Fix #258: preserve unsaved headless MCP work with opt-in checkpoints

crates/engine/src/commands/settings.rs

@@ -239,11 +239,12 @@ fn auto_save(s: &mut Session, _: &Value) -> Result<Value> {
 
 fn recovery_info(s: &mut Session, _: &Value) -> Result<Value> {
     let root = s.default_autosave_root();
-    let latest = crate::autosave::latest_in(s.file_ops(), &s.prefs, s.path.as_deref(), root.as_deref()).map(|p| p.to_string_lossy().to_string());
+    let prefs = s.autosave_prefs();
+    let latest = crate::autosave::latest_in(s.file_ops(), &prefs, s.path.as_deref(), root.as_deref()).map(|p| p.to_string_lossy().to_string());
     Ok(json!({
         "lastAutoSave": s.autosave.last_path,
         "latestOnDisk": latest,
-        "folder": crate::autosave::folder(&s.prefs, s.path.as_deref(), root.as_deref()).map(|p| p.to_string_lossy().to_string()),
+        "folder": crate::autosave::folder(&prefs, s.path.as_deref(), root.as_deref()).map(|p| p.to_string_lossy().to_string()),
         "recentProjects": s.prefs.recent_projects,
     }))
 }

crates/engine/src/lib.rs

@@ -396,6 +396,9 @@ pub struct Session {
     pub shortcut_table: std::sync::OnceLock<shortcuts::ShortcutTable>,
     /// Auto-save bookkeeping.
     pub autosave: autosave::AutoSaveState,
+    /// Host-owned isolation for all auto-saves (e.g. one MCP session). Independent of settings
+    /// and project bookkeeping, so opening/saving a project or changing preferences keeps it.
+    pub autosave_folder_override: Option<std::path::PathBuf>,
     /// The viewer snapshot (Take Snapshot / Show Snapshot).
     pub snapshot: Option<viewer::Snapshot>,
     /// The JavaScript scripting engine (set by the host that links `effectcraft-script`):
@@ -504,6 +507,7 @@ impl Default for Session {
             ui_commands: vec![],
             shortcut_table: std::sync::OnceLock::new(),
             autosave: autosave::AutoSaveState::default(),
+            autosave_folder_override: None,
             snapshot: None,
             script: None,
             plugin_loader: None,

crates/engine/src/session_settings.rs

@@ -200,6 +200,17 @@ impl Session {
         }
     }
 
+    /// Auto-save preferences with the host's isolation folder applied, without changing or
+    /// persisting the user's settings.
+    pub fn autosave_prefs(&self) -> crate::prefs::Prefs {
+        let mut prefs = self.prefs.clone();
+        if let Some(folder) = &self.autosave_folder_override {
+            prefs.auto_save.location = "custom".into();
+            prefs.auto_save.folder = folder.to_string_lossy().into_owned();
+        }
+        prefs
+    }
+
     /// Write an auto-save now (whether or not the project is dirty). Returns its path.
     ///
     /// With [`autosave::AutoSaveState::background`] the project is serialised and written on a
@@ -209,8 +220,9 @@ impl Session {
         // One write at a time: the previous one must land before its slot rotates.
         self.autosave_wait()?;
         let root = self.default_autosave_root();
+        let prefs = self.autosave_prefs();
         let fail = |e: std::io::Error| EngineError::Other(format!("auto-save failed: {e}"));
-        let plan = autosave::plan_in(self.file_ops(), &self.prefs, self.path.as_deref(), root.as_deref(), self.autosave.last_slot).map_err(fail)?;
+        let plan = autosave::plan_in(self.file_ops(), &prefs, self.path.as_deref(), root.as_deref(), self.autosave.last_slot).map_err(fail)?;
         if self.autosave.background && !cfg!(target_arch = "wasm32") {
             let project = self.project.clone();
             let config = self.config.clone();
@@ -267,7 +279,8 @@ impl Session {
         if !self.is_dirty() || self.autosave.saved_revision == Some(self.revision) || self.render_job.is_some() {
             return None;
         }
-        if self.path.is_none() && self.prefs.auto_save.location != "custom" && self.default_autosave_root().is_none() {
+        if self.path.is_none() && self.prefs.auto_save.location != "custom" && self.default_autosave_root().is_none() && self.autosave_folder_override.is_none()
+        {
             return None;
         }
         let r = self.autosave_now();