diff --git a/scripts/test_overlay.py b/scripts/test_overlay.py
new file mode 100644
index 0000000..9945e0d
--- /dev/null
+++ b/scripts/test_overlay.py
@@ -0,0 +1,32 @@
+"""Manual test: shows a pulsing border around Cursor for 10 seconds."""
+import sys
+import time
+
+from Cocoa import NSApplication, NSRunLoop, NSDate
+
+from cursor_flasher.config import Config
+from cursor_flasher.overlay import OverlayWindow
+from cursor_flasher.detector import CursorDetector
+
+app = NSApplication.sharedApplication()
+
+config = Config()
+overlay = OverlayWindow(config)
+detector = CursorDetector()
+
+pid = detector._find_cursor_pid()
+if pid is None:
+    print("Cursor not running")
+    sys.exit(1)
+
+print(f"Showing overlay for PID {pid} for 10 seconds...")
+overlay.show(pid)
+
+end_time = time.time() + 10
+while time.time() < end_time:
+    NSRunLoop.currentRunLoop().runUntilDate_(
+        NSDate.dateWithTimeIntervalSinceNow_(0.1)
+    )
+
+overlay.hide()
+print("Done.")
diff --git a/src/cursor_flasher/overlay.py b/src/cursor_flasher/overlay.py
new file mode 100644
index 0000000..9def2c0
--- /dev/null
+++ b/src/cursor_flasher/overlay.py
@@ -0,0 +1,178 @@
+"""Native macOS overlay window that draws a pulsing border around a target window."""
+import math
+
+import objc
+from Cocoa import (
+    NSApplication,
+    NSWindow,
+    NSBorderlessWindowMask,
+    NSColor,
+    NSView,
+    NSBezierPath,
+    NSTimer,
+    NSScreen,
+)
+from Foundation import NSInsetRect, NSMakeRect
+from Quartz import (
+    CGWindowListCopyWindowInfo,
+    kCGWindowListOptionOnScreenOnly,
+    kCGNullWindowID,
+    kCGWindowOwnerPID,
+    kCGWindowBounds,
+    kCGWindowLayer,
+)
+
+from cursor_flasher.config import Config
+
+
+def hex_to_nscolor(hex_str: str, alpha: float = 1.0) -> NSColor:
+    """Convert a hex color string to NSColor."""
+    hex_str = hex_str.lstrip("#")
+    r = int(hex_str[0:2], 16) / 255.0
+    g = int(hex_str[2:4], 16) / 255.0
+    b = int(hex_str[4:6], 16) / 255.0
+    return NSColor.colorWithCalibratedRed_green_blue_alpha_(r, g, b, alpha)
+
+
+class PulseBorderView(NSView):
+    """Custom view that draws a pulsing border rectangle."""
+
+    def initWithFrame_config_(self, frame, config):
+        self = objc.super(PulseBorderView, self).initWithFrame_(frame)
+        if self is None:
+            return None
+        self._config = config
+        self._phase = 0.0
+        return self
+
+    def drawRect_(self, rect):
+        opacity_range = self._config.pulse_opacity_max - self._config.pulse_opacity_min
+        alpha = self._config.pulse_opacity_min + opacity_range * (
+            0.5 + 0.5 * math.sin(self._phase)
+        )
+
+        color = hex_to_nscolor(self._config.pulse_color, alpha)
+        color.setStroke()
+
+        width = self._config.pulse_width
+        inset = width / 2.0
+        bounds = objc.super(PulseBorderView, self).bounds()
+        inset_rect = NSInsetRect(bounds, inset, inset)
+        path = NSBezierPath.bezierPathWithRect_(inset_rect)
+        path.setLineWidth_(width)
+        path.stroke()
+
+    def setPhase_(self, phase):
+        self._phase = phase
+        self.setNeedsDisplay_(True)
+
+
+class OverlayWindow:
+    """Manages the overlay window lifecycle."""
+
+    def __init__(self, config: Config):
+        self._config = config
+        self._window: NSWindow | None = None
+        self._view: PulseBorderView | None = None
+        self._timer: NSTimer | None = None
+        self._phase = 0.0
+        self._target_pid: int | None = None
+
+    def show(self, pid: int) -> None:
+        """Show the pulsing overlay around the window belonging to `pid`."""
+        self._target_pid = pid
+        frame = self._get_window_frame(pid)
+        if frame is None:
+            return
+
+        if self._window is None:
+            self._create_window(frame)
+        else:
+            self._window.setFrame_display_(frame, True)
+            self._view.setFrame_(((0, 0), (frame[1][0], frame[1][1])))
+
+        self._window.orderFrontRegardless()
+        self._start_animation()
+
+    def hide(self) -> None:
+        """Hide and clean up the overlay."""
+        self._stop_animation()
+        if self._window is not None:
+            self._window.orderOut_(None)
+
+    def update_position(self) -> None:
+        """Reposition overlay to match current Cursor window position."""
+        if self._target_pid is None or self._window is None:
+            return
+        frame = self._get_window_frame(self._target_pid)
+        if frame is not None:
+            self._window.setFrame_display_(frame, True)
+            self._view.setFrame_(((0, 0), (frame[1][0], frame[1][1])))
+
+    def _create_window(self, frame) -> None:
+        self._window = NSWindow.alloc().initWithContentRect_styleMask_backing_defer_(
+            frame, NSBorderlessWindowMask, 2, False  # NSBackingStoreBuffered
+        )
+        self._window.setOpaque_(False)
+        self._window.setBackgroundColor_(NSColor.clearColor())
+        self._window.setLevel_(25)  # Above normal windows
+        self._window.setIgnoresMouseEvents_(True)
+        self._window.setHasShadow_(False)
+
+        content_rect = ((0, 0), (frame[1][0], frame[1][1]))
+        self._view = PulseBorderView.alloc().initWithFrame_config_(
+            content_rect, self._config
+        )
+        self._window.setContentView_(self._view)
+
+    def _start_animation(self) -> None:
+        if self._timer is not None:
+            return
+        interval = 1.0 / 30.0  # 30 fps
+        self._timer = NSTimer.scheduledTimerWithTimeInterval_target_selector_userInfo_repeats_(
+            interval, self, "_tick:", None, True
+        )
+
+    def _stop_animation(self) -> None:
+        if self._timer is not None:
+            self._timer.invalidate()
+            self._timer = None
+        self._phase = 0.0
+
+    @objc.python_method
+    def _tick_impl(self):
+        speed = self._config.pulse_speed
+        step = (2.0 * math.pi) / (speed * 30.0)
+        self._phase += step
+        if self._view is not None:
+            self._view.setPhase_(self._phase)
+        self.update_position()
+
+    def _tick_(self, timer) -> None:
+        self._tick_impl()
+
+    def _get_window_frame(self, pid: int):
+        """Get the screen frame of the main window for the given PID."""
+        window_list = CGWindowListCopyWindowInfo(
+            kCGWindowListOptionOnScreenOnly, kCGNullWindowID
+        )
+        if not window_list:
+            return None
+
+        screen_height = NSScreen.mainScreen().frame().size.height
+
+        for info in window_list:
+            if info.get(kCGWindowOwnerPID) != pid:
+                continue
+            if info.get(kCGWindowLayer, 999) != 0:
+                continue
+            bounds = info.get(kCGWindowBounds)
+            if bounds is None:
+                continue
+            x = bounds["X"]
+            y = screen_height - bounds["Y"] - bounds["Height"]
+            w = bounds["Width"]
+            h = bounds["Height"]
+            return ((x, y), (w, h))
+
+        return None