An API key is a personal password for software. When a program — a script, an app, a tool — talks to an AI company's servers, the key tells the company who is asking, so they know whose account to bill and can shut the key off if it's stolen. It's a long random string like sk-ant-api03-x7Kd9… that you copy once and keep secret.

You need one when software has to talk to the AI on your behalf: you're building an app or script, a tool you installed asks for a key, you want to process things in bulk, or you're wiring AI into spreadsheets and automations.

You don't need one to simply chat — the consumer chat apps are easier and often cheaper for that. Note that ChatGPT Plus, Claude Pro, and similar subscriptions are separate products: they upgrade the chat experience but include no API access. And if a random website asks you to paste your key in, that's usually a scam.

Billing is pay-per-use, not a subscription — you're charged per token (roughly ¾ of a word). Casual tinkering costs cents, but always set a spending limit: a buggy script in a loop can rack up charges. Each walkthrough below points out where.

https://generativelanguage.googleapis.com/v1beta/models?key=YOUR_KEY

curl "https://generativelanguage.googleapis.com/v1beta/models/gemini-2.5-flash:generateContent?key=YOUR_KEY" \
  -H "Content-Type: application/json" \
  -d '{"contents":[{"parts":[{"text":"Say hello!"}]}]}'

curl https://api.openai.com/v1/chat/completions \
  -H "Authorization: Bearer YOUR_KEY" \
  -H "Content-Type: application/json" \
  -d '{"model":"gpt-5-mini","messages":[{"role":"user","content":"Say hello!"}]}'

curl https://api.anthropic.com/v1/messages \
  -H "x-api-key: YOUR_KEY" \
  -H "anthropic-version: 2023-06-01" \
  -H "Content-Type: application/json" \
  -d '{"model":"claude-sonnet-4-5","max_tokens":100,"messages":[{"role":"user","content":"Say hello!"}]}'

curl https://api.x.ai/v1/chat/completions \
  -H "Authorization: Bearer YOUR_KEY" \
  -H "Content-Type: application/json" \
  -d '{"model":"grok-4","messages":[{"role":"user","content":"Say hello!"}]}'

curl https://api.groq.com/openai/v1/chat/completions \
  -H "Authorization: Bearer YOUR_KEY" \
  -H "Content-Type: application/json" \
  -d '{"model":"llama-3.3-70b-versatile","messages":[{"role":"user","content":"Say hello!"}]}'

Treat a key like a credit card number — anyone who has it can spend your money. Never put keys in code you share; bots scan public repositories and find leaked keys within minutes. Use environment variables or a .env file instead, set spending limits before writing any code, and use one key per project so a leak only costs you that one. If a key does leak, revoke it in the console and make a new one — it takes thirty seconds.

# the standard safe pattern (Mac/Linux terminal)
export OPENAI_API_KEY="sk-..."
export ANTHROPIC_API_KEY="sk-ant-..."
export GEMINI_API_KEY="AIza..."
export XAI_API_KEY="xai-..."
# most tools and SDKs find these automatically