diff --git a/agent/agentfiles/files.go b/agent/agentfiles/files.go
index b526c60eba..b82b57ce6d 100644
--- a/agent/agentfiles/files.go
+++ b/agent/agentfiles/files.go
@@ -14,7 +14,6 @@ import (
 	"syscall"
 
 	"github.com/google/uuid"
-	"github.com/spf13/afero"
 	"golang.org/x/xerrors"
 
 	"cdr.dev/slog/v3"
@@ -335,69 +334,16 @@ func (api *API) writeFile(ctx context.Context, r *http.Request, path string) (HT
 
 	// Check if the target already exists so we can preserve its
 	// permissions on the temp file before rename.
-	var origMode os.FileMode
-	var haveOrigMode bool
+	var mode *os.FileMode
 	if stat, serr := api.filesystem.Stat(path); serr == nil {
 		if stat.IsDir() {
 			return http.StatusBadRequest, xerrors.Errorf("open %s: is a directory", path)
 		}
-		origMode = stat.Mode()
-		haveOrigMode = true
+		m := stat.Mode()
+		mode = &m
 	}
 
-	// Write to a temp file in the same directory so the rename is
-	// always on the same device (atomic).
-	tmpfile, err := afero.TempFile(api.filesystem, dir, filepath.Base(path))
-	if err != nil {
-		status := http.StatusInternalServerError
-		if errors.Is(err, os.ErrPermission) {
-			status = http.StatusForbidden
-		}
-		return status, err
-	}
-	tmpName := tmpfile.Name()
-
-	_, err = io.Copy(tmpfile, r.Body)
-	if err != nil && !errors.Is(err, io.EOF) {
-		_ = tmpfile.Close()
-		if rerr := api.filesystem.Remove(tmpName); rerr != nil {
-			api.logger.Warn(ctx, "unable to clean up temp file", slog.Error(rerr))
-		}
-		return http.StatusInternalServerError, xerrors.Errorf("write %s: %w", path, err)
-	}
-
-	// Close before rename to flush buffered data and catch write
-	// errors (e.g. delayed allocation failures).
-	if err := tmpfile.Close(); err != nil {
-		if rerr := api.filesystem.Remove(tmpName); rerr != nil {
-			api.logger.Warn(ctx, "unable to clean up temp file", slog.Error(rerr))
-		}
-		return http.StatusInternalServerError, xerrors.Errorf("write %s: %w", path, err)
-	}
-
-	// Set permissions on the temp file before rename so there is
-	// no window where the target has wrong permissions.
-	if haveOrigMode {
-		if err := api.filesystem.Chmod(tmpName, origMode); err != nil {
-			api.logger.Warn(ctx, "unable to set file permissions",
-				slog.F("path", path),
-				slog.Error(err),
-			)
-		}
-	}
-
-	if err := api.filesystem.Rename(tmpName, path); err != nil {
-		if rerr := api.filesystem.Remove(tmpName); rerr != nil {
-			api.logger.Warn(ctx, "unable to clean up temp file", slog.Error(rerr))
-		}
-		status := http.StatusInternalServerError
-		if errors.Is(err, os.ErrPermission) {
-			status = http.StatusForbidden
-		}
-		return status, err
-	}
-
-	return 0, nil
+	return api.atomicWrite(ctx, path, mode, r.Body)
 }
 
 func (api *API) HandleEditFiles(rw http.ResponseWriter, r *http.Request) {
@@ -500,45 +446,20 @@ func (api *API) editFile(ctx context.Context, path string, edits []workspacesdk.
 		}
 	}
 
-	// Create an adjacent file to ensure it will be on the same device and can be
-	// moved atomically.
-	tmpfile, err := afero.TempFile(api.filesystem, filepath.Dir(path), filepath.Base(path))
+	m := stat.Mode()
+	return api.atomicWrite(ctx, path, &m, strings.NewReader(content))
+}
+
+// atomicWrite writes content from r to path via a temp file in the
+// same directory. If the target exists, its permissions are preserved.
+// On failure the temp file is cleaned up and the original is
+// untouched.
+func (api *API) atomicWrite(ctx context.Context, path string, mode *os.FileMode, r io.Reader) (int, error) {
+	dir := filepath.Dir(path)
+	tmpName := filepath.Join(dir, fmt.Sprintf(".%s.tmp.%s", filepath.Base(path), uuid.New().String()[:8]))
+
+	tmpfile, err := api.filesystem.OpenFile(tmpName, os.O_WRONLY|os.O_CREATE|os.O_EXCL, 0o666)
 	if err != nil {
-		return http.StatusInternalServerError, err
-	}
-	tmpName := tmpfile.Name()
-
-	if _, err := tmpfile.Write([]byte(content)); err != nil {
-		_ = tmpfile.Close()
-		if rerr := api.filesystem.Remove(tmpName); rerr != nil {
-			api.logger.Warn(ctx, "unable to clean up temp file", slog.Error(rerr))
-		}
-		return http.StatusInternalServerError, xerrors.Errorf("edit %s: %w", path, err)
-	}
-
-	// Close before rename to flush buffered data and catch write
-	// errors (e.g. delayed allocation failures).
-	if err := tmpfile.Close(); err != nil {
-		if rerr := api.filesystem.Remove(tmpName); rerr != nil {
-			api.logger.Warn(ctx, "unable to clean up temp file", slog.Error(rerr))
-		}
-		return http.StatusInternalServerError, xerrors.Errorf("edit %s: %w", path, err)
-	}
-
-	// Set permissions on the temp file before rename so there is
-	// no window where the target has wrong permissions.
-	if err := api.filesystem.Chmod(tmpName, stat.Mode()); err != nil {
-		api.logger.Warn(ctx, "unable to set file permissions",
-			slog.F("path", path),
-			slog.Error(err),
-		)
-	}
-
-	err = api.filesystem.Rename(tmpName, path)
-	if err != nil {
-		if rerr := api.filesystem.Remove(tmpName); rerr != nil {
-			api.logger.Warn(ctx, "unable to clean up temp file", slog.Error(rerr))
-		}
 		status := http.StatusInternalServerError
 		if errors.Is(err, os.ErrPermission) {
 			status = http.StatusForbidden
@@ -546,6 +467,46 @@ func (api *API) editFile(ctx context.Context, path string, edits []workspacesdk.
 		return status, err
 	}
 
+	cleanup := func() {
+		if err := api.filesystem.Remove(tmpName); err != nil {
+			api.logger.Warn(ctx, "unable to clean up temp file", slog.Error(err))
+		}
+	}
+
+	_, err = io.Copy(tmpfile, r)
+	if err != nil {
+		_ = tmpfile.Close()
+		cleanup()
+		return http.StatusInternalServerError, xerrors.Errorf("write %s: %w", path, err)
+	}
+
+	// Close before rename to flush buffered data and catch write
+	// errors (e.g. delayed allocation failures).
+	if err := tmpfile.Close(); err != nil {
+		cleanup()
+		return http.StatusInternalServerError, xerrors.Errorf("write %s: %w", path, err)
+	}
+
+	// Set permissions on the temp file before rename so there is
+	// no window where the target has wrong permissions.
+	if mode != nil {
+		if err := api.filesystem.Chmod(tmpName, *mode); err != nil {
+			api.logger.Warn(ctx, "unable to set file permissions",
+				slog.F("path", path),
+				slog.Error(err),
+			)
+		}
+	}
+
+	if err := api.filesystem.Rename(tmpName, path); err != nil {
+		cleanup()
+		status := http.StatusInternalServerError
+		if errors.Is(err, os.ErrPermission) {
+			status = http.StatusForbidden
+		}
+		return status, xerrors.Errorf("write %s: %w", path, err)
+	}
+
 	return 0, nil
 }
 
diff --git a/agent/agentfiles/files_test.go b/agent/agentfiles/files_test.go
index a59204429e..d962b80da0 100644
--- a/agent/agentfiles/files_test.go
+++ b/agent/agentfiles/files_test.go
@@ -636,6 +636,8 @@ func TestEditFiles(t *testing.T) {
 			},
 			errCode: http.StatusInternalServerError,
 			errors:  []string{"rename failed"},
+			// Original file must survive the failed rename.
+			expected: map[string]string{failRenameFilePath: "foo bar"},
 		},
 		{
 			name:     "Edit1",