Post-Only Orders
Understand crypto post-only orders, maker-only intent, crossing prevention, rejection or repricing rules, queue risk and execution economics.
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A post-only instruction tells the venue that an order should add liquidity rather than immediately remove it. If the order would cross the book on arrival, the venue typically rejects, cancels or adjusts it according to its rules.
Maker intent as an order constraint
Without post-only, a limit order that crosses the opposite side can execute immediately and be classified as taker. Post-only adds a constraint: do not let this order take resting liquidity at acceptance.
A passive buy below the best ask will normally rest naturally. Post-only becomes important when prices move between quote calculation and order arrival, or when an algorithm continuously reprices close to the spread.
What if the order would cross?
| Possible venue rule | Outcome | Execution consequence |
|---|---|---|
| Reject | Order is not accepted | No fill; client must decide whether to resubmit |
| Cancel on entry | Venue accepts logic then cancels immediately | No resting order remains |
| Reprice/slide | Venue moves order to a non-crossing price | Price/queue position differs from submitted request |
| Venue-specific protection | Special maker-only semantics | Must be read from documentation |
Maker fee is only one part of execution quality
Maker rates can be lower than taker rates and may sometimes include rebates.
Passive liquidity often fills when incoming flow is willing to trade through your quote.
A post-only order can repeatedly miss while the market moves away.
Frequent cancel/repost behaviour can lose time priority and reduce fill probability.
The correct comparison is realised maker economics: fee/rebate plus price outcome after the fill, not fee schedule alone.
Worked example
Best bid is £79,990 and best ask is £80,000. You submit a post-only buy limit at £80,000.
Because that price would immediately match the resting ask, a venue with reject-on-cross post-only semantics rejects the order. You receive no fill and no maker queue position.
If instead you submit £79,990 and the order rests, it can qualify as maker if later hit. Suppose it fills and the mid-price falls to £79,900 seconds later. A small maker rebate does not compensate for a £90 adverse move.
Queue management and stale quotes
A post-only strategy often tries to remain close to the best bid or ask. Repricing every time the market moves can keep the quote visually competitive, but repeated cancel/repost activity normally resets time priority. A slightly less aggressive order with an older queue position can therefore fill before a newer order displayed at the same price.
There is also a stale-quote problem: by the time a post-only instruction reaches the venue, the opposite quote may have moved through it. The venue then rejects or slides the order instead of allowing an unintended taker fill. Latency therefore changes acceptance probability even when the strategy's intended price is valid at decision time.
Common mistakes and misunderstandings
- Assuming a post-only order is guaranteed to be accepted.
- Assuming post-only guarantees a fill at maker fees.
- Optimising for maker rebates while ignoring adverse selection.
- Repeatedly cancelling and reposting without considering lost queue priority.
- Not coding explicit handling for post-only rejection or venue repricing semantics.
Knowledge checkpoint
Q1. Why can a post-only buy at the current best ask be rejected?
Q2. What risk remains even after a post-only order successfully rests in the book?
Q3. Why can frequent repricing reduce fill probability on a price-time-priority venue?
Q4. Which metric would you examine alongside maker fee/rebate to assess realised passive execution quality?
FAQ
❓ Is post-only the same as a limit order?
It is usually an additional constraint on a limit order that prevents immediate liquidity-taking on entry.
❓ Will a post-only order always rest?
Not necessarily. If it would cross, the venue may reject, cancel or adjust it.
❓ Does post-only guarantee maker fees?
If it later executes as resting liquidity it is generally intended to qualify as maker, but venue fee rules should be verified.
❓ Why use post-only?
To enforce maker-only intent, commonly for strategies that explicitly value passive execution and maker fee treatment.
Summary
- Post-only prevents an order from intentionally taking liquidity on entry.
- Crossing post-only orders can be rejected, cancelled or repriced depending on venue rules.
- Maker fees do not remove adverse-selection, queue or non-fill risk.
- Evaluate realised passive execution using fees plus post-fill price behaviour.
This building block explains execution mechanics and risk. It is not a recommendation, signal or instruction to trade any cryptoasset.
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