200
The key works
A JSON body comes back. Nothing else to do.
Authentication
Agents authenticate against Tatum by sending an API key in the x-api-key header on every request. No token exchange, no refresh, no callback URL.
The whole contract
One header on every request, against every route on every chain. That is the entire auth surface.
GET /v4/data/wallet/portfolio?address=0x…&chain=ethereum-mainnet Host: api.tatum.io x-api-key: t-6890…c0b8 Where the key lives
Read the key from the environment everywhere. Never commit it, and never put it in client-side code.
ENV VAR
TATUM_API_KEY SURFACE LOCATION NOTE
Blockchain MCP
env in your MCP client JSON Lives in the editor config, never in the repo.
Backend app
.env or a secret manager Server side only. The key is not public.
CI
GitHub Actions · GitLab · Vercel · Cloudflare One key per environment, so you can rotate one at a time.
Local shell
export TATUM_API_KEY=… Fine for scratch work. Rotate the key when you are done.
Check it in one call
A JSON body means the key works. Anything else is one of three answers, and each one tells you exactly what to change.
$ curl -s -H "x-api-key: $TATUM_API_KEY" \ "https://api.tatum.io/v4/data/exchange-rate?currency=ETH&basePair=USD" {"value":"3241.08","basePair":"USD"} 401
Header missing or wrong
Check the spelling of x-api-key and that the variable resolved.
429
Rate limited
Back off and retry. Do not hammer the API in a loop.