#1830: require exporter support before skipping duplicate frames

crates/engine/src/output_guard.rs

@@ -494,7 +494,8 @@ impl Session {
         } else {
             Target::file(Path::new(path), by_identity)
         };
-        Write { item: m.id, path: path.to_string(), target, skips: m.settings.skip_existing }
+        let skips = m.settings.skip_existing && self.exporter.as_ref().is_some_and(|exporter| exporter.supports_skip_existing());
+        Write { item: m.id, path: path.to_string(), target, skips }
     }
 
     /// The report of `later` writing over `clash`.

crates/engine/src/render_queue.rs

@@ -57,6 +57,11 @@ pub struct ExportResult {
 pub trait Exporter: Send + Sync {
     /// Formats this build can write.
     fn formats(&self) -> Vec<OutputFormat>;
+    /// Whether Skip Existing Files preserves sequence frames written by earlier items. Exporters
+    /// without an existence check (such as memory sinks) must keep the conservative default.
+    fn supports_skip_existing(&self) -> bool {
+        false
+    }
     /// Render and write one item (blocking). `progress(done, total)` returns `false` to cancel;
     /// a cancelled export removes the files it opened for that output and returns `Err("cancelled")`.
     /// Completed earlier queue items and unrelated files must be preserved.

crates/engine/src/tests/render_overwrite.rs

@@ -28,6 +28,9 @@ impl Exporter for Writer {
     fn formats(&self) -> Vec<OutputFormat> {
         OutputFormat::ALL.to_vec()
     }
+    fn supports_skip_existing(&self) -> bool {
+        true
+    }
     fn export(&self, job: &ExportJob, _: &mut dyn FnMut(u64, u64) -> bool) -> Result<ExportResult, String> {
         self.jobs.lock().unwrap().push(job.path.to_string());
         let comp = job.project.comp(job.item.comp).ok_or("no comp")?;