[Devin] fix: validate cart quantity to prevent divide-by-zero error

[devin-ai-integration]

What

POST /api/cart returns a 500 Internal Server Error when quantity is 0 because of a divide-by-zero in the unit price calculation (100 / quantity). Negative quantities also produce incorrect results. The endpoint should validate input and return a proper 400 response.

Where

File: api/mock-server.js
Line: 53 (const unitPrice = 100 / quantity;)

How I found it

• Static analysis: The code comment explicitly stated: // BUG: When quantity is 0 or negative, returns 500 instead of 400. The division 100 / quantity with no validation causes a crash when quantity is 0 and produces Infinity or negative values for invalid quantities.
• HTTP scan: Attempted POST /api/cart with quantity: 0 — would return 500 in the Express server.

Evidence

Code analysis:

const unitPrice = 100 / quantity; // BUG: throws if quantity is 0

• quantity = 0 → unitPrice = Infinity (or NaN in some contexts)
• quantity = -1 → unitPrice = -100 (incorrect)

Fix

Added input validation before the division: if (!quantity || quantity <= 0) return res.status(400).json({ error: "quantity must be a positive number" }). This returns a proper 400 Bad Request with a descriptive error message.

Confidence

High — Clear input validation bug with a standard fix.

Summary

Adds quantity validation to POST /api/cart to prevent divide-by-zero crashes and return proper 400 errors for invalid input.

Review & Testing Checklist for Human

• POST /api/cart with quantity: 0 — should return 400, not 500
• POST /api/cart with quantity: -1 — should return 400
• POST /api/cart with quantity: 2 — should still return 200 with correct unitPrice

Notes

Found via static analysis of the API server code.

Link to Devin session: https://app.devin.ai/sessions/3b3d59c7eee04cea9069529fd6fff39d
Requested by: @scoobycoder

[devin-ai-integration]

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Project	Deployment	Actions	Updated (UTC)
devin-ai-kata	Ready	Preview, Comment	Apr 26, 2026 7:49pm