Liquidity, slippage, and order book vs AMM

The price you see on a market is a top-of-book quote — the price for the very next contract that trades. If your order is bigger than the depth sitting at that price, you'll fill at a worse average price than the headline. That gap is slippage, and it comes in two flavours depending on whether the market runs as an order book or as an automated market maker (AMM).

This guide covers what liquidity means in practice, how each mechanism turns size + price into a fill, how slippage shows up on each, and how to think about both when you're sizing.

If you're new to expected-value reasoning, read Reading prices and expected value first — slippage is one of the things that quietly erodes your edge.

Why liquidity matters

Liquidity, for a single contract, is the answer to one question: how much size can I trade right now without moving the price? That's the practical version. The textbook version sums depth across multiple price levels — but the trader's intuition is "how much can I push through cheaply."

A few specific consequences:

• Headline prices are tiny-trade prices. A 47¢ ask doesn't mean every contract fills at 47¢; it means the next one does (up to whatever size sits there).
• Edge erodes with size. A trade that's clearly +EV at $50 may be break-even or −EV at $5,000 once slippage is factored in.
• Liquidity changes by the hour. Same market, different depth, different slippage. Pre-event news cycles often see spreads widen.
• You can pay liquidity providers (LPs) to take the other side. That's exactly what spread is — the liquidity provider's margin for offering a tradable price.

How a CLOB works

A central limit order book (CLOB) is a literal list of resting buy and sell orders sorted by price. When you submit a market order, the matching engine pairs you with the best-priced counterparties first, then walks into worse prices if your size exceeds top-of-book depth.

What this means at order time:

• Buy market orders fill against asks, starting at the best (lowest) ask and walking up.
• Sell market orders fill against bids, starting at the best (highest) bid and walking down.
• Limit orders rest in the book at the price you specify and get matched if and when a counterparty crosses them.

Liquidity in a CLOB is discretionary — it shows up because human traders or market makers chose to post orders. It can disappear quickly when conditions change.

How an AMM works

An automated market maker (AMM) doesn't have resting orders. It holds inventory of each outcome's tokens and uses a formula — usually constant product (x × y = k) — to quote a price. When you buy YES tokens, the pool's YES inventory shrinks, the NO inventory effectively grows (relative to it), and the next quoted YES price moves up.

A few specific properties:

• Always a quote available. As long as the pool has any inventory, the AMM will quote a price. There's no "no liquidity" state until the pool is empty.
• Price impact scales non-linearly with size. Doubling your buy more than doubles the price impact. Large size on a small pool = large slippage.
• No bid/ask in the CLOB sense. There's a single quoted price that shifts continuously as you take size; the platform's UI usually shows the expected fill for the size you've entered rather than a static "ask."
• LPs are passive. Liquidity providers deposit on both sides and get paid (a share of trading fees) without having to actively manage orders. The trade-off is they bear "impermanent loss" if the market moves sharply between deposit and withdrawal.

How slippage shows up on each

The mechanism differs but the feel is similar: bigger order = worse average fill.

On a CLOB, slippage is a discrete walk through the book:

• Your order eats top-of-book depth at the best price first.
• Whatever size remains pushes into the next price level.
• Average fill = total cost ÷ total contracts.

The thick book in the diagram absorbs the entire $2,000 at the top-of-book price ($0.50). The thin book has only $400 at top of book; the rest of the order walks into deeper, worse prices, producing an 8.7¢ average-fill premium over the headline.

On an AMM, slippage is a continuous slide along the curve:

• Each unit you buy pushes the price up by a small amount.
• The bigger the pool relative to your trade, the smaller the slide.
• Most platforms show an expected fill and a price impact percentage before you confirm.

For the same percentage of pool drained, an AMM and a CLOB don't necessarily give the same fill — but the qualitative pattern is identical: thinner liquidity → more slippage.

Where each lives in practice

Among the venues EdgeLedger tracks today:

• Kalshi runs a CLOB end-to-end. Every market is matched against resting orders by the exchange's engine. When you hear "thin book" on Kalshi, it means the order book itself is thin; there's no AMM fallback.
• Polymarket runs a CLOB for most actively traded markets. Some markets — especially newer or lower-volume ones — quote from an AMM instead of a book. The UI typically tells you which mechanism is in play; the practical question is the same either way.

Most DeFi venues outside the prediction-market space (DEXes for spot tokens, perpetuals platforms, etc.) use AMMs heavily, so the mental model is portable beyond just predictive markets.

Practical takeaways

The short version, four bullets:

• Liquidity determines the price you actually fill at, not the price you see. Top-of-book is the tiny-trade price.
• Slippage erodes EV. Compute net EV with the realized fill, not the quoted top. Thin liquidity can flip a +EV trade to net negative.
• CLOB depth is discrete, AMM impact is continuous. Same pattern, different shape; both reward patience over urgency.
• For larger orders, limit-order modeling matters. Posting liquidity and taking liquidity have different timing, fill, and fee tradeoffs.

What EdgeLedger surfaces

EdgeLedger reads market-data quotes from supported venues, so the top-of-book prices you see in the dashboard are the same headline quotes the platforms show. Per-position realized P&L (visible on the trade-history view) reflects the actual fill prices on your trades — that is the number to compare against headline quotes when auditing how much slippage sizing has produced over time.

Where to go next

• Reading prices and expected value — how to compute net EV including the slippage you'll actually pay.
• Polymarket: Placing your first trade — the order-book section walks through bids/asks/spread/depth on a real CLOB.
• Kalshi: Placing your first contract trade — CLOB-only mechanics with cents pricing.