Time in Force

GTC, IOC, FOK, and Post-Only explained.

Time in Force (TIF) controls how long an order remains active and what happens to any unfilled quantity. Vela supports four TIF options, each suited to different trading strategies.

GTC — Good Till Cancelled

The default. A GTC order rests in the order book until it is fully filled or you explicitly cancel it.

Behavior:

• If the order can match immediately, it fills (partially or fully)
• Any unfilled remainder stays in the book indefinitely
• The order persists across trading sessions and server restarts
• You must actively cancel the order to remove it

When to use GTC:

• Standard limit orders where you’re willing to wait
• Setting target entry or exit prices
• Market making (quoting two-sided with resting limit orders)

Example: You place a GTC limit buy for 1 ETH at 3000 USDC when the market is trading at 3200. The order rests at the bottom of the book. Days later, if ETH drops to 3000, the order fills.

GTC orders count against your credit utilization if you are a market maker. Resting orders reserve both balance and credit quota. See MM Credit System.

IOC — Immediate or Cancel

An IOC order executes as much as possible immediately, then cancels any unfilled remainder. It never rests in the book.

Behavior:

• Matches against available orders at the specified price or better
• Any quantity that cannot fill immediately is cancelled
• Executes in a single engine pass — no waiting
• May result in zero fills if no matching orders exist

When to use IOC:

• When you want to take available liquidity at a price without leaving a resting order
• Algorithmic strategies that want precise control over order lifetime
• Avoiding the risk of a limit order filling later at an unfavorable time

Example: ETH is trading at 3200, with 0.5 ETH available at that price. You place an IOC limit buy for 1.0 ETH at 3200. The 0.5 ETH available fills; the remaining 0.5 ETH is immediately cancelled. You end up with 0.5 ETH rather than a resting order.

FOK — Fill or Kill

An FOK order must fill completely and immediately, or it is cancelled entirely. There are no partial fills.

Behavior:

• The engine checks if the full quantity can fill immediately
• If yes: the order executes fully in one atomic operation
• If no: the entire order is cancelled, zero fills occur

When to use FOK:

• When you need a specific quantity and partial fills are not acceptable
• Strategies that require an all-or-nothing guarantee
• Avoiding partial position building that complicates risk management

Example: You need exactly 2.0 ETH and place a FOK buy at market. The book has 1.5 ETH available at the current price. Since the full 2.0 ETH cannot fill immediately, the order is killed with zero fills.

FOK orders can fail silently if the book doesn’t have sufficient depth. Make sure to check the order book before relying on FOK for large orders.

Post-Only

A Post-Only order is guaranteed to be a maker order. If the order would match immediately as a taker, it is rejected. Only orders that will rest in the book are accepted.

Behavior:

• Accepted only if it would not immediately match any existing order
• If the order would be a taker (aggressive), it is rejected with no fills
• If accepted, rests in the book as a GTC order

When to use Post-Only:

• Market making — you always want maker status (and maker fees/rebates)
• Strategies that are extremely fee-sensitive
• Ensuring your limit order improves the market (tightens the spread)

Example: The best ask is 3200 USDC. You place a Post-Only buy at 3200. Since this would immediately match the resting ask, the order is rejected. You adjust to 3199, which doesn’t match — the order is accepted and rests in the book.

Post-Only combined with the MM credit system is the standard configuration for market makers on Vela. Post-Only ensures maker status; the credit system enables capital-efficient multi-market quoting.

Comparison Table

TIF	Rests in book	Partial fill	Immediate execution	Always maker
GTC	Yes	Yes	If price matches	No
IOC	No	Yes	Yes (or cancel)	No
FOK	No	No	Yes (or full cancel)	No
Post-Only	Yes (or reject)	Yes (once resting)	No	Yes

TIF and Order Types

Not all TIF options are valid with all order types:

Order Type	Valid TIF options
Limit	GTC, IOC, FOK, Post-Only
Market	IOC (implicitly — market orders are always immediate)

Submitting a market order with any TIF other than IOC is treated as IOC.

API Encoding

TIF values in the API are lowercase strings:

TIF	API value
Good Till Cancelled	"gtc"
Immediate or Cancel	"ioc"
Fill or Kill	"fok"
Post-Only	"post_only"