# Bid Selection

After all eligible creatives are collected for an impression, the system selects the winning bid.

## Selection algorithm

1. **Filter eligible creatives**: Only creatives that pass the delivery ratio check (`ratio > random`) or have a CPC segment match are kept
2. **Random shuffle**: All eligible creatives are randomly shuffled (for fairness when prices are equal)
3. **Sort by price**: Creatives are sorted by effective CPM in descending order, with currency conversion applied
4. **Select winner**: The highest-priced creative wins the impression

## Price priority

The effective CPM for a creative is determined by the first available value:

1. **Segment CPM** (from CPC segment match) -- highest priority
2. **Split CPM** (from the split's `cpm` field, if > 0)
3. **Line item rate** (from `delivery.rate`) -- base price

## Currency conversion

All prices are converted to the bid request's target currency (from `bidRequest.cur`, default `USD`) before comparison. The conversion uses rates from the Prebid currency CDN, updated every hour.

If conversion fails for a creative (unknown currency), that creative is deprioritized in the sort.

## Bid response

The winning creative is returned as an OpenRTB 2.5 bid response:

```json
{
  "id": "response-uuid",
  "cur": "USD",
  "seatbid": [
    {
      "seat": "Advertiser Name",
      "bid": [
        {
          "id": "bid-uuid",
          "impid": "impression-id",
          "price": 3.50,
          "crid": "creative-id",
          "adm": "<ad markup>",
          "adomain": ["advertiser.com"],
          "w": 300,
          "h": 250,
          "dealid": "deal-external-id",
          "ext": {
            "lineItemId": 12345,
            "splitId": 1,
            "splitName": "Split A",
            "segmentId": 10
          }
        }
      ]
    }
  ]
}
```

### Bid ext fields

| Field        | Presence          | Description                        |
| ------------ | ----------------- | ---------------------------------- |
| `lineItemId` | Always            | The line item that won             |
| `splitId`    | When from a split | The split identifier               |
| `splitName`  | When from a split | The split name                     |
| `segmentId`  | When CPC          | The matched CPC segment identifier |

## No bid

If no creative passes all checks for any impression, the DSP returns HTTP 204 (No Content).


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