Understanding Auctions on Bullish

An auction is a scheduled event where buy and sell orders are collected over a period of time and then matched all at once at a single clearing price. Unlike continuous trading, where orders match instantly as they arrive, the auction gathers all orders first and executes them together at the end. Bullish runs a daily auction for supported trading pairs at 4:00 PM ET.

The auction order book is fully separate from the continuous trading order book. Orders placed in the auction do not interact with orders on the continuous market, and vice versa.

The auction runs through four phases every day. Each phase has different rules about what you can do with your orders.

After the cross, the cycle repeats — accumulation begins again immediately for the next day's auction.

There are two types of auction orders. Both use Time in Force = GTX (Good Til Cross) and are kept in a separate order book from continuous trading.

Auction market order — An order to buy or sell at whatever the clearing price turns out to be. You do not specify a price. This gives you the highest chance of being filled, but you accept whatever price the auction determines.

Auction limit order — An order to buy or sell only if the clearing price meets your limit. For example, if you place an auction limit buy at 70,000 USDC, your order will only fill if the clearing price is 70,000 or below. This gives you price control, but your order may go unfilled if the clearing price moves beyond your limit.

Self-trade prevention applies to auction orders. The system checks for potential self-trades at the time you submit your order. If a conflict is detected, the incoming order is rejected and your existing order remains unchanged. For full details, see Understanding Self-Trade Prevention.

Bullish applies two layers of price protection to keep auction prices fair.

Price band (at order entry) — When you submit an auction limit order, the system checks that your limit price is within a reasonable range of the current Index Price. If your price is too far away, the order is rejected. This is the same price band check that applies to limit orders on the continuous order book.

1. priceBuffer = 10%

2. At the price entry, index price = 100,

   1. Max buy limit price = 100 * (1 + 10%) = 110

   2. Min sell limit price = 100 * (1 - 10%) = 90

Price collar (at the cross) — Even if your auction limit order was accepted at entry, the system applies a second check at cross time. If the Index Price has moved and your limit price now falls outside the collar range (Index Price +/- price collar percentage), your order is excluded from the cross. These orders receive an "Auction order excluded by price collar" status.

1. auctionPriceCollar = 5%

2. At the cross, index price = 100,

   1. final matched price will be capped between 95 - 105 and ignore all the price beyond the range.

To check the priceBuffer & auctionPriceCollar, you can refer to market API endpoint.

The system tests every auction limit price level in the order book and selects the price that produces the highest paired quantity — the total volume that can be matched between eligible buy and sell orders at that price. Auction market orders are eligible at every price level since they accept any price.

If multiple price levels produce the same paired quantity, the system applies tie-breakers in order:

These tie-breakers are applied in sequence, so a single clearing price is always produced.

During the 5-minute Lockdown phase, the system broadcasts an update every 10 seconds to help you assess the auction before the cross. Each update includes:

• Indicative price (Far price) — The price the auction would clear at if the cross happened right now.

• Paired quantity — The predicted matched volume at the indicative price.

• Imbalance direction — Whether there are more buyers or sellers (BUY, SELL, or NONE) at the indicative price (Far Price).

• Imbalance quantity — The size of the imbalance.

• Reference price — The last traded price on the continuous order book.

You can use this information to decide whether to submit additional orders to take advantage of the imbalance before the cross.

When there is more volume on one side than the other, not all orders can be fully filled. The system allocates fills in the following order:

1. Auction market orders first, by submission time (earliest first).

2. Auction limit orders next, by best price first, then by submission time.

This means auction market orders have a higher chance of being filled compared to auction limit orders. Any unfilled orders or remaining quantity from partially filled orders are automatically canceled after the cross.

After the cross executes, every auction order receives one of the following outcomes:

• Filled — Your entire order was matched at the clearing price.

• Partially filled — Part of your order was matched. The remaining quantity was not matched.

• Unfilled — Your order received no match.

• Unfilled (outside price collar) — Your auction limit order was excluded because its price fell outside the collar range. This does not apply to auction market orders.

All unfilled or partially filled orders are automatically closed after the cross. They do not carry over to the next auction.

Reference price: 99,000 USDC (last traded price on the continuous order book).

Orders submitted:

NOII during Lockdown:

At the cross (4:00 PM EST) — Clearing price: 101,000 USDC

Auction orders can be submitted via REST API, FIX, or the Web UI. For full API documentation including endpoints, request schemas, and WebSocket topics, see the Bullish API Documentation.

For a step-by-step guide on how to place auction orders and monitor the auction on the Bullish website, see How to view & trade auctions on Bullish.

At order entry, the exchange reserves funds (non-margin accounts) or available margin (margin-enabled accounts) to cover the worst-case clearing price. The reservation uses the Index Price plus a 10% buffer — the same Auction Price Collar Threshold used at the cross.

Buys lock quote currency; sells lock the base asset being delivered.

The 10% buffer only applies to MOC buys, because there is no limit price to anchor the lock.

30-minute repricing check (MOC buys only). Every 30 minutes the system rechecks that the locked quote currency still covers the current Index Price × 1.10. If not, the order is cancelled and the funds released. LOC orders and MOC sells are not subject to this check.

An auction order has two effects on your margin at order entry:

1. Available margin decrease — a buffer reserved on top of the order's normal margin requirement, symmetric for buy/sell and identical for LOC/MOC:

2. Spot margin requirement — the unfilled order also contributes to your overall BPM margin requirement. The orderPrice used to value it depends on the order type:

Margin scenario shifts (spot up/down, vol shifts, etc.) continue to use the Index Price.