What is a "post-only" order and what is its benefit?

Understanding Post-Only Orders

1. A post-only order is an instruction to place a limit order that will only be added to the order book and never execute immediately against existing orders.

2. If the price of the post-only order matches or improves upon the best available counterparty price, the exchange rejects the order instead of filling it.

3. This mechanism ensures the trader acts strictly as a liquidity provider, not a liquidity taker.

4. Exchanges enforce this behavior by checking the order’s potential execution path before acceptance.

5. The order remains passive until market prices move to meet its specified level, at which point it may begin matching with incoming aggressive orders.

Liquidity Provision Incentives

1. Many cryptocurrency exchanges offer reduced or zero maker fees for orders that add depth to the order book.

2. Post-only orders qualify automatically for maker fee treatment because they never remove liquidity upon submission.

3. Traders who consistently supply resting bids and asks earn fee rebates or tiered discounts based on their 30-day trading volume.

4. Some platforms extend additional benefits such as priority queue positioning in the price level or enhanced API rate limits for verified liquidity providers.

5. These incentives encourage tighter spreads and deeper markets, especially during low-volatility periods.

Risk Mitigation Features

1. Post-only orders prevent accidental market impact caused by unintentional aggressive execution.

2. During flash crashes or rapid price dislocations, traders avoid slippage by ensuring their orders remain passive regardless of short-term volatility.

3. They eliminate the possibility of partial fills at undesirable prices when placing large limit orders near the top of the book.

4. Arbitrageurs use them to maintain precise entry points without triggering cascading liquidations across correlated assets.

5. Algorithmic strategies relying on time-weighted average pricing incorporate post-only logic to preserve execution discipline amid fragmented venue landscapes.

Exchange-Specific Implementation Variants

1. Binance applies post-only logic exclusively to spot and futures limit orders but excludes stop-market and trailing-stop types.

2. Bybit enforces strict rejection with error code “PostOnlyOrderWouldMatch” if any portion would cross the spread upon placement.

3. Kraken implements a hybrid model where post-only orders convert to regular limit orders after initial rejection, provided no immediate match exists within 100ms.

4. OKX allows conditional activation via “Reduce-Only” flags alongside post-only constraints for hedging workflows.

5. Deribit restricts post-only usage to option contracts only, citing gamma exposure concerns in perpetual futures markets.

Frequently Asked Questions

Q: Can a post-only order ever become a taker order after being placed?A: No. Once accepted into the order book, it remains a maker order until fully executed or canceled. Its classification does not change even if market prices move through its level.

Q: Do post-only orders appear in the public order book immediately upon submission?A: Yes. They are published as visible resting orders unless the exchange supports hidden or iceberg functionality, which operates independently of post-only status.

Q: Is there a minimum size requirement for post-only orders on major exchanges?A: Most platforms impose no special minimums beyond standard lot size rules, though some require adherence to tick size precision for price fields to qualify.

Q: What happens if network latency causes a post-only order to arrive after a price shift?A: Execution eligibility depends solely on server-side timestamping and order book state at ingestion time—not client-side clock synchronization or transmission duration.