diff --git a/core/v3/session.go b/core/v3/session.go
index f9088e67..e7c53303 100644
--- a/core/v3/session.go
+++ b/core/v3/session.go
@@ -1005,18 +1005,22 @@ func isExplicitSessionCallSignature(sig SessionCallSignature) bool {
 }
 
 // --- Hashing call with replay protection ---
-//
-// hashCallWithReplayProtection computes the hash of a call with chain id, space and nonce.
-// It concatenates 32-byte representations of chainId, space and nonce with the call hash,
-// then computes the Keccak256 hash, returning it as a hex string.
-func hashCallWithReplayProtection(call PayloadCall, chainId, space, nonce *big.Int) (string, error) {
-	chainIdB := intToBytesBig(chainId, 32)
-	spaceB := intToBytesBig(space, 32)
-	nonceB := intToBytesBig(nonce, 32)
-	callHash := call.HashCall() // Assume call.HashCall() returns []byte in hex format? Adjust as needed.
-	data := ConcatBytes(chainIdB, spaceB, nonceB, callHash)
-	hash := keccak256(data)
-	return "0x" + hex.EncodeToString(hash), nil
+
+// HashPayloadCallIdx computes the digest that a session key signs for
+// the call at callIdx of the given payload. It matches
+// SessionSig.hashPayloadCallIdx in the v3 wallet contracts:
+// keccak256(Payload.hashFor(payload, wallet) ++ uint256(callIdx)), where the
+// payload hash is the EIP-712 digest with the wallet as verifying contract.
+// The session signature is recovered with plain ecrecover over this digest,
+// without an EIP-191 prefix.
+func HashPayloadCallIdx(payload CallsPayload, callIdx int) (common.Hash, error) {
+	if callIdx < 0 || callIdx >= len(payload.Calls) {
+		return common.Hash{}, fmt.Errorf("call index %v out of range [0, %v)", callIdx, len(payload.Calls))
+	}
+	payloadHash := payload.Digest().Hash
+	var idx [32]byte
+	big.NewInt(int64(callIdx)).FillBytes(idx[:])
+	return common.BytesToHash(keccak256(ConcatBytes(payloadHash.Bytes(), idx[:]))), nil
 }
 
 // --- Helper functions ---
@@ -1044,18 +1048,6 @@ func intToBytes(n int, size int) []byte {
 	return b
 }
 
-// intToBytesBig converts a *big.Int to a byte slice of a given size.
-// It left-pads the number with zeros.
-func intToBytesBig(n *big.Int, size int) []byte {
-	b := n.Bytes()
-	if len(b) > size {
-		return b[len(b)-size:]
-	}
-	padded := make([]byte, size)
-	copy(padded[size-len(b):], b)
-	return padded
-}
-
 // keccak256 computes the Keccak256 hash of the given data.
 func keccak256(data []byte) []byte {
 	h := sha3.NewLegacyKeccak256()
@@ -1063,14 +1055,6 @@ func keccak256(data []byte) []byte {
 	return h.Sum(nil)
 }
 
-// --- PayloadCall interface ---
-//
-// For hashing calls we assume a minimal interface.
-type PayloadCall interface {
-	// HashCall returns a []byte representing the call data to be hashed.
-	HashCall() []byte
-}
-
 // -----------------------------------------------------------------------------
 // Helper Functions
 // -----------------------------------------------------------------------------
diff --git a/core/v3/session_digest_test.go b/core/v3/session_digest_test.go
new file mode 100644
index 00000000..4b8f1774
--- /dev/null
+++ b/core/v3/session_digest_test.go
@@ -0,0 +1,86 @@
+package v3_test
+
+import (
+	"math/big"
+	"testing"
+
+	"github.com/0xsequence/ethkit/go-ethereum/common"
+	v3 "github.com/0xsequence/go-sequence/core/v3"
+)
+
+// The expected digests below are reference vectors produced by
+// SessionSig.hashPayloadCallIdx in 0xsequence/wallet-contracts-v3 (via a forge
+// test constructing the identical payloads), so this test asserts parity with
+// the on-chain verification.
+func TestHashPayloadCallIdx(t *testing.T) {
+	payload := v3.NewCallsPayload(
+		common.HexToAddress("0x1111111111111111111111111111111111111111"),
+		big.NewInt(42161),
+		[]v3.Call{
+			{
+				To:              common.HexToAddress("0x2222222222222222222222222222222222222222"),
+				Value:           new(big.Int).Exp(big.NewInt(10), big.NewInt(18), nil), // 1 ether
+				Data:            common.Hex2Bytes("deadbeef"),
+				GasLimit:        big.NewInt(100000),
+				BehaviorOnError: v3.BehaviorOnErrorRevert,
+			},
+			{
+				To:              common.HexToAddress("0x3333333333333333333333333333333333333333"),
+				DelegateCall:    true,
+				OnlyFallback:    true,
+				BehaviorOnError: v3.BehaviorOnErrorAbort,
+			},
+		},
+		big.NewInt(0),
+		big.NewInt(7),
+	)
+
+	expectedPayloadHash := common.HexToHash("0x9c891ce70c80739f54b60eb7f8a0d0e80e7f90f2c9aea51a0c4f34ec1cee067e")
+	if digest := payload.Digest().Hash; digest != expectedPayloadHash {
+		t.Errorf("payload digest mismatch: got %v, expected %v", digest, expectedPayloadHash)
+	}
+
+	expectedCallHashes := []common.Hash{
+		common.HexToHash("0x1957bab0f26824823a1f40bb132e9a6ab186db88c0adb7fb806351f6766ba66c"),
+		common.HexToHash("0xcb5346174462b1c9a0279b5acbdfe2458ba0825cb10d35eb912fa491c1ee5cdd"),
+	}
+	for i, expected := range expectedCallHashes {
+		hash, err := v3.HashPayloadCallIdx(payload, i)
+		if err != nil {
+			t.Fatalf("HashPayloadCallIdx(%v): %v", i, err)
+		}
+		if hash != expected {
+			t.Errorf("call %v hash mismatch: got %v, expected %v", i, hash, expected)
+		}
+	}
+
+	payload2 := v3.NewCallsPayload(
+		common.HexToAddress("0x5555555555555555555555555555555555555555"),
+		big.NewInt(1),
+		[]v3.Call{
+			{
+				To:              common.HexToAddress("0x4444444444444444444444444444444444444444"),
+				Data:            []byte{0x00},
+				BehaviorOnError: v3.BehaviorOnErrorIgnore,
+			},
+		},
+		big.NewInt(12345),
+		big.NewInt(0),
+	)
+
+	expected2 := common.HexToHash("0xde2ebba8ab9a581d22dbb5d066086ae1ab3d884f9becc493448b2875e7f3c6d4")
+	hash2, err := v3.HashPayloadCallIdx(payload2, 0)
+	if err != nil {
+		t.Fatalf("HashPayloadCallIdx: %v", err)
+	}
+	if hash2 != expected2 {
+		t.Errorf("vector 2 hash mismatch: got %v, expected %v", hash2, expected2)
+	}
+
+	if _, err := v3.HashPayloadCallIdx(payload2, 1); err == nil {
+		t.Errorf("expected out-of-range error for call index 1")
+	}
+	if _, err := v3.HashPayloadCallIdx(payload2, -1); err == nil {
+		t.Errorf("expected out-of-range error for call index -1")
+	}
+}