diff --git a/.gitignore b/.gitignore
index 5181c4c..33a420a 100644
--- a/.gitignore
+++ b/.gitignore
@@ -6,5 +6,7 @@ dist/
 agent.yml
 !cmd/theta-agent-helper/
 !cmd/theta-agent-helper/main.go
+!cmd/theta-agent-tray/
+!cmd/theta-agent-tray/main_test.go
 # Fetched by scripts/setup-build-env.ps1, pinned in installer/windows/vendor-manifest.json
 installer/windows/vendor/
diff --git a/cmd/theta-agent-tray/main_test.go b/cmd/theta-agent-tray/main_test.go
new file mode 100644
index 0000000..a579f48
--- /dev/null
+++ b/cmd/theta-agent-tray/main_test.go
@@ -0,0 +1,81 @@
+package main
+
+import (
+	"bytes"
+	"runtime"
+	"testing"
+)
+
+// pngToIco wraps a PNG in a Windows ICO with BMP (not PNG-compressed) entries,
+// because LoadImage cannot read PNG-in-ICO. Verify the container is well-formed
+// and every entry's DIB is a valid 32bpp bitmap with an AND mask.
+func TestPNGToIco(t *testing.T) {
+	ico := pngToIco(iconGreen)
+	if len(ico) < 6+16 {
+		t.Fatal("ico too short")
+	}
+
+	// ICONDIR
+	if ico[0] != 0 || ico[1] != 0 {
+		t.Errorf("reserved must be 0")
+	}
+	if ico[2] != 1 || ico[3] != 0 {
+		t.Errorf("type must be 1 (icon)")
+	}
+	count := int(ico[4]) | int(ico[5])<<8
+	if count != 3 {
+		t.Fatalf("expected 3 icon entries, got %d", count)
+	}
+
+	// Each ICONDIRENTRY: valid size, planes=1, bpp=32, DIB with BITMAPINFOHEADER.
+	for i := 0; i < count; i++ {
+		e := 6 + i*16
+		w := int(ico[e])
+		h := int(ico[e+1])
+		if w == 0 {
+			w = 256
+		}
+		if h == 0 {
+			h = 256
+		}
+		planes := int(ico[e+4]) | int(ico[e+5])<<8
+		bpp := int(ico[e+6]) | int(ico[e+7])<<8
+		size := int(ico[e+8]) | int(ico[e+9])<<8 | int(ico[e+10])<<16 | int(ico[e+11])<<24
+		offset := int(ico[e+12]) | int(ico[e+13])<<8 | int(ico[e+14])<<16 | int(ico[e+15])<<24
+
+		if planes != 1 || bpp != 32 {
+			t.Errorf("entry %d: want planes=1 bpp=32, got %d/%d", i, planes, bpp)
+		}
+		if offset+size > len(ico) {
+			t.Fatalf("entry %d: DIB out of range", i)
+		}
+
+		dib := ico[offset : offset+size]
+		// BITMAPINFOHEADER: biSize=40, biHeight = 2x for XOR+AND.
+		biSize := int(dib[0]) | int(dib[1])<<8 | int(dib[2])<<16 | int(dib[3])<<24
+		biW := int(dib[4]) | int(dib[5])<<8 | int(dib[6])<<16 | int(dib[7])<<24
+		biH := int(dib[8]) | int(dib[9])<<8 | int(dib[10])<<16 | int(dib[11])<<24
+		if biSize != 40 {
+			t.Errorf("entry %d: expected BITMAPINFOHEADER (40), got %d", i, biSize)
+		}
+		if biW != w || biH != h*2 {
+			t.Errorf("entry %d: DIB dims %dx%d, want %dx%d", i, biW, biH, w, h*2)
+		}
+	}
+}
+
+// toWindowsIcon passes the PNG through untouched on non-Windows and returns an
+// ICO on Windows (whose validity TestPNGToIco covers).
+func TestToWindowsIcon(t *testing.T) {
+	pngMagic := []byte{0x89, 'P', 'N', 'G'}
+	if runtime.GOOS == "windows" {
+		if bytes.HasPrefix(toWindowsIcon(iconRed), pngMagic) {
+			t.Error("windows must not receive a raw PNG")
+		}
+	} else {
+		if !bytes.HasPrefix(toWindowsIcon(iconRed), pngMagic) {
+			t.Error("non-windows must receive the PNG unchanged")
+		}
+	}
+}
+