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Perpetual futures

Post-only and reduce-only

Updated 2026-10-02 · 6 min read · Not financial advice

Ticket diagram of post-only, which rejects an order that would take liquidity, and reduce-only, which rejects an order that would open a new position.

Post-only protects your fee role. Reduce-only protects your position direction. You often want both on different orders, and you should not confuse them.

Post-only

Post-only means the order may enter the book as a maker and must not cross the spread. If the price you typed would trade immediately, the venue rejects the order or reprices it, according to its rule. You then get no fill, which is the point: you refused to pay the taker fee by accident.

Use it on entries where missing the trade is acceptable and paying the spread is not. Do not use it on an exit you need during a fast move. The reject is how the flag protects you, and an exit that gets rejected is still an open position.

Reduce-only

Reduce-only means the order may shrink a position and must not grow one or open the other side. If you are long 1, a reduce-only sell of 1.5 sells 1 and discards the rest, or is rejected beyond the position, according to the venue. If you are flat, a reduce-only order does nothing useful and is rejected.

In one-way mode this flag is the difference between closing a long and accidentally opening a short. In hedge mode it still matters, because an order aimed at the wrong side would otherwise add size. Put it on stops and on take-profit limits.

They compose

A post-only reduce-only limit is a passive exit: it will only rest, and it will only shrink the position. If the market runs away, it rests behind and does not chase. That is a good take-profit and a bad emergency stop.

A stop-market is neither post-only nor patient. Add reduce-only to it. Leave post-only off. The combination matches the job.

On the ticket

  1. On passive entries, turn post-only on and read the reject rule so you know whether the order disappears or reprices.
  2. On every exit, turn reduce-only on. Confirm the preview says the position will shrink.
  3. Leave post-only off on any order that must trade in a hurry.
  4. If a reduce-only order is rejected while you are flat, you are done. Do not remove the flag to force a fill.

Questions

Will post-only save me from slippage?

It saves you from crossing the spread. It does not get you a fill. Slippage is what you pay when you insist on the fill.

Does reduce-only lower the fee?

No. It changes the allowed position effect. The fee still follows maker or taker on the fill that happens.

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