diff --git a/pkg/display/FEATURES.md b/pkg/display/FEATURES.md
index 7da9e0c..1683875 100644
--- a/pkg/display/FEATURES.md
+++ b/pkg/display/FEATURES.md
@@ -31,6 +31,7 @@ This document tracks the implementation of display server features that enable A
 - [x] `window_title_get` - Get current window title (returns window name)
 - [x] `window_always_on_top` - Pin window above others
 - [x] `window_background_colour` - Set window background color with alpha (transparency)
+- [x] `window_opacity` - Set window opacity
 - [x] `window_fullscreen` - Enter/exit fullscreen mode
 
 ---
@@ -235,7 +236,6 @@ This document tracks the implementation of display server features that enable A
 - [x] `tray_info` - Get tray status
 
 ### Phase 8 - Remaining Features (Future)
-- [ ] window_opacity (true opacity if Wails adds support)
 - [ ] layout_beside_editor, layout_suggest
 - [ ] webview_devtools_open, webview_devtools_close
 - [ ] clipboard_read_image, clipboard_write_image
diff --git a/pkg/display/display.go b/pkg/display/display.go
index 2fd1a98..bfbf847 100644
--- a/pkg/display/display.go
+++ b/pkg/display/display.go
@@ -1168,6 +1168,15 @@ func (s *Service) SetWindowBackgroundColour(name string, r, g, b, a uint8) error
 	return err
 }
 
+// SetWindowOpacity updates a window's opacity via IPC.
+func (s *Service) SetWindowOpacity(name string, opacity float32) error {
+	_, _, err := s.Core().PERFORM(window.TaskSetOpacity{
+		Name:    name,
+		Opacity: opacity,
+	})
+	return err
+}
+
 // GetFocusedWindow returns the name of the currently focused window.
 func (s *Service) GetFocusedWindow() string {
 	infos := s.ListWindowInfos()
diff --git a/pkg/mcp/tools_window.go b/pkg/mcp/tools_window.go
index b735ed0..7604ba2 100644
--- a/pkg/mcp/tools_window.go
+++ b/pkg/mcp/tools_window.go
@@ -356,6 +356,24 @@ func (s *Subsystem) windowBackgroundColour(_ context.Context, _ *mcp.CallToolReq
 	return nil, WindowBackgroundColourOutput{Success: true}, nil
 }
 
+// --- window_opacity ---
+
+type WindowOpacityInput struct {
+	Name    string  `json:"name"`
+	Opacity float32 `json:"opacity"`
+}
+type WindowOpacityOutput struct {
+	Success bool `json:"success"`
+}
+
+func (s *Subsystem) windowOpacity(_ context.Context, _ *mcp.CallToolRequest, input WindowOpacityInput) (*mcp.CallToolResult, WindowOpacityOutput, error) {
+	_, _, err := s.core.PERFORM(window.TaskSetOpacity{Name: input.Name, Opacity: input.Opacity})
+	if err != nil {
+		return nil, WindowOpacityOutput{}, err
+	}
+	return nil, WindowOpacityOutput{Success: true}, nil
+}
+
 // --- window_fullscreen ---
 
 type WindowFullscreenInput struct {
@@ -394,5 +412,6 @@ func (s *Subsystem) registerWindowTools(server *mcp.Server) {
 	mcp.AddTool(server, &mcp.Tool{Name: "window_visibility", Description: "Show or hide a window"}, s.windowVisibility)
 	mcp.AddTool(server, &mcp.Tool{Name: "window_always_on_top", Description: "Pin a window above others"}, s.windowAlwaysOnTop)
 	mcp.AddTool(server, &mcp.Tool{Name: "window_background_colour", Description: "Set a window background colour"}, s.windowBackgroundColour)
+	mcp.AddTool(server, &mcp.Tool{Name: "window_opacity", Description: "Set a window opacity"}, s.windowOpacity)
 	mcp.AddTool(server, &mcp.Tool{Name: "window_fullscreen", Description: "Set a window to fullscreen mode"}, s.windowFullscreen)
 }
diff --git a/pkg/window/messages.go b/pkg/window/messages.go
index e2ed36c..755f4ee 100644
--- a/pkg/window/messages.go
+++ b/pkg/window/messages.go
@@ -107,6 +107,12 @@ type TaskSetBackgroundColour struct {
 	Alpha uint8
 }
 
+// TaskSetOpacity updates the window opacity as a value between 0 and 1.
+type TaskSetOpacity struct {
+	Name    string
+	Opacity float32
+}
+
 // TaskSetVisibility shows or hides a window.
 type TaskSetVisibility struct {
 	Name    string
diff --git a/pkg/window/mock_platform.go b/pkg/window/mock_platform.go
index d031f9e..293faf6 100644
--- a/pkg/window/mock_platform.go
+++ b/pkg/window/mock_platform.go
@@ -38,6 +38,7 @@ type MockWindow struct {
 	focused              bool
 	visible, alwaysOnTop bool
 	backgroundColor      [4]uint8
+	opacity              float32
 	closed               bool
 	eventHandlers        []func(WindowEvent)
 	fileDropHandlers     []func(paths []string, targetID string)
@@ -55,6 +56,7 @@ func (w *MockWindow) SetTitle(title string)                { w.title = title }
 func (w *MockWindow) SetPosition(x, y int)                 { w.x = x; w.y = y }
 func (w *MockWindow) SetSize(width, height int)            { w.width = width; w.height = height }
 func (w *MockWindow) SetBackgroundColour(r, g, b, a uint8) { w.backgroundColor = [4]uint8{r, g, b, a} }
+func (w *MockWindow) SetOpacity(opacity float32)           { w.opacity = opacity }
 func (w *MockWindow) SetVisibility(visible bool)           { w.visible = visible }
 func (w *MockWindow) SetAlwaysOnTop(alwaysOnTop bool)      { w.alwaysOnTop = alwaysOnTop }
 func (w *MockWindow) Maximise()                            { w.maximised = true; w.minimised = false; w.visible = true }
diff --git a/pkg/window/mock_test.go b/pkg/window/mock_test.go
index 442578d..452a1b9 100644
--- a/pkg/window/mock_test.go
+++ b/pkg/window/mock_test.go
@@ -37,6 +37,7 @@ type mockWindow struct {
 	focused              bool
 	visible, alwaysOnTop bool
 	backgroundColor      [4]uint8
+	opacity              float32
 	devtoolsOpen         bool
 	closed               bool
 	eventHandlers        []func(WindowEvent)
@@ -55,6 +56,7 @@ func (w *mockWindow) SetTitle(title string)                { w.title = title }
 func (w *mockWindow) SetPosition(x, y int)                 { w.x = x; w.y = y }
 func (w *mockWindow) SetSize(width, height int)            { w.width = width; w.height = height }
 func (w *mockWindow) SetBackgroundColour(r, g, b, a uint8) { w.backgroundColor = [4]uint8{r, g, b, a} }
+func (w *mockWindow) SetOpacity(opacity float32)           { w.opacity = opacity }
 func (w *mockWindow) SetVisibility(visible bool)           { w.visible = visible }
 func (w *mockWindow) SetAlwaysOnTop(alwaysOnTop bool)      { w.alwaysOnTop = alwaysOnTop }
 func (w *mockWindow) Maximise()                            { w.maximised = true; w.minimised = false; w.visible = true }
diff --git a/pkg/window/platform.go b/pkg/window/platform.go
index ecc1533..d277231 100644
--- a/pkg/window/platform.go
+++ b/pkg/window/platform.go
@@ -43,6 +43,7 @@ type PlatformWindow interface {
 	SetPosition(x, y int)
 	SetSize(width, height int)
 	SetBackgroundColour(r, g, b, a uint8)
+	SetOpacity(opacity float32)
 	SetVisibility(visible bool)
 	SetAlwaysOnTop(alwaysOnTop bool)
 
diff --git a/pkg/window/service.go b/pkg/window/service.go
index f52c710..c2e53f3 100644
--- a/pkg/window/service.go
+++ b/pkg/window/service.go
@@ -165,6 +165,8 @@ func (s *Service) handleTask(c *core.Core, t core.Task) (any, bool, error) {
 		return nil, true, s.taskSetAlwaysOnTop(t.Name, t.AlwaysOnTop)
 	case TaskSetBackgroundColour:
 		return nil, true, s.taskSetBackgroundColour(t.Name, t.Red, t.Green, t.Blue, t.Alpha)
+	case TaskSetOpacity:
+		return nil, true, s.taskSetOpacity(t.Name, t.Opacity)
 	case TaskSetVisibility:
 		return nil, true, s.taskSetVisibility(t.Name, t.Visible)
 	case TaskFullscreen:
@@ -371,6 +373,18 @@ func (s *Service) taskSetBackgroundColour(name string, red, green, blue, alpha u
 	return nil
 }
 
+func (s *Service) taskSetOpacity(name string, opacity float32) error {
+	if opacity < 0 || opacity > 1 {
+		return fmt.Errorf("opacity must be between 0 and 1")
+	}
+	pw, ok := s.manager.Get(name)
+	if !ok {
+		return fmt.Errorf("window not found: %s", name)
+	}
+	pw.SetOpacity(opacity)
+	return nil
+}
+
 func (s *Service) taskSetVisibility(name string, visible bool) error {
 	pw, ok := s.manager.Get(name)
 	if !ok {
diff --git a/pkg/window/service_test.go b/pkg/window/service_test.go
index 08d3f4d..f14ce9a 100644
--- a/pkg/window/service_test.go
+++ b/pkg/window/service_test.go
@@ -218,6 +218,29 @@ func TestTaskSetBackgroundColour_Good(t *testing.T) {
 	assert.Equal(t, [4]uint8{10, 20, 30, 40}, pw.(*mockWindow).backgroundColor)
 }
 
+func TestTaskSetOpacity_Good(t *testing.T) {
+	_, c := newTestWindowService(t)
+	_, _, _ = c.PERFORM(TaskOpenWindow{Opts: []WindowOption{WithName("test")}})
+
+	_, handled, err := c.PERFORM(TaskSetOpacity{Name: "test", Opacity: 0.65})
+	require.NoError(t, err)
+	assert.True(t, handled)
+
+	svc := core.MustServiceFor[*Service](c, "window")
+	pw, ok := svc.Manager().Get("test")
+	require.True(t, ok)
+	assert.InDelta(t, 0.65, pw.(*mockWindow).opacity, 0.0001)
+}
+
+func TestTaskSetOpacity_BadRange(t *testing.T) {
+	_, c := newTestWindowService(t)
+	_, _, _ = c.PERFORM(TaskOpenWindow{Opts: []WindowOption{WithName("test")}})
+
+	_, handled, err := c.PERFORM(TaskSetOpacity{Name: "test", Opacity: 1.5})
+	require.Error(t, err)
+	assert.True(t, handled)
+}
+
 func TestTaskStackWindows_Good(t *testing.T) {
 	_, c := newTestWindowService(t)
 	_, _, _ = c.PERFORM(TaskOpenWindow{Opts: []WindowOption{WithName("one")}})
diff --git a/pkg/window/wails.go b/pkg/window/wails.go
index 4a10646..b899d7b 100644
--- a/pkg/window/wails.go
+++ b/pkg/window/wails.go
@@ -72,6 +72,7 @@ func (ww *wailsWindow) SetSize(width, height int) { ww.w.SetSize(width, height)
 func (ww *wailsWindow) SetBackgroundColour(r, g, b, a uint8) {
 	ww.w.SetBackgroundColour(application.NewRGBA(r, g, b, a))
 }
+func (ww *wailsWindow) SetOpacity(opacity float32) { ww.w.SetOpacity(opacity) }
 func (ww *wailsWindow) SetVisibility(visible bool) {
 	if visible {
 		ww.w.Show()
diff --git a/stubs/wails/v3/pkg/application/application.go b/stubs/wails/v3/pkg/application/application.go
index c1969ae..54c7c14 100644
--- a/stubs/wails/v3/pkg/application/application.go
+++ b/stubs/wails/v3/pkg/application/application.go
@@ -127,6 +127,7 @@ func (w *WebviewWindow) SetSize(width, height int) {
 	w.width, w.height = width, height
 }
 func (w *WebviewWindow) SetBackgroundColour(colour RGBA) {}
+func (w *WebviewWindow) SetOpacity(opacity float32)      {}
 func (w *WebviewWindow) SetVisibility(visible bool)      { w.visible = visible }
 func (w *WebviewWindow) SetAlwaysOnTop(alwaysOnTop bool) { w.alwaysOnTop = alwaysOnTop }
 func (w *WebviewWindow) Maximise()                       { w.maximised = true; w.minimised = false; w.visible = true }