diff --git a/crates/engine/src/autosave_tests.rs b/crates/engine/src/autosave_tests.rs
index a02c661c..1e8ba30f 100644
--- a/crates/engine/src/autosave_tests.rs
+++ b/crates/engine/src/autosave_tests.rs
@@ -73,3 +73,52 @@ fn autosaves_keep_their_flag_in_the_file() {
     assert!(!autosave(&mut t));
     let _ = std::fs::remove_dir_all(&dir);
 }
+
+#[test]
+fn files_that_save_themselves_still_get_checkpoints() {
+    let dir = std::env::temp_dir().join(format!("figcraft-autosave-on-{}", std::process::id()));
+    std::fs::create_dir_all(&dir).unwrap();
+    let path = dir.join("auto.figcraft").to_string_lossy().to_string();
+    let mut s = Session::new();
+    s.execute("pref.autoSave", &json!({"on": true})).unwrap();
+    rect(&mut s, 0.0);
+    s.execute("file.save", &json!({"path": path})).unwrap();
+    assert!(autosave(&mut s), "saved, but changed since the last version");
+    assert!(!autosave(&mut s), "no change since the checkpoint");
+    // Just opened: nothing to checkpoint.
+    let mut t = Session::empty();
+    t.execute("pref.autoSave", &json!({"on": true})).unwrap();
+    t.execute("file.open", &json!({"path": path})).unwrap();
+    assert!(!autosave(&mut t));
+    let _ = std::fs::remove_dir_all(&dir);
+}
+
+#[test]
+fn a_file_encoded_elsewhere_is_saved_as_of_its_snapshot() {
+    use crate::cmd::file::FileSnapshot;
+    let dir = std::env::temp_dir().join(format!("figcraft-snapshot-{}", std::process::id()));
+    std::fs::create_dir_all(&dir).unwrap();
+    let path = dir.join("snap.figcraft").to_string_lossy().to_string();
+    let mut s = Session::new();
+    s.execute("file.save", &json!({"path": path})).unwrap();
+    rect(&mut s, 0.0);
+    let file = FileSnapshot::of(s.ds().unwrap());
+    let bytes = file.encode().unwrap();
+    rect(&mut s, 50.0);
+    assert!(file.save(&mut s, &bytes).unwrap());
+    assert!(s.ds().unwrap().is_dirty(), "the edit since the snapshot is still unsaved");
+    let mut t = Session::empty();
+    t.execute("file.open", &json!({"path": path})).unwrap();
+    assert_eq!(t.ds().unwrap().doc.page_content_bounds(t.ds().unwrap().page).unwrap().x1, 10.0, "the file is the snapshot");
+    // Saved meanwhile: the older bytes never overwrite the newer file.
+    let file = FileSnapshot::of(s.ds().unwrap());
+    let bytes = file.encode().unwrap();
+    rect(&mut s, 100.0);
+    s.execute("file.save", &json!({})).unwrap();
+    assert!(!file.save(&mut s, &bytes).unwrap());
+    assert!(!s.ds().unwrap().is_dirty());
+    let mut t = Session::empty();
+    t.execute("file.open", &json!({"path": path})).unwrap();
+    assert_eq!(t.ds().unwrap().doc.page_content_bounds(t.ds().unwrap().page).unwrap().x1, 110.0);
+    let _ = std::fs::remove_dir_all(&dir);
+}
diff --git a/crates/engine/src/cmd/file.rs b/crates/engine/src/cmd/file.rs
index 8122af42..5043ea40 100644
--- a/crates/engine/src/cmd/file.rs
+++ b/crates/engine/src/cmd/file.rs
@@ -195,7 +195,7 @@ pub fn specs() -> Vec<CommandSpec> {
         spec!(
             "version.autosave",
             "Autosave version",
-            "{}  save an automatic checkpoint when the document has unsaved changes since the last version (the app does this every 30 minutes); keeps the newest 20 autosaves → {saved, version?}",
+            "{}  save an automatic checkpoint when the document has unsaved changes since the last version (with `pref.autoSave` on: changes made since it was opened); the app does this every 30 minutes; keeps the newest 20 autosaves → {saved, version?}",
             false,
             autosave
         ),
@@ -257,8 +257,11 @@ pub const MAX_AUTOSAVES: usize = 20;
 
 /// `version.autosave`: a checkpoint of a document with unsaved changes since the last version.
 fn autosave(s: &mut Session, _: &Value) -> Result<Value> {
+    let auto_save = s.prefs.auto_save;
     let ds = s.ds_mut()?;
-    if ds.preview.is_some() || !ds.is_dirty() || !ds.changed_since_checkpoint() {
+    // A file that saves itself is rarely unsaved: edits since it was opened count then.
+    let edited = ds.is_dirty() || (auto_save && !ds.history.undo.is_empty());
+    if ds.preview.is_some() || !edited || !ds.changed_since_checkpoint() {
         return Ok(json!({"saved": false}));
     }
     Ok(match add_version(ds, AUTOSAVE, "", true) {
@@ -693,10 +696,71 @@ pub fn thumbnail(doc: &Document) -> Option<Vec<u8>> {
 
 /// The `.figcraft` file for a document: content, thumbnail and version history.
 pub fn file_bytes(ds: &DocState) -> Result<Vec<u8>> {
-    let mut doc = (*ds.doc).clone();
-    doc.current_page = Some(ds.page);
-    let thumb = thumbnail(&doc);
-    figcraft_format::save_bytes_with_history(&doc, thumb.as_deref(), &ds.versions, &ds.branches).map_err(|e| EngineError::Other(e.to_string()))
+    FileSnapshot::of(ds).encode()
+}
+
+/// A document's file as it is at one moment, so the slow part of saving it (the thumbnail and the
+/// encoding) can run on another thread while editing goes on: [`encode`](Self::encode) there, then
+/// [`save`](Self::save) the bytes back on the session's thread.
+pub struct FileSnapshot {
+    uid: u64,
+    path: Option<String>,
+    revision: u64,
+    doc: Arc<Document>,
+    /// The document as last saved when the snapshot was taken: another save since makes it stale.
+    saved: Arc<Document>,
+    page: NodeId,
+    versions: Vec<Version>,
+    branches: Vec<figcraft_format::Branch>,
+}
+
+impl FileSnapshot {
+    pub fn of(ds: &DocState) -> Self {
+        Self {
+            uid: ds.uid,
+            path: ds.path.clone(),
+            revision: ds.revision,
+            doc: ds.doc.clone(),
+            saved: ds.saved_doc.clone(),
+            page: ds.page,
+            versions: ds.versions.clone(),
+            branches: ds.branches.clone(),
+        }
+    }
+
+    /// The file's bytes: content, thumbnail and version history.
+    pub fn encode(&self) -> Result<Vec<u8>> {
+        let mut doc = (*self.doc).clone();
+        doc.current_page = Some(self.page);
+        let thumb = thumbnail(&doc);
+        figcraft_format::save_bytes_with_history(&doc, thumb.as_deref(), &self.versions, &self.branches)
+            .map_err(|e| EngineError::Other(e.to_string()))
+    }
+
+    /// Write `bytes` (this snapshot's [`encode`](Self::encode)) to the document's file: it is saved
+    /// as of the snapshot, and edits since stay unsaved. False (nothing written) when the document
+    /// was closed, saved, reverted or given another path since: the bytes are out of date.
+    pub fn save(self, s: &mut Session, bytes: &[u8]) -> Result<bool> {
+        let Some(ds) = s.docs.iter_mut().find(|d| d.uid == self.uid && d.path == self.path && Arc::ptr_eq(&d.saved_doc, &self.saved)) else {
+            return Ok(false);
+        };
+        write(self.path.as_deref().ok_or(EngineError::NoDocument)?, bytes)?;
+        // The history or branches may have changed since: then they still count as unsaved.
+        if ds.revision == self.revision {
+            ds.versions_changed = false;
+        }
+        ds.saved_doc = self.doc;
+        ds.revision += 1;
+        branch_tabs_saved(s, self.uid);
+        Ok(true)
+    }
+}
+
+/// The branch tabs of the file in tab `uid` were saved with it.
+fn branch_tabs_saved(s: &mut Session, uid: u64) {
+    for tab in s.docs.iter_mut().filter(|d| d.branch_of.as_ref().is_some_and(|(m, _)| *m == uid)) {
+        tab.mark_saved();
+    }
 }
 
 fn color_profile(s: &mut Session, p: &Value) -> Result<Value> {
@@ -721,9 +785,7 @@ fn save_to(s: &mut Session, path: String) -> Result<Value> {
     ds.mark_saved();
     ds.revision += 1;
     let uid = ds.uid;
-    for tab in s.docs.iter_mut().filter(|d| d.branch_of.as_ref().is_some_and(|(m, _)| *m == uid)) {
-        tab.mark_saved();
-    }
+    branch_tabs_saved(s, uid);
     remember(s, &path);
     Ok(json!({"path": path, "bytes": bytes.len()}))
 }
diff --git a/crates/engine/src/cmd/view.rs b/crates/engine/src/cmd/view.rs
index 13f638b9..0f09a6db 100644
--- a/crates/engine/src/cmd/view.rs
+++ b/crates/engine/src/cmd/view.rs
@@ -334,6 +334,7 @@ pub fn specs() -> Vec<CommandSpec> {
         spec!("pref.showDimensions", "Show dimensions on objects", "{on?}", false, |s, p| pref(s, p, |x| &mut x.show_dimensions)),
         spec!("pref.numberKeysOpacity", "Use number keys for opacity", "{on?}", false, |s, p| pref(s, p, |x| &mut x.number_keys_opacity)),
         spec!("pref.reduceMotion", "Reduce motion", "{on?}", false, |s, p| pref(s, p, |x| &mut x.reduce_motion)),
+        spec!("pref.autoSave", "Save files automatically", "{on?}", false, |s, p| pref(s, p, |x| &mut x.auto_save)),
         spec!("pref.nudge", "Nudge amount", "{small?, big?}", false, |s, p| {
             s.prefs.nudge_small = num(p, "small").unwrap_or(s.prefs.nudge_small).clamp(0.01, 1000.0);
             s.prefs.nudge_big = num(p, "big").unwrap_or(s.prefs.nudge_big).clamp(0.01, 10000.0);
diff --git a/crates/engine/src/lib.rs b/crates/engine/src/lib.rs
index 42423dbc..bf375be1 100644
--- a/crates/engine/src/lib.rs
+++ b/crates/engine/src/lib.rs
@@ -573,6 +573,9 @@ pub struct Prefs {
     /// Reduce motion: the interface's own animations are off (the system's setting turns them off
     /// too). Prototype transitions still play.
     pub reduce_motion: bool,
+    /// Files that have a path are saved by themselves a few seconds after they change (the app's
+    /// timer; off: only File ▸ Save writes them).
+    pub auto_save: bool,
     /// `figcraft://` invite links open this app (`collab.registerLinks`); `None`: on for installed
     /// builds, off for development builds.
     pub invite_links: Option<bool>,
@@ -625,6 +628,7 @@ impl Default for Prefs {
             language: "system".into(),
             user_name: cmd::comment::DEFAULT_AUTHOR.into(),
             reduce_motion: false,
+            auto_save: false,
             invite_links: None,
             plugins: Vec::new(),
             plugin_settings: BTreeMap::new(),
diff --git a/crates/ui-egui/locales/de.toml b/crates/ui-egui/locales/de.toml
index affda97d..d9641b85 100644
--- a/crates/ui-egui/locales/de.toml
+++ b/crates/ui-egui/locales/de.toml
@@ -302,6 +302,7 @@ status = "draft — needs native review"
 "Corner radius" = "Eckenradius"
 "Corner smoothing" = "Eckenglättung"
 "Couldn't reach {server}: showing what this computer remembers." = "{server} war nicht erreichbar: Es wird angezeigt, was dieser Computer gespeichert hat."
+"Couldn't save automatically: {error}" = "Automatisches Speichern fehlgeschlagen: {error}"
 "Count" = "Anzahl"
 "Create" = "Erstellen"
 "Create a variable collection first" = "Erstellen Sie zuerst eine Variablensammlung"
@@ -1071,6 +1072,7 @@ status = "draft — needs native review"
 "Save" = "Speichern"
 "Save as" = "Speichern unter"
 "Save as preset…" = "Als Vorgabe speichern…"
+"Save files automatically" = "Dateien automatisch speichern"
 "Save version  {key}" = "Version speichern  {key}"
 "Saved" = "Gespeichert"
 "Saved {name}" = "{name} gespeichert"
@@ -1785,6 +1787,7 @@ status = "draft — needs native review"
 "plugin.ui" = "Plugin-Bereich"
 "plugins.manage" = "Plugins verwalten…"
 "plugins.runLast" = "Letztes Plugin ausführen"
+"pref.autoSave" = "Dateien automatisch speichern"
 "pref.highlightHover" = "Ebenen beim Überfahren hervorheben"
 "pref.keepTool" = "Werkzeug nach Verwendung ausgewählt lassen"
 "pref.language" = "Sprache der Benutzeroberfläche"
diff --git a/crates/ui-egui/locales/en.toml b/crates/ui-egui/locales/en.toml
index 08ac64ba..73c41dcf 100644
--- a/crates/ui-egui/locales/en.toml
+++ b/crates/ui-egui/locales/en.toml
@@ -311,6 +311,7 @@ name = "English"
 "Corner radius" = "Corner radius"
 "Corner smoothing" = "Corner smoothing"
 "Couldn't reach {server}: showing what this computer remembers." = "Couldn't reach {server}: showing what this computer remembers."
+"Couldn't save automatically: {error}" = "Couldn't save automatically: {error}"
 "Count" = "Count"
 "Create" = "Create"
 "Create a variable collection first" = "Create a variable collection first"
@@ -1113,6 +1114,7 @@ name = "English"
 "Rulers" = "Rulers"
 "Run" = "Run"
 "Run {label}" = "Run {label}"
+"Save files automatically" = "Save files automatically"
 "Search templates" = "Search templates"
 "See all templates" = "See all templates"
 "Select layers and they space themselves as their content changes" = "Select layers and they space themselves as their content changes"
diff --git a/crates/ui-egui/locales/es.toml b/crates/ui-egui/locales/es.toml
index 3200e891..245284dd 100644
--- a/crates/ui-egui/locales/es.toml
+++ b/crates/ui-egui/locales/es.toml
@@ -312,6 +312,7 @@ status = "draft — needs native review"
 "Corner radius" = "Radio de esquina"
 "Corner smoothing" = "Suavizado de esquinas"
 "Couldn't reach {server}: showing what this computer remembers." = "No se pudo contactar con {server}: se muestra lo que este ordenador recuerda."
+"Couldn't save automatically: {error}" = "No se pudo guardar automáticamente: {error}"
 "Count" = "Número"
 "Create" = "Crear"
 "Create a variable collection first" = "Crea primero una colección de variables"
@@ -1114,6 +1115,7 @@ status = "draft — needs native review"
 "Rulers" = "Reglas"
 "Run" = "Ejecutar"
 "Run {label}" = "Ejecutar {label}"
+"Save files automatically" = "Guardar archivos automáticamente"
 "Search templates" = "Buscar plantillas"
 "See all templates" = "Ver todas las plantillas"
 "Select layers and they space themselves as their content changes" = "Selecciona capas y se espaciarán solas cuando cambie su contenido"
@@ -1866,6 +1868,7 @@ status = "draft — needs native review"
 "plugin.ui" = "Panel del plugin"
 "plugins.manage" = "Gestionar plugins…"
 "plugins.runLast" = "Ejecutar el último plugin"
+"pref.autoSave" = "Guardar archivos automáticamente"
 "pref.highlightHover" = "Resaltar capas al pasar el cursor"
 "pref.keepTool" = "Mantener la herramienta tras usarla"
 "pref.language" = "Idioma de la interfaz"
diff --git a/crates/ui-egui/locales/fr.toml b/crates/ui-egui/locales/fr.toml
index 69798340..efb13973 100644
--- a/crates/ui-egui/locales/fr.toml
+++ b/crates/ui-egui/locales/fr.toml
@@ -302,6 +302,7 @@ status = "draft — needs native review"
 "Corner radius" = "Rayon d’angle"
 "Corner smoothing" = "Lissage des angles"
 "Couldn't reach {server}: showing what this computer remembers." = "Impossible de joindre {server} : affichage de ce dont cet ordinateur se souvient."
+"Couldn't save automatically: {error}" = "Impossible d’enregistrer automatiquement : {error}"
 "Count" = "Nombre"
 "Create" = "Créer"
 "Create a variable collection first" = "Créez d’abord une collection de variables"
@@ -1071,6 +1072,7 @@ status = "draft — needs native review"
 "Save" = "Enregistrer"
 "Save as" = "Enregistrer sous"
 "Save as preset…" = "Enregistrer comme préréglage…"
+"Save files automatically" = "Enregistrer les fichiers automatiquement"
 "Save version  {key}" = "Enregistrer une version  {key}"
 "Saved" = "Enregistré"
 "Saved {name}" = "{name} enregistré"
@@ -1785,6 +1787,7 @@ status = "draft — needs native review"
 "plugin.ui" = "Panneau du plugin"
 "plugins.manage" = "Gérer les plugins…"
 "plugins.runLast" = "Exécuter le dernier plugin"
+"pref.autoSave" = "Enregistrer les fichiers automatiquement"
 "pref.highlightHover" = "Mettre en évidence les calques au survol"
 "pref.keepTool" = "Garder l’outil sélectionné après usage"
 "pref.language" = "Langue de l’interface"
diff --git a/crates/ui-egui/locales/ja.toml b/crates/ui-egui/locales/ja.toml
index fb81b042..8c01d3d9 100644
--- a/crates/ui-egui/locales/ja.toml
+++ b/crates/ui-egui/locales/ja.toml
@@ -314,6 +314,7 @@ status = "draft — needs native review"
 "Corner radius" = "角の半径"
 "Corner smoothing" = "角のスムージング"
 "Couldn't reach {server}: showing what this computer remembers." = "{server} に接続できない：このコンピューターが覚えている内容を表示中。"
+"Couldn't save automatically: {error}" = "自動保存できませんでした：{error}"
 "Count" = "数"
 "Create" = "作成"
 "Create a variable collection first" = "先に変数コレクションを作成"
@@ -1122,6 +1123,7 @@ status = "draft — needs native review"
 "Save" = "保存"
 "Save as" = "名前を付けて保存"
 "Save as preset…" = "プリセットとして保存…"
+"Save files automatically" = "ファイルを自動保存"
 "Save version  {key}" = "バージョンを保存  {key}"
 "Saved" = "保存済み"
 "Saved {name}" = "{name} を保存済み"
@@ -1868,6 +1870,7 @@ status = "draft — needs native review"
 "plugin.ui" = "プラグインパネル"
 "plugins.manage" = "プラグインを管理…"
 "plugins.runLast" = "最後のプラグインを実行"
+"pref.autoSave" = "ファイルを自動保存"
 "pref.highlightHover" = "ホバー時にレイヤーをハイライト"
 "pref.keepTool" = "使用後もツールを選択したままにする"
 "pref.language" = "インターフェースの言語"
diff --git a/crates/ui-egui/locales/ko.toml b/crates/ui-egui/locales/ko.toml
index aed3529a..7d0c415e 100644
--- a/crates/ui-egui/locales/ko.toml
+++ b/crates/ui-egui/locales/ko.toml
@@ -303,6 +303,7 @@ status = "draft — needs native review"
 "Corner radius" = "모서리 반경"
 "Corner smoothing" = "모서리 스무딩"
 "Couldn't reach {server}: showing what this computer remembers." = "{server}에 연결할 수 없음: 이 컴퓨터가 기억하는 내용을 표시함."
+"Couldn't save automatically: {error}" = "자동으로 저장할 수 없음: {error}"
 "Count" = "개수"
 "Create" = "만들기"
 "Create a variable collection first" = "먼저 변수 컬렉션을 만드세요"
@@ -1072,6 +1073,7 @@ status = "draft — needs native review"
 "Save" = "저장"
 "Save as" = "다른 이름으로 저장"
 "Save as preset…" = "프리셋으로 저장…"
+"Save files automatically" = "파일 자동 저장"
 "Save version  {key}" = "버전 저장  {key}"
 "Saved" = "저장됨"
 "Saved {name}" = "{name} 저장됨"
@@ -1786,6 +1788,7 @@ status = "draft — needs native review"
 "plugin.ui" = "플러그인 패널"
 "plugins.manage" = "플러그인 관리…"
 "plugins.runLast" = "마지막 플러그인 실행"
+"pref.autoSave" = "파일 자동 저장"
 "pref.highlightHover" = "마우스를 올리면 레이어 강조"
 "pref.keepTool" = "사용 후에도 도구 선택 유지"
 "pref.language" = "인터페이스 언어"
diff --git a/crates/ui-egui/locales/pt-BR.toml b/crates/ui-egui/locales/pt-BR.toml
index 97500236..0bbbc29f 100644
--- a/crates/ui-egui/locales/pt-BR.toml
+++ b/crates/ui-egui/locales/pt-BR.toml
@@ -301,6 +301,7 @@ status = "draft — needs native review"
 "Corner radius" = "Raio do canto"
 "Corner smoothing" = "Suavização de cantos"
 "Couldn't reach {server}: showing what this computer remembers." = "Não foi possível alcançar {server}: mostrando o que este computador lembra."
+"Couldn't save automatically: {error}" = "Não foi possível salvar automaticamente: {error}"
 "Count" = "Quantidade"
 "Create" = "Criar"
 "Create a variable collection first" = "Crie primeiro uma coleção de variáveis"
@@ -1070,6 +1071,7 @@ status = "draft — needs native review"
 "Save" = "Salvar"
 "Save as" = "Salvar como"
 "Save as preset…" = "Salvar como predefinição…"
+"Save files automatically" = "Salvar arquivos automaticamente"
 "Save version  {key}" = "Salvar versão  {key}"
 "Saved" = "Salvo"
 "Saved {name}" = "{name} salvo"
@@ -1784,6 +1786,7 @@ status = "draft — needs native review"
 "plugin.ui" = "Painel do plugin"
 "plugins.manage" = "Gerenciar plugins…"
 "plugins.runLast" = "Executar o último plugin"
+"pref.autoSave" = "Salvar arquivos automaticamente"
 "pref.highlightHover" = "Destacar camadas ao passar o cursor"
 "pref.keepTool" = "Manter a ferramenta selecionada após o uso"
 "pref.language" = "Idioma da interface"
diff --git a/crates/ui-egui/locales/zh-Hans.toml b/crates/ui-egui/locales/zh-Hans.toml
index d1887735..993f1c57 100644
--- a/crates/ui-egui/locales/zh-Hans.toml
+++ b/crates/ui-egui/locales/zh-Hans.toml
@@ -304,6 +304,7 @@ status = "draft — needs native review"
 "Corner radius" = "圆角半径"
 "Corner smoothing" = "圆角平滑"
 "Couldn't reach {server}: showing what this computer remembers." = "无法连接 {server}：显示的是这台电脑记录的内容。"
+"Couldn't save automatically: {error}" = "无法自动保存：{error}"
 "Count" = "数量"
 "Create" = "创建"
 "Create a variable collection first" = "请先创建变量集合"
@@ -1073,6 +1074,7 @@ status = "draft — needs native review"
 "Save" = "保存"
 "Save as" = "另存为"
 "Save as preset…" = "存储为预设…"
+"Save files automatically" = "自动保存文件"
 "Save version  {key}" = "保存版本  {key}"
 "Saved" = "已保存"
 "Saved {name}" = "已保存 {name}"
@@ -1787,6 +1789,7 @@ status = "draft — needs native review"
 "plugin.ui" = "插件面板"
 "plugins.manage" = "管理插件…"
 "plugins.runLast" = "运行上一个插件"
+"pref.autoSave" = "自动保存文件"
 "pref.highlightHover" = "悬停时高亮图层"
 "pref.keepTool" = "使用后保持工具选中"
 "pref.language" = "界面语言"
diff --git a/crates/ui-egui/src/dialogs.rs b/crates/ui-egui/src/dialogs.rs
index 24fc12a4..d881c882 100644
--- a/crates/ui-egui/src/dialogs.rs
+++ b/crates/ui-egui/src/dialogs.rs
@@ -163,6 +163,10 @@ pub fn show(app: &mut FigApp, ctx: &Context) {
             if app.services.reduce_motion() {
                 crate::widgets::label(ui, tr("Your system asks for reduced motion: animations stay off."), t.text_secondary);
             }
+            // The browser has no files to save to.
+            if !cfg!(target_arch = "wasm32") && crate::widgets::checkbox(ui, prefs.auto_save, tr("Save files automatically")) {
+                toggles.push("pref.autoSave");
+            }
             for c in toggles {
                 app.run(c, json!({}));
             }
diff --git a/crates/ui-egui/src/idle_tests.rs b/crates/ui-egui/src/idle_tests.rs
index f8f2f8af..c61699b5 100644
--- a/crates/ui-egui/src/idle_tests.rs
+++ b/crates/ui-egui/src/idle_tests.rs
@@ -324,3 +324,39 @@ fn a_hidden_window_answers_requests_that_need_no_frame() {
     assert_eq!(rx.try_recv().unwrap().id, json!(2));
     let _ = std::fs::remove_dir_all(&dir);
 }
+
+#[test]
+fn auto_save_writes_a_changed_file_then_rests() {
+    let dir = std::env::temp_dir().join(format!("figcraft-auto-save-{}", std::process::id()));
+    std::fs::create_dir_all(&dir).unwrap();
+    let path = dir.join("auto.figcraft").to_string_lossy().to_string();
+    let (mut h, _) = document();
+    h.run("file.save", json!({"path": path}));
+    h.run("pref.autoSave", json!({"on": true}));
+    h.assert_idle("a saved file");
+    h.run("shape.rectangle", json!({"x": 500, "y": 300, "width": 40, "height": 40}));
+    assert!(h.frame(vec![]) <= Duration::from_secs_f64(crate::AUTO_SAVE_SECS), "wakes up to save it");
+    assert!(h.app.session.ds().unwrap().is_dirty(), "not saved at once");
+    // Another change restarts the wait.
+    h.time += crate::AUTO_SAVE_SECS - 0.5;
+    h.run("shape.rectangle", json!({"x": 600, "y": 300, "width": 40, "height": 40}));
+    h.frame(vec![]);
+    h.time += 1.0;
+    h.frame(vec![]);
+    assert!(h.app.ui.auto_saving.is_empty() && h.app.session.ds().unwrap().is_dirty(), "waits for a pause in the changes");
+    h.time += crate::AUTO_SAVE_SECS;
+    // The file is encoded on a thread of its own, then written.
+    let start = Instant::now();
+    while h.app.session.ds().unwrap().is_dirty() {
+        assert!(start.elapsed() < Duration::from_secs(20), "saved");
+        let delay = h.frame(vec![]);
+        assert!(h.app.ui.auto_saving.is_empty() || delay <= Duration::from_millis(100), "looks again for the file being encoded");
+        std::thread::sleep(Duration::from_millis(5));
+    }
+    let mut t = Session::empty();
+    t.execute("file.open", &json!({"path": path})).unwrap();
+    let ds = t.ds().unwrap();
+    assert!(ds.doc.page_content_bounds(ds.page).unwrap().x1 >= 640.0, "the new rectangles are in the file");
+    h.assert_idle("an auto-saved file");
+    let _ = std::fs::remove_dir_all(&dir);
+}
diff --git a/crates/ui-egui/src/lib.rs b/crates/ui-egui/src/lib.rs
index 16cb4c20..4d75b4e4 100644
--- a/crates/ui-egui/src/lib.rs
+++ b/crates/ui-egui/src/lib.rs
@@ -305,6 +305,14 @@ pub enum TitleBar {
 /// How often open documents get an autosave checkpoint (seconds).
 pub const AUTOSAVE_SECS: f64 = 30.0 * 60.0;
 
+/// With auto-save on: how long after a file last changed it is saved, and how long a failed save
+/// waits before trying again (seconds).
+pub const AUTO_SAVE_SECS: f64 = 3.0;
+const AUTO_SAVE_RETRY_SECS: f64 = 60.0;
+
+/// A file being encoded for auto-save: its snapshot and, once done, its bytes.
+pub type AutoSaveEncoding = std::thread::JoinHandle<(figcraft_engine::cmd::file::FileSnapshot, figcraft_engine::Result<Vec<u8>>)>;
+
 /// How long a toast shows (seconds).
 const TOAST_SECS: f64 = 3.0;
 
@@ -440,6 +448,11 @@ pub struct UiState {
     pub last_copy: Option<String>,
     /// When (UI clock, seconds) open documents next get an autosave checkpoint.
     pub next_autosave: f64,
+    /// With auto-save on: when (UI clock, seconds) files with unsaved changes are next saved, and
+    /// the revisions of those files it waits on (a change restarts the wait).
+    pub auto_save_at: Option<(f64, u64)>,
+    /// Files being encoded for auto-save on threads of their own (tab `uid`).
+    pub auto_saving: Vec<(u64, AutoSaveEncoding)>,
     /// The Assets panel shows the libraries view (publish this file, enabled libraries).
     pub libraries: bool,
     /// Keyboard navigation: the region with the keyboard, keyboard mode (F6, Tab).
@@ -524,6 +537,8 @@ impl Default for UiState {
 
             last_copy: None,
             next_autosave: 0.0,
+            auto_save_at: None,
+            auto_saving: Vec::new(),
             libraries: false,
             nav: a11y::Nav::default(),
             plugins_window: false,
@@ -1066,6 +1081,79 @@ impl FigApp {
         repaint_in(ctx, AUTOSAVE_SECS);
     }
 
+    /// With auto-save on, files that have a path are saved [`AUTO_SAVE_SECS`] after they last
+    /// changed (not during a gesture, a live preview or while a version is shown). The file is
+    /// encoded on a thread of its own, so a big one doesn't hold up the window, and written here.
+    fn auto_save_tick(&mut self, ctx: &egui::Context) {
+        let now = ctx.input(|i| i.time);
+        let mut failed = self.auto_save_done();
+        // The browser has no files to save to.
+        let on = !cfg!(target_arch = "wasm32") && self.session.prefs.auto_save;
+        let saving = &self.ui.auto_saving;
+        let due = |d: &&figcraft_engine::DocState| {
+            d.path.is_some() && d.branch_of.is_none() && d.preview.is_none() && d.is_dirty() && !saving.iter().any(|(uid, _)| *uid == d.uid)
+        };
+        let changes = if on { self.session.docs.iter().filter(due).map(|d| d.revision).reduce(u64::wrapping_add) } else { None };
+        match changes {
+            None => self.ui.auto_save_at = None,
+            Some(changes) => {
+                let at = match self.ui.auto_save_at {
+                    Some((at, c)) if c == changes && failed.is_none() => at,
+                    _ => now + if failed.is_some() { AUTO_SAVE_RETRY_SECS } else { AUTO_SAVE_SECS },
+                };
+                self.ui.auto_save_at = Some((at, changes));
+                if now < at || self.session.docs.iter().any(|d| d.interaction.is_some() || d.text_preview.is_some()) {
+                    // Due but busy: look again in a second.
+                    repaint_in(ctx, if now < at { at - now } else { 1.0 });
+                } else {
+                    self.ui.auto_save_at = None;
+                    let files: Vec<_> =
+                        self.session.docs.iter().filter(due).map(|d| (d.uid, figcraft_engine::cmd::file::FileSnapshot::of(d))).collect();
+                    for (uid, file) in files {
+                        let encode = move || {
+                            let bytes = file.encode();
+                            (file, bytes)
+                        };
+                        match std::thread::Builder::new().name("figcraft-save".into()).spawn(encode) {
+                            Ok(h) => self.ui.auto_saving.push((uid, h)),
+                            Err(e) => {
+                                failed.get_or_insert(e.to_string());
+                                self.ui.auto_save_at = Some((now + AUTO_SAVE_RETRY_SECS, changes));
+                            }
+                        }
+                    }
+                }
+            }
+        }
+        if !self.ui.auto_saving.is_empty() {
+            // Look again shortly for the files being encoded.
+            repaint_in(ctx, 0.1);
+        }
+        if let Some(e) = failed {
+            self.toast(i18n::trf("Couldn't save automatically: {error}", &[("error", &e)]));
+        }
+    }
+
+    /// Write the files whose auto-save encoding finished; the first error, if one failed.
+    fn auto_save_done(&mut self) -> Option<String> {
+        if self.ui.auto_saving.is_empty() {
+            return None;
+        }
+        let mut failed = None;
+        let done: Vec<_> = self.ui.auto_saving.extract_if(.., |(_, h)| h.is_finished()).collect();
+        for (_, h) in done {
+            let saved = match h.join() {
+                Ok((snapshot, Ok(bytes))) => snapshot.save(&mut self.session, &bytes).map(|_| ()).map_err(|e| e.to_string()),
+                Ok((_, Err(e))) => Err(e.to_string()),
+                Err(_) => Err("the file couldn't be encoded".to_string()),
+            };
+            if let Err(e) = saved {
+                failed.get_or_insert(e);
+            }
+        }
+        failed
+    }
+
     /// Draw the whole UI: the main window (the other windows draw themselves).
     pub fn ui(&mut self, ui: &mut egui::Ui) {
         #[cfg(not(target_arch = "wasm32"))]
@@ -1076,6 +1164,7 @@ impl FigApp {
         let first = !self.fonts_installed;
         self.logic(&ctx);
         self.autosave_tick(&ctx);
+        self.auto_save_tick(&ctx);
         if first {
             // New fonts apply from the next frame; draw nothing until then.
             ctx.request_repaint();
diff --git a/docs/mcp.md b/docs/mcp.md
index 0704ed40..c16db025 100644
--- a/docs/mcp.md
+++ b/docs/mcp.md
@@ -349,6 +349,8 @@ on; `key` resolves keys the way the canvas does. Commands behind keys:
   nested layers), Escape or a click drops it.
 - `pref.reduceMotion {on?}`: reduced motion (the interface's own animations off; the system's
   accessibility setting turns it on too). Prototype transitions still play.
+- `pref.autoSave {on?}`: files that have a path save themselves a few seconds after they change
+  (off by default; the desktop app's timer does it, so scripts still call `file.save`).
 - `pref.language {language: system|en|cs|ru|es|es-419|fr|de|pt-BR|ja|ko|zh-Hans|…}`: the app's interface language, applied live ("system":
   the OS or browser language; a language without a catalog shows English). Command ids, labels and
   every result stay English.