feat: [US-034] - Add reverse mutual exclusivity check in DesktopRuntime

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/packages/sandbox-agent/src/desktop_errors.rs

@@ -62,6 +62,15 @@ impl DesktopProblem {
         )
     }
 
+    pub fn browser_conflict() -> Self {
+        Self::new(
+            409,
+            "Browser Conflict",
+            "desktop/browser-conflict",
+            "The browser runtime is currently active. Browser and desktop modes are mutually exclusive.",
+        )
+    }
+
     pub fn runtime_starting(message: impl Into<String>) -> Self {
         Self::new(
             409,

server/packages/sandbox-agent/src/desktop_runtime.rs

@@ -2,7 +2,7 @@ use std::collections::HashMap;
 use std::fs::{self, OpenOptions};
 use std::path::{Path, PathBuf};
 use std::process::{Output, Stdio};
-use std::sync::Arc;
+use std::sync::{Arc, OnceLock};
 use std::time::Duration;
 
 use tokio::process::{Child, Command};
@@ -50,6 +50,7 @@ pub struct DesktopRuntime {
     recording_manager: DesktopRecordingManager,
     streaming_manager: DesktopStreamingManager,
     inner: Arc<Mutex<DesktopRuntimeStateData>>,
+    browser_runtime: Arc<OnceLock<Arc<crate::browser_runtime::BrowserRuntime>>>,
 }
 
 #[derive(Debug, Clone)]
@@ -179,9 +180,17 @@ impl DesktopRuntime {
                 recording_fps: None,
             })),
             config,
+            browser_runtime: Arc::new(OnceLock::new()),
         }
     }
 
+    pub fn set_browser_runtime(
+        &self,
+        browser_runtime: Arc<crate::browser_runtime::BrowserRuntime>,
+    ) {
+        let _ = self.browser_runtime.set(browser_runtime);
+    }
+
     pub async fn status(&self) -> DesktopStatusResponse {
         let mut state = self.inner.lock().await;
         self.refresh_status_locked(&mut state).await;
@@ -204,6 +213,14 @@ impl DesktopRuntime {
         &self,
         request: DesktopStartRequest,
     ) -> Result<DesktopStatusResponse, DesktopProblem> {
+        // Check mutual exclusivity with browser runtime
+        if let Some(browser_rt) = self.browser_runtime.get() {
+            let browser_status = browser_rt.status().await;
+            if browser_status.state == crate::browser_types::BrowserState::Active {
+                return Err(DesktopProblem::browser_conflict());
+            }
+        }
+
         let mut state = self.inner.lock().await;
 
         if !self.platform_supported() {

server/packages/sandbox-agent/src/router.rs

@@ -125,6 +125,7 @@ impl AppState {
             process_runtime.clone(),
             desktop_runtime.clone(),
         ));
+        desktop_runtime.set_browser_runtime(browser_runtime.clone());
         Self {
             auth,
             agent_manager,