Bid Extension (Anti-Sniping)

Technical documentation for the bid extension protection feature that automatically extends auction item deadlines when late bids arrive.

Database Schema

Auction Model (defaults)

model Auction {
  defaultAntiSnipe             Boolean  @default(false)
  defaultAntiSnipeThreshold    Int      @default(300)  // 60-3600 seconds
  defaultAntiSnipeExtension    Int      @default(300)  // 5-3600 seconds
}

AuctionItem Model (per-item)

model AuctionItem {
  antiSnipeEnabled          Boolean   @default(false)
  antiSnipeThresholdSeconds Int       @default(300)  // 60-3600 seconds
  antiSnipeExtensionSeconds Int       @default(300)  // 5-3600 seconds
}

Validation Rules

Both frontend and backend enforce these ranges via Zod schemas:

Backend Logic

The core anti-snipe logic lives in src/lib/services/bid.service.ts inside the placeBid() function:

1. After creating a bid, check if item.antiSnipeEnabled is true and item.endDate exists
2. Calculate milliseconds until end: endTime - now
3. If the bid is within the threshold window (msUntilEnd > 0 && msUntilEnd <= thresholdMs), compute a new end date
4. Update the item’s endDate in the database
5. Include newEndDate in the BidNewEvent realtime event

if (item.antiSnipeEnabled && item.endDate) {
  const now = new Date();
  const endTime = new Date(item.endDate);
  const msUntilEnd = endTime.getTime() - now.getTime();
  const thresholdMs = item.antiSnipeThresholdSeconds * 1000;
 
  if (msUntilEnd > 0 && msUntilEnd <= thresholdMs) {
    newEndDate = new Date(
      endTime.getTime() + item.antiSnipeExtensionSeconds * 1000,
    );
  }
}

Realtime Events

The BidNewEvent interface includes an optional newEndDate field:

interface BidNewEvent {
  // ... existing fields
  newEndDate?: string; // ISO string, set when anti-snipe extends end time
}

The item detail page listens for this field and updates the displayed end date in real-time via SWR mutation.

API Endpoints

Item Create/Update

Anti-snipe fields are optional in both create and update payloads:

{
  "antiSnipeEnabled": true,
  "antiSnipeThresholdSeconds": 300,
  "antiSnipeExtensionSeconds": 300
}

Auction Update

Auction-level defaults use different field names:

{
  "defaultAntiSnipe": true,
  "defaultAntiSnipeThreshold": 300,
  "defaultAntiSnipeExtension": 300
}

Inheritance Flow

1. Auction owner sets defaults in auction settings
2. When creating a new item, the create form pre-fills with auction defaults
3. Item owner can override per-item
4. At bid time, only the item-level values are checked

Files Modified