3 Financial APIs for Building Your First Algorithmic Trading Models

If you are building machine learning models around stock or crypto data, you need reliable APIs that give you market access without enterprise pricing. Mariana Antaya walks through the three financial APIs she uses most, with honest takes on the tradeoffs of each. The recommendations are aimed at beginners who want to start paper trading and prototyping algorithmic strategies.

1. Robin Stocks (Unofficial Robinhood API)

The first recommendation is Robin Stocks, an unofficial Python library that wraps the Robinhood platform. If you already have a Robinhood account, this is the fastest way to start pulling financial data programmatically.

The tradeoffs are real, though. Because the library is unofficial, it breaks whenever Robinhood pushes app updates. There is no paper trading support, which means you cannot safely test strategies without risking actual money. For experimentation and data exploration it works fine, but it is not a production-grade foundation for live trading bots.

2. Alpaca Markets

Alpaca is Mariana's top recommendation for learning algorithmic trading. The key differentiator is that Alpaca offers paper trading accounts out of the box, so you can test strategies against live market data without putting capital at risk. This is where she personally started when building her first trading strategies.

The historical data for stocks and crypto is free, which removes a major barrier for beginners. The main limitation is that Alpaca only supports U.S. equities, so if you need international market data, you will have to look elsewhere.

3. Public API

The Public API stands out for having the broadest asset class coverage of the three. It supports stocks, options, crypto, and bonds in a single platform. Public also pays rebates on options contracts, which is a tangible financial incentive that the other two platforms do not offer.

Mariana highlights two additional strengths. First, the documentation is clean and well-organized, which matters when you are integrating a new API into your workflow. Second, Public recently launched a built-in AI agent mechanism that lets you define a trading strategy without coding the agent infrastructure yourself. She is actively experimenting with building AI agents inside the Public system and plans to share results.

Choosing the Right One

Each API fits a different stage of the learning curve:

Robin Stocks works if you already use Robinhood and want to quickly pull data into a Python notebook. Treat it as a scratchpad, not a trading system.
Alpaca Markets is the best starting point for anyone who wants to build and test real strategies with paper trading before committing money.
Public API is the most complete option for traders who want multi-asset coverage, options rebates, and are interested in the emerging AI agent tooling.

Key Takeaways

Robin Stocks is fast to set up but unofficial, fragile on updates, and lacks paper trading.
Alpaca Markets offers free historical data and paper accounts, making it the best learning environment for U.S. equities.
Public API covers the widest range of asset classes (stocks, options, crypto, bonds) and includes a new AI agent framework for strategy automation.
All three are free or low-cost entry points for beginners building ML trading models.
Paper trading should be your first step before deploying any live algorithmic strategy.