diff --git a/src/transaction/endpoint.rs b/src/transaction/endpoint.rs
index 7e0440a8..2eaa8f11 100644
--- a/src/transaction/endpoint.rs
+++ b/src/transaction/endpoint.rs
@@ -49,6 +49,13 @@ pub struct EndpointOption {
     /// (RFC 7989) needs no switch: dialogs participate only when the
     /// application supplies a UUID or the peer sends a Session-ID header.
     pub history_info_enabled: bool,
+    /// Whether a client INVITE transaction ACKs a 2xx final itself (default `true`).
+    /// With `false` a 2xx moves the transaction straight to
+    /// Terminated: no automatic ACK, no stored ACK.
+    /// RFC 3261 section 17.1.1.2: the 2xx ACK is the TU's, end-to-end;
+    /// proxies (section 16) must not ACK a 2xx and must see retransmitted 2xx.
+    /// `true` keeps the UA behavior the dialog layer relies on.
+    pub auto_ack_2xx: bool,
 }
 
 impl Default for EndpointOption {
@@ -61,6 +68,7 @@ impl Default for EndpointOption {
             callid_suffix: None,
             callid_format: CallIdFormat::default(),
             history_info_enabled: false,
+            auto_ack_2xx: true,
         }
     }
 }
diff --git a/src/transaction/transaction.rs b/src/transaction/transaction.rs
index 82fcd89f..0abc7607 100644
--- a/src/transaction/transaction.rs
+++ b/src/transaction/transaction.rs
@@ -746,7 +746,16 @@ impl Transaction {
             }
             _ => {
                 if self.transaction_type == TransactionType::ClientInvite {
-                    TransactionState::Completed
+                    if resp.status_code.kind() == StatusCodeKind::Successful
+                        && !self.endpoint_inner.option.auto_ack_2xx
+                    {
+                        // auto_ack_2xx = false (proxy mode): a 2xx terminates
+                        // the client INVITE transaction at once; the ACK is
+                        // the TU's job (RFC 3261 section 17.1.1.2).
+                        TransactionState::Terminated
+                    } else {
+                        TransactionState::Completed
+                    }
                 } else {
                     TransactionState::Terminated
                 }
@@ -1105,8 +1114,17 @@ impl Transaction {
                     ) && self.last_ack.is_none()
                     {
                         if let Some(ref resp) = self.last_response {
-                            if let Ok(ack) = self.endpoint_inner.make_ack(&self.original, resp) {
-                                self.last_ack.replace(ack);
+                            // auto_ack_2xx = false (proxy mode): store no ACK
+                            // for a 2xx, so retransmitted 2xx fall through
+                            // to the TU instead of being absorbed or re-ACKed
+                            // here (RFC 3261 section 17.1.1.2).
+                            let auto_ack = self.endpoint_inner.option.auto_ack_2xx
+                                || resp.status_code.kind() != StatusCodeKind::Successful;
+                            if auto_ack {
+                                if let Ok(ack) = self.endpoint_inner.make_ack(&self.original, resp)
+                                {
+                                    self.last_ack.replace(ack);
+                                }
                             }
                         }
                     }