1. Check your plan
The REST API is a Growth and Scale feature (a Free-plan key is refused with RU-1003). The MCP server is more permissive: Free plans get read-only tool access, with mutating tools paid (RU-1005).
2. Generate the key
From your account's API & MCP access section, generate a key, choosing read-only or read-write permission. Copy it immediately: it is stored hashed and never shown again.
3. Authenticate
Send it as Authorization: Bearer <api key> on every request to /api/v1, or configure it in your MCP client for mcp.realuptime.io. Missing or wrong credentials answer 401 with codes RU-1001 or RU-1002.
4. Mind the budgets
Reads and writes have per-minute budgets (RU-2002 and RU-2003 when exceeded); honor the Retry-After header and the limits take care of themselves.
Go deeper
The full reference lives in the docs: API documentation. Error codes named above are each explained in the error-code reference.