiple requests can run at the same time.
And they all see the same balance.
🧠 What Happens in Reality
If 3 requests come together:
Balance = 100
Each request checks → “Yes, enough balance”
Then all of them deduct.
Now balance becomes negative.
This is called a race condition.
🤯 What I Observed
When AI models tried this:
- Some ignored concurrency
- Some gave correct logic but unsafe system
- Very few handled real-world consistency
The code works.
But under load, it breaks.
🔥 Try It Yourself
I created this challenge to test real backend thinking.
👉 Try it here:
https://vibecodearena.ai/duel/b1d5e8f8-a24f-41dc-bf9d-0170142d1fa7
Can you:
- Fix race condition?
- Ensure safe balance updates?
- Design a reliable system?
💡 Final Thought
Bugs don’t appear in single requests.
They appear under pressure.
And that’s where real engineering starts.
Would you trust AI with concurrency-heavy systems?
Let’s discuss 👇
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