diff --git a/apps/photocraft/src/services.rs b/apps/photocraft/src/services.rs
index d4e7a01553..92ce4840f5 100644
--- a/apps/photocraft/src/services.rs
+++ b/apps/photocraft/src/services.rs
@@ -298,6 +298,26 @@ fn write_atomic_progress(path: &Path, bytes: &[u8], progress: &dyn Fn(f32)) -> R
     photocraft_format::atomic_write_progress(path, bytes, progress).map_err(|e| e.to_string())
 }
 
+/// Whether `a` and `b` reach the same file: the same device and inode on Unix, the same final path
+/// on Windows (where it holds the name's letter case as stored). False when either can't be read.
+fn same_file(a: &Path, b: &Path) -> bool {
+    #[cfg(unix)]
+    {
+        use std::os::unix::fs::MetadataExt;
+        match (std::fs::metadata(a), std::fs::metadata(b)) {
+            (Ok(a), Ok(b)) => a.dev() == b.dev() && a.ino() == b.ino(),
+            _ => false,
+        }
+    }
+    #[cfg(not(unix))]
+    {
+        match (std::fs::canonicalize(a), std::fs::canonicalize(b)) {
+            (Ok(a), Ok(b)) => a == b,
+            _ => false,
+        }
+    }
+}
+
 fn export_options(settings: &photocraft_ui_egui::ExportSettings) -> photocraft_io::ExportOptions {
     settings.io_options()
 }
@@ -511,6 +531,7 @@ pub fn native(automation: Option<photocraft_automation::AuthorizedWorkspace>) ->
             let meta = std::fs::metadata(path).ok()?;
             Some(photocraft_ui_egui::FileStamp { len: meta.len(), modified: meta.modified().ok() })
         })),
+        same_file: Some(Box::new(|a: &str, b: &str| same_file(Path::new(a), Path::new(b)))),
         automation_read,
         automation_write,
         automation_open_pcraft,
@@ -888,6 +909,22 @@ mod tests {
         assert_eq!(dialog_file_name("", true), None);
     }
 
+    /// A name in another letter case reaches the same file where the file system ignores case
+    /// (Windows, and macOS by default), and only there. Two files with the same contents are two.
+    #[test]
+    fn same_file_follows_the_file_system() {
+        let dir = std::env::temp_dir().join(format!("photocraft-same-file-{}", std::process::id()));
+        std::fs::create_dir_all(&dir).unwrap();
+        let (cat, dog, upper) = (dir.join("CAT.psd"), dir.join("dog.psd"), dir.join("CAT.PSD"));
+        std::fs::write(&cat, b"same").unwrap();
+        std::fs::write(&dog, b"same").unwrap();
+        assert!(same_file(&cat, &cat));
+        assert!(!same_file(&cat, &dog));
+        assert_eq!(same_file(&upper, &cat), upper.exists(), "the same file only where letter case is ignored");
+        assert!(!same_file(&dir.join("missing.psd"), &cat));
+        std::fs::remove_dir_all(&dir).unwrap();
+    }
+
     #[test]
     fn block_on_waits_for_a_wake_from_another_thread() {
         let (tx, rx) = std::sync::mpsc::channel::<std::task::Waker>();
diff --git a/crates/ui-egui/src/dialogs.rs b/crates/ui-egui/src/dialogs.rs
index 43b4f3d3f9..0d76025828 100644
--- a/crates/ui-egui/src/dialogs.rs
+++ b/crates/ui-egui/src/dialogs.rs
@@ -322,6 +322,7 @@ pub fn show(app: &mut PhotocraftApp, ctx: &egui::Context) {
                 DialogKind::Command if crate::shape_dialog::owns(&fields) => crate::shape_dialog::body(app, ui, &mut fields),
                 DialogKind::Command if crate::delete_layer_prompt::owns(&fields) => crate::delete_layer_prompt::body(ui, &mut fields),
                 DialogKind::Command if crate::disk_change_prompt::owns(&fields) => crate::disk_change_prompt::body(ui, &fields),
+                DialogKind::Command if crate::replace_prompt::owns(&fields) => crate::replace_prompt::body(ui, &fields),
                 DialogKind::Command if crate::actions::owns_insert_stop(&fields) => crate::actions::insert_stop_body(ui, &mut fields),
                 DialogKind::Command if crate::new_layer_ui::owns(&fields) => crate::new_layer_ui::body(ui, &mut fields),
                 DialogKind::Command if crate::mask_trash_prompt::owns(&fields) => crate::mask_trash_prompt::body(ui, &fields),
@@ -405,7 +406,7 @@ pub fn show(app: &mut PhotocraftApp, ctx: &egui::Context) {
                             tl!("Create")
                         } else if crate::delete_layer_prompt::owns(&d.fields) {
                             tl!("Delete")
-                        } else if crate::disk_change_prompt::owns(&d.fields) {
+                        } else if crate::disk_change_prompt::owns(&d.fields) || crate::replace_prompt::owns(&d.fields) {
                             tl!("Replace")
                         } else if d.fields.contains_key("__export") {
                             tl!("Export")
@@ -621,6 +622,7 @@ pub fn confirm(app: &mut PhotocraftApp, id: u64) -> Result<Value, String> {
         DialogKind::Command if crate::shape_dialog::owns(&d.fields) => crate::shape_dialog::confirm(app, &d.fields),
         DialogKind::Command if crate::delete_layer_prompt::owns(&d.fields) => crate::delete_layer_prompt::confirm(app, &d.fields),
         DialogKind::Command if crate::disk_change_prompt::owns(&d.fields) => crate::disk_change_prompt::confirm(app, &d.fields),
+        DialogKind::Command if crate::replace_prompt::owns(&d.fields) => crate::replace_prompt::confirm(app),
         DialogKind::Command if crate::actions::owns_insert_stop(&d.fields) => crate::actions::insert_stop_confirm(app, &d.fields),
         DialogKind::Command if crate::new_layer_ui::owns(&d.fields) => crate::new_layer_ui::confirm(app, &d.fields),
         DialogKind::Command if crate::mask_trash_prompt::owns(&d.fields) => crate::mask_trash_prompt::confirm(app, &d.fields),
@@ -679,6 +681,10 @@ pub fn cancel(app: &mut PhotocraftApp, id: u64) -> Result<Value, String> {
     if crate::open_prompt::owns(&closed.fields) {
         crate::open_prompt::answer(app, None)?;
     }
+    // Cancel on the question whether to replace a file writes nothing.
+    if crate::replace_prompt::owns(&closed.fields) {
+        crate::replace_prompt::cancel(app)?;
+    }
     app.ui.dialogs.retain(|d| !crate::layer_style::color_picker::child_of(&d.fields, id));
     app.filter_preview = None;
     app.color_range = None;
diff --git a/crates/ui-egui/src/discard_ui.rs b/crates/ui-egui/src/discard_ui.rs
index 3dbf33c905..ac9d3b030e 100644
--- a/crates/ui-egui/src/discard_ui.rs
+++ b/crates/ui-egui/src/discard_ui.rs
@@ -878,6 +878,53 @@ mod tests {
         assert!(h.state().session.documents().is_empty());
     }
 
+    /// Save on closing an untitled document, answered with `kept` while `kept.psd` exists: the
+    /// prompt waits for the question whether to replace it. Cancel brings the prompt back with
+    /// nothing written; Replace saves and closes the document.
+    #[test]
+    fn saving_from_the_prompt_waits_for_the_question_whether_to_replace() {
+        use crate::{FileDialogAnswer, FileDialogReply};
+        use egui_kittest::kittest::Queryable;
+        use std::{cell::RefCell, rc::Rc};
+        let written: Rc<RefCell<Vec<String>>> = Rc::default();
+        let (w, waiting): (_, Rc<RefCell<Option<FileDialogReply>>>) = (written.clone(), Rc::default());
+        let held = waiting.clone();
+        let mut app = app_with_docs(1);
+        make_dirty(&mut app, 0);
+        app.services.export = Some(Box::new(|_: &photocraft_doc::Document, _: &str, _: &crate::ExportSettings| Ok((b"out".to_vec(), Vec::new()))));
+        app.services.write = Some(Box::new(move |p: &str, _: &[u8]| {
+            w.borrow_mut().push(p.to_string());
+            Ok(())
+        }));
+        app.services.file_stamp = Some(Box::new(|p: &str| (p == "/pics/kept.psd").then_some(crate::FileStamp { len: 1, modified: None })));
+        app.services.file_dialog = Some(Box::new(move |_, _, reply| *held.borrow_mut() = Some(reply)));
+        let mut h = prompt_for(app, "file.close", egui::os::OperatingSystem::Windows);
+        let save = |h: &mut Prompted| {
+            h.key_press(Key::Y);
+            h.run_steps(3);
+            waiting.borrow_mut().take().expect("the save dialog is open").send(Some(FileDialogAnswer::SaveTo("/pics/kept".into())));
+            h.run_steps(2);
+            h.state().ui.dialogs.iter().find(|d| crate::replace_prompt::owns(&d.fields)).map(|d| d.id).expect("the save asks first")
+        };
+        let id = save(&mut h);
+        assert!(h.state().file_dialog_open());
+        assert!(h.query_by_label("(Y)es").is_none(), "the prompt waits behind the question");
+        assert!(written.borrow().is_empty());
+
+        crate::dialogs::cancel(h.state_mut(), id).unwrap();
+        h.run_steps(2);
+        assert!(written.borrow().is_empty(), "Cancel writes nothing");
+        assert_eq!(docs_left(&h), Some(1));
+        assert_eq!(focused(&h, &["(Y)es", "(N)o", "(C)ancel"]), Some("(Y)es"), "the prompt comes back");
+
+        let id = save(&mut h);
+        crate::dialogs::confirm(h.state_mut(), id).unwrap();
+        h.run_steps(2);
+        assert_eq!(*written.borrow(), ["/pics/kept.psd"]);
+        assert!(h.state().discard.is_none());
+        assert!(h.state().session.documents().is_empty());
+    }
+
     #[test]
     fn saving_from_the_prompt_waits_for_a_background_save_then_closes() {
         use egui_kittest::kittest::Queryable;
diff --git a/crates/ui-egui/src/file_dialog.rs b/crates/ui-egui/src/file_dialog.rs
index 7cdff50af9..7c9e83a0bb 100644
--- a/crates/ui-egui/src/file_dialog.rs
+++ b/crates/ui-egui/src/file_dialog.rs
@@ -73,12 +73,20 @@ pub type FileDialogFn = Box<dyn FnMut(FileDialogRequest, Option<&eframe::Frame>,
 /// What to do with the answer (`None`: cancelled).
 type Then = Box<dyn FnOnce(&mut PhotocraftApp, Option<FileDialogAnswer>) -> Result<Value, String>>;
 
+/// The path a save writes to for the path the save dialog answered (see [`saved_path`]).
+type SavedPath = Box<dyn Fn(&PhotocraftApp, &str) -> String>;
+
 /// The file dialog in progress.
 pub(crate) struct Pending {
     /// Not shown yet: it is shown at the end of the frame, which has the window to parent it to.
     request: Option<FileDialogRequest>,
     answer: Option<Receiver<Option<FileDialogAnswer>>>,
     then: Then,
+    /// For a save dialog, the path the save writes to for the dialog's answer.
+    saved_path: Option<SavedPath>,
+    /// The save dialog's answer while the app asks whether to replace the file it writes to
+    /// (`replace_prompt`).
+    held: Option<FileDialogAnswer>,
 }
 
 /// The error of a cancelled dialog: not reported, since backing out is the user's choice.
@@ -115,7 +123,7 @@ impl PhotocraftApp {
             Some(FileDialogAnswer::Unavailable(why)) => Err(why),
             Some(a) => then(app, a),
         });
-        self.file_dialog = Some(Pending { request: Some(request), answer: None, then });
+        self.file_dialog = Some(Pending { request: Some(request), answer: None, then, saved_path: None, held: None });
         Ok(json!({ "fileDialog": kind }))
     }
 
@@ -130,7 +138,9 @@ impl PhotocraftApp {
     }
 
     /// Ask where to save: `then` gets the chosen path, with a lowercase extension when
-    /// Preferences ▸ File Handling › Lowercase Extension is on (its default).
+    /// Preferences ▸ File Handling › Lowercase Extension is on (its default). When that, or the
+    /// extension added to a bare name, changes the path to one where a file exists, the app asks
+    /// whether to replace it before `then` runs (`replace_prompt`).
     ///
     /// A bare file name (export dialogs, an untitled document) starts in the folder the active
     /// document was last saved or exported to through a dialog, else beside the document
@@ -153,13 +163,10 @@ impl PhotocraftApp {
         // extensions are hidden, #5039) gets the suggested file's extension, as Photoshop's
         // Save As appends the extension of the chosen file type.
         let suggested_ext = suggested.extension().and_then(|e| e.to_str()).map(|e| e.to_ascii_lowercase());
-        self.ask_file(FileDialogRequest::Save { suggested: suggested.to_string_lossy().into_owned() }, move |app, answer| match answer {
+        let ext = suggested_ext.clone();
+        let asked = self.ask_file(FileDialogRequest::Save { suggested: suggested.to_string_lossy().into_owned() }, move |app, answer| match answer {
             FileDialogAnswer::SaveTo(path) => {
-                let path = Self::lowercased_extension(app, path);
-                let path = match &suggested_ext {
-                    Some(ext) if Path::new(&path).extension().is_none() => Path::new(&path).with_extension(ext).to_string_lossy().into_owned(),
-                    _ => path,
-                };
+                let path = saved_path(app, &path, suggested_ext.as_deref());
                 // The web answers a download name, which has no folder to remember.
                 if let Some(doc) = doc
                     && let Some(dir) = Path::new(&path).parent().filter(|p| !p.as_os_str().is_empty())
@@ -169,7 +176,32 @@ impl PhotocraftApp {
                 then(app, path)
             }
             _ => Err(UNEXPECTED.into()),
-        })
+        })?;
+        if let Some(p) = self.file_dialog.as_mut() {
+            p.saved_path = Some(Box::new(move |app, path| saved_path(app, path, ext.as_deref())));
+        }
+        Ok(asked)
+    }
+
+    /// Whether a save to `path` replaces a file the save dialog didn't ask about: the dialog
+    /// answered `answered`, and [`saved_path`] changed it. A file at `path` that
+    /// [`Services::same_file`](crate::Services::same_file) finds is also the file at `answered` (the
+    /// same name in another letter case on a file system that ignores case, as Windows and macOS do
+    /// by default) counts as asked about. Never on the web, which has no file stamps.
+    fn replaces_unasked(&self, answered: &str, path: &str) -> bool {
+        let Some(stamp) = self.services.file_stamp.as_ref() else { return false };
+        answered != path && stamp(path).is_some() && !self.services.same_file.as_ref().is_some_and(|same| same(answered, path))
+    }
+
+    /// The question whether to replace a file is answered. Replace writes the save dialog's answer
+    /// as if the question had not been asked and returns what the save returns, its failure
+    /// included. Cancel ends the save like the dialog's Cancel.
+    pub(crate) fn answer_replace(&mut self, replace: bool) -> Result<Value, String> {
+        let held = self.file_dialog.as_mut().and_then(|p| p.held.take()).ok_or("no save is waiting to replace a file")?;
+        match self.finish_file_dialog(replace.then_some(held)) {
+            Err(e) if e == CANCELLED => Ok(json!({ "replaced": false })),
+            r => r,
+        }
     }
 
     /// `X.PSD` → `X.psd`, only the extension: the stem and the folders keep the user's spelling.
@@ -275,6 +307,14 @@ impl PhotocraftApp {
     /// continues the action that asked once the answer is in. Its errors go to the status bar.
     pub fn poll_file_dialog(&mut self, ctx: &egui::Context, parent: Option<&eframe::Frame>) {
         let Some(p) = self.file_dialog.as_mut() else { return };
+        if p.held.is_some() {
+            // The question whether to replace answers through `answer_replace`. Closed without an
+            // answer, it counts as Cancel.
+            if !self.ui.dialogs.iter().any(|d| crate::replace_prompt::owns(&d.fields)) {
+                let _ = self.answer_replace(false);
+            }
+            return;
+        }
         if let Some(request) = p.request.take() {
             let (tx, rx) = mpsc::channel();
             p.answer = Some(rx);
@@ -288,13 +328,43 @@ impl PhotocraftApp {
             // The reply was dropped unanswered: the dialog couldn't be shown.
             Some(Err(TryRecvError::Disconnected)) | None => None,
         };
-        let Some(Pending { then, .. }) = self.file_dialog.take() else { return };
-        if let Err(e) = then(self, answer)
+        if let Some(FileDialogAnswer::SaveTo(answered)) = &answer
+            && let Some(path) = self.file_dialog.as_ref().and_then(|p| p.saved_path.as_ref()).map(|written| written(self, answered))
+            && self.replaces_unasked(answered, &path)
+        {
+            crate::replace_prompt::open(self, answered, &path);
+            if let Some(p) = self.file_dialog.as_mut() {
+                p.answer = None;
+                p.held = answer;
+            }
+            return;
+        }
+        let _ = self.finish_file_dialog(answer);
+    }
+
+    /// Ends the file dialog in progress: its action continues with `answer`, and its errors go to
+    /// the status bar. Returns the action's result.
+    fn finish_file_dialog(&mut self, answer: Option<FileDialogAnswer>) -> Result<Value, String> {
+        let Some(Pending { then, .. }) = self.file_dialog.take() else { return Err(CANCELLED.into()) };
+        let r = then(self, answer);
+        if let Err(e) = &r
             && e != CANCELLED
         {
-            self.ui.status = e;
+            self.ui.status = e.clone();
             self.ui.status_error = true;
         }
+        r
+    }
+}
+
+/// The path a save writes to for the path the save dialog answered: its extension lowercased
+/// when Preferences ▸ File Handling › Lowercase Extension is on, and a name without one given
+/// `suggested_ext`, the suggested file's.
+fn saved_path(app: &PhotocraftApp, answered: &str, suggested_ext: Option<&str>) -> String {
+    let path = PhotocraftApp::lowercased_extension(app, answered.to_string());
+    match suggested_ext {
+        Some(ext) if Path::new(&path).extension().is_none() => Path::new(&path).with_extension(ext).to_string_lossy().into_owned(),
+        _ => path,
     }
 }
 
diff --git a/crates/ui-egui/src/file_dialog/tests.rs b/crates/ui-egui/src/file_dialog/tests.rs
index 6a3ccfcf9e..e08d576c01 100644
--- a/crates/ui-egui/src/file_dialog/tests.rs
+++ b/crates/ui-egui/src/file_dialog/tests.rs
@@ -433,6 +433,108 @@ fn save_paths_without_an_extension_get_the_suggested_one() {
     assert_eq!(*written.borrow(), ["/pics/cat.psd", "/pics/dog.png"]);
 }
 
+/// The question whether to replace a file (`replace_prompt`), when one is open.
+fn replace_prompt(app: &PhotocraftApp) -> Option<u64> {
+    app.ui.dialogs.iter().find(|d| crate::replace_prompt::owns(&d.fields)).map(|d| d.id)
+}
+
+/// File › Save As, answered with `path`.
+fn save_as(app: &mut PhotocraftApp, open: &Open, path: &str) {
+    let ctx = egui::Context::default();
+    menus::invoke(app, &ctx, "file.saveAs", json!({})).unwrap();
+    app.poll_file_dialog(&ctx, None);
+    answer(open, Some(FileDialogAnswer::SaveTo(path.into())));
+    app.poll_file_dialog(&ctx, None);
+}
+
+/// The save dialog asked only about `cat`. The extension added to it names a file that exists,
+/// so the save asks before replacing that file: Cancel writes nothing, Replace writes it.
+#[test]
+fn a_save_asks_before_replacing_the_file_its_added_extension_names() {
+    let ctx = egui::Context::default();
+    let (mut app, open, written) = app();
+    let disk = on_disk(&mut app);
+    changed_by_another_app(&disk, "/pics/cat.psd");
+    save_as(&mut app, &open, "/pics/cat");
+    let id = replace_prompt(&app).expect("the save asks first");
+    let fields = &app.ui.dialogs.iter().find(|d| d.id == id).unwrap().fields;
+    assert!(fields["message"].as_str().unwrap().contains("“cat.psd”"), "{fields:?}");
+    assert!(fields["detail"].as_str().unwrap().contains("“cat”"), "{fields:?}");
+    assert!(written.borrow().is_empty(), "nothing is written before the answer");
+    app.poll_file_dialog(&ctx, None);
+    assert!(app.file_dialog_open(), "the save waits for the answer");
+    assert!(written.borrow().is_empty());
+
+    crate::dialogs::cancel(&mut app, id).unwrap();
+    assert!(written.borrow().is_empty(), "Cancel writes nothing");
+    assert!(!app.file_dialog_open() && !app.ui.status_error);
+    assert_eq!(app.session.active().unwrap().path, None);
+
+    // A question closed without an answer counts as Cancel.
+    save_as(&mut app, &open, "/pics/cat");
+    app.ui.close_dialog(replace_prompt(&app).expect("asked again"));
+    app.poll_file_dialog(&ctx, None);
+    assert!(written.borrow().is_empty() && !app.file_dialog_open());
+
+    // Replace answers with the save's failure.
+    save_as(&mut app, &open, "/pics/cat");
+    let write = app.services.write.replace(Box::new(|_: &str, _: &[u8]| Err("disk full".into())));
+    let id = replace_prompt(&app).expect("asked again");
+    let e = crate::dialogs::confirm(&mut app, id).unwrap_err();
+    assert!(e.contains("disk full") && app.ui.status_error, "{e}");
+    app.services.write = write;
+
+    save_as(&mut app, &open, "/pics/cat");
+    let id = replace_prompt(&app).expect("asked again");
+    crate::dialogs::confirm(&mut app, id).unwrap();
+    assert_eq!(*written.borrow(), ["/pics/cat.psd"]);
+    assert_eq!(app.session.active().unwrap().path.as_deref(), Some("/pics/cat.psd"));
+    assert!(!app.file_dialog_open() && replace_prompt(&app).is_none());
+}
+
+/// Lowercase Extension changes `CAT.PSD` to `CAT.psd`. Where letter case tells two files apart
+/// (Linux), the dialog asked only about `CAT.PSD`; where it doesn't (Windows and macOS by
+/// default), both names reach the same file, which the dialog asked about.
+#[test]
+fn a_save_asks_before_replacing_the_file_its_lowercased_extension_names() {
+    {
+        // Two files, even with the same size and modification time.
+        let (mut app, open, written) = app();
+        let disk = on_disk(&mut app);
+        for name in ["/pics/CAT.psd", "/pics/CAT.PSD"] {
+            changed_by_another_app(&disk, name);
+        }
+        app.services.same_file = Some(Box::new(|a: &str, b: &str| a == b));
+        save_as(&mut app, &open, "/pics/CAT.PSD");
+        let id = replace_prompt(&app).expect("the save asks first");
+        assert!(written.borrow().is_empty());
+        crate::dialogs::confirm(&mut app, id).unwrap();
+        assert_eq!(*written.borrow(), ["/pics/CAT.psd"]);
+    }
+    let (mut app, open, written) = app();
+    let disk = on_disk(&mut app);
+    for name in ["/pics/CAT.psd", "/pics/CAT.PSD"] {
+        changed_by_another_app(&disk, name);
+    }
+    app.services.same_file = Some(Box::new(|a: &str, b: &str| a.eq_ignore_ascii_case(b)));
+    save_as(&mut app, &open, "/pics/CAT.PSD");
+    assert!(replace_prompt(&app).is_none(), "the dialog asked about this file");
+    assert_eq!(*written.borrow(), ["/pics/CAT.psd"]);
+}
+
+/// The save dialog asked about the file it answered: a bare name with no file under the added
+/// extension, and a full name, are written at once.
+#[test]
+fn a_save_writes_at_once_when_the_dialog_asked_about_the_file() {
+    let (mut app, open, written) = app();
+    let disk = on_disk(&mut app);
+    changed_by_another_app(&disk, "/pics/dog.png");
+    save_as(&mut app, &open, "/pics/cat");
+    save_as(&mut app, &open, "/pics/dog.png");
+    assert!(replace_prompt(&app).is_none());
+    assert_eq!(*written.borrow(), ["/pics/cat.psd", "/pics/dog.png"]);
+}
+
 #[test]
 fn save_and_export_dialogs_start_beside_the_document() {
     let (mut app, open, _) = app();
diff --git a/crates/ui-egui/src/i18n/cs.tsv b/crates/ui-egui/src/i18n/cs.tsv
index f50c0249b7..d12c09fb5c 100644
--- a/crates/ui-egui/src/i18n/cs.tsv
+++ b/crates/ui-egui/src/i18n/cs.tsv
@@ -3818,3 +3818,5 @@ navigator	Zoom	Zvětšení
 	GPU Memory (MB, 0 = Automatic)	Paměť GPU (MB, 0 = Automaticky)
 	{name} was changed by another app after it was opened or saved here.	Soubor {name} změnila jiná aplikace poté, co byl zde otevřen nebo uložen.
 	Replace it with this document, or cancel and use File › Save As to keep both.	Nahraďte ho tímto dokumentem, nebo zrušte a použijte Soubor › Uložit jako, abyste zachovali obojí.
+	“{name}” already exists. Replace it?	„{name}“ již existuje. Nahradit?
+	The name in the save dialog was “{name}”.	Název v dialogu ukládání byl „{name}“.
diff --git a/crates/ui-egui/src/i18n/de.tsv b/crates/ui-egui/src/i18n/de.tsv
index b1ea8977e2..05c2a8cb7e 100644
--- a/crates/ui-egui/src/i18n/de.tsv
+++ b/crates/ui-egui/src/i18n/de.tsv
@@ -3212,3 +3212,5 @@ curves	Smooth	Glätten
 	GPU Memory (MB, 0 = Automatic)	GPU-Speicher (MB, 0 = Automatisch)
 	{name} was changed by another app after it was opened or saved here.	{name} wurde von einer anderen App geändert, nachdem es hier geöffnet oder gespeichert wurde.
 	Replace it with this document, or cancel and use File › Save As to keep both.	Ersetze die Datei durch dieses Dokument oder brich ab und nutze Datei › Speichern unter, um beide zu behalten.
+	“{name}” already exists. Replace it?	„{name}“ ist bereits vorhanden. Ersetzen?
+	The name in the save dialog was “{name}”.	Der Name im Speichern-Dialog war „{name}“.
diff --git a/crates/ui-egui/src/i18n/el.tsv b/crates/ui-egui/src/i18n/el.tsv
index 8229806307..03d45ed037 100644
--- a/crates/ui-egui/src/i18n/el.tsv
+++ b/crates/ui-egui/src/i18n/el.tsv
@@ -3910,3 +3910,5 @@ curves	Smooth	Εξομάλυνση
 	GPU Memory (MB, 0 = Automatic)	Μνήμη GPU (MB, 0 = Αυτόματο)
 	{name} was changed by another app after it was opened or saved here.	Το {name} άλλαξε από άλλη εφαρμογή αφού ανοίχτηκε ή αποθηκεύτηκε εδώ.
 	Replace it with this document, or cancel and use File › Save As to keep both.	Αντικαταστήστε το με αυτό το έγγραφο ή ακυρώστε και χρησιμοποιήστε Αρχείο › Αποθήκευση ως για να κρατήσετε και τα δύο.
+	“{name}” already exists. Replace it?	Το «{name}» υπάρχει ήδη. Να αντικατασταθεί;
+	The name in the save dialog was “{name}”.	Το όνομα στο παράθυρο αποθήκευσης ήταν «{name}».
diff --git a/crates/ui-egui/src/i18n/es.tsv b/crates/ui-egui/src/i18n/es.tsv
index 6f3d85002b..9e5158bf67 100644
--- a/crates/ui-egui/src/i18n/es.tsv
+++ b/crates/ui-egui/src/i18n/es.tsv
@@ -4172,3 +4172,5 @@ navigator	Zoom	Zoom
 	GPU Memory (MB, 0 = Automatic)	Memoria de GPU (MB, 0 = Automático)
 	{name} was changed by another app after it was opened or saved here.	Otra aplicación cambió {name} después de abrirlo o guardarlo aquí.
 	Replace it with this document, or cancel and use File › Save As to keep both.	Reemplázalo con este documento, o cancela y usa Archivo › Guardar como para conservar ambos.
+	“{name}” already exists. Replace it?	«{name}» ya existe. ¿Reemplazarlo?
+	The name in the save dialog was “{name}”.	El nombre en el cuadro de diálogo Guardar era «{name}».
diff --git a/crates/ui-egui/src/i18n/fr.tsv b/crates/ui-egui/src/i18n/fr.tsv
index 4a819b0e1b..139db79602 100644
--- a/crates/ui-egui/src/i18n/fr.tsv
+++ b/crates/ui-egui/src/i18n/fr.tsv
@@ -3216,3 +3216,5 @@ curves	Smooth	Lisser
 	Replace	Remplacer
 	{name} was changed by another app after it was opened or saved here.	{name} a été modifié par une autre application après avoir été ouvert ou enregistré ici.
 	Replace it with this document, or cancel and use File › Save As to keep both.	Remplacez-le par ce document, ou annulez et utilisez Fichier › Enregistrer sous pour garder les deux.
+	“{name}” already exists. Replace it?	« {name} » existe déjà. Le remplacer ?
+	The name in the save dialog was “{name}”.	Le nom dans la boîte de dialogue d'enregistrement était « {name} ».
diff --git a/crates/ui-egui/src/i18n/hu.tsv b/crates/ui-egui/src/i18n/hu.tsv
index 86a6b30acb..7705a01e67 100644
--- a/crates/ui-egui/src/i18n/hu.tsv
+++ b/crates/ui-egui/src/i18n/hu.tsv
@@ -4129,3 +4129,5 @@ cameraRaw	Yellow	Sárga
 	GPU Memory (MB, 0 = Automatic)	GPU-memória (MB, 0 = Automatikus)
 	{name} was changed by another app after it was opened or saved here.	A(z) {name} fájlt egy másik alkalmazás módosította, miután itt megnyitották vagy mentették.
 	Replace it with this document, or cancel and use File › Save As to keep both.	Cserélje le erre a dokumentumra, vagy válassza a Mégse gombot, és a Fájl › Mentés másként paranccsal tartsa meg mindkettőt.
+	“{name}” already exists. Replace it?	„{name}” már létezik. Lecseréli?
+	The name in the save dialog was “{name}”.	A mentési párbeszédpanelen megadott név: „{name}”.
diff --git a/crates/ui-egui/src/i18n/id.tsv b/crates/ui-egui/src/i18n/id.tsv
index 49777913c1..1ce1b3ab20 100644
--- a/crates/ui-egui/src/i18n/id.tsv
+++ b/crates/ui-egui/src/i18n/id.tsv
@@ -3203,3 +3203,5 @@ curves	Smooth	Haluskan
 	Replace	Ganti
 	{name} was changed by another app after it was opened or saved here.	{name} diubah oleh aplikasi lain setelah dibuka atau disimpan di sini.
 	Replace it with this document, or cancel and use File › Save As to keep both.	Ganti dengan dokumen ini, atau batalkan dan gunakan Berkas › Simpan Sebagai untuk menyimpan keduanya.
+	“{name}” already exists. Replace it?	“{name}” sudah ada. Ganti?
+	The name in the save dialog was “{name}”.	Nama di dialog simpan adalah “{name}”.
diff --git a/crates/ui-egui/src/i18n/it.tsv b/crates/ui-egui/src/i18n/it.tsv
index f1144c0fdd..3e4d805f3d 100644
--- a/crates/ui-egui/src/i18n/it.tsv
+++ b/crates/ui-egui/src/i18n/it.tsv
@@ -4331,3 +4331,5 @@ shape	Appearance	Aspetto
 	GPU Memory (MB, 0 = Automatic)	Memoria GPU (MB, 0 = Automatico)
 	{name} was changed by another app after it was opened or saved here.	{name} è stato modificato da un'altra app dopo essere stato aperto o salvato qui.
 	Replace it with this document, or cancel and use File › Save As to keep both.	Sostituiscilo con questo documento, oppure annulla e usa File › Salva con nome per tenerli entrambi.
+	“{name}” already exists. Replace it?	“{name}” esiste già. Sostituirlo?
+	The name in the save dialog was “{name}”.	Il nome nella finestra di salvataggio era “{name}”.
diff --git a/crates/ui-egui/src/i18n/ja.tsv b/crates/ui-egui/src/i18n/ja.tsv
index 351699ea89..11532fe9e7 100644
--- a/crates/ui-egui/src/i18n/ja.tsv
+++ b/crates/ui-egui/src/i18n/ja.tsv
@@ -3887,3 +3887,5 @@ curves	Smooth	スムーズ
 	Replace	置き換え
 	{name} was changed by another app after it was opened or saved here.	{name} は、ここで開くか保存した後に別のアプリで変更されました。
 	Replace it with this document, or cancel and use File › Save As to keep both.	この書類で置き換えるか、キャンセルしてファイル › 名前を付けて保存で両方を残してください。
+	“{name}” already exists. Replace it?	「{name}」は既に存在します。置き換えますか？
+	The name in the save dialog was “{name}”.	保存ダイアログで指定した名前は「{name}」でした。
diff --git a/crates/ui-egui/src/i18n/ko.tsv b/crates/ui-egui/src/i18n/ko.tsv
index 3eaf143979..ad6ba85342 100644
--- a/crates/ui-egui/src/i18n/ko.tsv
+++ b/crates/ui-egui/src/i18n/ko.tsv
@@ -3389,3 +3389,5 @@ curves	Smooth	매끄럽게
 	Replace	대치
 	{name} was changed by another app after it was opened or saved here.	{name}이(가) 여기서 열거나 저장한 뒤 다른 앱에서 변경되었습니다.
 	Replace it with this document, or cancel and use File › Save As to keep both.	이 문서로 대치하거나, 취소하고 파일 › 다른 이름으로 저장을 사용해 둘 다 보관하세요.
+	“{name}” already exists. Replace it?	“{name}”이(가) 이미 있습니다. 대치하시겠습니까?
+	The name in the save dialog was “{name}”.	저장 대화상자에 입력한 이름: “{name}”
diff --git a/crates/ui-egui/src/i18n/nl.tsv b/crates/ui-egui/src/i18n/nl.tsv
index 09dd568009..4bb345d1e6 100644
--- a/crates/ui-egui/src/i18n/nl.tsv
+++ b/crates/ui-egui/src/i18n/nl.tsv
@@ -3970,3 +3970,5 @@ curves	Smooth	Vloeiend maken
 	GPU Memory (MB, 0 = Automatic)	GPU-geheugen (MB, 0 = Automatisch)
 	{name} was changed by another app after it was opened or saved here.	{name} is door een andere app gewijzigd nadat het hier is geopend of bewaard.
 	Replace it with this document, or cancel and use File › Save As to keep both.	Vervang het door dit document, of annuleer en gebruik Bestand › Opslaan als om beide te houden.
+	“{name}” already exists. Replace it?	“{name}” bestaat al. Vervangen?
+	The name in the save dialog was “{name}”.	De naam in het venster Opslaan was “{name}”.
diff --git a/crates/ui-egui/src/i18n/pl.tsv b/crates/ui-egui/src/i18n/pl.tsv
index c4827aa829..d2446750ee 100644
--- a/crates/ui-egui/src/i18n/pl.tsv
+++ b/crates/ui-egui/src/i18n/pl.tsv
@@ -3809,3 +3809,5 @@ curves	Smooth	Wygładź
 	Replace	Zastąp
 	{name} was changed by another app after it was opened or saved here.	Plik {name} został zmieniony przez inną aplikację po otwarciu lub zapisaniu go tutaj.
 	Replace it with this document, or cancel and use File › Save As to keep both.	Zastąp go tym dokumentem albo anuluj i użyj Plik › Zapisz jako, aby zachować oba.
+	“{name}” already exists. Replace it?	„{name}” już istnieje. Zastąpić?
+	The name in the save dialog was “{name}”.	Nazwa w oknie zapisywania: „{name}”.
diff --git a/crates/ui-egui/src/i18n/pt-br.tsv b/crates/ui-egui/src/i18n/pt-br.tsv
index f96542d642..75a9640695 100644
--- a/crates/ui-egui/src/i18n/pt-br.tsv
+++ b/crates/ui-egui/src/i18n/pt-br.tsv
@@ -4000,3 +4000,5 @@ layerLabel	No Color	Sem cor
 	GPU Memory (MB, 0 = Automatic)	Memória da GPU (MB, 0 = Automático)
 	{name} was changed by another app after it was opened or saved here.	{name} foi alterado por outro app depois de ser aberto ou salvo aqui.
 	Replace it with this document, or cancel and use File › Save As to keep both.	Substitua-o por este documento, ou cancele e use Arquivo › Salvar como para manter os dois.
+	“{name}” already exists. Replace it?	“{name}” já existe. Substituir?
+	The name in the save dialog was “{name}”.	O nome na caixa de diálogo Salvar era “{name}”.
diff --git a/crates/ui-egui/src/i18n/ru.tsv b/crates/ui-egui/src/i18n/ru.tsv
index f49200b973..f95a6a6cee 100644
--- a/crates/ui-egui/src/i18n/ru.tsv
+++ b/crates/ui-egui/src/i18n/ru.tsv
@@ -3181,3 +3181,5 @@ curves	Smooth	Сгладить
 	Replace	Заменить
 	{name} was changed by another app after it was opened or saved here.	Файл {name} был изменён другим приложением после того, как его открыли или сохранили здесь.
 	Replace it with this document, or cancel and use File › Save As to keep both.	Замените его этим документом или отмените и используйте Файл › Сохранить как, чтобы сохранить оба.
+	“{name}” already exists. Replace it?	«{name}» уже существует. Заменить?
+	The name in the save dialog was “{name}”.	В окне сохранения было указано имя «{name}».
diff --git a/crates/ui-egui/src/i18n/sr.tsv b/crates/ui-egui/src/i18n/sr.tsv
index c207facb72..a83e28b7ef 100644
--- a/crates/ui-egui/src/i18n/sr.tsv
+++ b/crates/ui-egui/src/i18n/sr.tsv
@@ -4283,3 +4283,5 @@ shape	Appearance	Izgled
 	GPU Memory (MB, 0 = Automatic)	GPU memorija (MB, 0 = Automatski)
 	{name} was changed by another app after it was opened or saved here.	Datoteku {name} je izmenila druga aplikacija nakon što je ovde otvorena ili sačuvana.
 	Replace it with this document, or cancel and use File › Save As to keep both.	Zameni je ovim dokumentom ili otkaži i koristi Datoteka › Sačuvaj kao da zadržiš obe.
+	“{name}” already exists. Replace it?	„{name}“ već postoji. Zameniti?
+	The name in the save dialog was “{name}”.	Ime u dijalogu za čuvanje je bilo „{name}“.
diff --git a/crates/ui-egui/src/i18n/tr.tsv b/crates/ui-egui/src/i18n/tr.tsv
index a8aa0c3c58..9cf46749b6 100644
--- a/crates/ui-egui/src/i18n/tr.tsv
+++ b/crates/ui-egui/src/i18n/tr.tsv
@@ -3940,3 +3940,5 @@ cameraData	Lens	Lens
 	GPU Memory (MB, 0 = Automatic)	GPU Belleği (MB, 0 = Otomatik)
 	{name} was changed by another app after it was opened or saved here.	{name}, burada açıldıktan veya kaydedildikten sonra başka bir uygulama tarafından değiştirildi.
 	Replace it with this document, or cancel and use File › Save As to keep both.	Bu belgeyle değiştirin ya da iptal edip ikisini de korumak için Dosya › Farklı Kaydet'i kullanın.
+	“{name}” already exists. Replace it?	'{name}' zaten var. Değiştirilsin mi?
+	The name in the save dialog was “{name}”.	Kaydetme penceresindeki ad '{name}' idi.
diff --git a/crates/ui-egui/src/i18n/uk.tsv b/crates/ui-egui/src/i18n/uk.tsv
index 48fb60343c..28d4aa961c 100644
--- a/crates/ui-egui/src/i18n/uk.tsv
+++ b/crates/ui-egui/src/i18n/uk.tsv
@@ -3910,3 +3910,5 @@ curves	Smooth	Згладити
 	Replace	Замінити
 	{name} was changed by another app after it was opened or saved here.	Файл {name} змінила інша програма після того, як його тут відкрили або зберегли.
 	Replace it with this document, or cancel and use File › Save As to keep both.	Замініть його цим документом або скасуйте й скористайтеся Файл › Зберегти як, щоб зберегти обидва.
+	“{name}” already exists. Replace it?	«{name}» уже існує. Замінити?
+	The name in the save dialog was “{name}”.	У вікні збереження було вказано назву «{name}».
diff --git a/crates/ui-egui/src/i18n/vi.tsv b/crates/ui-egui/src/i18n/vi.tsv
index f33ef15011..112b44aa1e 100644
--- a/crates/ui-egui/src/i18n/vi.tsv
+++ b/crates/ui-egui/src/i18n/vi.tsv
@@ -3697,3 +3697,5 @@ curves	Smooth	Làm mượt
 	Replace	Thay thế
 	{name} was changed by another app after it was opened or saved here.	{name} đã bị một ứng dụng khác thay đổi sau khi được mở hoặc lưu ở đây.
 	Replace it with this document, or cancel and use File › Save As to keep both.	Thay thế bằng tài liệu này, hoặc hủy và dùng Tệp › Lưu thành để giữ cả hai.
+	“{name}” already exists. Replace it?	“{name}” đã tồn tại. Thay thế?
+	The name in the save dialog was “{name}”.	Tên trong hộp thoại lưu là “{name}”.
diff --git a/crates/ui-egui/src/i18n/zh-hans.tsv b/crates/ui-egui/src/i18n/zh-hans.tsv
index 75eacc994d..988d3faad0 100644
--- a/crates/ui-egui/src/i18n/zh-hans.tsv
+++ b/crates/ui-egui/src/i18n/zh-hans.tsv
@@ -2827,3 +2827,5 @@ curves	Smooth	平滑
 	Replace	替换
 	{name} was changed by another app after it was opened or saved here.	{name} 在此处打开或存储后已被其他应用更改。
 	Replace it with this document, or cancel and use File › Save As to keep both.	用此文档替换它，或取消并使用“文件 › 另存为”以保留两者。
+	“{name}” already exists. Replace it?	“{name}”已存在。要替换它吗？
+	The name in the save dialog was “{name}”.	存储对话框中的名称是“{name}”。
diff --git a/crates/ui-egui/src/i18n/zh-hant.tsv b/crates/ui-egui/src/i18n/zh-hant.tsv
index 95a11dc593..75be2a7289 100644
--- a/crates/ui-egui/src/i18n/zh-hant.tsv
+++ b/crates/ui-egui/src/i18n/zh-hant.tsv
@@ -2812,3 +2812,5 @@ curves	Smooth	平滑
 	Replace	取代
 	{name} was changed by another app after it was opened or saved here.	{name} 在此處開啟或儲存後已被其他 App 更改。
 	Replace it with this document, or cancel and use File › Save As to keep both.	以此文件取代它，或取消並使用「檔案 › 另存新檔」以保留兩者。
+	“{name}” already exists. Replace it?	「{name}」已存在。要取代它嗎？
+	The name in the save dialog was “{name}”.	儲存對話框中的名稱是「{name}」。
diff --git a/crates/ui-egui/src/lib.rs b/crates/ui-egui/src/lib.rs
index 04c37282e6..8bb3bdc586 100644
--- a/crates/ui-egui/src/lib.rs
+++ b/crates/ui-egui/src/lib.rs
@@ -173,6 +173,7 @@ pub mod proxy;
 pub mod puppet_ui;
 pub mod quick_pick;
 pub mod rasterize_prompt;
+pub mod replace_prompt;
 pub mod retouch_ui;
 mod rgb_histogram;
 pub mod rotate_view;
@@ -360,6 +361,8 @@ pub struct FileStamp {
 }
 /// The [`FileStamp`] of the file at a path, `None` when it can't be read.
 pub type FileStampFn = Box<dyn Fn(&str) -> Option<FileStamp>>;
+/// Whether two paths reach the same file on disk; false when either can't be read.
+pub type SameFileFn = Box<dyn Fn(&str, &str) -> bool>;
 /// Encode a document and write it to a path, reporting the stage to the job; returns the export
 /// warnings. Runs on a worker thread (see [`Services::save_file`]). It must call
 /// [`jobs_ui::SaveCtl::commit`] right before replacing the file, and write nothing if that fails.
@@ -460,6 +463,10 @@ pub struct Services {
     /// Size and modification time of a file (native); `None` on the web, where Save never
     /// writes back to a file.
     pub file_stamp: Option<FileStampFn>,
+    /// Whether two paths reach the same file (native): a save dialog's answer and the path the
+    /// save writes to after Lowercase Extension, on a file system that ignores letter case. A save
+    /// asks before replacing a file it finds different (`replace_prompt`).
+    pub same_file: Option<SameFileFn>,
     /// Encode and write in one step on a worker thread, so a large save doesn't freeze the
     /// window (#2017). With background jobs on, saves use it instead of `export` and `write`.
     pub save_file: Option<SaveFileFn>,
diff --git a/crates/ui-egui/src/replace_prompt.rs b/crates/ui-egui/src/replace_prompt.rs
new file mode 100644
index 0000000000..7f301db8e7
--- /dev/null
+++ b/crates/ui-egui/src/replace_prompt.rs
@@ -0,0 +1,46 @@
+//! A save asks before replacing a file that the save dialog did not ask about. The dialog asks
+//! only about the name it answers, and the app can change that name afterwards: it gives a name
+//! without an extension the suggested file's (#5039), and lowercases the extension when
+//! Preferences ▸ File Handling › Lowercase Extension is on. When a file exists under the changed
+//! name, Replace writes over it and Cancel ends the save like the save dialog's Cancel.
+//!
+//! The prompt is an ordinary dialog in the UI state: `ui.dialog.confirm` replaces the file and
+//! `ui.dialog.cancel` keeps it. The file dialog stays open until it is answered
+//! ([`PhotocraftApp::file_dialog_open`]).
+
+use serde_json::{Map, Value, json};
+
+use crate::PhotocraftApp;
+use crate::file_open::display_name;
+use crate::state::DialogKind;
+
+const MARK: &str = "__replaceSaved";
+
+pub fn owns(fields: &Map<String, Value>) -> bool {
+    fields.contains_key(MARK)
+}
+
+/// Opens the prompt about `path`, the file a save writes to for `answered`, the save dialog's
+/// answer.
+pub(crate) fn open(app: &mut PhotocraftApp, answered: &str, path: &str) {
+    let message = crate::i18n::fmt(tl!("“{name}” already exists. Replace it?"), &[("name", display_name(path).as_str())]);
+    let detail = crate::i18n::fmt(tl!("The name in the save dialog was “{name}”."), &[("name", display_name(answered).as_str())]);
+    let fields = json!({MARK: true, "__label": "Save", "message": message, "detail": detail, "path": path});
+    app.ui.open_dialog(DialogKind::Command, fields.as_object().cloned().unwrap_or_default());
+}
+
+pub fn body(ui: &mut egui::Ui, fields: &Map<String, Value>) {
+    ui.label(fields.get("message").and_then(Value::as_str).unwrap_or_default());
+    ui.add_space(6.0);
+    ui.label(fields.get("detail").and_then(Value::as_str).unwrap_or_default());
+}
+
+/// Replace: the save the prompt held back.
+pub fn confirm(app: &mut PhotocraftApp) -> Result<Value, String> {
+    app.answer_replace(true)
+}
+
+/// Cancel: nothing is written.
+pub fn cancel(app: &mut PhotocraftApp) -> Result<Value, String> {
+    app.answer_replace(false)
+}
diff --git a/crates/ui-egui/src/swatches_ui.rs b/crates/ui-egui/src/swatches_ui.rs
index a18ddee640..e73ec1d440 100644
--- a/crates/ui-egui/src/swatches_ui.rs
+++ b/crates/ui-egui/src/swatches_ui.rs
@@ -442,6 +442,32 @@ mod tests {
         assert!(out[0].1.starts_with(b"ASEF"));
     }
 
+    /// The Linux save dialog shows the export's name without its extension, `Swatches`, and asks
+    /// only about that name. The export adds `.aco`, so with `Swatches.aco` in the folder it asks
+    /// before replacing it.
+    #[test]
+    fn export_asks_before_replacing_the_file_its_added_extension_names() {
+        use crate::file_dialog::FileDialogAnswer;
+        let written: std::rc::Rc<std::cell::RefCell<Vec<String>>> = Default::default();
+        let w = written.clone();
+        let services = crate::Services {
+            file_dialog: Some(crate::file_dialog::fake(vec![Some(FileDialogAnswer::SaveTo("/swatches/Swatches".into()))]).0),
+            write: Some(Box::new(move |p: &str, _: &[u8]| {
+                w.borrow_mut().push(p.to_string());
+                Ok(())
+            })),
+            file_stamp: Some(Box::new(|p: &str| (p == "/swatches/Swatches.aco").then_some(crate::FileStamp { len: 1, modified: None }))),
+            ..Default::default()
+        };
+        let mut app = PhotocraftApp::new(photocraft_engine::Session::new(), services);
+        export_dialog(&mut app, "aco").unwrap();
+        app.poll_file_dialog(&egui::Context::default(), None);
+        let id = app.ui.dialogs.iter().find(|d| crate::replace_prompt::owns(&d.fields)).map(|d| d.id).expect("the export asks first");
+        assert!(written.borrow().is_empty(), "nothing is written before the answer");
+        crate::dialogs::confirm(&mut app, id).unwrap();
+        assert_eq!(*written.borrow(), ["/swatches/Swatches.aco"]);
+    }
+
     /// #3658: dragging a group header reorders the folders, and the new order sticks in the UI
     /// state (UI state is serde, so it saves and reloads with the rest of the panel state).
     #[test]
diff --git a/docs/control-protocol.md b/docs/control-protocol.md
index 15334ac3a2..873dcb5f60 100644
--- a/docs/control-protocol.md
+++ b/docs/control-protocol.md
@@ -66,6 +66,7 @@ The transport is `apps/photocraft/src/control_server.rs`, and the handlers are i
   - `ui.dialog.open {kind:"colorPicker", textStyle:{kind:"character"|"paragraph", id:u32}}` opens the shared Color Picker for an explicitly enabled Color attribute in Style Options. It opens the matching style panel/editor after validation; inherited colors must first be enabled through the existing style command. The picker reports its bound document/revision/style under `__textStyleColor` in `ui.inspect`. Changes preview in an isolated document; OK runs the existing style `set` command once, Cancel changes nothing. Document edits/switches or closed/changed Style Options discard the picker. `textStyle` cannot be combined with `parent`, `effect`, `field` or `target`.
   - The shell's own dialogs (Window › Workspace › New Workspace / Delete Workspace, View › Pixel Aspect Ratio › Custom Pixel Aspect Ratio…, View › Show › Show Extras Options…, View › 32-bit Preview Options…) are listed in `ui.inspect`'s `dialogs` with the id `"shell"` (at most one is open). `ui.dialog.set`, `ui.dialog.confirm` and `ui.dialog.cancel` take `{"dialog": "shell"}`; OK runs the dialog's command and keeps the dialog open if it fails
   - The unsaved-changes prompt (closing a document, Revert, quitting with unsaved work) is listed in `ui.inspect`'s `dialogs` with the id `"discard"` while it is up: `fields` gives the waiting `command` (`file.close`, `file.exit`…), the `document` it asks about, how many are `remaining`, and its `choices`. `ui.dialog.confirm` takes `{"dialog": "discard", "choice": "save" | "dontSave" | "cancel"}` (default `save`; Revert takes `dontSave` or `cancel`) and answers it as the window's buttons do, one document at a time; `ui.dialog.cancel` with `{"dialog": "discard"}` cancels it. Save of an untitled document opens the save dialog first, so the reply's `fileDialog` is true and the prompt stays open until that is answered.
+  - A save dialog's answer that PhotoCraft changes, by giving a name without an extension the suggested file's or by lowercasing the extension (Preferences › File Handling › Lowercase Extension), opens a replace prompt when a file exists under the changed name: a dialog with `__replaceSaved`, `message`, `detail` and `path` (the file the save would replace). `ui.dialog.confirm` writes the file; `ui.dialog.cancel` ends the save without writing, as Cancel in the save dialog does. The file dialog counts as open until the prompt is answered. A name that differs only in letter case and reaches the same file (Windows and macOS by default) does not open it.
   - Layer Style (`layerStyle {effect?}` opens on that effect kind): the fields hold the dialog's whole state, so agents read and drive it like a user. `effects` lists every effect instance in the layer's order as `{id, kind, on, params, fx}` (`fx` is the effect snapshot the dialog loaded; OK edits that snapshot with `params`, so anything the dialog doesn't model survives). `selected` picks the page (`blendingOptions` or an instance id), `preview` (default on) gates the live canvas preview, `p:blendingOptions` edits blend mode/opacity/fill opacity, and `patternList` names the usable patterns. OK replaces the layer's effects in one step (`layer.layerStyle.replace`); Cancel discards. "New Style…" saves the pending state via `style.presets.new {effects}`, a swatch click applies a style preset, and Make/Reset Default use `layer.layerStyle.makeDefault` / `layer.layerStyle.defaultFor`
   - Preferences (`ui.menu.invoke {id: "edit.preferences.interface"}`) exposes the sections in its `values` field. Apply (`ui.dialog.apply`) and OK (`ui.dialog.confirm`) on Preferences are refused over control because automation denies whole-preferences updates and filesystem-bearing preference sections. A refused `ui.dialog.apply` returns an error and keeps the dialog open; a refused `ui.dialog.confirm` returns an error but closes and removes the dialog. `ui.dialog.cancel` closes the dialog without saving. Individual safe preferences can instead be changed with `engine.execute` using `prefs.set` (e.g. `{"command": "prefs.set", "params": {"path": "interface.showTooltips", "value": false}}`). Settings marked for the next launch still require a restart.
   - Edit › Fill… (`ui.menu.invoke {id: "edit.fill"}`, Shift+F5, Shift+Backspace) opens the Fill dialog; its fields are `edit.fill`'s params (`contents`, `color`, `pattern`, `colorAdaptation`, `mode`, `opacity`, `preserveTransparency`), and OK remembers them in the preferences (`dialogs["edit.fill"]`).
diff --git a/docs/ui-dialogs.md b/docs/ui-dialogs.md
index 032edfde5d..8767f59714 100644
--- a/docs/ui-dialogs.md
+++ b/docs/ui-dialogs.md
@@ -50,6 +50,7 @@ Dispatch order matches `dialogs::show` in `dialogs.rs` (check earlier rows befor
 | `__stroke` | `stroke_ui.rs` | Edit › Stroke… |
 | `__createShape` | `shape_dialog.rs` | Click-with-shape-tool creation (rectangle, ellipse, …) |
 | `__deleteLayer` | `delete_layer_prompt.rs` | Layers panel trash confirmation |
+| `__replaceSaved` | `replace_prompt.rs` | Replace prompt when a save's added or lowercased extension names an existing file |
 | `__newLayer` | `new_layer_ui.rs` | Layer › New › Layer… (⇧⌘N; ⌥⇧⌘N makes the layer without it) |
 | `__rasterize` | `rasterize_prompt.rs` | Rasterize-before-paint prompt |
 | `__resetAllTools` | `tool_reset.rs` | Reset All Tools confirmation |
@@ -101,6 +102,7 @@ actions with their marker already set.
 | `__stroke` | `stroke_ui` |
 | `__createShape` | `shape_dialog` |
 | `__deleteLayer` | `delete_layer_prompt` |
+| `__replaceSaved` | `replace_prompt` |
 | `__newLayer` | `new_layer_ui` |
 | `__rasterize` | `rasterize_prompt` |
 | `__resetAllTools` | `tool_reset` |