diff --git a/crates/engine/src/cmd/prop.rs b/crates/engine/src/cmd/prop.rs
index b4432dd2f..9c2515dd8 100644
--- a/crates/engine/src/cmd/prop.rs
+++ b/crates/engine/src/cmd/prop.rs
@@ -501,6 +501,7 @@ fn set(s: &mut Session, p: &Value) -> Result<Value> {
     }
     let v = p.get("value").ok_or_else(|| bad("prop.set", "missing `value`"))?.clone();
     let at = snapped_key_time(s, cid, lid, p, "prop.set")?;
+    let selected: Vec<Tick> = s.state.selected_keys.iter().filter(|k| k.layer == lid && k.prop == uid).map(|k| k.time).collect();
     let out = with_prop(s, "Change Property", merge_p(p), cid, lid, uid, |pr, lt| {
         let cur = pr.value_at(lt);
         let mut nv = cur.coerce_json(&v).ok_or_else(|| bad("prop.set", format!("can't use {v} for a {} property", cur.kind_name())))?;
@@ -510,7 +511,23 @@ fn set(s: &mut Session, p: &Value) -> Result<Value> {
         {
             nv = effectcraft_keyframe::Value::Scalar(x.clamp(min, max));
         }
-        pr.set_value_at(at.unwrap_or(lt), nv.clone());
+        let target = at.unwrap_or(lt);
+        // After Effects: editing the value with the CTI on one of several selected keys moves all
+        // the selected keys by the same change, measured from the stored key (even a muted one); between keys, or at an explicit
+        // other time, it only edits that key.
+        if selected.len() > 1 && target == lt && selected.contains(&target) && key_at(&pr.keys, target).is_some() && nv.interpolates() {
+            let stored = key_at(&pr.keys, target).and_then(|i| pr.keys.get(i)).map(|k| k.value.components()).unwrap_or_default();
+            let delta: Vec<f64> = nv.components().iter().zip(&stored).map(|(n, o)| n - o).collect();
+            let range = if let effectcraft_project::ParamUi::Slider { min, max, .. } = pr.ui { Some((min, max)) } else { None };
+            for k in pr.keys.iter_mut().filter(|k| k.time != target && selected.contains(&k.time)) {
+                let moved: Vec<f64> = k.value.components().iter().zip(&delta).map(|(c, d)| c + d).collect();
+                k.value = k.value.with_components(&moved);
+                if let (Some((min, max)), effectcraft_keyframe::Value::Scalar(x)) = (range, &k.value) {
+                    k.value = effectcraft_keyframe::Value::Scalar(x.clamp(min, max));
+                }
+            }
+        }
+        pr.set_value_at(target, nv.clone());
         Ok(nv.to_json())
     })?;
     if p.get("timeBase").is_none() {
diff --git a/crates/engine/src/tests.rs b/crates/engine/src/tests.rs
index 013d2f24c..9497dbad6 100644
--- a/crates/engine/src/tests.rs
+++ b/crates/engine/src/tests.rs
@@ -45,6 +45,7 @@ pub(crate) mod roto;
 mod scale_comp;
 mod sequence;
 mod session;
+mod set_selected_keys;
 mod settings;
 mod shared_layers;
 mod shortcut_cmds;
diff --git a/crates/engine/src/tests/set_selected_keys.rs b/crates/engine/src/tests/set_selected_keys.rs
new file mode 100644
index 000000000..03ce6b2dd
--- /dev/null
+++ b/crates/engine/src/tests/set_selected_keys.rs
@@ -0,0 +1,115 @@
+//! `prop.set` with the CTI on one of several selected keys moves all the selected keys by the same change.
+
+use effectcraft_project::LayerId;
+use serde_json::json;
+
+use crate::{KeyRef, Session};
+
+fn setup() -> (Session, u64, u64) {
+    let mut s = Session::default();
+    s.execute_checked("comp.new", json!({"width": 64, "height": 64, "frameRate": 30, "duration": 4})).unwrap();
+    let layer = s.execute_checked("layer.newSolid", json!({"name": "Box", "color": "#fff"})).unwrap()["layer"].as_u64().unwrap();
+    for t in 0..3 {
+        s.execute_checked("prop.addKey", json!({"layer": layer, "path": "transform/opacity", "time": t, "value": t * 25})).unwrap();
+    }
+    let prop = s.active_comp().unwrap().layer(LayerId(layer)).unwrap().props.prop("transform/opacity").unwrap().uid;
+    (s, layer, prop)
+}
+
+fn values(s: &Session, layer: u64, prop: u64) -> Vec<f64> {
+    s.active_comp().unwrap().layer(LayerId(layer)).unwrap().props.find(prop).unwrap().keys.iter().map(|k| k.value.as_f64()).collect()
+}
+
+fn select_all(s: &mut Session, layer: u64, prop: u64) {
+    let keys = s.active_comp().unwrap().layer(LayerId(layer)).unwrap().props.find(prop).unwrap().keys.iter().map(|k| k.time).collect::<Vec<_>>();
+    s.state.selected_keys = keys.into_iter().map(|time| KeyRef { layer: LayerId(layer), prop, time }).collect();
+}
+
+#[test]
+fn value_edit_on_a_selected_key_offsets_all_selected_keys() {
+    let (mut s, layer, prop) = setup();
+    select_all(&mut s, layer, prop);
+    s.execute_checked("time.set", json!({"time": 1.0})).unwrap();
+    s.execute_checked("prop.set", json!({"layer": layer, "path": "transform/opacity", "value": 40})).unwrap();
+    assert_eq!(values(&s, layer, prop), vec![15.0, 40.0, 65.0]);
+}
+
+#[test]
+fn one_undo_restores_every_offset_key() {
+    let (mut s, layer, prop) = setup();
+    select_all(&mut s, layer, prop);
+    s.execute_checked("time.set", json!({"time": 1.0})).unwrap();
+    s.execute_checked("prop.set", json!({"layer": layer, "path": "transform/opacity", "value": 40})).unwrap();
+    s.execute("edit.undo", json!({})).unwrap();
+    assert_eq!(values(&s, layer, prop), vec![0.0, 25.0, 50.0]);
+}
+
+#[test]
+fn offset_selected_keys_stay_in_the_slider_range() {
+    let (mut s, layer, prop) = setup();
+    select_all(&mut s, layer, prop);
+    s.execute_checked("time.set", json!({"time": 1.0})).unwrap();
+    s.execute_checked("prop.set", json!({"layer": layer, "path": "transform/opacity", "value": 90})).unwrap();
+    assert_eq!(values(&s, layer, prop), vec![65.0, 90.0, 100.0]);
+}
+
+#[test]
+fn value_edit_between_keys_only_adds_a_key() {
+    let (mut s, layer, prop) = setup();
+    select_all(&mut s, layer, prop);
+    s.execute_checked("time.set", json!({"time": 0.5})).unwrap();
+    s.execute_checked("prop.set", json!({"layer": layer, "path": "transform/opacity", "value": 40})).unwrap();
+    assert_eq!(values(&s, layer, prop), vec![0.0, 40.0, 25.0, 50.0]);
+}
+
+#[test]
+fn explicit_time_off_the_playhead_only_edits_that_key() {
+    let (mut s, layer, prop) = setup();
+    select_all(&mut s, layer, prop);
+    s.execute_checked("time.set", json!({"time": 0.5})).unwrap();
+    s.execute_checked("prop.set", json!({"layer": layer, "path": "transform/opacity", "time": 1.0, "value": 40})).unwrap();
+    assert_eq!(values(&s, layer, prop), vec![0.0, 40.0, 50.0]);
+}
+
+#[test]
+fn value_edit_leaves_unselected_keys_unchanged() {
+    let (mut s, layer, prop) = setup();
+    let times = s.active_comp().unwrap().layer(LayerId(layer)).unwrap().props.find(prop).unwrap().keys.iter().take(2).map(|k| k.time).collect::<Vec<_>>();
+    s.state.selected_keys = times.into_iter().map(|time| KeyRef { layer: LayerId(layer), prop, time }).collect();
+    s.execute_checked("time.set", json!({"time": 1.0})).unwrap();
+    s.execute_checked("prop.set", json!({"layer": layer, "path": "transform/opacity", "value": 40})).unwrap();
+    assert_eq!(values(&s, layer, prop), vec![15.0, 40.0, 50.0]);
+}
+
+#[test]
+fn offset_is_measured_from_a_muted_target_key() {
+    let mut s = Session::default();
+    s.execute_checked("comp.new", json!({"width": 64, "height": 64, "frameRate": 30, "duration": 4})).unwrap();
+    let layer = s.execute_checked("layer.newSolid", json!({"name": "Box", "color": "#fff"})).unwrap()["layer"].as_u64().unwrap();
+    for (t, v) in [(0, 0), (1, 10), (2, 50)] {
+        s.execute_checked("prop.addKey", json!({"layer": layer, "path": "transform/opacity", "time": t, "value": v})).unwrap();
+    }
+    let prop = s.active_comp().unwrap().layer(LayerId(layer)).unwrap().props.prop("transform/opacity").unwrap().uid;
+    s.execute_checked("keys.select", json!({"keys": [{"layer": layer, "path": "transform/opacity", "time": 1.0}]})).unwrap();
+    s.execute_checked("keys.setMuted", json!({"muted": true})).unwrap();
+    select_all(&mut s, layer, prop);
+    s.execute_checked("time.set", json!({"time": 1.0})).unwrap();
+    s.execute_checked("prop.set", json!({"layer": layer, "path": "transform/opacity", "value": 40})).unwrap();
+    assert_eq!(values(&s, layer, prop), vec![30.0, 40.0, 80.0]);
+}
+
+#[test]
+fn value_edit_offsets_every_component_of_a_vector() {
+    let mut s = Session::default();
+    s.execute_checked("comp.new", json!({"width": 64, "height": 64, "frameRate": 30, "duration": 4})).unwrap();
+    let layer = s.execute_checked("layer.newSolid", json!({"name": "Box", "color": "#fff"})).unwrap()["layer"].as_u64().unwrap();
+    for (t, v) in [(0, [10, 20]), (1, [20, 40]), (2, [30, 60])] {
+        s.execute_checked("prop.addKey", json!({"layer": layer, "path": "transform/position", "time": t, "value": v})).unwrap();
+    }
+    let prop = s.active_comp().unwrap().layer(LayerId(layer)).unwrap().props.prop("transform/position").unwrap().uid;
+    select_all(&mut s, layer, prop);
+    s.execute_checked("time.set", json!({"time": 1.0})).unwrap();
+    s.execute_checked("prop.set", json!({"layer": layer, "path": "transform/position", "value": [25, 45]})).unwrap();
+    let keys = s.active_comp().unwrap().layer(LayerId(layer)).unwrap().props.find(prop).unwrap().keys.iter().map(|k| k.value.as_vec2()).collect::<Vec<_>>();
+    assert_eq!(keys, vec![[15.0, 25.0], [25.0, 45.0], [35.0, 65.0]]);
+}