diff --git a/src/app/api/chat/route.ts b/src/app/api/chat/route.ts
index 0de65ba..c3a488f 100644
--- a/src/app/api/chat/route.ts
+++ b/src/app/api/chat/route.ts
@@ -641,16 +641,20 @@ async function processCompletionServerSide(
       console.log('[chat API] Server-side checkin completion succeeded');
     }
 
-    // Clear hookTriggeredSessionId directly (no HTTP needed)
+    // Clear hookTriggeredSessionId directly (no HTTP needed).
+    // CAS: only clear if we are still the owner — prevents wiping another
+    // tab's legitimate lock when completions arrive out of order.
     try {
       const { loadState, saveState } = await import('@/lib/assistant-workspace');
       const { getSetting: getSettingDirect } = await import('@/lib/db');
       const wsPath = getSettingDirect('assistant_workspace_path');
       if (wsPath) {
         const state = loadState(wsPath);
-        state.hookTriggeredSessionId = undefined;
-        state.hookTriggeredAt = undefined;
-        saveState(wsPath, state);
+        if (state.hookTriggeredSessionId === sessionId || !state.hookTriggeredSessionId) {
+          state.hookTriggeredSessionId = undefined;
+          state.hookTriggeredAt = undefined;
+          saveState(wsPath, state);
+        }
       }
     } catch {
       // Best effort
diff --git a/src/app/api/workspace/hook-triggered/route.ts b/src/app/api/workspace/hook-triggered/route.ts
index 688e3d5..00ee14c 100644
--- a/src/app/api/workspace/hook-triggered/route.ts
+++ b/src/app/api/workspace/hook-triggered/route.ts
@@ -8,8 +8,19 @@ export async function POST(request: NextRequest) {
     if (!workspacePath) {
       return NextResponse.json({ error: 'No workspace path configured' }, { status: 400 });
     }
-    const { sessionId } = await request.json();
+    const { sessionId, expectedOwner } = await request.json();
     const state = loadState(workspacePath);
+
+    // Compare-and-swap: if expectedOwner is provided, only proceed when the
+    // current owner matches.  This prevents two tabs that both see the same
+    // stale owner from racing through clear → set sequences.
+    if (expectedOwner !== undefined) {
+      const currentOwner = state.hookTriggeredSessionId ?? null;
+      if (currentOwner !== expectedOwner) {
+        return NextResponse.json({ success: false, reason: 'owner_mismatch', currentOwner });
+      }
+    }
+
     // '__clear__' sentinel clears the field (used after onboarding/checkin completes)
     if (sessionId === '__clear__') {
       state.hookTriggeredSessionId = undefined;
diff --git a/src/app/chat/page.tsx b/src/app/chat/page.tsx
index f413131..a6d66ae 100644
--- a/src/app/chat/page.tsx
+++ b/src/app/chat/page.tsx
@@ -47,16 +47,20 @@ export default function NewChatPage() {
   const [thinkingMode, setThinkingMode] = useState<string>('adaptive');
   const [context1m, setContext1m] = useState(false);
 
-  // Fetch provider-specific options
+  // Fetch provider-specific options (with abort to prevent stale responses on fast switch)
   useEffect(() => {
     const pid = currentProviderId || 'env';
-    fetch(`/api/providers/options?providerId=${encodeURIComponent(pid)}`)
+    const controller = new AbortController();
+    fetch(`/api/providers/options?providerId=${encodeURIComponent(pid)}`, { signal: controller.signal })
       .then(r => r.ok ? r.json() : null)
       .then(data => {
-        setThinkingMode(data?.options?.thinking_mode || 'adaptive');
-        setContext1m(!!data?.options?.context_1m);
+        if (!controller.signal.aborted) {
+          setThinkingMode(data?.options?.thinking_mode || 'adaptive');
+          setContext1m(!!data?.options?.context_1m);
+        }
       })
       .catch(() => {});
+    return () => controller.abort();
   }, [currentProviderId]);
 
   const stopStreaming = useCallback(() => {
diff --git a/src/components/chat/ChatView.tsx b/src/components/chat/ChatView.tsx
index 32b438a..ceae1b7 100644
--- a/src/components/chat/ChatView.tsx
+++ b/src/components/chat/ChatView.tsx
@@ -59,16 +59,20 @@ export function ChatView({ sessionId, initialMessages = [], initialHasMore = fal
   useEffect(() => { if (modelName) setCurrentModel(modelName); }, [modelName]);
   useEffect(() => { if (providerId) setCurrentProviderId(providerId); }, [providerId]);
 
-  // Fetch provider-specific options (thinking mode + 1M context)
+  // Fetch provider-specific options (with abort to prevent stale responses on fast switch)
   useEffect(() => {
     const pid = currentProviderId || 'env';
-    fetch(`/api/providers/options?providerId=${encodeURIComponent(pid)}`)
+    const controller = new AbortController();
+    fetch(`/api/providers/options?providerId=${encodeURIComponent(pid)}`, { signal: controller.signal })
       .then(r => r.ok ? r.json() : null)
       .then(data => {
-        setThinkingMode(data?.options?.thinking_mode || 'adaptive');
-        setContext1m(!!data?.options?.context_1m);
+        if (!controller.signal.aborted) {
+          setThinkingMode(data?.options?.thinking_mode || 'adaptive');
+          setContext1m(!!data?.options?.context_1m);
+        }
       })
       .catch(() => {});
+    return () => controller.abort();
   }, [currentProviderId]);
   useEffect(() => { if (initialPermissionProfile) setPermissionProfile(initialPermissionProfile); }, [initialPermissionProfile]);
 
diff --git a/src/hooks/useAssistantTrigger.ts b/src/hooks/useAssistantTrigger.ts
index 0e46b42..c2522d5 100644
--- a/src/hooks/useAssistantTrigger.ts
+++ b/src/hooks/useAssistantTrigger.ts
@@ -140,13 +140,24 @@ export function useAssistantTrigger({
         if (isOwnerAlive(state.hookTriggeredSessionId)) {
           return; // Owning tab is still open, don't interfere
         }
-        // Owner tab is gone — clear the stale lock and proceed
+        // Owner tab is gone — atomically clear the stale lock (CAS: only if owner
+        // is still the stale session we observed).  If another tab already swapped
+        // in, the server returns owner_mismatch and we bail out.
         try {
-          await fetch('/api/workspace/hook-triggered', {
+          const clearRes = await fetch('/api/workspace/hook-triggered', {
             method: 'POST',
             headers: { 'Content-Type': 'application/json' },
-            body: JSON.stringify({ sessionId: '__clear__' }),
+            body: JSON.stringify({
+              sessionId: '__clear__',
+              expectedOwner: state.hookTriggeredSessionId,
+            }),
           });
+          if (clearRes.ok) {
+            const clearData = await clearRes.json();
+            if (!clearData.success) return; // Another tab won the race
+          } else {
+            return;
+          }
         } catch {
           return; // Can't clear, err on the safe side
         }
@@ -181,7 +192,10 @@ export function useAssistantTrigger({
                 await fetch('/api/workspace/hook-triggered', {
                   method: 'POST',
                   headers: { 'Content-Type': 'application/json' },
-                  body: JSON.stringify({ sessionId: '__clear__' }),
+                  body: JSON.stringify({
+                    sessionId: '__clear__',
+                    expectedOwner: state.hookTriggeredSessionId || null,
+                  }),
                 }).catch(() => {});
                 console.log('[useAssistantTrigger] Onboarding compensation succeeded, skipping re-trigger');
                 return; // Don't re-trigger onboarding
@@ -212,12 +226,38 @@ export function useAssistantTrigger({
       stopHeartbeatRef.current?.();
       stopHeartbeatRef.current = startHeartbeat(sessionId);
 
-      // Mark in persistent state to prevent duplicate triggers across page reloads
-      await fetch('/api/workspace/hook-triggered', {
-        method: 'POST',
-        headers: { 'Content-Type': 'application/json' },
-        body: JSON.stringify({ sessionId }),
-      });
+      // Mark in persistent state to prevent duplicate triggers across page reloads.
+      // CAS: only set owner if currently unowned (null).  If another tab set itself
+      // as owner between our clear and this call, the server rejects and we bail.
+      try {
+        const setRes = await fetch('/api/workspace/hook-triggered', {
+          method: 'POST',
+          headers: { 'Content-Type': 'application/json' },
+          body: JSON.stringify({ sessionId, expectedOwner: null }),
+        });
+        // Bail on any non-2xx (including 500) or CAS rejection — never
+        // proceed to startStream without a confirmed lock.
+        if (!setRes.ok) {
+          assistantTriggerFiredRef.current = false;
+          stopHeartbeatRef.current?.();
+          stopHeartbeatRef.current = null;
+          return;
+        }
+        const setData = await setRes.json();
+        if (!setData.success) {
+          // Lost race — another tab claimed ownership
+          assistantTriggerFiredRef.current = false;
+          stopHeartbeatRef.current?.();
+          stopHeartbeatRef.current = null;
+          return;
+        }
+      } catch {
+        // Network error — bail out
+        assistantTriggerFiredRef.current = false;
+        stopHeartbeatRef.current?.();
+        stopHeartbeatRef.current = null;
+        return;
+      }
 
       // Use autoTrigger: the message is invisible (no user bubble, no title update)
       const triggerMsg = needsOnboarding
diff --git a/src/lib/db.ts b/src/lib/db.ts
index 2fbfb53..73dfc6f 100644
--- a/src/lib/db.ts
+++ b/src/lib/db.ts
@@ -893,7 +893,7 @@ export function updateMessageBySessionAndHint(
   // Strategy 1: Exact match on the raw ```image-gen-request...``` block content.
   // This is unambiguous even when multiple requests share the same prompt.
   if (rawRequestBlock) {
-    const escaped = rawRequestBlock.replace(/%/g, '\\%').replace(/_/g, '\\_');
+    const escaped = rawRequestBlock.replace(/\\/g, '\\\\').replace(/%/g, '\\%').replace(/_/g, '\\_');
     const row = db.prepare(
       "SELECT id FROM messages WHERE session_id = ? AND role = 'assistant' AND content LIKE ? ESCAPE '\\' AND content NOT LIKE '%image-gen-result%' ORDER BY created_at DESC LIMIT 1"
     ).get(sessionId, `%${escaped}%`) as { id: string } | undefined;