diff --git a/backend/protocol_rpc/transactions_parser.py b/backend/protocol_rpc/transactions_parser.py
index 1a0f85126..93e754682 100644
--- a/backend/protocol_rpc/transactions_parser.py
+++ b/backend/protocol_rpc/transactions_parser.py
@@ -604,17 +604,22 @@ def decode_method_send_data(self, data: str) -> DecodedMethodSendData:
     def decode_method_call_data(self, data: str) -> DecodedMethodCallData:
         raw_bytes = eth_utils.hexadecimal.decode_hex(data)
 
-        # Remove the null byte
-        if raw_bytes[-1] == 0:
+        # Newer clients send rlp([calldata, leader_only]). leader_only is a
+        # single byte, so it survives RLP encoding as a literal 0x00 or 0x01.
+        if len(raw_bytes) > 1 and raw_bytes[-1] in (0, 1):
             raw_bytes = raw_bytes[:-1]
 
-            # Try to decode the outer list first
-            if raw_bytes[0] >= 0xF8:  # Long list
-                raw_bytes = raw_bytes[2:]  # Skip list prefix and length
-            elif raw_bytes[0] >= 0xC0:  # Short list
-                raw_bytes = raw_bytes[1:]  # Skip list prefix
-
-            # Now try to decode the inner string
+            # Strip the outer list header.
+            prefix = raw_bytes[0]
+            if prefix >= 0xF8:
+                # Long list: 0xF7 + number of length bytes, so the header is
+                # 1 + (prefix - 0xF7) bytes. It is 3 bytes once the payload
+                # reaches 256 bytes, not always 2.
+                raw_bytes = raw_bytes[1 + (prefix - 0xF7) :]
+            elif prefix >= 0xC0:  # Short list
+                raw_bytes = raw_bytes[1:]
+
+            # Now decode the inner string
             raw_bytes = rlp.decode(raw_bytes)
 
         return DecodedMethodCallData(raw_bytes)
diff --git a/tests/unit/test_transactions_parser.py b/tests/unit/test_transactions_parser.py
index 8c102d4fa..0f9fc032a 100644
--- a/tests/unit/test_transactions_parser.py
+++ b/tests/unit/test_transactions_parser.py
@@ -579,3 +579,48 @@ def test_decode_signed_transaction_submit_appeal_uses_value_as_bond(monkeypatch)
     assert decoded.data.tx_id == tx_id
     assert decoded.data.fees_distribution is None
     assert decoded.data.top_up_and_submit is False
+
+
+# --- decode_method_call_data -------------------------------------------------
+# Wire format produced by genlayer-js readContract / eth_call is
+# rlp([calldata, leader_only]); see genlayer-js src/contracts/actions.ts.
+
+
+def _call_wire(calldata_bytes: bytes, leader_only: bool = False) -> str:
+    payload = encode([calldata_bytes, b"\x01" if leader_only else b"\x00"])
+    return "0x" + payload.hex()
+
+
+def _opaque_calldata(size: int) -> bytes:
+    return bytes([(3 << 3) | 6]) + b"\x41" * (size - 1)
+
+
+@pytest.mark.parametrize("size", [1, 5, 100, 252, 253, 300, 1000, 70000])
+def test_decode_method_call_data_roundtrip(transaction_parser, size):
+    calldata_bytes = _opaque_calldata(size)
+    decoded = transaction_parser.decode_method_call_data(_call_wire(calldata_bytes))
+    assert decoded.calldata == calldata_bytes
+
+
+@pytest.mark.parametrize("size", [5, 253, 1000])
+def test_decode_method_call_data_leader_only(transaction_parser, size):
+    calldata_bytes = _opaque_calldata(size)
+    decoded = transaction_parser.decode_method_call_data(
+        _call_wire(calldata_bytes, leader_only=True)
+    )
+    assert decoded.calldata == calldata_bytes
+
+
+def test_decode_method_call_data_long_list_header_boundary(transaction_parser):
+    # RLP switches from a 2-byte to a 3-byte list header at 256 payload bytes,
+    # which lands between these two calldata sizes.
+    for size in (252, 253):
+        calldata_bytes = _opaque_calldata(size)
+        decoded = transaction_parser.decode_method_call_data(_call_wire(calldata_bytes))
+        assert decoded.calldata == calldata_bytes
+
+
+@pytest.mark.parametrize("data", ["0x", "0x00", "0x01"])
+def test_decode_method_call_data_degenerate_input(transaction_parser, data):
+    # Too short to be a wrapped payload; must not raise.
+    transaction_parser.decode_method_call_data(data)