diff --git a/internal/filesystems/filesystems_test.go b/internal/filesystems/filesystems_test.go
new file mode 100644
index 000000000..a78ecf07c
--- /dev/null
+++ b/internal/filesystems/filesystems_test.go
@@ -0,0 +1,173 @@
+package filesystems
+
+import (
+	"io/fs"
+	"testing"
+	"testing/fstest"
+)
+
+func TestFileSystemMapDefaultKey(t *testing.T) {
+	m := &FileSystemMap{}
+
+	// Empty key should map to default
+	if m.key("") != DefaultFileSystemKey {
+		t.Errorf("empty key should map to %q, got %q", DefaultFileSystemKey, m.key(""))
+	}
+
+	// Non-empty key should be returned as-is
+	if m.key("custom") != "custom" {
+		t.Errorf("non-empty key should be returned as-is, got %q", m.key("custom"))
+	}
+}
+
+func TestFileSystemMapRegisterAndGet(t *testing.T) {
+	m := &FileSystemMap{}
+	testFS := fstest.MapFS{
+		"hello.txt": &fstest.MapFile{Data: []byte("hello")},
+	}
+
+	m.Register("test", testFS)
+
+	got, ok := m.Get("test")
+	if !ok {
+		t.Fatal("expected to find registered filesystem")
+	}
+	if got == nil {
+		t.Fatal("expected non-nil filesystem")
+	}
+
+	// Verify the filesystem works
+	f, err := got.Open("hello.txt")
+	if err != nil {
+		t.Fatalf("Open() error = %v", err)
+	}
+	f.Close()
+}
+
+func TestFileSystemMapGetNonExistent(t *testing.T) {
+	m := &FileSystemMap{}
+
+	_, ok := m.Get("nonexistent")
+	if ok {
+		t.Error("expected Get to return false for nonexistent key")
+	}
+}
+
+func TestFileSystemMapDefault(t *testing.T) {
+	m := &FileSystemMap{}
+
+	d := m.Default()
+	if d == nil {
+		t.Fatal("Default() should never return nil")
+	}
+}
+
+func TestFileSystemMapGetDefaultLazyInit(t *testing.T) {
+	m := &FileSystemMap{}
+
+	// Getting the default key before any registration should
+	// auto-initialize to DefaultFileSystem
+	got, ok := m.Get(DefaultFileSystemKey)
+	if !ok {
+		t.Fatal("expected default filesystem to be auto-initialized")
+	}
+	if got == nil {
+		t.Fatal("expected non-nil default filesystem")
+	}
+}
+
+func TestFileSystemMapUnregister(t *testing.T) {
+	m := &FileSystemMap{}
+	testFS := fstest.MapFS{}
+
+	m.Register("test", testFS)
+	m.Unregister("test")
+
+	_, ok := m.Get("test")
+	if ok {
+		t.Error("expected filesystem to be unregistered")
+	}
+}
+
+func TestFileSystemMapUnregisterDefault(t *testing.T) {
+	m := &FileSystemMap{}
+	customFS := fstest.MapFS{}
+
+	// Override default
+	m.Register("", customFS)
+	// Unregister default should reset to OsFS, not delete
+	m.Unregister("")
+
+	d := m.Default()
+	if d == nil {
+		t.Fatal("unregistering default should reset it, not delete it")
+	}
+}
+
+func TestFileSystemMapRegisterNil(t *testing.T) {
+	m := &FileSystemMap{}
+	testFS := fstest.MapFS{}
+
+	// Register then register nil (should unregister)
+	m.Register("test", testFS)
+	m.Register("test", nil)
+
+	_, ok := m.Get("test")
+	if ok {
+		t.Error("registering nil should unregister the filesystem")
+	}
+}
+
+func TestFileSystemMapEmptyKeyIsDefault(t *testing.T) {
+	m := &FileSystemMap{}
+	testFS := fstest.MapFS{
+		"test.txt": &fstest.MapFile{Data: []byte("test")},
+	}
+
+	// Register with empty key should register as default
+	m.Register("", testFS)
+
+	got, ok := m.Get("")
+	if !ok {
+		t.Fatal("expected to find filesystem registered with empty key")
+	}
+
+	// Should also be accessible via default key
+	got2, ok := m.Get(DefaultFileSystemKey)
+	if !ok {
+		t.Fatal("expected to find filesystem via default key")
+	}
+
+	// Both should work
+	if got == nil || got2 == nil {
+		t.Fatal("expected non-nil filesystems")
+	}
+}
+
+func TestFileSystemMapGetTrimsWhitespace(t *testing.T) {
+	m := &FileSystemMap{}
+	testFS := fstest.MapFS{}
+
+	m.Register("test", testFS)
+
+	// Get with whitespace-padded key should match
+	got, ok := m.Get("test ")
+	if !ok {
+		t.Fatal("expected Get to trim whitespace from key")
+	}
+	if got == nil {
+		t.Fatal("expected non-nil filesystem")
+	}
+}
+
+func TestOsFSInterfaces(t *testing.T) {
+	var osFS OsFS
+
+	// Verify interface compliance at compile time (already done with var _ checks)
+	// but test that the methods exist and are callable
+	var _ fs.FS = osFS
+	var _ fs.StatFS = osFS
+	var _ fs.GlobFS = osFS
+	var _ fs.ReadDirFS = osFS
+	var _ fs.ReadFileFS = osFS
+}