diff --git a/NucleicRemote/NucleicRemote/Views/SessionDetailView.swift b/NucleicRemote/NucleicRemote/Views/SessionDetailView.swift
index 343ca8a..1e2ca5e 100644
--- a/NucleicRemote/NucleicRemote/Views/SessionDetailView.swift
+++ b/NucleicRemote/NucleicRemote/Views/SessionDetailView.swift
@@ -123,17 +123,37 @@ struct SessionDetailView: View {
             set: { store.setSessionEffort(sessionID, $0) })
     }
 
+    /// Memo for the context-occupancy scan below. The backward scan usually stops at the last
+    /// turn's usage event, but a stream with sparse (or no) usage reporting walks the whole
+    /// transcript — and `body` re-evaluates on every store change and keystroke, so an O(N) scan
+    /// per evaluation quietly compounds on long sessions. `(count, lastSeq)` pins the stream, as
+    /// in the projection cache; a class held in `@State` so updating it from `body` doesn't
+    /// itself invalidate the view.
+    private final class ContextScanCache {
+        var count = -1
+        var lastSeq: UInt64 = 0
+        var used: Int?
+    }
+    @State private var contextScanCache = ContextScanCache()
+
     /// Live context-window occupancy (newest turn's input tokens ÷ the model's window), read
-    /// from the transcript exactly as the Mac header does.
+    /// from the transcript exactly as the Mac header does. Only the token scan is memoized —
+    /// the window division stays live, so a model switch reflects immediately.
     private var contextPercent: Int? {
-        let used = store.openEvents.reversed().lazy.compactMap { event -> Int? in
-            switch event.kind {
-            case .turnCompleted(let turn): return turn.usage?.contextInputTokens
-            case .usage(let usage): return usage.contextInputTokens
-            default: return nil
-            }
-        }.first { $0 > 0 }
-        guard let used else { return nil }
+        let events = store.openEvents
+        let cache = contextScanCache
+        if cache.count != events.count || cache.lastSeq != events.last?.seq ?? 0 {
+            cache.count = events.count
+            cache.lastSeq = events.last?.seq ?? 0
+            cache.used = events.reversed().lazy.compactMap { event -> Int? in
+                switch event.kind {
+                case .turnCompleted(let turn): return turn.usage?.contextInputTokens
+                case .usage(let usage): return usage.contextInputTokens
+                default: return nil
+                }
+            }.first { $0 > 0 }
+        }
+        guard let used = cache.used else { return nil }
         let window = store.modelCatalog.contextWindow(summary?.model)
         guard window > 0 else { return nil }
         return min(100, Int((Double(used) / Double(window)) * 100))