Definition
A master account can approve API wallets to sign on its own behalf or for any of its sub-accounts. They are only used to sign: account data must be queried with the real address of the account, and the documentation names querying the agent address a common pitfall, since it returns an empty result rather than an error. An account can hold one unnamed approved wallet and up to three named ones, with two more named agents allowed per sub-account, and an expiry set with valid_until can be at most 180 days in the future.
What such a key can and cannot do was measured rather than assumed. On 28 September 2026, on mainnet, a test account approved a fresh agent for one hour and had it sign every action that moves money for 1,000 USDC against a balance of about 60, so nothing could move and only the reason for each answer counted. Refused on the signer: withdraw3, usdSend, spotSend, sendAsset, usdClassTransfer, a second approveAgent and approveBuilderFee, where the account itself met a balance error instead. Accepted: placing and cancelling an order. And one movement of funds was accepted, vaultTransfer, with which 5 USDC signed by the agent key reached a Hyperliquid vault.
One operational warning belongs with the definition. Nonces are tracked per signer, and an API wallet's nonce state may be pruned when the wallet is deregistered by the approval of a new one, when it expires, or when the account that registered it no longer has funds. The documentation says previously signed actions can then be replayed, and strongly suggests never reusing an agent address: generate a new one instead.
In our own setup the agent key lives on the computer, sealed on disk under a password and held in memory by the guardian only while it is unlocked. The guardian signs a closed list of trading actions and nothing else, the key expires on its own with warnings 30, 14, 7, 3 and 1 days before, and lpa agent revoke retires it with your wallet. No file on a computer stops a thief holding both the key and its password, so the bound that holds is the account: what it contains is the most that can be lost.
Example
An example, in round figures. An account funded with 1,000 USDC approves one agent key, with an expiry 180 days ahead, the ceiling Hyperliquid allows. That key can open and close positions against the 1,000 USDC and set a market's leverage and margin mode; it cannot withdraw a cent of it, send it to another address or move it between the spot and the perpetuals balance. The whole of what it can reach is those 1,000 USDC, which is why a dedicated account is the bound that matters.