diff --git a/src/dialog/dialog.rs b/src/dialog/dialog.rs
index 9f62fc2a..81db5aa0 100644
--- a/src/dialog/dialog.rs
+++ b/src/dialog/dialog.rs
@@ -1250,7 +1250,17 @@ impl DialogInner {
                         if method == Method::Invite {
                             self.handle_provisional_response(&resp).await?;
                         }
-                        self.transition(DialogState::Early(self.id.lock().clone(), resp))?;
+                        // RFC 3261 §12: a dialog moves from early to confirmed
+                        // and never back. A 1xx to a mid-dialog request (re-INVITE,
+                        // UPDATE, ...) must not regress an established dialog to
+                        // Early, or BYE is refused and hangup() tries to CANCEL.
+                        // The provisional is still notified so the caller sees it.
+                        let state = DialogState::Early(self.id.lock().clone(), resp);
+                        if self.can_cancel() {
+                            self.transition(state)?;
+                        } else {
+                            self.state_sender.send(state).ok();
+                        }
                         continue;
                     }
 
diff --git a/src/dialog/tests/mod.rs b/src/dialog/tests/mod.rs
index 52a59d7a..433615b7 100644
--- a/src/dialog/tests/mod.rs
+++ b/src/dialog/tests/mod.rs
@@ -5,6 +5,7 @@ mod test_client_dialog;
 mod test_connection_affinity;
 mod test_dialog_layer;
 mod test_dialog_states;
+mod test_in_dialog_provisional;
 mod test_in_dialog_via;
 mod test_invite_auth_challenge;
 mod test_prack;
diff --git a/src/dialog/tests/test_in_dialog_provisional.rs b/src/dialog/tests/test_in_dialog_provisional.rs
new file mode 100644
index 00000000..40b82bbc
--- /dev/null
+++ b/src/dialog/tests/test_in_dialog_provisional.rs
@@ -0,0 +1,218 @@
+//! A provisional response to an in-dialog request must not move a confirmed
+//! dialog back to the early state (RFC 3261 §12: a dialog goes early →
+//! confirmed and never back; a 1xx to a mid-dialog request does not create
+//! early state). Otherwise `bye()` refuses to run on the dialog and
+//! `hangup()` falls through to a CANCEL of the long-completed INVITE.
+use crate::dialog::{
+    dialog::{DialogState, DialogStateReceiver},
+    dialog_layer::DialogLayer,
+    invitation::InviteOption,
+    invite_dialog::InviteDialog,
+};
+use crate::sip::{prelude::HeadersExt, Method, Request, SipMessage, Uri};
+use crate::transport::{udp::UdpConnection, TransportLayer};
+use crate::EndpointBuilder;
+use std::net::SocketAddr;
+use std::time::Duration;
+use tokio::net::UdpSocket;
+use tokio::sync::mpsc::unbounded_channel;
+use tokio_util::sync::CancellationToken;
+
+const PEER_TAG: &str = "peer-tag";
+
+/// Receive the next request with `method` on the raw peer socket, skipping
+/// anything else (retransmissions, ACKs we are not waiting for).
+async fn recv_request(socket: &UdpSocket, method: Method) -> (Request, SocketAddr) {
+    let mut buf = vec![0u8; 4096];
+    loop {
+        let (len, from) = tokio::time::timeout(Duration::from_secs(2), socket.recv_from(&mut buf))
+            .await
+            .unwrap_or_else(|_| panic!("timeout waiting for {method}"))
+            .expect("recv_from failed");
+        let text = std::str::from_utf8(&buf[..len]).expect("non utf-8 SIP message");
+        if let Ok(SipMessage::Request(req)) = SipMessage::try_from(text) {
+            if req.method == method {
+                return (req, from);
+            }
+        }
+    }
+}
+
+/// Build a response to `req` as the peer UA, adding the peer's To tag when
+/// the request does not carry one yet.
+fn response(req: &Request, code: u16, reason: &str, contact: &str) -> String {
+    let to = req.to_header().unwrap().value().to_string();
+    let to = if to.contains(";tag=") {
+        to
+    } else {
+        format!("{to};tag={PEER_TAG}")
+    };
+    format!(
+        "SIP/2.0 {code} {reason}\r\n\
+         Via: {}\r\n\
+         From: {}\r\n\
+         To: {to}\r\n\
+         Call-ID: {}\r\n\
+         CSeq: {}\r\n\
+         Contact: <{contact}>\r\n\
+         Content-Length: 0\r\n\r\n",
+        req.via_header().unwrap().value(),
+        req.from_header().unwrap().value(),
+        req.call_id_header().unwrap().value(),
+        req.cseq_header().unwrap().value(),
+    )
+}
+
+async fn reply(socket: &UdpSocket, to: SocketAddr, req: &Request, code: u16, reason: &str) {
+    let contact = format!("sip:bob@{}", socket.local_addr().unwrap());
+    socket
+        .send_to(response(req, code, reason, &contact).as_bytes(), to)
+        .await
+        .expect("send_to failed");
+}
+
+fn drain_states(rx: &mut DialogStateReceiver) -> Vec<DialogState> {
+    let mut states = Vec::new();
+    while let Ok(state) = rx.try_recv() {
+        states.push(state);
+    }
+    states
+}
+
+/// Set up a UAC endpoint and a raw UDP peer, and establish a dialog whose
+/// initial INVITE is answered 100 → 183 → 200. Returns the confirmed dialog.
+async fn establish(
+    token: &CancellationToken,
+) -> crate::Result<(InviteDialog, DialogStateReceiver, UdpSocket)> {
+    let peer = UdpSocket::bind("127.0.0.1:0").await?;
+    let peer_port = peer.local_addr()?.port();
+
+    let transport_layer = TransportLayer::new(token.child_token());
+    let udp = UdpConnection::create_connection(
+        "127.0.0.1:0".parse().unwrap(),
+        None,
+        Some(token.child_token()),
+    )
+    .await?;
+    let uac_addr = udp.get_addr().addr.clone();
+    transport_layer.add_transport(udp.into());
+    let endpoint = EndpointBuilder::new()
+        .with_user_agent("rsipstack-test")
+        .with_transport_layer(transport_layer)
+        .with_cancel_token(token.child_token())
+        .build();
+    let endpoint_inner = endpoint.inner.clone();
+    tokio::spawn(async move {
+        let _ = endpoint_inner.serve().await;
+    });
+    let dialog_layer = DialogLayer::new(endpoint.inner.clone());
+
+    let (state_sender, mut state_receiver) = unbounded_channel();
+    let invite_option = InviteOption {
+        caller: Uri::try_from("sip:alice@example.com")?,
+        callee: Uri::try_from(format!("sip:bob@127.0.0.1:{peer_port};transport=udp").as_str())?,
+        contact: Uri::try_from(format!("sip:alice@{uac_addr}").as_str())?,
+        ..Default::default()
+    };
+    let invite = tokio::spawn(async move {
+        dialog_layer
+            .do_invite(invite_option, state_sender)
+            .await
+            .map(|(dialog, _)| dialog)
+    });
+
+    let (req, uac) = recv_request(&peer, Method::Invite).await;
+    reply(&peer, uac, &req, 100, "Trying").await;
+    reply(&peer, uac, &req, 183, "Session Progress").await;
+    reply(&peer, uac, &req, 200, "OK").await;
+    recv_request(&peer, Method::Ack).await;
+
+    let dialog = invite.await.expect("do_invite task panicked")?;
+    assert!(dialog.state().is_confirmed(), "initial INVITE must confirm");
+
+    // The initial INVITE's 183 still produces early state, as before.
+    let states = drain_states(&mut state_receiver);
+    assert!(
+        states.iter().any(|s| matches!(s, DialogState::Early(_, _))),
+        "a 183 to the initial INVITE must report Early, got {states:?}"
+    );
+    assert!(
+        matches!(states.last(), Some(DialogState::Confirmed(_, _))),
+        "initial INVITE must end Confirmed, got {states:?}"
+    );
+    Ok((dialog, state_receiver, peer))
+}
+
+/// Send `method` in-dialog, answer it 100 → 183 → 200 from the peer, and
+/// check the dialog stays Confirmed throughout and can still send BYE.
+async fn assert_provisional_keeps_confirmed(method: Method) -> crate::Result<()> {
+    let token = CancellationToken::new();
+    let (dialog, mut states, peer) = establish(&token).await?;
+
+    let requester = dialog.clone();
+    let pending = tokio::spawn(async move {
+        match method {
+            Method::Invite => requester.reinvite(None, None).await,
+            Method::Update => requester.update(None, None).await,
+            _ => unreachable!(),
+        }
+    });
+
+    let (req, uac) = recv_request(&peer, method).await;
+    reply(&peer, uac, &req, 100, "Trying").await;
+    reply(&peer, uac, &req, 183, "Session Progress").await;
+    // Let the dialog process the provisionals while the request is pending.
+    tokio::time::sleep(Duration::from_millis(100)).await;
+    assert!(
+        dialog.state().is_confirmed(),
+        "a 183 to an in-dialog {method} must not leave Confirmed, state: {}",
+        dialog.state()
+    );
+
+    reply(&peer, uac, &req, 200, "OK").await;
+    let resp = tokio::time::timeout(Duration::from_secs(2), pending)
+        .await
+        .expect("in-dialog request did not complete")
+        .expect("request task panicked")?
+        .expect("no final response");
+    assert_eq!(resp.status_code, crate::sip::StatusCode::OK);
+    assert!(
+        dialog.state().is_confirmed(),
+        "dialog must be Confirmed after the in-dialog {method} completes, state: {}",
+        dialog.state()
+    );
+    // The 183 is still delivered to the caller, it just does not change the
+    // dialog's state.
+    let states = drain_states(&mut states);
+    assert!(
+        states.iter().any(|s| matches!(
+            s,
+            DialogState::Early(_, r) if r.status_code == crate::sip::StatusCode::SessionProgress
+        )),
+        "the 183 to an in-dialog {method} must still be notified, got {states:?}"
+    );
+
+    // The call can still be hung up with a BYE.
+    let hanger = dialog.clone();
+    let bye = tokio::spawn(async move { hanger.bye().await });
+    let (bye_req, uac) = recv_request(&peer, Method::Bye).await;
+    reply(&peer, uac, &bye_req, 200, "OK").await;
+    tokio::time::timeout(Duration::from_secs(2), bye)
+        .await
+        .expect("bye did not complete")
+        .expect("bye task panicked")?;
+    assert!(dialog.state().is_terminated());
+
+    token.cancel();
+    Ok(())
+}
+
+#[tokio::test]
+async fn test_reinvite_provisional_keeps_dialog_confirmed() -> crate::Result<()> {
+    assert_provisional_keeps_confirmed(Method::Invite).await
+}
+
+#[tokio::test]
+async fn test_update_provisional_keeps_dialog_confirmed() -> crate::Result<()> {
+    assert_provisional_keeps_confirmed(Method::Update).await
+}