# Cross-Platform Price Monitor

Use this script to compare market pricing across multiple platforms.

### What this script does

* connects to Kalshi, Polymarket, and Outpoll
* subscribes to live order book data
* shows the current best bid in one terminal view

Start with [Setup & Helpers](/api/rest-api/python-examples/setup-and-helpers.md) if you still need the shared prerequisites.

{% code title="cross\_platform\_price\_monitor.py" %}

```python
#!/usr/bin/env python3
"""Compare best bids across Kalshi, Polymarket, and Outpoll in real time."""
import asyncio, json, time
import websockets
from cryptography.hazmat.primitives import serialization, hashes
from cryptography.hazmat.primitives.asymmetric import padding
import base64

# ─── Configuration ───────────────────────────────────────────────────
# Kalshi (requires RSA key pair — see https://docs.kalshi.com)
KALSHI_KEY_ID = "your_kalshi_api_key_id"
KALSHI_PRIVATE_KEY_PATH = "path/to/kalshi_private_key.pem"
KALSHI_MARKET_TICKER = "KXBTC-25DEC31-T100000"  # example: BTC $100k by end 2025
KALSHI_WS = "wss://api.elections.kalshi.com/trade-api/ws/v2"

# Polymarket (no auth for market data)
POLYMARKET_ASSET_ID = "12345..."  # YES token asset_id from gamma-api.polymarket.com/markets
POLYMARKET_WS = "wss://ws-subscriptions-clob.polymarket.com/ws/market"

# Outpoll (no auth for order book)
OUTPOLL_EVENT_ID = "54ccea1a-16fd-469c-8018-84b375243e8a"
OUTPOLL_OUTCOME_ID = "b21f6fd8-b9d1-4b9f-bb79-ef141e3dcb76"  # YES runner
OUTPOLL_WS = "wss://order-book.outpoll.com/event/ws"

# ─── Shared state ────────────────────────────────────────────────────
prices = {
    "Kalshi": None,
    "Polymarket": None,
    "Outpoll": None,
}


def display():
    """Print the current best bids."""
    print("\033[H\033[J", end="")  # clear terminal
    print("═══ Cross-Platform Best Bid ═══\n")
    for platform, bid in prices.items():
        if bid is not None:
            print(f"  {platform:12s} Best Bid: ${bid:.2f}")
        else:
            print(f"  {platform:12s} Best Bid: waiting...")
    print(f"\n  Updated: {time.strftime('%H:%M:%S')}")


# ─── Kalshi ──────────────────────────────────────────────────────────
async def watch_kalshi():
    with open(KALSHI_PRIVATE_KEY_PATH, "rb") as f:
        private_key = serialization.load_pem_private_key(f.read(), password=None)

    # Sign handshake
    ts = str(int(time.time() * 1000))
    msg_to_sign = ts + "GET" + "/trade-api/ws/v2"
    sig = private_key.sign(
        msg_to_sign.encode(),
        padding.PSS(mgf=padding.MGF1(hashes.SHA256()), salt_length=padding.PSS.DIGEST_LENGTH),
        hashes.SHA256(),
    )
    headers = {
        "KALSHI-ACCESS-KEY": KALSHI_KEY_ID,
        "KALSHI-ACCESS-SIGNATURE": base64.b64encode(sig).decode(),
        "KALSHI-ACCESS-TIMESTAMP": ts,
    }

    async with websockets.connect(KALSHI_WS, additional_headers=headers) as ws:
        # Subscribe to orderbook
        await ws.send(json.dumps({
            "id": 1,
            "cmd": "subscribe",
            "params": {"channels": ["orderbook_delta"], "market_ticker": KALSHI_MARKET_TICKER},
        }))

        yes_book = {}  # price_str -> size

        async for raw in ws:
            data = json.loads(raw)
            msg_type = data.get("type")

            if msg_type == "orderbook_snapshot":
                yes_book = {p: float(s) for p, s in data["msg"].get("yes_dollars_fp", [])}
            elif msg_type == "orderbook_delta" and data["msg"]["side"] == "yes":
                p = data["msg"]["price_dollars"]
                delta = float(data["msg"]["delta_fp"])
                yes_book[p] = yes_book.get(p, 0) + delta
                if yes_book[p] <= 0:
                    yes_book.pop(p, None)

            if yes_book:
                prices["Kalshi"] = float(max(yes_book, key=float))
                display()


# ─── Polymarket ──────────────────────────────────────────────────────
async def watch_polymarket():
    async with websockets.connect(POLYMARKET_WS) as ws:
        await ws.send(json.dumps({
            "assets_ids": [POLYMARKET_ASSET_ID],
            "type": "market",
            "custom_feature_enabled": True,
        }))

        # Heartbeat
        async def heartbeat():
            while True:
                await asyncio.sleep(10)
                try:
                    await ws.send("PING")
                except Exception:
                    break

        hb = asyncio.create_task(heartbeat())

        try:
            async for raw in ws:
                if raw == "PONG":
                    continue
                events = json.loads(raw)
                if not isinstance(events, list):
                    events = [events]

                for event in events:
                    if event.get("event_type") == "book":
                        bids = event.get("bids", [])
                        if bids:
                            prices["Polymarket"] = float(max(bids, key=lambda x: float(x["price"]))["price"])
                            display()

                    elif event.get("event_type") == "price_change":
                        for pc in event.get("price_changes", []):
                            if pc.get("best_bid"):
                                prices["Polymarket"] = float(pc["best_bid"])
                                display()
        finally:
            hb.cancel()


# ─── Outpoll ─────────────────────────────────────────────────────────
async def watch_outpoll():
    async with websockets.connect(OUTPOLL_WS) as ws:
        await ws.send(json.dumps({"t": "SUBSCRIBE", "e": OUTPOLL_EVENT_ID}))

        async for raw in ws:
            data = json.loads(raw)
            if data["t"] not in ("PRICES_DATA", "PRICES_DATA_DELTA"):
                continue
            for p in data["d"]:
                if p["o"] == OUTPOLL_OUTCOME_ID:
                    prices["Outpoll"] = p["b"]
                    display()


# ─── Main ────────────────────────────────────────────────────────────
async def main():
    display()
    await asyncio.gather(
        watch_kalshi(),
        watch_polymarket(),
        watch_outpoll(),
    )

asyncio.run(main())
```

{% endcode %}

### Example output

{% code title="terminal.txt" %}

```
═══ Cross-Platform Best Bid ═══

  Kalshi       Best Bid: $0.44
  Polymarket   Best Bid: $0.43
  Outpoll      Best Bid: $0.44

  Updated: 14:32:05
```

{% endcode %}

{% hint style="info" %}
For a meaningful cross-platform comparison, Kalshi and Polymarket IDs must point to the same underlying event. Use the [Kalshi API](https://docs.kalshi.com) and [Polymarket CLOB API](https://docs.polymarket.com) to resolve matching market identifiers.
{% endhint %}

### Related pages

* [Order Book](/api/rest-api/websockets/order-book.md)
* [WebSocket API Reference](/api/rest-api/websockets/websocket-api-reference.md)


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