diff --git a/crates/engine/src/cmd/masks.rs b/crates/engine/src/cmd/masks.rs
index 171435e22b..06168989a6 100644
--- a/crates/engine/src/cmd/masks.rs
+++ b/crates/engine/src/cmd/masks.rs
@@ -98,6 +98,10 @@ fn resolve_ai(s: &mut Session, shape: &mut MaskShape, c: &str) -> Result<Later>
     match shape {
         MaskShape::Object { seg: Some(_), .. } | MaskShape::Prompt { seg: Some(_), .. } => {}
         MaskShape::Object { hint, exclude, .. } if !hint.is_empty() && background => {
+            // refused here, not when the worker later fails at the model's limit after "pending"
+            if hint.len() + exclude.len() > crate::segment::MAX_CLICKS {
+                return Err(ai_err(format!("an Object selection takes at most {} clicks", crate::segment::MAX_CLICKS)));
+            }
             s.segmenter.require_model()?;
             return Ok(Later::Clicks(std::mem::take(hint), std::mem::take(exclude)));
         }
diff --git a/crates/engine/src/segment.rs b/crates/engine/src/segment.rs
index 3f95c310bd..0f99636122 100644
--- a/crates/engine/src/segment.rs
+++ b/crates/engine/src/segment.rs
@@ -498,6 +498,9 @@ impl Session {
         if !clicks.iter().any(|c| c.include) {
             return Err("click the object to include it first".into());
         }
+        if clicks.len() > MAX_CLICKS {
+            return Err(format!("an Object selection takes at most {MAX_CLICKS} clicks"));
+        }
         #[cfg(feature = "sam")]
         {
             self.segment_wait(id, worker::Kind::Clicks(Self::model_clicks(clicks)), None)?.ok_or_else(|| "nothing was selected".to_string())
diff --git a/crates/engine/src/tests/segment.rs b/crates/engine/src/tests/segment.rs
index b649dbdd7e..51d6e26e8f 100644
--- a/crates/engine/src/tests/segment.rs
+++ b/crates/engine/src/tests/segment.rs
@@ -240,6 +240,25 @@ fn object_clicks_are_capped() {
     let _ = std::fs::remove_dir_all(&dir);
 }
 
+/// In background mode a new Object mask with too many clicks is refused up front, not queued.
+#[test]
+fn background_object_clicks_over_the_cap_are_refused_before_queueing() {
+    if !Segmenter::AVAILABLE {
+        return;
+    }
+    let mut s = Session::with_demo();
+    s.segmenter.background = true;
+    let dir = damaged_model("later-clicks");
+    s.segmenter.dir = Some(dir.clone());
+    let id = s.active().unwrap();
+    let points: Vec<_> = (0..=crate::segment::MAX_CLICKS).map(|_| json!([0.5, 0.5])).collect();
+    let e = s.execute("mask.add", &json!({"kind": "object", "points": points})).unwrap_err().to_string();
+    assert!(e.contains("at most"), "{e}");
+    assert!(s.develop_of(id).unwrap().masks.is_empty());
+    assert!(!s.segmenter.busy());
+    let _ = std::fs::remove_dir_all(&dir);
+}
+
 #[cfg(feature = "sam")]
 #[test]
 fn a_running_model_download_shows_in_activity_and_cancels() {
diff --git a/crates/segment/src/lib.rs b/crates/segment/src/lib.rs
index 6fd077ff96..fba28b00e9 100644
--- a/crates/segment/src/lib.rs
+++ b/crates/segment/src/lib.rs
@@ -76,6 +76,8 @@ pub type Result<T> = std::result::Result<T, Error>;
 pub const INPUT: usize = 1008;
 /// Side of the low-resolution masks the model predicts.
 pub const MASK_SIDE: usize = 288;
+/// Most clicks one Object selection may carry (the engine's `MAX_CLICKS`).
+pub const MAX_CLICKS: usize = 64;
 #[cfg(not(target_arch = "wasm32"))]
 const _: () = assert!(MASK_SIDE == tracker::LOW_RES && INPUT == vit::IMAGE);
 
@@ -203,6 +205,10 @@ impl Sam3 {
         if clicks.is_empty() {
             return Err(Error::Model("no clicks".into()));
         }
+        // the prompt grows with every click: the editor takes at most this many (the engine's `MAX_CLICKS`)
+        if clicks.len() > MAX_CLICKS {
+            return Err(Error::Model(format!("at most {MAX_CLICKS} clicks, not {}", clicks.len())));
+        }
         let (neck, tracker) = self.tracker_parts()?;
         if enc.tracker.is_none() {
             let levels = neck.forward(&enc.backbone, 3)?;