Order types

Arcus runs a central limit order book (CLOB). You place orders against it with an order type and a time-in-force that together control how aggressively the order executes and how long it rests.

Order types

Time-in-force

Time-in-force (TIF) controls how long an order stays active: A few rules worth knowing at the concept level (the API enforces the exact constraints):

• Every order carries a goodTilTime expiry, set at least one month ahead — including IOC and FOK orders, where it acts as a replay-protection timestamp. See POST /v1/placeOrder for the exact rule.
• Market orders must use IOC.
• Order reads report a resting GTT order’s time-in-force as GTC, the legacy name for the same bucket; treat the two as equivalent.

Modifying orders and queue priority

A modifyOrder changes a resting order’s price or size. The order keeps its orderId, its createdAt and its fill state (filledSize, avgFillPrice).

• Size (quantity) is the new total, including any amount already filled. A total at or below the filled amount is rejected with MODIFY_SIZE_ALREADY_FILLED.
• Same price, smaller size — the order keeps its queue priority.
• Any price change, or a size increase — the order loses its queue priority and rejoins at the back of the queue at its (new) price.
• If the engine rejects the modified order (for example, an ALO modify that would cross, or MODIFY_SIZE_ALREADY_FILLED), the original order is removed too. A request that fails validation with HTTP 400 leaves the original order untouched.

On the orders channel, a modify arrives as two consecutive updates for the same orderId: CANCELED with cancelReason: "MODIFY_CANCELED", carrying the pre-modify price and size, then the order’s new state (OPEN, a fill, or REJECTED). Use these orders channel updates, not the HTTP response, for the outcome of a modify.

Take-profit / stop-loss

Take-profit and stop-loss (TP/SL) orders protect a position. They only close or reduce it, so they are always reduce-only (reduceOnly: true). A reduce-only order can only shrink a position, never flip or grow it. A TP/SL is a trigger order. It sets tpslType (STOP_LOSS or TAKE_PROFIT) and a trigger price, stopPrice. It sits dormant off the order book, reported with status UNTRIGGERED, until the mark price reaches stopPrice. Then it submits a regular order: a market leg (IOC) or a limit leg (GTT). A trigger price on the wrong side of the current mark price, one that would fire at once, is rejected.

• A stop-loss caps a loss. It sits below a long or above a short.
• A take-profit locks in a gain. It sits on the other side. A TAKE_PROFIT fires in the limit direction: a sell take-profit when the mark price rises to stopPrice, a buy take-profit when it falls to it. Unlike a limit order it stays off the book until it fires. A TAKE_PROFIT is always reduce-only.

You set TP/SL on an open position, or attach it to a new entry order. In the Arcus app, you set it on a position or add it when you place an order. A single POST /v1/placeOrder with tpslType set places one standalone TP/SL order. To bundle legs or bind them to an entry, use POST /v1/batchPlaceOrders with a TPSL grouping:

• positionTpsl closes the full open position. Each leg is resized to the whole position when it fires, so it still fits after you add to or trim the position.
• partialTpsl closes a specific quantity.
• entryTpsl is an entry order plus its bound TP/SL child legs. The legs arm on the entry’s first fill, including a fill against a liquidation, so they cannot fire while the entry is still unfilled. If the position closes while the entry still rests on the book, the legs stay attached and re-arm on the entry’s next fill. Canceling the entry cancels its legs.

Every TP/SL leg in a batch grouping sets reduceOnly: true.

Stop market and stop limit (advanced orders)

Stop market and stop limit orders are advanced orders. They are entry stops: they open a position, or add to one, rather than protect it. In the Arcus app they are under Advanced in the trade form. An entry stop is a STOP_LOSS order with reduceOnly: false, placed on its own with POST /v1/placeOrder. It uses the same trigger mechanism as TP/SL: it stays off the book as UNTRIGGERED until the mark price reaches stopPrice, then submits its order. Unlike TP/SL it is not reduce-only, so once triggered it opens or adds to a position in the trigger direction. Use it for a breakout or breakdown entry, for example to buy once the price rises through a resistance level. Only a STOP_LOSS can set reduceOnly: false. The reduce-only TP/SL above is unchanged. The order a stop submits is either a market or a limit order, and it follows the TIF rules for that type:

• Stop market (a market leg, IOC) fills at whatever the book offers once triggered. Its price is still a protective bound, checked against stopPrice under the 10% rule above.
• Stop limit (a limit leg, GTT) caps the price you accept. In a fast move the price can pass your limit, and the order then rests unfilled or partly filled.

Choose a market leg when getting in matters more than the price. Choose a limit leg when the price matters more than the fill.

Stops versus limit orders

A limit order and a stop wait for opposite moves: A limit order waits for a better price. A stop waits for the market to move through a level, then acts. A buy limit placed above the market is not a stop: it is marketable and fills at once. To buy only after the price rises through a level, you need a stop.