feat: [US-042] - Handle CDP connection death on Chromium crash

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

scripts/ralph/prd.json

@@ -684,7 +684,7 @@
         "Typecheck passes"
       ],
       "priority": 42,
-      "passes": false,
+      "passes": true,
       "notes": "If Chromium crashes, RestartPolicy::Always restarts it but the CdpClient holds a dead WebSocket. State stays Active but all CDP operations fail. No recovery without manual stop+start."
     },
     {

scripts/ralph/progress.txt

@@ -716,3 +716,13 @@ Started: Tue Mar 17 04:32:06 AM PDT 2026
   - PUT /v1/fs/file auto-creates parent directories, so no need for separate mkdir calls
   - BrowserProblem extensions are flattened into the ProblemDetails JSON response (e.g. `parsed["code"]` not `parsed["extensions"]["code"]`)
 ---
+
+## 2026-03-17 - US-042
+- Added `is_alive()` method to CdpClient that checks if the reader_task JoinHandle is finished
+- Updated `refresh_status_locked()` in BrowserRuntime to check CDP client liveness after the Chromium process check; transitions to Failed state if the CDP WebSocket connection has died
+- Changed Chromium's RestartPolicy from Always to Never so a crash transitions cleanly to Failed state instead of auto-restarting with a stale CDP connection
+- Files changed: `browser_cdp.rs`, `browser_runtime.rs`
+- **Learnings for future iterations:**
+  - `JoinHandle::is_finished()` is a non-blocking way to check if a spawned task has exited - useful for health checks without await
+  - RestartPolicy is declared but not actually enforced by ProcessRuntime (the field has `#[allow(dead_code)]`), so changing it is primarily a signal for future implementation and prevents accidental auto-restart if it gets wired up
+---

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

@@ -158,6 +158,14 @@ impl CdpClient {
         rx
     }
 
+    /// Check whether the CDP WebSocket connection is still alive.
+    ///
+    /// Returns `true` if the background reader task is still running, which
+    /// means the WebSocket connection has not been closed or errored.
+    pub fn is_alive(&self) -> bool {
+        !self.reader_task.is_finished()
+    }
+
     /// Close the CDP connection and stop the reader task.
     pub async fn close(&self) {
         self.reader_task.abort();

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

@@ -757,6 +757,17 @@ impl BrowserRuntime {
                 }
             }
         }
+
+        // Check CDP WebSocket connection is alive
+        if let Some(ref cdp) = state.cdp_client {
+            if !cdp.is_alive() {
+                let problem =
+                    BrowserProblem::cdp_error("CDP WebSocket connection died unexpectedly");
+                self.record_problem_locked(state, &problem);
+                state.state = BrowserState::Failed;
+                return;
+            }
+        }
     }
 
     fn snapshot_locked(&self, state: &BrowserRuntimeStateData) -> BrowserStatusResponse {
@@ -910,7 +921,7 @@ impl BrowserRuntime {
                 tty: false,
                 interactive: false,
                 owner: ProcessOwner::Desktop,
-                restart_policy: Some(RestartPolicy::Always),
+                restart_policy: Some(RestartPolicy::Never),
             })
             .await
             .map_err(|err| {