fix(desktop): stop button sends interrupt to wrong session + stale events re-arm busy

apps/desktop/src/app/session/hooks/use-message-stream/gateway-event.ts

@@ -249,6 +249,15 @@ export function useGatewayEventHandler(deps: GatewayEventDeps) {
               }
 
               if (busy) {
+                // Don't re-arm busy from a stale session.info if the user
+                // just clicked Stop (interrupted=true). The backend's
+                // cooperative interrupt may not have propagated yet, so
+                // running is still true in the heartbeat. The turn's
+                // finally block will emit running=false to clear busy.
+                if (state.interrupted) {
+                  return state
+                }
+
                 return {
                   ...state,
                   busy,
@@ -307,14 +316,27 @@ export function useGatewayEventHandler(deps: GatewayEventDeps) {
           triggerHaptic('streamStart')
         }
 
-        updateSessionState(sessionId, state => ({
-          ...state,
-          busy: true,
-          awaitingResponse: true,
-          sawAssistantPayload: false,
-          interrupted: false,
-          turnStartedAt: Date.now()
-        }))
+        updateSessionState(sessionId, state => {
+          // If the user clicked Stop (cancelRun set interrupted=true), don't
+          // let a stale message.start from a chained turn (goal follow-up,
+          // completion drain) or an in-flight LLM response re-arm busy.
+          // The interrupt is user intent — the backend's cooperative cancel
+          // may not have propagated yet, so its events are stale. The turn's
+          // finally block will emit session.info with running=false to clear
+          // busy for real once the agent loop actually exits.
+          if (state.interrupted) {
+            return state
+          }
+
+          return {
+            ...state,
+            busy: true,
+            awaitingResponse: true,
+            sawAssistantPayload: false,
+            interrupted: false,
+            turnStartedAt: Date.now()
+          }
+        })
 
         if (isActiveEvent) {
           setTurnStartedAt(Date.now())

apps/desktop/src/app/session/hooks/use-prompt-actions/index.ts

@@ -514,7 +514,16 @@ export function usePromptActions({
   )
 
   const cancelRun = useCallback(async () => {
-    const sessionId = activeSessionId || activeSessionIdRef.current
+    // Read from the ref, not the closure-captured `activeSessionId`. The
+    // actions bag is a stable ref mutated in place (Object.assign on each
+    // ContribWiring render), and ChatRoutesSurface is memoized on that stable
+    // ref — so it does NOT re-render when activeSessionId changes, which means
+    // the ChatView element's onCancel prop holds a stale cancelRun closure.
+    // The closure's `activeSessionId` can be a previous session's id (or null
+    // from a new-chat draft), sending session.interrupt to the wrong session.
+    // The ref is updated via useEffect on every activeSessionId change, so it
+    // always reflects the current session — same pattern submitText uses.
+    const sessionId = activeSessionIdRef.current
 
     const releaseBusy = () => {
       setMutableRef(busyRef, false)
@@ -589,7 +598,6 @@ export function usePromptActions({
       notifyError(stopError, copy.stopFailed)
     }
   }, [
-    activeSessionId,
     activeSessionIdRef,
     busyRef,
     copy.stopFailed,