diff --git a/src/optimized_transaction.rs b/src/optimized_transaction.rs
index 4fdeddb..2e030c3 100644
--- a/src/optimized_transaction.rs
+++ b/src/optimized_transaction.rs
@@ -6,6 +6,7 @@ use crate::types::{
     SmartTransactionConfig, Timeout,
 };
 use crate::Helius;
+use std::collections::HashSet;
 use std::str::FromStr;
 use std::sync::Arc;
 
@@ -64,6 +65,42 @@ const MIN_TIP_LAMPORTS_DUAL: u64 = 200_000; // 0.0002 SOL
 /// Minimum tip: 0.000005 SOL (5,000 lamports).
 const MIN_TIP_LAMPORTS_SWQOS: u64 = 5_000; // 0.000005 SOL
 
+fn collect_unique_signers(signers: &[Arc<dyn Signer>], fee_payer: Option<&Arc<dyn Signer>>) -> Vec<Arc<dyn Signer>> {
+    let mut all_signers: Vec<Arc<dyn Signer>> = Vec::with_capacity(signers.len() + usize::from(fee_payer.is_some()));
+    let mut seen: HashSet<Pubkey> = HashSet::with_capacity(all_signers.capacity());
+
+    if let Some(fee_payer) = fee_payer {
+        if seen.insert(fee_payer.pubkey()) {
+            all_signers.push(fee_payer.clone());
+        }
+    }
+
+    for signer in signers {
+        if seen.insert(signer.pubkey()) {
+            all_signers.push(signer.clone());
+        }
+    }
+
+    all_signers
+}
+
+fn collect_unique_keypair_refs<'a>(signers: &'a [Keypair], fee_payer: &'a Keypair) -> Vec<&'a Keypair> {
+    let mut all_signers: Vec<&Keypair> = Vec::with_capacity(signers.len() + 1);
+    let mut seen: HashSet<Pubkey> = HashSet::with_capacity(all_signers.capacity());
+
+    if seen.insert(fee_payer.pubkey()) {
+        all_signers.push(fee_payer);
+    }
+
+    for signer in signers {
+        if seen.insert(signer.pubkey()) {
+            all_signers.push(signer);
+        }
+    }
+
+    all_signers
+}
+
 /// URL to fetch current Jito bundle tip floor prices.
 ///
 /// This endpoint returns the minimum tip amounts required for different
@@ -355,8 +392,8 @@ impl Helius {
                     {
                         return Ok(txt_sig);
                     }
-                    if status.err.is_some() {
-                        return Err(HeliusError::TransactionError(status.err.unwrap()));
+                    if let Some(err) = status.err {
+                        return Err(HeliusError::TransactionError(err));
                     }
                 }
                 None => {
@@ -451,12 +488,13 @@ impl Helius {
         final_instructions.push(compute_budget_ix);
 
         // Get the optimal compute units
+        let all_signers: Vec<Arc<dyn Signer>> = collect_unique_signers(&config.signers, config.fee_payer.as_ref());
         let units: Option<u64> = self
             .get_compute_units(
                 updated_instructions,
                 payer_pubkey,
                 config.lookup_tables.clone().unwrap_or_default(),
-                Some(&config.signers),
+                Some(&all_signers),
             )
             .await?;
 
@@ -491,26 +529,9 @@ impl Helius {
             let v0_message: v0::Message =
                 v0::Message::try_compile(&payer_pubkey, &final_instructions, lookup_tables, recent_blockhash)?;
             let versioned_message: VersionedMessage = VersionedMessage::V0(v0_message);
-
-            let all_signers: Vec<Arc<dyn Signer>> = if let Some(fee_payer) = config.fee_payer.as_ref() {
-                let mut all_signers = config.signers.clone();
-                if !all_signers.iter().any(|signer| signer.pubkey() == fee_payer.pubkey()) {
-                    all_signers.push(fee_payer.clone());
-                }
-                all_signers
-            } else {
-                config.signers.clone()
-            };
-
-            let signatures: Vec<Signature> = all_signers
-                .iter()
-                .map(|signer| signer.try_sign_message(versioned_message.serialize().as_slice()))
-                .collect::<std::result::Result<Vec<_>, _>>()?;
-
-            let versioned_transaction = VersionedTransaction {
-                signatures,
-                message: versioned_message,
-            };
+            let versioned_transaction: VersionedTransaction =
+                VersionedTransaction::try_new(versioned_message, all_signers.as_slice())
+                    .map_err(|e| HeliusError::InvalidInput(format!("Signing error: {:?}", e)))?;
 
             Ok((
                 SmartTransaction::Versioned(versioned_transaction),
@@ -518,11 +539,7 @@ impl Helius {
             ))
         } else {
             let mut tx: Transaction = Transaction::new_with_payer(&final_instructions, Some(&payer_pubkey));
-            tx.try_partial_sign(&config.signers, recent_blockhash)?;
-
-            if let Some(fee_payer) = config.fee_payer.as_ref() {
-                tx.try_partial_sign(&[fee_payer], recent_blockhash)?;
-            }
+            tx.try_partial_sign(&all_signers, recent_blockhash)?;
 
             Ok((SmartTransaction::Legacy(tx), last_valid_block_hash))
         }
@@ -637,16 +654,8 @@ impl Helius {
         let versioned_message: VersionedMessage = VersionedMessage::V0(v0_message);
 
         let transaction: VersionedTransaction = if let Some(keypairs) = keypairs {
-            let mut tx = VersionedTransaction {
-                signatures: vec![Signature::default(); keypairs.len()],
-                message: versioned_message.clone(),
-            };
-
-            for (i, keypair) in keypairs.iter().enumerate() {
-                tx.signatures[i] = keypair.sign_message(&versioned_message.serialize());
-            }
-
-            tx
+            VersionedTransaction::try_new(versioned_message, keypairs)
+                .map_err(|e| HeliusError::InvalidInput(format!("Signing error: {:?}", e)))?
         } else {
             VersionedTransaction {
                 signatures: vec![],
@@ -759,14 +768,7 @@ impl Helius {
         let mut test_instructions: Vec<Instruction> = final_instructions.clone();
         test_instructions.extend(create_config.instructions.clone());
 
-        let mut all_signers: Vec<&Keypair> = vec![&fee_payer];
-        let mut seen: std::collections::HashSet<Pubkey> = std::collections::HashSet::new();
-        seen.insert(fee_payer.pubkey());
-        for kp in &keypairs {
-            if seen.insert(kp.pubkey()) {
-                all_signers.push(kp);
-            }
-        }
+        let all_signers: Vec<&Keypair> = collect_unique_keypair_refs(&keypairs, &fee_payer);
 
         let units: Option<u64> = self
             .get_compute_units_thread_safe(
@@ -804,29 +806,13 @@ impl Helius {
             )?;
 
             let versioned_message: VersionedMessage = VersionedMessage::V0(message);
-
-            let fee_payer_copy: Keypair = fee_payer.insecure_clone();
-            let mut all_signers: Vec<Keypair> = vec![fee_payer_copy];
-            all_signers.extend(keypairs.into_iter().filter(|k| k.pubkey() != fee_payer.pubkey()));
-
-            let mut tx: VersionedTransaction = VersionedTransaction {
-                signatures: vec![Signature::default(); all_signers.len()],
-                message: versioned_message.clone(),
-            };
-
-            // Sign message with all keypairs
-            for (i, keypair) in all_signers.iter().enumerate() {
-                tx.signatures[i] = keypair.sign_message(&versioned_message.serialize());
-            }
+            let tx: VersionedTransaction = VersionedTransaction::try_new(versioned_message, all_signers.as_slice())
+                .map_err(|e| HeliusError::InvalidInput(format!("Signing error: {:?}", e)))?;
 
             SmartTransaction::Versioned(tx)
         } else {
             let mut tx: Transaction = Transaction::new_with_payer(&final_instructions, Some(&fee_payer.pubkey()));
-
-            let mut signers: Vec<&Keypair> = vec![&fee_payer];
-            signers.extend(keypairs.iter().filter(|k| k.pubkey() != fee_payer.pubkey()));
-
-            tx.sign(&signers, recent_blockhash);
+            tx.sign(&all_signers, recent_blockhash);
 
             SmartTransaction::Legacy(tx)
         };
@@ -1210,3 +1196,190 @@ impl Helius {
         }
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::{collect_unique_keypair_refs, collect_unique_signers};
+    use solana_sdk::{
+        hash::Hash,
+        instruction::{AccountMeta, Instruction},
+        message::{v0, VersionedMessage},
+        pubkey::Pubkey,
+        signature::{Keypair, Signature, Signer},
+        transaction::{Transaction, VersionedTransaction},
+    };
+    use std::sync::Arc;
+
+    fn build_versioned_message(
+        payer: &Keypair,
+        writable_signer: &Keypair,
+        readonly_signer: &Keypair,
+    ) -> VersionedMessage {
+        let instruction = Instruction {
+            program_id: Pubkey::new_unique(),
+            accounts: vec![
+                AccountMeta::new(writable_signer.pubkey(), true),
+                AccountMeta::new_readonly(readonly_signer.pubkey(), true),
+            ],
+            data: vec![],
+        };
+
+        VersionedMessage::V0(
+            v0::Message::try_compile(&payer.pubkey(), &[instruction], &[], Hash::new_unique()).unwrap(),
+        )
+    }
+
+    #[test]
+    fn collect_unique_signers_includes_fee_payer_once() {
+        let fee_payer: Arc<dyn Signer> = Arc::new(Keypair::new());
+        let signer: Arc<dyn Signer> = Arc::new(Keypair::new());
+
+        let signers: Vec<Arc<dyn Signer>> = vec![signer.clone(), fee_payer.clone(), signer.clone()];
+        let all_signers = collect_unique_signers(&signers, Some(&fee_payer));
+
+        let signer_pubkeys: Vec<Pubkey> = all_signers.iter().map(|signer| signer.pubkey()).collect();
+        assert_eq!(signer_pubkeys, vec![fee_payer.pubkey(), signer.pubkey()]);
+    }
+
+    #[test]
+    fn collect_unique_keypair_refs_includes_fee_payer_once() {
+        let fee_payer = Keypair::new();
+        let signer = Keypair::new();
+        let signers = vec![
+            signer.insecure_clone(),
+            fee_payer.insecure_clone(),
+            signer.insecure_clone(),
+        ];
+
+        let all_signers = collect_unique_keypair_refs(&signers, &fee_payer);
+        let signer_pubkeys: Vec<Pubkey> = all_signers.iter().map(|signer| signer.pubkey()).collect();
+
+        assert_eq!(signer_pubkeys, vec![fee_payer.pubkey(), signer.pubkey()]);
+    }
+
+    #[test]
+    fn versioned_try_new_reorders_arc_signers_to_match_message() {
+        let fee_payer = Keypair::new();
+        let writable_signer = Keypair::new();
+        let readonly_signer = Keypair::new();
+        let fee_payer_signer: Arc<dyn Signer> = Arc::new(fee_payer.insecure_clone());
+        let writable_signer_arc: Arc<dyn Signer> = Arc::new(writable_signer.insecure_clone());
+        let readonly_signer_arc: Arc<dyn Signer> = Arc::new(readonly_signer.insecure_clone());
+
+        let message = build_versioned_message(&fee_payer, &writable_signer, &readonly_signer);
+        let signers: Vec<Arc<dyn Signer>> = vec![readonly_signer_arc.clone(), writable_signer_arc.clone()];
+        let all_signers = collect_unique_signers(&signers, Some(&fee_payer_signer));
+        let tx = VersionedTransaction::try_new(message.clone(), all_signers.as_slice()).unwrap();
+        let message_bytes = message.serialize();
+
+        assert_eq!(
+            tx.signatures,
+            vec![
+                Signature::from(fee_payer.sign_message(&message_bytes)),
+                Signature::from(writable_signer.sign_message(&message_bytes)),
+                Signature::from(readonly_signer.sign_message(&message_bytes)),
+            ]
+        );
+    }
+
+    #[test]
+    fn manual_fee_payer_appended_signature_order_fails_verification() {
+        let fee_payer = Keypair::new();
+        let writable_signer = Keypair::new();
+        let readonly_signer = Keypair::new();
+        let message = build_versioned_message(&fee_payer, &writable_signer, &readonly_signer);
+        let message_bytes = message.serialize();
+
+        // This mirrors the pre-fix separate fee payer path: caller signers first, fee payer appended last.
+        let manual_signatures = [
+            readonly_signer.sign_message(&message_bytes),
+            writable_signer.sign_message(&message_bytes),
+            fee_payer.sign_message(&message_bytes),
+        ];
+
+        let verification_results: Vec<bool> = manual_signatures
+            .iter()
+            .zip(message.static_account_keys().iter())
+            .map(|(signature, pubkey)| signature.verify(pubkey.as_ref(), &message_bytes))
+            .collect();
+
+        assert_eq!(verification_results, vec![false, true, false]);
+    }
+
+    #[test]
+    fn manual_non_payer_caller_order_can_fail_verification() {
+        let fee_payer = Keypair::new();
+        let writable_signer = Keypair::new();
+        let readonly_signer = Keypair::new();
+        let message = build_versioned_message(&fee_payer, &writable_signer, &readonly_signer);
+        let message_bytes = message.serialize();
+
+        // This mirrors the pre-fix seed path: fee payer first, remaining signers left in caller order.
+        let manual_signatures = [
+            fee_payer.sign_message(&message_bytes),
+            readonly_signer.sign_message(&message_bytes),
+            writable_signer.sign_message(&message_bytes),
+        ];
+
+        let verification_results: Vec<bool> = manual_signatures
+            .iter()
+            .zip(message.static_account_keys().iter())
+            .map(|(signature, pubkey)| signature.verify(pubkey.as_ref(), &message_bytes))
+            .collect();
+
+        assert_eq!(verification_results, vec![true, false, false]);
+    }
+
+    #[test]
+    fn versioned_try_new_reorders_keypair_signers_to_match_message() {
+        let fee_payer = Keypair::new();
+        let writable_signer = Keypair::new();
+        let readonly_signer = Keypair::new();
+
+        let message = build_versioned_message(&fee_payer, &writable_signer, &readonly_signer);
+        let signers = vec![readonly_signer.insecure_clone(), writable_signer.insecure_clone()];
+        let all_signers = collect_unique_keypair_refs(&signers, &fee_payer);
+        let tx = VersionedTransaction::try_new(message.clone(), all_signers.as_slice()).unwrap();
+        let message_bytes = message.serialize();
+
+        assert_eq!(
+            tx.signatures,
+            vec![
+                Signature::from(fee_payer.sign_message(&message_bytes)),
+                Signature::from(writable_signer.sign_message(&message_bytes)),
+                Signature::from(readonly_signer.sign_message(&message_bytes)),
+            ]
+        );
+    }
+
+    #[test]
+    fn legacy_try_partial_sign_reorders_keypairs_to_match_message() {
+        let fee_payer = Keypair::new();
+        let writable_signer = Keypair::new();
+        let readonly_signer = Keypair::new();
+        let recent_blockhash = Hash::new_unique();
+        let instruction = Instruction {
+            program_id: Pubkey::new_unique(),
+            accounts: vec![
+                AccountMeta::new(writable_signer.pubkey(), true),
+                AccountMeta::new_readonly(readonly_signer.pubkey(), true),
+            ],
+            data: vec![],
+        };
+        let mut tx = Transaction::new_with_payer(&[instruction], Some(&fee_payer.pubkey()));
+        let signers = vec![readonly_signer.insecure_clone(), writable_signer.insecure_clone()];
+        let all_signers = collect_unique_keypair_refs(&signers, &fee_payer);
+
+        tx.try_partial_sign(&all_signers, recent_blockhash).unwrap();
+
+        let message_bytes = tx.message_data();
+        assert_eq!(
+            tx.signatures,
+            vec![
+                fee_payer.sign_message(&message_bytes),
+                writable_signer.sign_message(&message_bytes),
+                readonly_signer.sign_message(&message_bytes),
+            ]
+        );
+    }
+}