diff --git a/core/src/operator.rs b/core/src/operator.rs
index dec65c9d..c8612e3b 100644
--- a/core/src/operator.rs
+++ b/core/src/operator.rs
@@ -11,6 +11,7 @@ use crate::{script_builder, utils, EVMAddress, UTXO};
 use bitcoin::address::NetworkUnchecked;
 use bitcoin::consensus::deserialize;
 use bitcoin::hashes::Hash;
+use bitcoin::script::PushBytesBuf;
 use bitcoin::sighash::SighashCache;
 use bitcoin::{Address, OutPoint, TapSighash, Transaction, TxOut, Txid};
 use bitcoin_mock_rpc::RpcApiWrapper;
@@ -266,7 +267,9 @@ where
             &Message::from_digest_slice(sighash.as_byte_array()).expect("should be hash"),
             &user_xonly_pk,
         )?;
-        let op_return_txout = script_builder::op_return_txout(self.idx.to_be_bytes());
+        let mut push_bytes = PushBytesBuf::new();
+        push_bytes.extend_from_slice(&utils::usize_to_var_len_bytes(self.idx)).unwrap();
+        let op_return_txout = script_builder::op_return_txout(push_bytes);
         tx.output.push(op_return_txout.clone());
         let funded_tx = self
             .rpc
diff --git a/core/src/transaction_builder.rs b/core/src/transaction_builder.rs
index 3e1b2d0d..d9c636e6 100644
--- a/core/src/transaction_builder.rs
+++ b/core/src/transaction_builder.rs
@@ -290,13 +290,7 @@ impl TransactionBuilder {
             network,
         );
         let mut op_return_script: Vec<u8> = hex::decode(move_txid.to_string()).unwrap();
-        let usize_bytes = (usize::BITS / 8) as usize;
-        let bits = operator_idx.max(1).ilog2() + 1;
-        let len = ((bits + 7) / 8) as usize;
-        let empty = usize_bytes - len;
-        let op_idx_bytes = operator_idx.to_be_bytes();
-        let op_idx_bytes = &op_idx_bytes[empty..];
-        op_return_script.extend(op_idx_bytes);
+        op_return_script.extend(utils::usize_to_var_len_bytes(operator_idx));
         let mut push_bytes = PushBytesBuf::new();
         push_bytes.extend_from_slice(&op_return_script).unwrap();
         let op_return_txout = script_builder::op_return_txout(push_bytes);
diff --git a/core/src/utils.rs b/core/src/utils.rs
index 49359110..faa28a2a 100644
--- a/core/src/utils.rs
+++ b/core/src/utils.rs
@@ -58,6 +58,16 @@ pub fn parse_hex_to_btc_tx(
     }
 }
 
+pub fn usize_to_var_len_bytes(x: usize) -> Vec<u8> {
+    let usize_bytes = (usize::BITS / 8) as usize;
+    let bits = x.max(1).ilog2() + 1;
+    let len = ((bits + 7) / 8) as usize;
+    let empty = usize_bytes - len;
+    let op_idx_bytes = x.to_be_bytes();
+    let op_idx_bytes = &op_idx_bytes[empty..];
+    op_idx_bytes.to_vec()
+}
+
 pub fn handle_taproot_witness_new<T: AsRef<[u8]>>(
     tx: &mut TxHandler,
     witness_elements: &[T],
diff --git a/core/tests/flow.rs b/core/tests/flow.rs
index 7af4ce4f..d31fe92a 100644
--- a/core/tests/flow.rs
+++ b/core/tests/flow.rs
@@ -52,15 +52,17 @@ async fn test_honest_operator_takes_refund() {
         .new_withdrawal_sig_rpc(0, user_sig, empty_utxo, withdrawal_tx_out)
         .await
         .unwrap();
-    println!("{:?}", withdrawal_provide_txid);
+    println!("Withdrawal provide: {:?}", withdrawal_provide_txid);
     let txs_to_be_sent = operators[0]
         .0
         .withdrawal_proved_on_citrea_rpc(0, deposit_outpoint)
         .await
         .unwrap();
+    tracing::debug!("txs_to_be_sent: {:#?}", txs_to_be_sent);
 
     for tx in txs_to_be_sent.iter().take(txs_to_be_sent.len() - 1) {
-        rpc.send_raw_transaction(tx.clone()).unwrap();
+        let outpoint = rpc.send_raw_transaction(tx.clone()).unwrap();
+        tracing::debug!("outpoint: {:#?}", outpoint);
     }
     rpc.mine_blocks(OPERATOR_TAKES_AFTER as u64).unwrap();
     // send the last tx