diff --git a/crates/engine/src/lib.rs b/crates/engine/src/lib.rs
index c6d9f4503..54866374a 100644
--- a/crates/engine/src/lib.rs
+++ b/crates/engine/src/lib.rs
@@ -811,10 +811,30 @@ fn clamp_selection(st: &mut DocState) {
 /// Default for [`History::byte_limit`].
 pub const UNDO_BYTE_BUDGET: usize = 256 << 20;
 
+/// A rough size of a text body in memory: its paragraphs and runs.
+fn text_cost(t: &deckcraft_model::text::TextBody) -> usize {
+    t.paragraphs.iter().map(|p| 96 + p.runs.iter().map(|r| 128 + r.text.len()).sum::<usize>()).sum()
+}
+
+/// A rough size of comments in memory, replies included.
+fn comments_cost(cs: &[deckcraft_model::Comment], depth: usize) -> usize {
+    let kids = |c: &deckcraft_model::Comment| if depth < 64 { comments_cost(&c.replies, depth + 1) } else { 0 };
+    cs.iter().map(|c| 256 + c.author.len() + c.initials.len() + c.text.len() + c.date.len() + kids(c)).sum()
+}
+
+/// A rough size of a slide's own data besides its shapes: notes, comments, tags and kept XML,
+/// all copied with the slide when an edit changes it.
+fn slide_extra_cost(s: &deckcraft_model::Slide) -> usize {
+    text_cost(&s.notes)
+        + comments_cost(&s.comments, 0)
+        + s.name.len()
+        + s.tags.iter().map(|(k, v)| 48 + k.len() + v.len()).sum::<usize>()
+        + s.raw_ext.as_ref().map_or(0, String::len)
+}
+
 /// A rough size of a shape in memory: the struct, its text and kept XML, its children.
 fn shape_cost(sh: &deckcraft_model::Shape, depth: usize) -> usize {
-    let text: usize =
-        sh.text.as_ref().map_or(0, |t| t.paragraphs.iter().map(|p| 96 + p.runs.iter().map(|r| 128 + r.text.len()).sum::<usize>()).sum());
+    let text: usize = sh.text.as_ref().map_or(0, text_cost);
     let kids: usize = if depth < 64 { sh.children().iter().map(|c| shape_cost(c, depth + 1)).sum() } else { 0 };
     768 + text + sh.raw_ext.as_ref().map_or(0, String::len) + kids
 }
@@ -832,7 +852,7 @@ pub fn undo_cost(doc: &Presentation, newer: &Presentation) -> usize {
     let media: HashSet<*const Vec<u8>> = newer.media.iter().map(|m| Arc::as_ptr(&m.data)).collect();
     let mut n = 64 * doc.slides.len();
     for s in doc.slides.iter().filter(|s| !slides.contains(&Arc::as_ptr(s))) {
-        n += 512 + shapes_cost(&s.shapes);
+        n += 512 + shapes_cost(&s.shapes) + slide_extra_cost(s);
     }
     for m in doc.masters.iter().filter(|m| !masters.contains(&Arc::as_ptr(m))) {
         n += 4096 + shapes_cost(&m.shapes) + m.layouts.iter().map(|l| 512 + shapes_cost(&l.shapes)).sum::<usize>();
diff --git a/crates/engine/src/tests.rs b/crates/engine/src/tests.rs
index 68de13418..750e06304 100644
--- a/crates/engine/src/tests.rs
+++ b/crates/engine/src/tests.rs
@@ -3338,6 +3338,29 @@ fn undo_history_is_capped_by_memory_as_well_as_steps() {
     assert_eq!(s.doc().unwrap().history.undo.len(), 120);
 }
 
+/// A slide's notes count toward the undo budget: small edits to a slide with large notes keep a
+/// copy of those notes per step, and must not slip past the budget as if the slide were empty.
+#[test]
+fn undo_history_counts_slide_notes() {
+    let mut s = Session::with_new();
+    let notes = "n".repeat(1 << 20);
+    s.edit(|doc, _| {
+        std::sync::Arc::make_mut(&mut doc.slides[0]).notes = deckcraft_model::text::TextBody::from_text(&notes);
+        Ok(())
+    })
+    .unwrap();
+    s.doc_mut().unwrap().history = History { limit: 500, byte_limit: 8 << 20, ..Default::default() };
+    for i in 0..40 {
+        s.edit(|doc, _| {
+            std::sync::Arc::make_mut(&mut doc.slides[0]).name = format!("v{i}");
+            Ok(())
+        })
+        .unwrap();
+    }
+    let n = s.doc().unwrap().history.undo.len();
+    assert!((1..=8).contains(&n), "{n} steps of 1 MB notes kept under an 8 MB budget");
+}
+
 #[test]
 fn picture_styles_replace_each_other_in_one_step() {
     const PNG: &[u8] = &[