diff --git a/packages/ui/src/components/chat/hooks/useChatTimelineController.ts b/packages/ui/src/components/chat/hooks/useChatTimelineController.ts
index a47dd207..047a4717 100644
--- a/packages/ui/src/components/chat/hooks/useChatTimelineController.ts
+++ b/packages/ui/src/components/chat/hooks/useChatTimelineController.ts
@@ -313,6 +313,20 @@ export const useChatTimelineController = ({
         setActiveTurnId(null);
     }, [sessionId]);
 
+    React.useLayoutEffect(() => {
+        if (!isPinned) {
+            return;
+        }
+        const latestTurnId = turnWindowModel.turnIds[turnWindowModel.turnIds.length - 1];
+        if (!latestTurnId) {
+            return;
+        }
+        // A sent prompt updates the timeline model before its new DOM node is
+        // measurable by the scroll spy. While pinned, the latest turn is
+        // authoritative and keeps the rail in sync for that interval.
+        setActiveTurnId((current) => current === latestTurnId ? current : latestTurnId);
+    }, [isPinned, turnWindowModel.turnIds]);
+
     const resolvePendingRenderWaiters = React.useCallback(() => {
         const resolvers = pendingRenderResolversRef.current;
         if (resolvers.length === 0) {
diff --git a/packages/ui/src/components/chat/lib/scroll/scrollSpy.test.ts b/packages/ui/src/components/chat/lib/scroll/scrollSpy.test.ts
new file mode 100644
index 00000000..864ab85b
--- /dev/null
+++ b/packages/ui/src/components/chat/lib/scroll/scrollSpy.test.ts
@@ -0,0 +1,40 @@
+import { describe, expect, test } from 'bun:test';
+
+import { createScrollSpy } from './scrollSpy';
+
+describe('createScrollSpy', () => {
+    test('activates the final turn throughout the chat bottom spacer', () => {
+        const frames: FrameRequestCallback[] = [];
+        const activeTurnIds: string[] = [];
+        const spy = createScrollSpy({
+            onActive: (turnId) => activeTurnIds.push(turnId),
+            raf: (callback) => {
+                frames.push(callback);
+                return frames.length;
+            },
+            caf: () => {},
+        });
+        const container = {
+            scrollHeight: 1_000,
+            scrollTop: 870,
+            clientHeight: 100,
+            getBoundingClientRect: () => ({ top: 0 }),
+        } as HTMLDivElement;
+        const previousTurn = {
+            getBoundingClientRect: () => ({ top: -120 }),
+        } as HTMLElement;
+        const finalTurn = {
+            // Its top is still below the reading line at this scroll position.
+            getBoundingClientRect: () => ({ top: 120 }),
+        } as HTMLElement;
+
+        spy.setContainer(container);
+        spy.register(previousTurn, 'previous');
+        spy.register(finalTurn, 'final');
+        while (frames.length > 0) {
+            frames.shift()?.(0);
+        }
+
+        expect(activeTurnIds).toEqual(['final']);
+    });
+});
diff --git a/packages/ui/src/components/chat/lib/scroll/scrollSpy.ts b/packages/ui/src/components/chat/lib/scroll/scrollSpy.ts
index a68b3930..f6562f00 100644
--- a/packages/ui/src/components/chat/lib/scroll/scrollSpy.ts
+++ b/packages/ui/src/components/chat/lib/scroll/scrollSpy.ts
@@ -16,10 +16,16 @@ type ScrollSpyInput = {
 // stable while scrolling inside a long turn (no visibility-ratio flip-flop).
 const READ_LINE_OFFSET_PX = 100;
 
-// When the container is scrolled to (or almost to) the bottom, the last turn
-// is what the user is reading even if it is too short for its top edge to
-// ever cross the reading line — force-activate it in that case.
-const BOTTOM_ANCHOR_EPSILON_PX = 8;
+// The chat treats its bottom spacer as part of the pinned zone. Match that
+// boundary here so the rail reaches the final prompt even when its top never
+// crosses the reading line before the viewport enters that spacer.
+const BOTTOM_ANCHOR_MIN_PX = 48;
+const BOTTOM_ANCHOR_VIEWPORT_FACTOR = 0.1;
+
+const isInBottomAnchorZone = (container: HTMLDivElement): boolean => {
+    const distanceFromBottom = container.scrollHeight - container.scrollTop - container.clientHeight;
+    return distanceFromBottom <= Math.max(BOTTOM_ANCHOR_MIN_PX, container.clientHeight * BOTTOM_ANCHOR_VIEWPORT_FACTOR);
+};
 
 const pickOffsetTurnId = (list: OffsetTurn[], cutoff: number): string | undefined => {
     if (list.length === 0) {
@@ -103,8 +109,7 @@ export const createScrollSpy = (input: ScrollSpyInput) => {
             refreshOffsets();
         }
 
-        const distanceFromBottom = container.scrollHeight - container.scrollTop - container.clientHeight;
-        const next = distanceFromBottom <= BOTTOM_ANCHOR_EPSILON_PX
+        const next = isInBottomAnchorZone(container)
             ? offsets[offsets.length - 1]?.id
             : pickOffsetTurnId(offsets, container.scrollTop + READ_LINE_OFFSET_PX);
         if (!next || next === active) {