Stock API Price Alerts Need State as Well as a Price Threshold

Summary: A stock API can supply prices for an alert, but a successful subscription does not define when a notification should fire. A useful alert needs an observation type, a trigger rule, remembered state, and a policy for missing data. In a synthetic five-update example, checking whether the price exceeds $100 produces three matches, while detecting an observed upward crossing produces two events. The distinction prevents repeated updates above the threshold from becoming repeated crossing alerts.

What the API contributes to a price alert

A stock API gives software access to stock-related observations, such as recent prices, trades, quotes, or historical bars. A price alert is one application of that data. The API supplies observations; the application decides which observations qualify, whether the rule has changed state, and what message to deliver.

That division matters even for a small watchlist.…