The core logic of a Pos t Only order is "add liquidity only, never fill immediately." If you place a Post Only order but it would be a taker, it won't execute as a taker; instead, the system will cancel it directly.
Here are the key scenarios where a Post Only order is canceled.
Step 1: Check if Your Order Price "Overlaps" with the Order Book
The main reason a Post Only order gets canceled is that your quote conflicts with existing orders on the market—meaning if the order were to be placed, it would fill immediately.
What to do: Check whether your order price "crosses" the current best opposite side of the book.
How to do it:
Scenario: Your price is too "good"
When buying: If your limit buy price is higher than or equal to the current best ask, the Post Only order will be canceled.
When selling: If your limit sell price is lower than or equal to the current best bid, the Post Only order will be canceled.
The logic behind it: The purpose of Post Only is to "contribute" to the order book and increase depth. When your price can directly take the existing orders in the book, you become a "taker," which goes against the design of Post Only. Therefore, the system directly rejects the order rather than converting it into a taker fill.
When is it completed: You can confirm that your price is set to "wait in the book for someone to take," not to "immediately take from the book."
Common reason for failure: In a fast-moving market, trying to "grab" a buy or sell at the current best price while having Post Only enabled causes the order to be submitted and immediately canceled.
Step 2: Check if the Order Triggers "Immediate Execution" Logic
Even if the price is appropriate, the order can be canceled if some of its attributes contain an "immediate" nature.
What to do: Confirm you aren't using any order combination that could be "immediately filled."
How to do it:
Perpetual futures case: If the user has enabled the Retail Price Improvement (RPI) feature for futures, its order type is essentially Post Only as well; it ensures the order only adds liquidity and never executes as a taker.
During system upgrades: During specific exchange system upgrades (such as coin-margined contract upgrades), the market may enter a "cancel-only" phase, and any Post Only orders you submit or modify will be restricted.
Trying a partial fill: A Post Only order does not support an "immediate fill of partial quantity." If you want both a maker portion and an immediate fill portion, the Post Only mechanism does not support this hybrid mode and will cancel the order to ensure its purity.
When is it completed: You have confirmed that in the current trading context (standard spot or specific contract mode) the Post Only function is normally enabled.
How to Verify Your Operation is Correct?
When your Post Only order is canceled, you can check the specific reason in the "Order History" or "Cancellation Record" at the bottom of the trading interface. If you see a message like "Post Only order would be filled immediately, order cancelled," it means the cancellation was the expected behavior.
If you must buy or sell at that price, we suggest unchecking "Post Only" and using a regular limit order or a market order, but be aware that you will pay a higher taker fee.
