Polymarket API & Data Access Guide

Purpose: Comprehensive reference for connecting to, understanding, and using Polymarket APIs
Last Updated: 2026-03-23

🎯 What This Skill Covers

1. ✅ Connect to Polymarket - API endpoints, authentication
2. ✅ Find Markets - Search, filter, time-based pattern discovery
3. ✅ Get Real-time Data - WebSocket usage and event types
4. ✅ Access Order Books - Liquidity, bid/ask spreads, depth
5. ✅ Place Orders - CLOB SDK, market/limit orders, decimal precision
6. ✅ Market Mechanics - How 5-min markets work, winner determination, timing
7. ✅ Known Pitfalls - Data reliability, timing issues, common bugs

This skill does NOT include:

• ❌ Trading strategies
• ❌ Signal analysis methodology

🏗️ Architecture Overview

API Components

Data Flow for 5-Minute BTC Up/Down Markets

1. Generate timestamp → slug: btc-updown-5m-{timestamp}
2. Fetch market info from Gamma API → get asset IDs
3. Subscribe to WebSocket with asset IDs → real-time bid/ask
4. Place orders via CLOB SDK → buy UP or DOWN tokens
5. Market closes (5 min) → Polymarket resolves winner
6. Verify winner via Gamma API outcomePrices (1.0 = winner)

🔍 Finding Markets

5-Minute Markets — Timestamp Pattern

import time

def get_current_interval(interval_seconds=300):
    """Calculate current 5-minute interval timestamp"""
    current_time = int(time.time())
    return (current_time // interval_seconds) * interval_seconds

def generate_market_timestamps(num_markets=8):
    """Generate N market timestamps: current + next (N-1) intervals"""
    base = get_current_interval()
    return [base + i * 300 for i in range(num_markets)]

# Supported cryptos: btc, eth, sol, xrp
# Slug pattern: {crypto}-updown-5m-{timestamp}
# Example: btc-updown-5m-1772699400

API Search Methods

import requests

# By slug (most reliable for 5-min markets)
resp = requests.get("https://gamma-api.polymarket.com/markets", params={"slug": "btc-updown-5m-1772699400"})

# By keyword search
resp = requests.get("https://gamma-api.polymarket.com/markets", params={"_s": "bitcoin"})

# Active markets
resp = requests.get("https://gamma-api.polymarket.com/markets", params={"active": "true", "limit": 100})

🆔 Asset IDs (Token IDs)

Critical: Each outcome (UP/DOWN) has a unique asset ID required for WS subscriptions and trading.

Getting Asset IDs

import json

def get_asset_ids(market):
    """Get asset IDs from clobTokenIds field (preferred)"""
    clob_token_ids_str = market.get('clobTokenIds')
    if clob_token_ids_str:
        return json.loads(clob_token_ids_str)  # List of 2 IDs: [UP_ID, DOWN_ID]
    return []

Important:

• ✅ clobTokenIds is a JSON string, not an array — must json.loads()
• ✅ Index 0 = UP, Index 1 = DOWN (for btc-updown markets)
• ⚠️ Fall back to tokens[].token_id only if clobTokenIds missing

📡 WebSocket — Real-time Data

Connection & Subscription

import aiohttp, asyncio, json, time

async def monitor_market(asset_ids):
    async with aiohttp.ClientSession() as session:
        ws = await session.ws_connect(
            "wss://ws-subscriptions-clob.polymarket.com/ws/market",
            heartbeat=None,
            timeout=aiohttp.ClientWSTimeout(ws_close=30.0)
        )
        # Subscribe
        await ws.send_json({
            "assets_ids": asset_ids,
            "type": "market",
            "custom_feature_enabled": True
        })
        # Heartbeat every 10 seconds
        last_ping = time.time()
        async for msg in ws:
            if msg.data == "PONG": continue
            if time.time() - last_ping >= 10:
                await ws.send_str("PING")
                last_ping = time.time()
            if msg.type == aiohttp.WSMsgType.TEXT:
                data = json.loads(msg.data)
                # Handle events...

⚠️ CRITICAL: Event Type Reliability

🔥 Key Lesson: price_change = past trades, NOT current orderbook state. Using it as real-time bid/ask causes impossible states (ask \x3C bid). Only use book and best_bid_ask.

⚠️ CRITICAL: Book Event Structure

# ❌ WRONG: bids/asks are NOT under 'book' key
bids = data.get('book', {}).get('bids', [])  # EMPTY!

# ✅ CORRECT: bids/asks at TOP LEVEL
bids = data.get('bids', [])
asks = data.get('asks', [])

Order Book Fields

# Each bid/ask entry:
{
    'price': '0.4550',  # Price level (0-1)
    'size': '7270'      # Size in USD at this price
}

# Liquidity = sum of all bid sizes + sum of all ask sizes
total_liquidity = sum(float(b.get('size', 0)) for b in bids) + sum(float(a.get('size', 0)) for a in asks)

💰 Placing Orders — CLOB SDK

Authentication

from py_clob_client.client import ClobClient
from py_clob_client.clob_types import ApiCreds

host = "https://clob.polymarket.com"
chain_id = 137  # Polygon

client = ClobClient(host, key=key, chain_id=chain_id)
client.set_api_creds(ApiCreds(
    api_key="YOUR_API_KEY",
    api_secret="YOUR_API_SECRET",
    api_passphrase="YOUR_API_PASSPHRASE"
))

🔥 CRITICAL: Decimal Precision Rules

Rule: Calculate tokens FIRST, then derive USD

# ✅ CORRECT: Explicit precision control
raw_tokens = amount_usd / price           # e.g. 3.33 / 0.31 = 10.74193548
tokens = round(raw_tokens, 4)             # 10.7419 (max 4 decimals)
usd_amount = round(tokens * price, 2)     # 3.33 (max 2 decimals)
price_rounded = round(price, 4)           # 0.3100 (max 4 decimals)

# ❌ WRONG: Let SDK auto-calculate → may produce > 4 decimal tokens
# ❌ WRONG: Market order with SDK auto-calculation → same issue

Limit Order (Recommended — More Control)

from py_clob_client.order_builder.constants import BUY

order_args = OrderArgs(
    token_id=asset_id,
    price=price_rounded,    # 0.3100 (max 4 decimals)
    size=tokens,            # 10.7419 (max 4 decimals)
    side=BUY,
    expiration=0,           # GTC (Good Till Cancelled)
)
# A limit order at market price = instant fill with full precision control

Market Order (From Official Docs)

market_order = client.create_market_order(
    token_id="TOKEN_ID",
    side=BUY,
    amount=100,      # Dollar amount! SDK calculates tokens
    price=0.50,      # Slippage protection (can be 0)
    order_type=OrderType.FOK
)
client.post_order(market_order, OrderType.FOK)

Market Order Notes:

• amount = USD amount (SDK calculates tokens internally)
• price = slippage protection ceiling, NOT exact execution price
• ⚠️ SDK may produce >4 decimal tokens → prefer limit orders for safety

⚠️ ALWAYS Read Official Docs Before Trading

• https://docs.polymarket.com/trading/orders/create
• https://docs.polymarket.com/trading/orders/overview

Never guess API behavior — read docs first. Multiple failed versions were caused by not reading official documentation.

⏰ 5-Minute Market Mechanics

How Markets Work

1. New market every 5 minutes — timestamp-based: btc-updown-5m-{unix_ts}
2. Market duration: Exactly 5 minutes (300 seconds)
3. Winner determination: Polymarket resolves based on BTC price movement during the 5-minute interval
   • UP wins → outcomePrices: UP=1.0, DOWN=0.0
   • DOWN wins → outcomePrices: UP=0.0, DOWN=1.0
4. Market appears on Polymarket: ~30-60 seconds before interval start
5. Orderbook becomes active: Shortly after market appears

Timing — What Happens When

⚠️ CRITICAL: Winner Resolution Timing

# ❌ WRONG: Fetch winner immediately after market close
# Gamma API returns trading prices (0.60/0.40), NOT resolved prices (1.0/0.0)
# Can take 60+ seconds to resolve

# ✅ CORRECT: Wait for outcomePrices to show 1.0/0.0
# After market close, poll Gamma API until resolved
# outcomePrices [1.00, 0.00] = UP winner confirmed
# outcomePrices [0.00, 1.00] = DOWN winner confirmed

📊 Data Reliability — What to Trust

Price Data Hierarchy

⚠️ best_bid_ask Ask Price ≠ Market Probability

• Ask price = lowest sell order in orderbook
• This is not the market's consensus probability
• It reflects orderbook dynamics, not market sentiment
• All versions using ask price as probability showed ~50% accuracy

🔄 Market Rotation

Interval-Based Rotation (Correct Method)

def get_current_interval():
    """Get current 5-minute interval boundary"""
    current_time = int(time.time())
    return (current_time // 300) * 300

def should_rotate(current_interval, last_interval):
    """Check if we've entered a new interval"""
    return current_interval != last_interval

# ❌ WRONG: Time-based (rotates at wrong time)
# time_since_rotation = time.time() - last_rotation
# if time_since_rotation >= 300: rotate()  # Off by up to 15 seconds!

# ✅ CORRECT: Interval-based (rotates exactly when interval changes)
# if get_current_interval() != current_base_timestamp: rotate()

⚠️ Known Pitfalls & Lessons Learned

1. Never Use price_change as Orderbook Data

• price_change = past trade records
• BUY in price_change = someone bought at this price IN THE PAST
• This is NOT the current best ask
• Using it causes impossible states (ask \x3C bid)

2. Decimal Precision Will Break Your Orders

• Maker amount (tokens): max 4 decimals
• Taker amount (USD): max 2 decimals
• Calculate tokens first, derive USD second
• Never trust SDK auto-calculation
• Use limit orders for full control

3. Read Official Docs Before Implementing

• Multiple wasted versions from guessing API behavior
• Official docs: https://docs.polymarket.com/trading/orders/create

4. Winner Resolution Takes Time

• Gamma API doesn't return 1.0/0.0 immediately after close
• Poll until outcomePrices resolves to 1.0/0.0
• Verify with Gamma API outcomePrices post-session

⚠️ Pitfall #5: Market Resolution Timing

• Gamma API doesn't return 1.0/0.0 immediately after close
• Can take 60+ seconds for outcomePrices to resolve
• Poll until you see 1.0/0.0, don't assume immediate resolution

6. Market Appearance Timing is Variable

• Markets don't always appear exactly 60s before start
• Sometimes 30s, sometimes 90s
• Fetch ask immediately when market appears — don't delay

7. Ask Price at Market Start ≈ 0.50

• Fresh market ask is ~0.50 (fair odds)
• Ask drifts quickly (within seconds) based on BTC movement
• If you see ask = 0.99, market is already closed/ending

📚 API Reference

Gamma API Endpoints

WebSocket Events

Rate Limits

🔧 CLOB SDK Quick Reference

from py_clob_client.client import ClobClient
from py_clob_client.clob_types import OrderArgs, OrderType, ApiCreds
from py_clob_client.order_builder.constants import BUY

# Initialize
client = ClobClient("https://clob.polymarket.com", key=key, chain_id=137)
client.set_api_creds(ApiCreds(api_key=..., api_secret=..., api_passphrase=...))

# Limit order (recommended)
order = OrderArgs(token_id=ASSET_ID, price=0.50, size=10.0, side=BUY, expiration=0)
# size = tokens (max 4 decimals), price (max 4 decimals)

# Market order
market_order = client.create_market_order(token_id=ASSET_ID, side=BUY, amount=100, price=0, order_type=OrderType.FOK)
# amount = USD, price = slippage ceiling

# Nonces: Must be unique per order. Use counter or UUID.

Official Documentation

• Orders: https://docs.polymarket.com/trading/orders/create
• Overview: https://docs.polymarket.com/trading/orders/overview

📝 Version History

• v2.0.0 (2026-03-23):

  • ✅ Added CLOB SDK trading section (limit/market orders, decimal precision)
  • ✅ Added 5-min market mechanics (timing, winner determination)
  • ✅ Added data reliability guide (what to trust, what not to)
  • ✅ Added known pitfalls section (7 critical lessons)
  • ✅ Added winner verification timing guide
  • ✅ Added winner verification timing guide
  • ✅ Added market rotation (interval-based vs time-based)
  • ✅ Updated WebSocket reliability table
  • ✅ Removed arbitrage scanner example (not applicable to 5-min markets)

• v1.2.0 (2026-03-05):

  • ✅ Fixed WebSocket book event structure
  • ✅ Added real-time liquidity tracking
  • ✅ Added multi-cryptocurrency monitoring

• v1.0.0 (2026-03-03):

  • Initial release