Kalshi API

Kalshi API

CFTC-regulated US event exchange. USD-denominated binary contracts settle at $1.00 (YES wins) or $0.00 (NO wins). REST + WebSocket, RSA-PSS authentication on every request.

For contract semantics and settlement rules, see the kalshi-weather-markets and kalshi-crypto-index-markets skills. For strategy, sizing, and backtesting, see prediction-market-strategy.

VERIFY BEFORE CODING. The Kalshi API has broken backward compatibility before: the host changed (old trading-api.kalshi.com → dead), and the order schema changed (integer cents → dollar strings). Always smoke-test signing and order bodies against a live response before shipping.

Canonical sources:

• API reference: https://docs.kalshi.com (legacy mirror: https://trading-api.readme.io)
• Official Python starter: https://github.com/Kalshi/kalshi-starter-code-python

Overview

• Base URL: https://api.elections.kalshi.com/trade-api/v2
• Auth: RSA-PSS on every request — there are no public/unauthenticated endpoints
• No demo parity: the demo environment (demo-api.kalshi.co) has a near-empty book; use production even for read-only pulls
• Contracts: $0.01–$0.99 per contract; pay price if YES wins, lose price if NO wins; max payout = $1.00

Quick Start

1. Credentials

KALSHI_KEY_ID=<your-key-uuid>
KALSHI_PRIVATE_KEY_PATH=~/.kalshi/private.pem

Generate the key in the Kalshi dashboard. Store secrets in environment variables or a secrets manager — never in code.

2. Install

pip install httpx cryptography

3. Host + auth (the part everyone gets wrong)

The host and signature format are where implementations break. Three common failures:

1. Using the old trading-api.kalshi.com host → 401
2. Including the query string in the signed path → 401
3. Signing with seconds instead of milliseconds → 401

import os, time, base64, httpx
from cryptography.hazmat.primitives import hashes, serialization
from cryptography.hazmat.primitives.asymmetric import padding

BASE = "https://api.elections.kalshi.com/trade-api/v2"
KEY_ID = os.environ["KALSHI_KEY_ID"]
with open(os.environ["KALSHI_PRIVATE_KEY_PATH"], "rb") as f:
    PRIV = serialization.load_pem_private_key(f.read(), password=None)

def _headers(method: str, path: str) -> dict:
    """path must include /trade-api/v2 prefix and exclude query string."""
    ts = str(int(time.time() * 1000))             # milliseconds
    msg = f"{ts}{method}{path}".encode()
    sig = PRIV.sign(
        msg,
        padding.PSS(mgf=padding.MGF1(hashes.SHA256()),
                    salt_length=padding.PSS.DIGEST_LENGTH),
        hashes.SHA256(),
    )
    return {
        "KALSHI-ACCESS-KEY": KEY_ID,
        "KALSHI-ACCESS-TIMESTAMP": ts,
        "KALSHI-ACCESS-SIGNATURE": base64.b64encode(sig).decode(),
    }

def get(path: str, params=None):
    # Sign the path only — query goes into params, not the signature
    r = httpx.get(BASE + path, params=params,
                  headers=_headers("GET", "/trade-api/v2" + path))
    r.raise_for_status()
    return r.json()

def post(path: str, body: dict):
    r = httpx.post(BASE + path, json=body,
                   headers=_headers("POST", "/trade-api/v2" + path))
    r.raise_for_status()
    return r.json()

# Example: open markets in a series
markets = get("/markets", params={"series_ticker": "KXHIGHNY", "status": "open"})

Signature spec: RSA-PSS, MGF1 over SHA-256, salt length = PSS.DIGEST_LENGTH. String to sign: {timestamp_ms}{METHOD}{path} where path includes /trade-api/v2 and excludes the query string.

Headers: KALSHI-ACCESS-KEY (UUID), KALSHI-ACCESS-TIMESTAMP (ms), KALSHI-ACCESS-SIGNATURE (base64).

4. Candlestick history

# Returns OHLC for yes_bid / yes_ask + volume + open_interest
# Values are dollar strings: {"close": "0.42"}
candles = get(
    f"/series/KXHIGHNY/markets/{ticker}/candlesticks",
    params={"start_ts": start_epoch, "end_ts": end_epoch, "period_interval": 60},
)

period_interval is in minutes: 1, 60, or 1440.

YES/NO Order-Book Convention

On Kalshi, yes and no are both resting BID ladders — there is no separate ask book. To take the other side you lift the opposing bid:

no_ask  = 1 − best_yes_bid    # cost to buy NO right now (lift YES bids)
yes_ask = 1 − best_no_bid     # cost to buy YES right now (lift NO bids)
P(YES) mid = (best_yes_bid + (1 − best_no_bid)) / 2

Getting this backwards silently inverts every signal. Use the helpers in scripts/kalshi_orderbook.py.

Orderbook response comes in two variants depending on API tier — normalize before using:

{"orderbook": {"yes": [[price, size], ...], "no": [[price, size], ...]}}

If a price value is > 1.0, it is integer cents — divide by 100.

Order Schema

POST /trade-api/v2/portfolio/orders uses fixed-point dollar STRINGS, not integers. The old schema (integer cents, count, yes_price) returns 400 invalid_parameters.

{
  "ticker": "KXHIGHNY-26JUN02-B75.5",
  "action": "buy",
  "side": "yes",
  "count_fp": "1.00",
  "yes_price_dollars": "0.01",
  "client_order_id": "my-strategy-001",
  "time_in_force": "good_till_canceled"
}

Critical field rules — each violation returns 400:

For the full order lifecycle (amend, decrease, cancel, batch) and strike_type gotchas, see references/auth-and-orders.md.

Endpoint Summary

Full endpoint surface, market metadata fields, and rate limits: references/endpoints-and-marketdata.md. WebSocket discovery pipeline: references/websocket.md.

Files

References

• references/auth-and-orders.md — RSA-PSS spec, dollar-string order schema, time_in_force, client_order_id sanitization, amend/decrease/cancel lifecycle, strike_type gotcha, fees
• references/endpoints-and-marketdata.md — Full endpoint table, orderbook variants, market metadata fields (result, open_time, close_time, series_ticker), candlesticks, rate limits
• references/websocket.md — WS host, discovery pipeline, channels, signing the WS upgrade

Scripts

• scripts/kalshi_orderbook.py — YES/NO bid-ladder helpers (no_ask, yes_ask, p_yes_mid, overround, kalshi_fee). Pure stdlib, no keys, runs offline.