Fee tiers

Every send action takes fee as an exact amount or a named tier: 'min' | 'low' | 'mid' | 'high'. Tiers map to the node's three estimations; 'min' is the minimum relay fee (1 uSTX per serialized byte), computed offline with no round-trip. Omitting fee estimates mid. When the node answers NoEstimateAvailable (a quiet chain or fresh devnet with no fee history), the SDK falls back to 'min' instead of failing; resolveFee returns { fee, tier } so you can see which happened. Any other estimator failure (timeout, 5xx, auth) throws rather than silently under-paying, and the nonce reserved for that send is handed back.

await client.transferStx({ to, amount: 1000n, fee: "low" });
await client.callContract({ contract, functionName: "mint", fee: "min" });