diff --git a/tools/tht/internal/backup/preflight.go b/tools/tht/internal/backup/preflight.go
index 6ca1c2a6..00b0203a 100644
--- a/tools/tht/internal/backup/preflight.go
+++ b/tools/tht/internal/backup/preflight.go
@@ -287,7 +287,7 @@ func (result PreflightResult) StageArchive(ctx context.Context) (_ *stagedArchiv
 		return nil, errors.New("protect private restore staging directory")
 	}
 	path := filepath.Join(directory, "archive.zip")
-	file, err := os.OpenFile(path, os.O_RDWR|os.O_CREATE|os.O_EXCL, 0o600)
+	file, err := safeio.CreateCanonicalNewPrivateFile(path)
 	if err != nil {
 		_ = os.Remove(directory)
 		return nil, errors.New("create private restore staging archive")
diff --git a/tools/tht/internal/backup/preflight_test.go b/tools/tht/internal/backup/preflight_test.go
index a720ac91..ae9c58dc 100644
--- a/tools/tht/internal/backup/preflight_test.go
+++ b/tools/tht/internal/backup/preflight_test.go
@@ -455,6 +455,37 @@ func TestPreflightStagesArchiveIntoImmutablePrivateBytes(t *testing.T) {
 	}
 }
 
+func TestStageArchiveCleansPrivateFileAfterStreamingFailure(t *testing.T) {
+	installation := preflightTestInstallation(t)
+	if err := os.MkdirAll(installation.ControlDirectory(), 0o700); err != nil {
+		t.Fatal(err)
+	}
+	archive := filepath.Join(t.TempDir(), "checked.zip")
+	writePreflightArchive(t, archive, preflightArchiveSpec{
+		entries: []preflightArchiveEntry{{path: "configuration/operator.env", body: []byte("before")}},
+	})
+
+	result, err := Preflight(context.Background(), installation, PreflightRequest{Archive: archive, Confirm: true}, permissivePreflightDependencies())
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer result.CloseArchive()
+	result.freeBytes = func(string) (uint64, error) {
+		result.archive.digest = "after-preflight-mismatch"
+		return 1024, nil
+	}
+	if _, err := result.StageArchive(context.Background()); err == nil || !strings.Contains(err.Error(), "changed") {
+		t.Fatalf("StageArchive() error = %v, want streaming size refusal", err)
+	}
+	entries, err := os.ReadDir(result.stagingRoot)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(entries) != 0 {
+		t.Fatalf("private staging leftovers = %v, want none", entries)
+	}
+}
+
 func TestPreflightStagingRejectsInPlaceArchiveHashMutation(t *testing.T) {
 	installation := preflightTestInstallation(t)
 	archive := filepath.Join(t.TempDir(), "checked.zip")
diff --git a/tools/tht/internal/backup/preflight_unix_test.go b/tools/tht/internal/backup/preflight_unix_test.go
index 05c8d79e..ce691fe7 100644
--- a/tools/tht/internal/backup/preflight_unix_test.go
+++ b/tools/tht/internal/backup/preflight_unix_test.go
@@ -10,6 +10,7 @@ import (
 	"testing"
 
 	"github.com/aritmolab/thothii/tools/tht/internal/config"
+	"github.com/aritmolab/thothii/tools/tht/internal/safeio"
 )
 
 func TestStageArchiveRejectsSymlinkedInstallationAncestor(t *testing.T) {
@@ -46,3 +47,35 @@ func TestStageArchiveRejectsSymlinkedInstallationAncestor(t *testing.T) {
 		t.Fatalf("StageArchive() error = %v, want unsafe symlinked staging-root rejection", err)
 	}
 }
+
+func TestStageArchiveCreatesUnixPrivateRegularFile(t *testing.T) {
+	installation := preflightTestInstallation(t)
+	if err := os.MkdirAll(installation.ControlDirectory(), 0o700); err != nil {
+		t.Fatal(err)
+	}
+	archive := filepath.Join(t.TempDir(), "valid.zip")
+	writePreflightArchive(t, archive, preflightArchiveSpec{
+		entries: []preflightArchiveEntry{{path: "configuration/operator.env", body: []byte("safe")}},
+	})
+	result, err := Preflight(context.Background(), installation, PreflightRequest{Archive: archive, Confirm: true}, permissivePreflightDependencies())
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer result.CloseArchive()
+
+	staged, err := result.StageArchive(context.Background())
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer staged.Close()
+	if err := safeio.ValidatePrivateRegular(staged.path); err != nil {
+		t.Fatalf("staged archive privacy = %v, want owner-private regular file", err)
+	}
+	info, err := os.Lstat(staged.path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got := info.Mode().Perm(); got != 0o600 {
+		t.Fatalf("staged archive mode = %#o, want 0600", got)
+	}
+}
diff --git a/tools/tht/internal/backup/preflight_windows_test.go b/tools/tht/internal/backup/preflight_windows_test.go
index 48cde248..5968e83b 100644
--- a/tools/tht/internal/backup/preflight_windows_test.go
+++ b/tools/tht/internal/backup/preflight_windows_test.go
@@ -11,7 +11,7 @@ import (
 	"github.com/aritmolab/thothii/tools/tht/internal/safeio"
 )
 
-func TestStageArchiveProtectsWindowsStagingDirectoriesWithOwnerOnlyACLs(t *testing.T) {
+func TestStageArchiveProtectsWindowsStagingArtifactsWithOwnerOnlyACLs(t *testing.T) {
 	installation := preflightTestInstallation(t)
 	if err := os.MkdirAll(installation.ControlDirectory(), 0o700); err != nil {
 		t.Fatal(err)
@@ -37,4 +37,7 @@ func TestStageArchiveProtectsWindowsStagingDirectoriesWithOwnerOnlyACLs(t *testi
 	if err := safeio.ValidatePrivateDirectory(staged.directory); err != nil {
 		t.Fatalf("staged archive directory ACL = %v, want owner-only", err)
 	}
+	if err := safeio.ValidatePrivateRegular(staged.path); err != nil {
+		t.Fatalf("staged archive ACL = %v, want owner-only", err)
+	}
 }
diff --git a/tools/tht/internal/safeio/files.go b/tools/tht/internal/safeio/files.go
index 33b0a7fd..36b9d11c 100644
--- a/tools/tht/internal/safeio/files.go
+++ b/tools/tht/internal/safeio/files.go
@@ -275,25 +275,24 @@ func WriteCanonicalNewPrivateFile(path string, contents []byte, mode os.FileMode
 	return writeCanonicalNewFile(path, contents, mode, true)
 }
 
+// CreateCanonicalNewPrivateFile exclusively creates an owner-private regular file under an
+// already private parent and returns a read/write handle for streamed contents. Callers must close
+// the returned handle and remove the file if their stream fails.
+func CreateCanonicalNewPrivateFile(path string) (*os.File, error) {
+	if err := validateCanonicalNewFile(path, true); err != nil {
+		return nil, err
+	}
+	file, err := createCanonicalNewPrivateParentReadWriteFile(path, 0o600)
+	if err != nil {
+		return nil, ErrUnsafeFile
+	}
+	return file, nil
+}
+
 func writeCanonicalNewFile(path string, contents []byte, mode os.FileMode, requirePrivateParent bool) error {
-	if err := ValidateCanonicalPath(path); err != nil {
+	if err := validateCanonicalNewFile(path, requirePrivateParent); err != nil {
 		return err
 	}
-	parent := filepath.Dir(path)
-	if err := requireCanonicalDirectory(parent); err != nil {
-		return err
-	}
-	if requirePrivateParent && ValidatePrivateDirectory(parent) != nil {
-		return ErrUnsafeFile
-	}
-	if info, err := os.Lstat(path); err == nil {
-		if !info.Mode().IsRegular() || info.Mode()&os.ModeSymlink != 0 || info.Mode()&os.ModeType != 0 {
-			return ErrUnsafeFile
-		}
-		return ErrUnsafeFile
-	} else if !errors.Is(err, os.ErrNotExist) {
-		return ErrUnsafeFile
-	}
 	var (
 		file *os.File
 		err  error
@@ -323,6 +322,28 @@ func writeCanonicalNewFile(path string, contents []byte, mode os.FileMode, requi
 	return nil
 }
 
+func validateCanonicalNewFile(path string, requirePrivateParent bool) error {
+	if err := ValidateCanonicalPath(path); err != nil {
+		return err
+	}
+	parent := filepath.Dir(path)
+	if err := requireCanonicalDirectory(parent); err != nil {
+		return err
+	}
+	if requirePrivateParent && ValidatePrivateDirectory(parent) != nil {
+		return ErrUnsafeFile
+	}
+	if info, err := os.Lstat(path); err == nil {
+		if !info.Mode().IsRegular() || info.Mode()&os.ModeSymlink != 0 || info.Mode()&os.ModeType != 0 {
+			return ErrUnsafeFile
+		}
+		return ErrUnsafeFile
+	} else if !errors.Is(err, os.ErrNotExist) {
+		return ErrUnsafeFile
+	}
+	return nil
+}
+
 // ReplaceCanonicalRegular durably replaces one existing private regular file without following
 // symlinked path components. Platform implementations keep the temporary file in the target
 // directory and use the platform's atomic replace primitive.
diff --git a/tools/tht/internal/safeio/files_test.go b/tools/tht/internal/safeio/files_test.go
index 82170172..3bcd3efa 100644
--- a/tools/tht/internal/safeio/files_test.go
+++ b/tools/tht/internal/safeio/files_test.go
@@ -3,6 +3,7 @@ package safeio
 import (
 	"errors"
 	"fmt"
+	"io"
 	"os"
 	"path/filepath"
 	"strings"
@@ -97,6 +98,57 @@ func TestWriteCanonicalNewFileRejectsExistingTargets(t *testing.T) {
 	}
 }
 
+func TestCreateCanonicalNewPrivateFileProvidesPrivateStreamingWriter(t *testing.T) {
+	temporaryRoot, err := filepath.EvalSymlinks(os.TempDir())
+	if err != nil {
+		t.Fatal(err)
+	}
+	directory, err := os.MkdirTemp(temporaryRoot, "tht-safeio-private-stream-")
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(func() { _ = os.RemoveAll(directory) })
+	if err := EnsurePrivateDirectory(directory); err != nil {
+		t.Fatal(err)
+	}
+
+	path := filepath.Join(directory, "archive.zip")
+	file, err := CreateCanonicalNewPrivateFile(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, err := file.Write([]byte("streamed archive")); err != nil {
+		_ = file.Close()
+		t.Fatal(err)
+	}
+	if _, err := file.Seek(0, io.SeekStart); err != nil {
+		_ = file.Close()
+		t.Fatal(err)
+	}
+	streamed := make([]byte, len("streamed archive"))
+	if _, err := io.ReadFull(file, streamed); err != nil {
+		_ = file.Close()
+		t.Fatal(err)
+	}
+	if string(streamed) != "streamed archive" {
+		_ = file.Close()
+		t.Fatalf("streamed contents through open file = %q, want %q", streamed, "streamed archive")
+	}
+	if err := file.Close(); err != nil {
+		t.Fatal(err)
+	}
+	if err := ValidatePrivateRegular(path); err != nil {
+		t.Fatalf("ValidatePrivateRegular() = %v, want owner-private streamed file", err)
+	}
+	contents, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(contents) != "streamed archive" {
+		t.Fatalf("streamed contents = %q, want %q", contents, "streamed archive")
+	}
+}
+
 func TestPreflightPrivateDirectoryAllowsOnlyAMissingFinalComponentWithoutMutation(t *testing.T) {
 	temporaryRoot, err := filepath.EvalSymlinks(os.TempDir())
 	if err != nil {
diff --git a/tools/tht/internal/safeio/private_unix.go b/tools/tht/internal/safeio/private_unix.go
index 2e3d8769..322f1ce9 100644
--- a/tools/tht/internal/safeio/private_unix.go
+++ b/tools/tht/internal/safeio/private_unix.go
@@ -89,7 +89,11 @@ func ProtectPrivateRegular(path string) error {
 }
 
 func createCanonicalNewPrivateFile(path string, mode os.FileMode) (*os.File, error) {
-	file, err := os.OpenFile(path, os.O_WRONLY|os.O_CREATE|os.O_EXCL, mode)
+	return createCanonicalNewPrivateFileWithFlags(path, mode, os.O_WRONLY)
+}
+
+func createCanonicalNewPrivateFileWithFlags(path string, mode os.FileMode, flags int) (*os.File, error) {
+	file, err := os.OpenFile(path, flags|os.O_CREATE|os.O_EXCL, mode)
 	if err != nil {
 		return nil, err
 	}
@@ -108,6 +112,13 @@ func createCanonicalNewPrivateParentFile(path string, mode os.FileMode) (*os.Fil
 	return createCanonicalNewPrivateFile(path, mode)
 }
 
+func createCanonicalNewPrivateParentReadWriteFile(path string, mode os.FileMode) (*os.File, error) {
+	if err := ValidatePrivateDirectory(filepath.Dir(path)); err != nil {
+		return nil, ErrUnsafeFile
+	}
+	return createCanonicalNewPrivateFileWithFlags(path, mode, os.O_RDWR)
+}
+
 // ValidatePrivateRegular requires a canonical, single-link private regular file.
 func ValidatePrivateRegular(path string) error {
 	if err := ValidateCanonicalPath(path); err != nil {
diff --git a/tools/tht/internal/safeio/private_windows.go b/tools/tht/internal/safeio/private_windows.go
index dc916e83..18a30744 100644
--- a/tools/tht/internal/safeio/private_windows.go
+++ b/tools/tht/internal/safeio/private_windows.go
@@ -96,14 +96,18 @@ func ProtectPrivateRegular(path string) error {
 // createCanonicalNewPrivateFile installs the owner-only protected DACL in the CreateFile call, so
 // another mutation can never observe a newly-created lock with an inherited/default DACL.
 func createCanonicalNewPrivateFile(path string, mode os.FileMode) (*os.File, error) {
-	return createCanonicalNewFile(path, mode, false)
+	return createCanonicalNewFile(path, mode, false, windows.GENERIC_WRITE)
 }
 
 func createCanonicalNewPrivateParentFile(path string, mode os.FileMode) (*os.File, error) {
-	return createCanonicalNewFile(path, mode, true)
+	return createCanonicalNewFile(path, mode, true, windows.GENERIC_WRITE)
 }
 
-func createCanonicalNewFile(path string, mode os.FileMode, requirePrivateParent bool) (*os.File, error) {
+func createCanonicalNewPrivateParentReadWriteFile(path string, mode os.FileMode) (*os.File, error) {
+	return createCanonicalNewFile(path, mode, true, windows.GENERIC_READ|windows.GENERIC_WRITE)
+}
+
+func createCanonicalNewFile(path string, mode os.FileMode, requirePrivateParent bool, access uint32) (*os.File, error) {
 	parents, target, err := openCanonicalWindowsParent(path)
 	if err != nil || len(parents.handles) == 0 || (requirePrivateParent && validateOwnerOnlyDACL(parents.handles[len(parents.handles)-1]) != nil) {
 		if parents != nil {
@@ -123,7 +127,7 @@ func createCanonicalNewFile(path string, mode os.FileMode, requirePrivateParent
 	}
 	handle, err := windows.CreateFile(
 		windows.StringToUTF16Ptr(filepath.Join(parents.directory, target)),
-		windows.GENERIC_WRITE,
+		access,
 		windowsRetainedHandleShareMode,
 		attributes,
 		windows.CREATE_NEW,