diff --git a/hal/dx12/d3d12/types.go b/hal/dx12/d3d12/types.go
index 62055fb..504b56b 100644
--- a/hal/dx12/d3d12/types.go
+++ b/hal/dx12/d3d12/types.go
@@ -597,7 +597,11 @@ type D3D12_COMMAND_SIGNATURE_DESC struct {
 type D3D12_INDIRECT_ARGUMENT_DESC struct {
 	Type D3D12_INDIRECT_ARGUMENT_TYPE
 	// Union for different argument types
-	Union [8]byte
+	// The largest member (D3D12_INDIRECT_ARGUMENT_DESC_CONSTANT) is three
+	// uint32 values, so the C union occupies 12 bytes and the full descriptor
+	// is 16 bytes including Type. Keeping the exact size matters because
+	// CreateCommandSignature reads the native descriptor layout directly.
+	Union [12]byte
 }
 
 // D3D12_DISCARD_REGION describes a discard region.
diff --git a/hal/dx12/d3d12/types_test.go b/hal/dx12/d3d12/types_test.go
new file mode 100644
index 0000000..16d5052
--- /dev/null
+++ b/hal/dx12/d3d12/types_test.go
@@ -0,0 +1,17 @@
+//go:build windows && !(js && wasm)
+
+package d3d12
+
+import (
+	"testing"
+	"unsafe"
+)
+
+func TestIndirectArgumentDescABI(t *testing.T) {
+	if got := unsafe.Sizeof(D3D12_INDIRECT_ARGUMENT_DESC{}); got != 16 {
+		t.Fatalf("D3D12_INDIRECT_ARGUMENT_DESC size = %d, want 16", got)
+	}
+	if got := unsafe.Offsetof(D3D12_INDIRECT_ARGUMENT_DESC{}.Union); got != 4 {
+		t.Fatalf("D3D12_INDIRECT_ARGUMENT_DESC.Union offset = %d, want 4", got)
+	}
+}