One public profile, normalised into the same envelope as every other platform. Source: the account's own instance (official API, no key). Costs 1 credit; the answer is cached for an hour and a cache hit costs nothing (cached: true, credits_used: 0).

user@instance, and the instance is required. Mastodon is federated: Gargron alone identifies nobody, because a different person can hold that name on every server. A handle without an instance is refused with 400 rather than completed with a default server — completing it would return somebody else's profile with success: true. The instance must be a public DNS name: IP literals, ports and paths are refused. The handle comes back fully qualified, so the response value is a valid next request.