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Theta Labs routes spot trades through Polymarket’s native order book and fills options trades against its own dealer desk. Understanding how each executes helps you get better prices and avoid unexpected costs.

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Market orders

All spot trades on Theta Labs are market orders — they execute immediately at the best available price. The ticket footer says it plainly: “Market order · fills at the best available price.” When you place a trade, the platform:
  1. Checks the current best ask (for buys) or best bid (for sells)
  2. Submits your order to the exchange
  3. Fills as much as possible at the current price, then continues at the next available price level if needed
Market orders prioritize speed and certainty of execution over getting the exact price you saw. The actual fill price may differ slightly from the price displayed — this difference is called slippage. Theta Labs applies an automatic slippage guard of about 2% on your behalf: if the market moves further than that mid-order, the order won’t fill at the worse price.
A Limit option is visible in the trade ticket but not yet enabled — limit orders are coming soon.

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The orderbook

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Polymarket (CLOB)

Polymarket uses a Central Limit Order Book (CLOB). Orders sit in the book as bids and asks. When you place a market order to buy, it matches against the lowest available ask. The CLOB is off-chain for matching but settles on-chain on Polygon.

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Reading the spread

The bid-ask spread is the difference between what sellers want and what buyers are offering:
  • Bid = the highest price someone is willing to buy at
  • Ask = the lowest price someone is willing to sell at
  • A narrow spread (e.g., 39¢ bid / 41¢ ask) means a liquid market — you can trade near the fair value
  • A wide spread (e.g., 30¢ bid / 50¢ ask) means the market is thin — large orders will move the price significantly
You can see the live order book — bids and asks at each price level, plus the spread — on every market page.

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Slippage

Slippage occurs when your order fills at a different price than expected. It happens when:
  • The market moves between when you click and when the order reaches the exchange
  • Your order is large enough to consume multiple price levels in the orderbook, so later fills occur at worse prices

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Minimizing slippage

Trade in liquid markets. High-volume markets (shown by their volume figure on the market card) have tighter spreads and can absorb larger orders without significant price impact.

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Position sizing

A few practical rules for sizing your trades:
  • Never risk more than you’re prepared to lose entirely. Even high-probability markets can resolve unexpectedly.
  • Spread across markets rather than concentrating a large position in one. This reduces the impact of a single bad outcome.
  • Start small on new markets. Until you understand a market’s liquidity and spread behavior, keep initial positions small.
  • Account for the spread cost. If you buy at 41¢ and the bid is 39¢, you’re starting 2¢ in the hole. Factor this into your expected return.

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How options orders execute

Options trades work differently from spot:
  • Orders fill instantly against the platform dealer at the quoted price — there is no orderbook and no slippage. The price in the ticket is the price you pay.
  • Quotes include a market-making spread (see Options Trading).
  • Supply is finite: each strike is backed by real collateral, so a strike can show Sold out once its capacity is taken.