6 min · +90 XP
What an API is
How your tools can talk to the services you already use.
By the end of this lesson
You will be able to tell whether a service can be connected to your tool and how much effort that takes.
You do not go into the kitchen. You order from the menu and get a dish back. An API is the menu a service offers to programs: what can be asked for and how. Your tool orders from it and gets back a forecast, a message sent, a row added to a table.
Most services you use every day have such a menu. You do not need to study them: Claude knows many of them and can read the documentation of the others. Your part is to decide which connection you want.
Services you may already use, and how hard they are to connect
- 1Telegram: the easiest start. A bot is created in a couple of minutes and can send messages to you or to people who started it. Free.
- 2Google Sheets, Calendar, Gmail, YouTube: possible, but the setup is longer (a project in Google Cloud Console, a consent screen). Plan half an hour with Claude guiding you, or start with exporting and importing CSV files.
- 3Instagram: the API is open only to business or creator accounts and needs an app review. For a personal tool, skip it.
- 4Weather, currency rates, timetables, translation: many services have a free tier with a key.
- 5Notion, Trello, Todoist and similar: your notes and tasks can be read and written.
- 6Payments (Stripe, PayPal and others): they need a verified business account and are not available in every country. Leave them for later.
“Does Telegram have an API, is there a free tier, and can my tool use it to send me a message when I go over budget? Check the service's current documentation and answer in three sentences.”
Put your own service and task in place of the example. Services change their rules, so the request to check the current documentation matters.
If you want your own tool to call Claude, for example to summarise texts on its own, that is the Claude API. It needs a separate account in the Claude Console (platform.claude.com) and is paid per use. Your Pro or Max subscription covers your work in Claude Code, not the API calls your tool makes.
What connections make possible
- 1A weather API: a morning note on what to wear.
- 2A Telegram bot: your budget tool tells you when you go over the food budget.
- 3Google Sheets: receipts go into the table your accountant already uses.
- 4A calendar API: a booking page that creates events by itself.
›What happens when a tool "calls an API"
Your tool sends a short message over the internet to the service's address, for example "forecast for Berlin, tomorrow", together with your key. The service answers with data in a format programs read easily. Claude writes this part of the code; you only see the result in your tool.
Before you build on an API, ask Claude to check its rate limits, the free-tier quotas and the terms of use for your case. Prefer the official SDK where one exists, and webhooks over frequent polling where the service offers them. Ask for error handling and retries with backoff from the start, so a slow API does not break the whole tool.
You want your expense tool to send you a message when you go over budget. Which connection is the simplest start?
Facts checked against the official Claude Code docs October 24, 2026 · source
Author
Evgenii Arsentev
PhD · Chief Executive Officer, digital health