diff --git a/internal/update/inuse_dll_windows_test.go b/internal/update/inuse_dll_windows_test.go
new file mode 100644
index 0000000..984f01d
--- /dev/null
+++ b/internal/update/inuse_dll_windows_test.go
@@ -0,0 +1,126 @@
+//go:build windows
+
+package update
+
+import (
+	"context"
+	"os"
+	"path/filepath"
+	"strings"
+	"syscall"
+	"testing"
+
+	"modavd/internal/errs"
+)
+
+// loadDLL puts a copy of a small system DLL at path and loads it into this process, the way MAA loads
+// mumu-shim\shell\sdk\external_renderer_ipc.dll; freed at the end of the test (or by the returned func).
+func loadDLL(t *testing.T, path string) (free func()) {
+	t.Helper()
+	b, err := os.ReadFile(filepath.Join(os.Getenv("SystemRoot"), "System32", "msimg32.dll"))
+	if err != nil {
+		t.Skipf("no system DLL to load: %v", err)
+	}
+	if err := os.MkdirAll(filepath.Dir(path), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(path, b, 0o644); err != nil {
+		t.Fatal(err)
+	}
+	h, err := syscall.LoadLibrary(path)
+	if err != nil {
+		t.Fatalf("LoadLibrary %s: %v", path, err)
+	}
+	freed := false
+	free = func() {
+		if !freed {
+			freed = true
+			syscall.FreeLibrary(h)
+		}
+	}
+	t.Cleanup(free)
+	return free
+}
+
+func TestFileInUseSeesLoadedDLL(t *testing.T) {
+	dir := t.TempDir()
+	dll := filepath.Join(dir, "external_renderer_ipc.dll")
+	free := loadDLL(t, dll)
+	if !fileInUse(dll) {
+		t.Error("a loaded DLL is not in use")
+	}
+	free()
+	if fileInUse(dll) {
+		t.Error("a freed DLL is still in use")
+	}
+	plain := filepath.Join(dir, "plain.txt")
+	os.WriteFile(plain, []byte("x"), 0o644)
+	readOnly := filepath.Join(dir, "readonly.txt")
+	os.WriteFile(readOnly, []byte("x"), 0o444)
+	for _, p := range []string{plain, readOnly, filepath.Join(dir, "missing.dll")} {
+		if fileInUse(p) {
+			t.Errorf("%s counts as in use", p)
+		}
+	}
+	if b, _ := os.ReadFile(plain); string(b) != "x" {
+		t.Errorf("the check changed the file: %q", b)
+	}
+}
+
+// TestInstallOverThirdPartyShim: AUTO-MAS copies its own external_renderer_ipc.dll to the same path, so the root's
+// file differs from the root manifest. A full package install takes the package's copy (the other one is kept in the
+// backup until the install is done) and does not fail.
+func TestInstallOverThirdPartyShim(t *testing.T) {
+	const shim = "mumu-shim/shell/sdk/external_renderer_ipc.dll"
+	withSettings(t, SourceGitHub, "")
+	r := writeRoot(t, 36, map[string]string{shim: "shim 36"})
+	if err := os.WriteFile(filepath.Join(r.Dir, filepath.FromSlash(shim)), []byte("AUTO-MAS copy"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	f := newSources(t, 37, zipOf(t, 37, pkgFiles(37, map[string]string{shim: "shim 36"}), false))
+	u := f.updater(r)
+	u.scan = func(bool) ([]processUse, error) { return nil, nil }
+	if st, err := u.Install(context.Background()); err != nil || !st.Updated {
+		t.Fatalf("install: %+v %v", st, err)
+	}
+	if got := read(t, r, shim); got != "shim 36" {
+		t.Errorf("shim = %q", got)
+	}
+}
+
+// TestInstallBlockedByLoadedDLL: MAA holding the MuMu shell (or extras\modavd_extras.dll) loaded stops the install
+// before anything is swapped, with a message that says which file and to close MAA; the stage stays installable and
+// the next install, once MAA is closed, goes through.
+func TestInstallBlockedByLoadedDLL(t *testing.T) {
+	const shim = "mumu-shim/shell/sdk/external_renderer_ipc.dll"
+	withSettings(t, SourceGitHub, "")
+	r := writeRoot(t, 36, map[string]string{shim: "shim 36"})
+	f := newSources(t, 37, zipOf(t, 37, pkgFiles(37, map[string]string{shim: "shim 37"}), false))
+	u := f.updater(r)
+	u.scan = func(bool) ([]processUse, error) { return nil, nil }
+	free := loadDLL(t, filepath.Join(r.Dir, filepath.FromSlash(shim)))
+	_, err := u.Install(context.Background())
+	if errs.CodeOf(err) != errs.Busy || !strings.Contains(err.Error(), `mumu-shim\shell\sdk\external_renderer_ipc.dll`) ||
+		!strings.Contains(err.Error(), "MAA") || !strings.Contains(err.Error(), "什么都没动") {
+		t.Fatalf("blocked: %v", err)
+	}
+	if st := u.Status(); st.Phase != phaseStaged || st.PreparedVersion != "v0.37.0" {
+		t.Fatalf("after: %+v", st)
+	}
+	for rel, want := range map[string]string{"sdk/emulator/emulator.exe": "emulator mas-36", "说明.txt": "ModAVD 0.36.0\n",
+		"tools/modavd-bridge.ps1": "# bridge 36\n"} {
+		if got := read(t, r, rel); got != want {
+			t.Errorf("%s = %q after the blocked install", rel, got)
+		}
+	}
+	if lexists(u.l.previous()) {
+		t.Error("previous\\ left behind")
+	}
+	free()
+	if st, err := u.Install(context.Background()); err != nil || !st.Updated {
+		t.Fatalf("install after MAA closed: %+v %v", st, err)
+	}
+	if got := read(t, r, shim); got != "shim 37" {
+		t.Errorf("shim = %q", got)
+	}
+}
diff --git a/internal/update/state_other.go b/internal/update/state_other.go
index 0631eba..90f6af4 100644
--- a/internal/update/state_other.go
+++ b/internal/update/state_other.go
@@ -6,3 +6,6 @@ import "os"
 
 func isReparsePoint(st os.FileInfo) bool { return st.Mode()&os.ModeSymlink != 0 }
 func removeLink(p string) bool           { return os.Remove(p) == nil }
+
+// fileInUse: a loaded library or an open file never stops a rename or an unlink here.
+func fileInUse(string) bool { return false }
diff --git a/internal/update/state_windows.go b/internal/update/state_windows.go
index 07daeac..78ae678 100644
--- a/internal/update/state_windows.go
+++ b/internal/update/state_windows.go
@@ -3,11 +3,28 @@
 package update
 
 import (
+	"errors"
 	"os"
 	"syscall"
 )
 
-const fileAttributeReparsePoint = 0x400
+const (
+	fileAttributeReparsePoint = 0x400
+	errorSharingViolation     = syscall.Errno(32)
+	errorLockViolation        = syscall.Errno(33)
+)
+
+// fileInUse: another process holds path so that it can be neither replaced nor deleted, e.g. a DLL MAA has loaded
+// (a mapped image refuses write access) or a file opened without sharing. Opened for writing without truncating and
+// closed at once; any other error (read-only, missing) is not "in use": the swap reports those itself.
+func fileInUse(path string) bool {
+	f, err := os.OpenFile(path, os.O_WRONLY, 0)
+	if err == nil {
+		f.Close()
+		return false
+	}
+	return errors.Is(err, errorSharingViolation) || errors.Is(err, errorLockViolation)
+}
 
 func isReparsePoint(st os.FileInfo) bool {
 	sys, ok := st.Sys().(*syscall.Win32FileAttributeData)
diff --git a/internal/update/swap.go b/internal/update/swap.go
index cbd4005..4c21c01 100644
--- a/internal/update/swap.go
+++ b/internal/update/swap.go
@@ -243,6 +243,22 @@ func (l layout) applyRootFiles(p rootPlan) error {
 	return nil
 }
 
+// rootFileInUse is the first root file the plan replaces or deletes that another process holds (fileInUse), "" when
+// none. The running program itself is left out: applyRootFiles renames it away first.
+func (l layout) rootFileInUse(p rootPlan) string {
+	self := selfExe()
+	for _, rel := range append(append([]string{}, p.replace...), p.delete...) {
+		dst := l.at(rel)
+		if inFold(p.created, rel) || (self != "" && strings.EqualFold(dst, self)) || !isFile(dst) {
+			continue
+		}
+		if fileInUse(dst) {
+			return rel
+		}
+	}
+	return ""
+}
+
 // restoreRootFiles undoes applyRootFiles from previous\root.
 func (l layout) restoreRootFiles(p rootPlan) error {
 	for _, rel := range p.created {
@@ -312,6 +328,13 @@ func (u *Updater) swapIn() (string, error) {
 	if err := l.backupRootFiles(plan); err != nil {
 		return "", err
 	}
+	// A root file another program holds (MAA loads extras\modavd_extras.dll and
+	// mumu-shim\shell\sdk\external_renderer_ipc.dll) cannot be replaced: found out after the renames, it would leave
+	// the root half swapped until that program exits. Checked while nothing has been changed yet.
+	if rel := l.rootFileInUse(plan); rel != "" {
+		l.backToStaged(st)
+		return "", swapBlocked("%s 正被别的程序占用（多半是 MAA 之类载入了它），什么都没动。请先关掉它再装", filepath.FromSlash(rel))
+	}
 	st.Set("phase", jsonord.NewString(phaseSwapping))
 	st.Set("plan", plan.value())
 	st.Set("swap", stepVal(stepBegin))