Slippage can quietly destroy a profitable Polymarket trading strategy.

Your signal may correctly identify a YES token as undervalued, but that does not mean you can actually buy the required position at the price used by your model. If the order book has limited liquidity, a large order may consume multiple ask levels. The deeper your order walks through the book, the worse your average execution price becomes.

For a Polymarket bot, slippage must be part of the trading decision—not something calculated after the trade.

In this guide, you will learn how to estimate Polymarket slippage from the CLOB order book, reject trades that exceed an execution budget, split large orders, and monitor the difference between expected and actual execution.

What You'll Learn

• How Polymarket slippage occurs in the CLOB
• How to calculate expected average execution price
• How to estimate order-book depth and price impact
• How to create a maximum slippage rule
• How to reduce slippage through order sizing
• How to monitor expected versus actual fills
• Common execution mistakes in automated Polymarket trading

What Is Polymarket Slippage?

For a BUY order, slippage occurs when your actual average fill price is higher than the price your strategy expected.

For a SELL order, slippage occurs when your average fill price is lower.

A simple model is:

BUY slippage = average_fill_price - expected_price

For example:

Expected buy price: 0.52
Average fill price: 0.54

Slippage = 0.02

That two-cent difference matters because Polymarket prices represent implied probabilities between 0 and 1.

The Polymarket CLOB exposes order book bids and asks with price and size information. Bids are sorted from highest to lowest price, while asks are sorted from lowest to highest. The book response also includes information such as the market tick size and minimum order size.

The important point is simple:

Your bot should evaluate the depth of the book for the size it intends to trade.

Looking only at the best ask is not enough.

The Real Cost of Walking the Order Book

Imagine the ask side looks like this:

If your bot wants to buy 300 shares:

• 100 shares fill at 0.50
• 150 shares fill at 0.51
• 50 shares fill at 0.53

The average execution price is:

(100 × 0.50 + 150 × 0.51 + 50 × 0.53) / 300
= 0.5117

The best ask was 0.50, but the realistic average price for the complete order is approximately 0.5117.

This is why backtests that assume every order fills at the top of book can significantly overstate strategy quality.

Polymarket's current CLOB client documentation also exposes a market-price calculation method designed to estimate the market price for a given token, side, amount, and order type. That is useful when your bot needs an execution estimate before submitting an order.

Recommended Bot Architecture

flowchart LR
    A[Trading Signal] --> B[Calculate Fair Value]
    B --> C[Fetch Order Book]
    C --> D[Simulate Execution]
    D --> E{Slippage Acceptable?}
    E -->|No| F[Reduce Size or Skip]
    E -->|Yes| G[Submit Order]
    G --> H[Monitor Fill]
    H --> I[Compare Expected vs Actual]
    I --> J[Update Execution Metrics]

The key design principle is that signal generation and execution validation should be separate.

Your model can say:

Fair value = 0.60

But the execution engine should independently decide:

Can we actually buy this position below our maximum acceptable price?

Step 1: Simulate Order Book Execution in Python

The following example calculates the expected average price by walking through the available order book levels.

from decimal import Decimal
from typing import List, Dict


def simulate_buy_order(
    asks: List[Dict[str, str]],
    target_size: Decimal
) -> dict:
    remaining = target_size
    total_cost = Decimal("0")

    for level in asks:
        price = Decimal(level["price"])
        available = Decimal(level["size"])

        fill_size = min(remaining, available)

        total_cost += fill_size * price
        remaining -= fill_size

        if remaining <= 0:
            break

    filled = target_size - remaining

    if filled == 0:
        return {
            "filled": Decimal("0"),
            "average_price": None,
            "complete": False,
        }

    return {
        "filled": filled,
        "average_price": total_cost / filled,
        "complete": remaining <= 0,
    }

Example usage:

asks = [
    {"price": "0.50", "size": "100"},
    {"price": "0.51", "size": "150"},
    {"price": "0.53", "size": "300"},
]

result = simulate_buy_order(
    asks=asks,
    target_size=Decimal("300")
)

print(result)

The complete field is important.

If the visible order book cannot fill the requested size, your bot should not assume the remaining liquidity will appear. Treat insufficient depth as execution risk.

Step 2: Add a Maximum Slippage Budget

Suppose your strategy expects to buy at:

Reference price = 0.50
Maximum slippage = 0.01

Your maximum acceptable average price is:

max_acceptable_price = reference_price + max_slippage

Example:

reference_price = Decimal("0.50")
max_slippage = Decimal("0.01")

max_acceptable_price = reference_price + max_slippage

result = simulate_buy_order(
    asks=asks,
    target_size=Decimal("300")
)

if not result["complete"]:
    print("Skip trade: insufficient visible liquidity")

elif result["average_price"] > max_acceptable_price:
    print("Skip trade: slippage too high")

else:
    print("Trade is within execution budget")

This should happen before the bot creates and submits an order.

A useful strategy-level rule is:

Expected edge
- estimated slippage
- spread cost
- applicable fees
= remaining execution edge

If the remaining edge is too small, skip the trade.

Polymarket also exposes best-price, midpoint, spread, and order-book market-data functionality. The spread is defined as the difference between the best ask and best bid. These values can be useful execution signals, but midpoint alone is not a guaranteed executable price.

Step 3: Use Dynamic Position Sizing

A fixed position size is rarely ideal.

A better approach is to size orders according to available liquidity.

For example:

High liquidity + low slippage → larger size
Low liquidity + high slippage → smaller size
Insufficient liquidity → no trade

You can also find the largest size that remains below a maximum average execution price:

def max_affordable_size(
    asks,
    max_average_price: Decimal
) -> Decimal:
    size = Decimal("0")
    cost = Decimal("0")

    for level in asks:
        price = Decimal(level["price"])
        available = Decimal(level["size"])

        if price > max_average_price:
            break

        size += available
        cost += available * price

    return size

For production, use the actual current order book and validate the resulting average price rather than relying only on individual price levels.

Step 4: Limit Adverse Selection

Slippage is not always caused by your own order size.

Sometimes the order book changes before execution.

Your bot may observe:

Best ask: 0.50

Then, after your signal is generated:

Best ask: 0.53

This is execution latency and market movement.

A robust workflow is:

1. Generate signal.
2. Fetch or maintain the latest order book.
3. Simulate the proposed order.
4. Set a maximum acceptable execution price.
5. Submit the order.
6. Monitor actual fill information.
7. Cancel or stop according to your execution policy when the remaining risk is no longer acceptable.

Do not blindly retry an order using progressively worse prices. A retry loop without a new price validation can turn a temporary execution failure into a bad trade.

Production Considerations

Use Fresh Market Data

Stale order-book data creates false confidence.

If your bot maintains an internal book, track timestamps and detect gaps in updates. If your execution system cannot verify that its local state is sufficiently fresh, fall back to a new snapshot or skip the trade.

The official order-book response includes a timestamp and book hash, which can help with state tracking and reconciliation.

Do Not Hardcode Tick Sizes

Markets can expose their own tick size and minimum order size through order-book metadata. Read these values from the market data instead of assuming a universal price increment.

Use Decimal for Price Math

Avoid Python floating-point arithmetic for execution thresholds.

Use:

Decimal("0.51")

instead of:

0.51

Small rounding differences can become important when comparing an estimated price with a strict slippage threshold.

Failure Modes and Common Mistakes

1. Using the midpoint as the execution price

The midpoint is useful as a reference, but a large marketable order may not execute at that price.

2. Checking only the best ask

The first level may contain only a small amount of liquidity.

3. Ignoring partial fills

A partially filled position changes your risk. Your bot must know exactly how much exposure it has acquired before deciding what to do next.

4. Retrying without re-pricing

Every retry should re-evaluate the current execution conditions.

5. Backtesting with perfect fills

Historical strategies should model spread, depth assumptions, slippage, fees where applicable, and incomplete execution.

Monitoring and Observability

Track these metrics for every order:

• expected price
• maximum allowed price
• actual average fill price
• estimated slippage
• realized slippage
• requested size
• filled size
• unfilled size
• order-book snapshot time
• decision-to-submission time

A simple realized slippage calculation:

realized_slippage = actual_average_price - expected_price

Then monitor whether actual execution is consistently worse than your model predicts.

If it is, the problem may be:

• stale market data
• underestimated depth consumption
• aggressive order behavior
• market movement
• model assumptions

Advanced Improvements

The next version of a Polymarket execution engine can add:

Order slicing

Split a large position into smaller pieces and re-evaluate the book between slices.

Liquidity-aware signals

Require a minimum amount of executable liquidity before a trading signal becomes valid.

Slippage-adjusted expected value

Instead of ranking trades by theoretical edge:

fair value - market price

rank them by estimated executable edge:

fair value
- estimated average execution price
- fees
- execution risk

This is usually closer to the economics your live bot actually experiences.

Frequently Asked Questions

What is Polymarket slippage?

Polymarket slippage is the difference between the expected execution price and the actual average fill price of a trade.

How do I calculate slippage in a Polymarket bot?

Fetch the relevant order-book side, simulate consuming liquidity for your intended order size, calculate the weighted average execution price, and compare it with your reference price.

Can the best ask be used as the expected fill price?

Only for very small orders when sufficient liquidity exists at that level. Larger orders may consume multiple price levels.

Should my bot always trade when there is enough liquidity?

No. Liquidity alone does not make the trade attractive. Your expected edge must still exceed estimated execution costs and risk.

How can I reduce slippage?

Use smaller position sizes, liquidity-aware sizing, maximum acceptable prices, updated order-book data, and execution logic that re-prices before retrying.

Conclusion

The best way to handle Polymarket slippage is to treat execution as part of the trading strategy.

Before placing a trade, your bot should know:

• how much size it wants,
• how much visible liquidity exists,
• the estimated average execution price,
• the maximum acceptable price,
• and what action to take if the market moves.

A trading signal without execution analysis is only half a strategy.

The strongest Polymarket bots model the actual order book, calculate expected fill quality, and refuse trades when slippage destroys the expected edge.

Educational and trading-risk disclaimer: Automated prediction-market trading involves substantial financial and technical risk. Slippage, spreads, fees, partial fills, liquidity changes, latency, model errors, and adverse market movement can materially affect results. No strategy is guaranteed to be profitable.

Useful Resources

• Polymarket Official Website — Official platform information.
• Polymarket Developer Documentation — Primary technical reference for APIs and trading infrastructure.
• Official CLOB Order Book Documentation — Relevant for retrieving bids, asks, tick size, minimum order size, and book metadata.
• Official Polymarket X Account — Useful for official platform announcements.
• Polymarket API Help Article — Official starting point for developers looking for API resources.

No third-party Medium, DEV.to, or YouTube resource was included because a genuinely relevant resource for this specific slippage implementation could not be confidently verified at publication time.

Related Articles

1. How Polymarket CLOB Works

Suggested anchor: Polymarket CLOB order book
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2. Polymarket Order Book Explained

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3. Building a Basic Limit Order Integration

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4. How to Build a Polymarket Trading Bot

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6. Polymarket Market Making Bot

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7. Polymarket CLOB API Guide

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About the Author

Nagi777

I write about Polymarket trading bots, prediction-market infrastructure, algorithmic trading, Python automation, Web3 development, and quantitative strategies.

Contact:
Github: https://github.com/nagi777x/Polymarket-Trading-Bot
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