diff --git a/crates/develop/src/lib.rs b/crates/develop/src/lib.rs
index ea7ed1bd0..9c5afa696 100644
--- a/crates/develop/src/lib.rs
+++ b/crates/develop/src/lib.rs
@@ -161,9 +161,20 @@ impl DevelopSettings {
         }
     }
 
-    /// Next free mask id.
+    /// Next free mask id: one past the highest, or, when that is `u32::MAX` (ids come from files,
+    /// pastes and agents), the lowest one no mask uses.
     pub fn next_mask_id(&self) -> u32 {
-        self.masks.iter().map(|m| m.id).max().map_or(1, |m| m + 1)
+        match self.masks.iter().map(|m| m.id).max() {
+            None => 1,
+            // fewer masks than ids: one of the first `masks.len() + 1` is free
+            Some(max) => max
+                .checked_add(1)
+                .or_else(|| {
+                    let used: std::collections::HashSet<u32> = self.masks.iter().map(|m| m.id).collect();
+                    (1..=u32::MAX).find(|id| !used.contains(id))
+                })
+                .unwrap_or(0),
+        }
     }
 }
 
@@ -411,4 +422,16 @@ mod tests {
         s.reset_section(Section::Detail);
         assert_eq!(s.enhance.denoise_on, None);
     }
+
+    #[test]
+    fn next_mask_id_after_the_highest_id_is_unused() {
+        // ids come from files, pastes and agents: one at the limit must not overflow (a panic in
+        // debug builds) or wrap to an id another mask has
+        let mut s = DevelopSettings::default();
+        assert_eq!(s.next_mask_id(), 1);
+        s.masks = vec![Mask { id: u32::MAX, ..Default::default() }, Mask { id: 1, ..Default::default() }];
+        let next = s.next_mask_id();
+        assert!(!s.masks.iter().any(|m| m.id == next), "{next}");
+        assert_eq!(next, 2);
+    }
 }
diff --git a/crates/develop/src/presets.rs b/crates/develop/src/presets.rs
index b3edc801c..d5a658f51 100644
--- a/crates/develop/src/presets.rs
+++ b/crates/develop/src/presets.rs
@@ -220,14 +220,12 @@ impl Preset {
             o.remove("masks");
         }
         let mut out = apply_partial(s, &rest, amount);
-        let mut next = out.masks.iter().map(|m| m.id).max().unwrap_or(0);
         for m in masks {
             let Ok(mut m) = serde_json::from_value::<crate::Mask>(m.clone()) else { continue };
             if out.masks.iter().any(|q| q.name == m.name && q.components == m.components) {
                 continue;
             }
-            next += 1;
-            m.id = next;
+            m.id = out.next_mask_id();
             m.adjust.amount = (m.adjust.amount * amount).clamp(0.0, 200.0);
             out.masks.push(m);
         }
@@ -293,4 +291,23 @@ mod tests {
         deep_merge(&mut a, &json!({"x": {"b": 3}, "y": [2, 3]}));
         assert_eq!(a, json!({"x": {"a": 1, "b": 3}, "y": [2, 3]}));
     }
+
+    #[test]
+    fn preset_masks_get_unused_ids_after_a_mask_id_at_the_limit() {
+        let s = DevelopSettings { masks: vec![crate::Mask { id: u32::MAX, name: "Theirs".into(), ..Default::default() }], ..Default::default() };
+        let p = Preset {
+            id: "p".into(),
+            name: "P".into(),
+            group: "User".into(),
+            settings: json!({"masks": [{"id": 7, "name": "A"}, {"id": 7, "name": "B"}]}),
+            favorite: false,
+            builtin: false,
+        };
+        let out = p.apply(&s, 1.0);
+        let mut ids: Vec<u32> = out.masks.iter().map(|m| m.id).collect();
+        assert_eq!(ids.len(), 3);
+        ids.sort_unstable();
+        ids.dedup();
+        assert_eq!(ids.len(), 3, "{:?}", out.masks);
+    }
 }
diff --git a/crates/engine/src/tests.rs b/crates/engine/src/tests.rs
index ae03859d2..db77c3cba 100644
--- a/crates/engine/src/tests.rs
+++ b/crates/engine/src/tests.rs
@@ -412,6 +412,17 @@ fn connected_brush_strokes_start_where_the_last_one_ended() {
     assert_eq!(strokes(&s).len(), 2);
 }
 
+#[test]
+fn a_mask_id_at_the_limit_does_not_overflow_the_next_masks() {
+    let mut s = demo();
+    s.execute("develop.merge", &json!({"settings": {"masks": [{"id": u32::MAX, "name": "Pasted"}]}})).unwrap();
+    s.execute("mask.add", &json!({"kind": "radial"})).unwrap();
+    s.execute("mask.duplicate", &json!({})).unwrap();
+    let ids: Vec<u32> = active_dev(&s).masks.iter().map(|m| m.id).collect();
+    assert_eq!(ids.len(), 3);
+    assert!(ids.iter().enumerate().all(|(i, id)| !ids[..i].contains(id)), "distinct ids: {ids:?}");
+}
+
 #[test]
 fn brush_strokes_keep_pen_pressure_and_refuse_bad_lists() {
     let mut s = demo();
diff --git a/docs/whats-new.md b/docs/whats-new.md
index 7d43b2ec8..b494eb8cf 100644
--- a/docs/whats-new.md
+++ b/docs/whats-new.md
@@ -15,6 +15,11 @@
   place, AI masks failed with "unsupported dtype C64". LightCraft now says that the file is Meta's original
   checkpoint and which file it needs instead.
 
+### Reliability
+- A photo with a mask id at the 32-bit limit (from a damaged or hand-edited sidecar, a paste or an agent) no longer
+  breaks the masks added after it: new masks, duplicates and the masks of a preset each get an id no other mask has.
+  Before, the next id wrapped around to 0, so a second new mask shared the first one's id (debug builds crashed).
+
 ## 2026-10-29
 
 ### Object and Describe masks show their pins