Broker API Comparison
| Broker | Markets | Paper Trading | Commission | Python SDK | Rate Limit | Best For |
|---|---|---|---|---|---|---|
| Alpaca | US Stocks, Crypto | Free, built-in | $0 | alpaca-py | 200 req/min | First agent, clean API |
| Interactive Brokers | Global: stocks, options, futures, forex | Separate account | $0–$1/trade | ib_insync | 50 msg/sec | Advanced traders, global markets |
| Coinbase Adv. Trade | Crypto (US-regulated) | Sandbox available | 0.05–0.6% | coinbase-advanced-py | 10 req/sec | US crypto traders |
| Binance | Crypto (international) | Testnet available | 0.02–0.1% | python-binance | 1200 req/min | International crypto, high frequency |
Alpaca — The Standard Starting Point
Alpaca is built for algorithmic traders. The REST API is clean, well-documented, and returns JSON. Paper trading is enabled by default — no separate account needed, just use your paper trading API keys instead of live keys.
Setup (5 minutes)
- Create a free account at alpaca.markets
- Go to Paper Trading → API Keys → Generate
- Install the SDK:
pip install alpaca-py - Store your API key and secret in environment variables (never hard-code credentials)
What Your Agent Can Do
- Market orders: Buy/sell at current market price — simplest execution
- Limit orders: Set a maximum buy price or minimum sell price
- Stop-loss orders: Automatic exit when price falls below a level — critical for risk management
- Bracket orders: Entry + take-profit + stop-loss in a single API call — the most useful order type for agents
- Portfolio queries: Check current positions, buying power, P&L — needed for the "observe" step of the agent loop
Interactive Brokers — For Advanced Agents
IBKR provides access to 150+ markets globally: US/international stocks, options, futures, forex, bonds, and crypto. The API is more complex than Alpaca (gateway-based rather than pure REST) but gives your agent access to instruments that no other retail broker API offers.
Key Differences from Alpaca
- TWS Gateway: IBKR requires running their Trader Workstation (TWS) or IB Gateway locally. Your Python script connects to this local process via socket, not directly to IBKR's servers. This adds operational complexity but provides a reliable connection.
- ib_insync library: The community-maintained
ib_insynclibrary wraps the raw API in a Pythonic interface. Highly recommended over the official IBKR Python API. - Paper trading: Requires a separate paper trading login. Configure in IBKR Account Management → Settings → Paper Trading Account.
- Rate limits: Stricter than Alpaca — 50 messages per second. Agents that poll frequently need to batch requests.
Crypto: Coinbase & Binance
For crypto trading agents, the two dominant APIs are Coinbase Advanced Trade (US-regulated, smaller selection, higher fees) and Binance (international, largest selection, lower fees). Both offer testnet/sandbox environments for paper trading.
Crypto API advantages for agent builders: 24/7 markets (your agent can trade weekends and holidays), fractional orders (buy 0.001 BTC), and lower minimum account sizes (no $25K PDT rule). The main risk is higher volatility — your agent's risk controls need to be tighter in crypto.
Safety Mechanisms — Mandatory for All Brokers
- API key permissions: Use the most restrictive permissions available. On Alpaca, you can limit keys to paper trading only. On IBKR, use the Financial Advisor (FA) account model to restrict what the API can do.
- Order validation layer: Before sending any order to the broker, validate it: Is the ticker in the allowed watchlist? Does the position size comply with limits? Does this order push total exposure above the max? Reject orders that fail any check.
- Kill switch: A function that cancels all open orders and (optionally) closes all positions. Trigger it if: daily loss exceeds threshold, the agent throws an unhandled exception, or you manually send a stop signal.
- Rate limiting: Implement client-side rate limiting even if the broker has server-side limits. An agent in a retry loop can burn through rate limits and get your API key banned.
- Logging everything: Log every API request, every response, every order status update. When something goes wrong at 3 AM, the logs are the only way to understand what happened.
- Never store credentials in code. Use environment variables or a secrets manager. Your API key has the power to place trades with your real money.
Frequently Asked Questions
Which broker API is best for a first AI trading agent?
Alpaca. It has the cleanest REST API, a free paper trading environment, zero commissions, a well-maintained Python SDK (alpaca-py), and a generous rate limit (200 requests/minute). Most AI trading agent tutorials and open-source projects are built on Alpaca.
Can I paper trade on Interactive Brokers?
Yes. IBKR offers a paper trading account that mirrors their full API. Create a separate paper trading account in Account Management. The API endpoints are the same — you connect to a different port (4002 for paper vs 4001 for live). IBKR's API is more complex than Alpaca but gives access to global markets, options, futures, and forex.
Is automated trading through broker APIs legal?
Yes. Automated trading through retail broker APIs is legal in all major markets. Brokers like Alpaca, Interactive Brokers, and Coinbase explicitly support API-based trading. You remain responsible for compliance with Pattern Day Trader rules (if applicable), tax reporting, and your broker's terms of service.
Disclaimer: This content is for educational purposes only. Automated trading involves risk. Always paper trade before using real capital. Broker API access carries inherent risks — improper implementation can result in financial loss. Not financial advice.