diff --git a/config.go b/config.go
index 9b7856b..3c28c3c 100644
--- a/config.go
+++ b/config.go
@@ -56,7 +56,8 @@ type Config struct {
 
 	// HyperLiquid API
 	HLPrivateKey string // ed25519 private key hex
-	HLAddress    string // wallet address
+	HLAddress    string // main account address
+	HLAPIAddress string // API wallet address (signer, auto-derived if empty)
 }
 
 // jsonConfig maps config.json fields (non-secret defaults checked into git).
@@ -157,6 +158,7 @@ func LoadConfig() *Config {
 
 		HLPrivateKey: getEnv("HL_PRIVATE_KEY", ""),
 		HLAddress:    getEnv("HL_ADDRESS", ""),
+		HLAPIAddress: getEnv("HL_API_ADDRESS", ""),
 	}
 }
 
diff --git a/exchange/hyperliquid_trade.go b/exchange/hyperliquid_trade.go
index b78156f..2752042 100644
--- a/exchange/hyperliquid_trade.go
+++ b/exchange/hyperliquid_trade.go
@@ -39,6 +39,7 @@ type HLSignedAction struct {
 	Action      HLOrderAction `json:"action"`
 	Nonce       int64         `json:"nonce"`
 	Signature   string        `json:"signature"`
+	Address     string        `json:"address"` // API wallet address (signer)
 	VaultAddress string       `json:"vaultAddress,omitempty"`
 }
 
@@ -51,12 +52,13 @@ type HLOrderResponse struct {
 type HyperLiquidTrade struct {
 	PrivateKey ed25519.PrivateKey
 	Address    string
+	APIAddress string // API wallet address (signer), derived from private key
 	client     *http.Client
 	lastNonce  int64
 	nonceMu    sync.Mutex // protect lastNonce++ (Issue #4)
 }
 
-func NewHyperLiquidTrade(privateKeyHex, address string) (*HyperLiquidTrade, error) {
+func NewHyperLiquidTrade(privateKeyHex, mainAddress, apiAddress string) (*HyperLiquidTrade, error) {
 	if privateKeyHex == "" {
 		return &HyperLiquidTrade{client: &http.Client{Timeout: 10 * time.Second}}, nil
 	}
@@ -81,9 +83,17 @@ func NewHyperLiquidTrade(privateKeyHex, address string) (*HyperLiquidTrade, erro
 		return nil, fmt.Errorf("invalid private key length: %d (expected 32 or %d)", len(keyBytes), ed25519.PrivateKeySize)
 	}
 
+	// If apiAddress not provided, derive from private key
+	if apiAddress == "" {
+		privKey := ed25519.PrivateKey(keyBytes)
+		pubKey := privKey.Public().(ed25519.PublicKey)
+		apiAddress = "0x" + hex.EncodeToString(pubKey)
+	}
+
 	return &HyperLiquidTrade{
 		PrivateKey: ed25519.PrivateKey(keyBytes),
-		Address:    address,
+		Address:    mainAddress,
+		APIAddress: apiAddress,
 		client:     &http.Client{Timeout: 10 * time.Second},
 	}, nil
 }
@@ -130,6 +140,7 @@ func (h *HyperLiquidTrade) PlaceMarketOrder(coin, side, sz string) (string, erro
 		Action:    action,
 		Nonce:     nonce,
 		Signature: sig,
+		Address:   h.APIAddress,
 	}
 
 	bodyJSON, _ := json.Marshal(signed)
diff --git a/trader.go b/trader.go
index c3168c8..0d0a1af 100644
--- a/trader.go
+++ b/trader.go
@@ -191,7 +191,7 @@ func NewTrader(cfg *Config, database *db.DB) *Trader {
 	if cfg.BitgetAPIKey != "" {
 		bt = exchange.NewBitgetTrade(cfg.BitgetAPIKey, cfg.BitgetAPISecret, cfg.BitgetPassphrase)
 	}
-	hl, _ := exchange.NewHyperLiquidTrade(cfg.HLPrivateKey, cfg.HLAddress)
+	hl, _ := exchange.NewHyperLiquidTrade(cfg.HLPrivateKey, cfg.HLAddress, cfg.HLAPIAddress)
 
 	t := &Trader{
 		cfg:            cfg,