diff --git a/agent/src/main.rs b/agent/src/main.rs
index de3d9d1..3e2132b 100644
--- a/agent/src/main.rs
+++ b/agent/src/main.rs
@@ -23,6 +23,11 @@ use anyhow::Result;
 use tracing::{info, error, Level};
 use tracing_subscriber::FmtSubscriber;
 
+#[cfg(windows)]
+use windows::Win32::UI::WindowsAndMessaging::{MessageBoxW, MB_OK, MB_ICONINFORMATION};
+#[cfg(windows)]
+use windows::core::PCWSTR;
+
 /// Extract a 6-digit support code from the executable's filename.
 /// Looks for patterns like "GuruConnect-123456.exe" or "123456.exe"
 fn extract_code_from_filename() -> Option<String> {
@@ -50,6 +55,37 @@ fn extract_code_from_filename() -> Option<String> {
     None
 }
 
+/// Show a message box to the user (Windows only)
+#[cfg(windows)]
+fn show_message_box(title: &str, message: &str) {
+    use std::ffi::OsStr;
+    use std::os::windows::ffi::OsStrExt;
+
+    // Convert strings to wide strings for Windows API
+    let title_wide: Vec<u16> = OsStr::new(title)
+        .encode_wide()
+        .chain(std::iter::once(0))
+        .collect();
+    let message_wide: Vec<u16> = OsStr::new(message)
+        .encode_wide()
+        .chain(std::iter::once(0))
+        .collect();
+
+    unsafe {
+        MessageBoxW(
+            None,
+            PCWSTR(message_wide.as_ptr()),
+            PCWSTR(title_wide.as_ptr()),
+            MB_OK | MB_ICONINFORMATION,
+        );
+    }
+}
+
+#[cfg(not(windows))]
+fn show_message_box(_title: &str, _message: &str) {
+    // No-op on non-Windows platforms
+}
+
 #[tokio::main]
 async fn main() -> Result<()> {
     // Initialize logging
@@ -85,6 +121,7 @@ async fn main() -> Result<()> {
 async fn run_agent(config: config::Config) -> Result<()> {
     // Create session manager
     let mut session = session::SessionManager::new(config.clone());
+    let is_support_session = config.support_code.is_some();
 
     // Connect to server and run main loop
     loop {
@@ -96,15 +133,47 @@ async fn run_agent(config: config::Config) -> Result<()> {
 
                 // Run session until disconnect
                 if let Err(e) = session.run().await {
+                    let error_msg = e.to_string();
+
+                    // Check if this is a cancellation
+                    if error_msg.contains("SESSION_CANCELLED") {
+                        info!("Session was cancelled by technician");
+                        show_message_box(
+                            "Support Session Ended",
+                            "The support session was cancelled by the technician.\n\nThis window will close automatically.",
+                        );
+                        // Exit cleanly without reconnecting
+                        return Ok(());
+                    }
+
                     error!("Session error: {}", e);
                 }
             }
             Err(e) => {
+                let error_msg = e.to_string();
+
+                // Check if connection was rejected due to cancelled code
+                if error_msg.contains("cancelled") {
+                    info!("Support code was cancelled before connection");
+                    show_message_box(
+                        "Support Session Cancelled",
+                        "This support session has been cancelled.\n\nPlease contact your technician for a new support code.",
+                    );
+                    // Exit cleanly without reconnecting
+                    return Ok(());
+                }
+
                 error!("Connection failed: {}", e);
             }
         }
 
-        // Wait before reconnecting
+        // For support sessions, don't reconnect if something goes wrong
+        if is_support_session {
+            info!("Support session ended, not reconnecting");
+            return Ok(());
+        }
+
+        // Wait before reconnecting (only for persistent agent connections)
         info!("Reconnecting in 5 seconds...");
         tokio::time::sleep(tokio::time::Duration::from_secs(5)).await;
     }
diff --git a/agent/src/session/mod.rs b/agent/src/session/mod.rs
index 1c3daa1..7837b5e 100644
--- a/agent/src/session/mod.rs
+++ b/agent/src/session/mod.rs
@@ -202,6 +202,10 @@ impl SessionManager {
 
             Some(message::Payload::Disconnect(disc)) => {
                 tracing::info!("Disconnect requested: {}", disc.reason);
+                // Check if this is a cancellation
+                if disc.reason.contains("cancelled") {
+                    return Err(anyhow::anyhow!("SESSION_CANCELLED: {}", disc.reason));
+                }
                 return Err(anyhow::anyhow!("Disconnect: {}", disc.reason));
             }
 
diff --git a/server/src/relay/mod.rs b/server/src/relay/mod.rs
index da5e94f..8882fab 100644
--- a/server/src/relay/mod.rs
+++ b/server/src/relay/mod.rs
@@ -73,6 +73,28 @@ async fn handle_agent_connection(
 ) {
     info!("Agent connected: {} ({})", agent_name, agent_id);
 
+    let (mut ws_sender, mut ws_receiver) = socket.split();
+
+    // If a support code was provided, check if it's valid
+    if let Some(ref code) = support_code {
+        // Check if the code is cancelled or invalid
+        if support_codes.is_cancelled(code).await {
+            warn!("Agent tried to connect with cancelled code: {}", code);
+            // Send disconnect message to agent
+            let disconnect_msg = proto::Message {
+                payload: Some(proto::message::Payload::Disconnect(proto::Disconnect {
+                    reason: "Support session was cancelled by technician".to_string(),
+                })),
+            };
+            let mut buf = Vec::new();
+            if prost::Message::encode(&disconnect_msg, &mut buf).is_ok() {
+                let _ = ws_sender.send(Message::Binary(buf.into())).await;
+            }
+            let _ = ws_sender.close().await;
+            return;
+        }
+    }
+
     // Register the agent and get channels
     let (session_id, frame_tx, mut input_rx) = sessions.register_agent(agent_id.clone(), agent_name.clone()).await;
 
@@ -85,12 +107,16 @@ async fn handle_agent_connection(
         support_codes.link_session(code, session_id).await;
     }
 
-    let (mut ws_sender, mut ws_receiver) = socket.split();
+    // Use Arc<Mutex> for sender so we can use it from multiple places
+    let ws_sender = std::sync::Arc::new(tokio::sync::Mutex::new(ws_sender));
+    let ws_sender_input = ws_sender.clone();
+    let ws_sender_cancel = ws_sender.clone();
 
     // Task to forward input events from viewers to agent
     let input_forward = tokio::spawn(async move {
         while let Some(input_data) = input_rx.recv().await {
-            if ws_sender.send(Message::Binary(input_data.into())).await.is_err() {
+            let mut sender = ws_sender_input.lock().await;
+            if sender.send(Message::Binary(input_data.into())).await.is_err() {
                 break;
             }
         }
@@ -99,6 +125,34 @@ async fn handle_agent_connection(
     let sessions_cleanup = sessions.clone();
     let support_codes_cleanup = support_codes.clone();
     let support_code_cleanup = support_code.clone();
+    let support_code_check = support_code.clone();
+    let support_codes_check = support_codes.clone();
+
+    // Task to check for cancellation every 2 seconds
+    let cancel_check = tokio::spawn(async move {
+        let mut interval = tokio::time::interval(std::time::Duration::from_secs(2));
+        loop {
+            interval.tick().await;
+            if let Some(ref code) = support_code_check {
+                if support_codes_check.is_cancelled(code).await {
+                    info!("Support code {} was cancelled, disconnecting agent", code);
+                    // Send disconnect message
+                    let disconnect_msg = proto::Message {
+                        payload: Some(proto::message::Payload::Disconnect(proto::Disconnect {
+                            reason: "Support session was cancelled by technician".to_string(),
+                        })),
+                    };
+                    let mut buf = Vec::new();
+                    if prost::Message::encode(&disconnect_msg, &mut buf).is_ok() {
+                        let mut sender = ws_sender_cancel.lock().await;
+                        let _ = sender.send(Message::Binary(buf.into())).await;
+                        let _ = sender.close().await;
+                    }
+                    break;
+                }
+            }
+        }
+    });
 
     // Main loop: receive frames from agent and broadcast to viewers
     while let Some(msg) = ws_receiver.next().await {
@@ -135,12 +189,15 @@ async fn handle_agent_connection(
 
     // Cleanup
     input_forward.abort();
+    cancel_check.abort();
     sessions_cleanup.remove_session(session_id).await;
 
-    // Mark support code as completed if one was used
+    // Mark support code as completed if one was used (unless cancelled)
     if let Some(ref code) = support_code_cleanup {
-        support_codes_cleanup.mark_completed(code).await;
-        info!("Support code {} marked as completed", code);
+        if !support_codes_cleanup.is_cancelled(code).await {
+            support_codes_cleanup.mark_completed(code).await;
+            info!("Support code {} marked as completed", code);
+        }
     }
 
     info!("Session {} ended", session_id);
diff --git a/server/src/support_codes.rs b/server/src/support_codes.rs
index ae119ea..5207a5e 100644
--- a/server/src/support_codes.rs
+++ b/server/src/support_codes.rs
@@ -176,11 +176,11 @@ impl SupportCodeManager {
         }
     }
 
-    /// Cancel a code
+    /// Cancel a code (works for both pending and connected)
     pub async fn cancel_code(&self, code: &str) -> bool {
         let mut codes = self.codes.write().await;
         if let Some(support_code) = codes.get_mut(code) {
-            if support_code.status == CodeStatus::Pending {
+            if support_code.status == CodeStatus::Pending || support_code.status == CodeStatus::Connected {
                 support_code.status = CodeStatus::Cancelled;
                 return true;
             }
@@ -188,6 +188,18 @@ impl SupportCodeManager {
         false
     }
 
+    /// Check if a code is cancelled
+    pub async fn is_cancelled(&self, code: &str) -> bool {
+        let codes = self.codes.read().await;
+        codes.get(code).map(|c| c.status == CodeStatus::Cancelled).unwrap_or(false)
+    }
+
+    /// Check if a code is valid for connection (exists and is pending)
+    pub async fn is_valid_for_connection(&self, code: &str) -> bool {
+        let codes = self.codes.read().await;
+        codes.get(code).map(|c| c.status == CodeStatus::Pending).unwrap_or(false)
+    }
+
     /// List all codes (for dashboard)
     pub async fn list_codes(&self) -> Vec<SupportCode> {
         let codes = self.codes.read().await;