diff --git a/crates/engine/src/cmd/features.rs b/crates/engine/src/cmd/features.rs
index 64b6ff50..186b635e 100644
--- a/crates/engine/src/cmd/features.rs
+++ b/crates/engine/src/cmd/features.rs
@@ -23,10 +23,10 @@ pub static COMMANDS: &[CommandSpec] = &[
         .icon("sweep")
         .params("sketch: profile sketch; profiles?; path_sketch: sketch; path: [curve ids in order]; operation?, targets?, name?, body_name?"),
     CommandSpec::new("solid.loft", "Loft", loft).at("SOLID", "CREATE").icon("loft").params("sections: [{sketch, profiles?} | {sketch, point: id} (an apex)] (2 or more, in order); operation?, targets?, name?, body_name?"),
-    CommandSpec::new("solid.box", "Box", prim_box).at("SOLID", "CREATE").icon("box").params("length, width, height: expr; corner?: [x,y,z] | center?: [x,y,z] (numbers in mm or length expressions; default: the origin); operation?, name?, body_name?"),
-    CommandSpec::new("solid.cylinder", "Cylinder", prim_cylinder).at("SOLID", "CREATE").icon("cylinder").params("radius | diameter, height: expr; base?: [x,y,z] (centre of the bottom: numbers in mm or length expressions; default: the origin); axis?: X|Y|Z|[x,y,z] (default Z); operation?, name?, body_name?"),
-    CommandSpec::new("solid.sphere", "Sphere", prim_sphere).at("SOLID", "CREATE").icon("sphere").params("radius | diameter: expr; center?: [x,y,z] (numbers in mm or length expressions; default: the origin); operation?, name?, body_name?"),
-    CommandSpec::new("solid.torus", "Torus", prim_torus).at("SOLID", "CREATE").icon("torus").params("major, minor: expr (radii); center?: [x,y,z] (numbers in mm or length expressions; default: the origin); operation?, name?, body_name?"),
+    CommandSpec::new("solid.box", "Box", prim_box).at("SOLID", "CREATE").icon("box").params("length, width, height: expr; corner?: [x,y,z] | center?: [x,y,z] (numbers in mm or length expressions; default: the origin); operation?, targets?: [body], name?, body_name?"),
+    CommandSpec::new("solid.cylinder", "Cylinder", prim_cylinder).at("SOLID", "CREATE").icon("cylinder").params("radius | diameter, height: expr; base?: [x,y,z] (centre of the bottom: numbers in mm or length expressions; default: the origin); axis?: X|Y|Z|[x,y,z] (default Z); operation?, targets?: [body], name?, body_name?"),
+    CommandSpec::new("solid.sphere", "Sphere", prim_sphere).at("SOLID", "CREATE").icon("sphere").params("radius | diameter: expr; center?: [x,y,z] (numbers in mm or length expressions; default: the origin); operation?, targets?: [body], name?, body_name?"),
+    CommandSpec::new("solid.torus", "Torus", prim_torus).at("SOLID", "CREATE").icon("torus").params("major, minor: expr (radii); center?: [x,y,z] (numbers in mm or length expressions; default: the origin); operation?, targets?: [body], name?, body_name?"),
     CommandSpec::new("solid.fillet", "Fillet", fillet)
         .at("SOLID", "MODIFY")
         .icon("fillet")
@@ -413,9 +413,14 @@ fn add_feature_in(s: &mut Session, p: &Value, kind: FeatureKind, world: bool) ->
             solvecraft_doc::naming::face_names_at(&s.model.state(), &pts)
         }
     };
-    let participants = string_list(p, "participants");
+    // Primitives have no `targets` of their own: their `targets` are the bodies taking part (#259).
+    let primitive = matches!(kind, FeatureKind::Box { .. } | FeatureKind::Cylinder { .. } | FeatureKind::Sphere { .. } | FeatureKind::Torus { .. });
+    let (participants, key) = match string_list(p, "participants") {
+        v if v.is_empty() && primitive => (string_list(p, "targets"), "targets"),
+        v => (v, "participants"),
+    };
     if participants.len() > 10_000 || participants.iter().any(|b| b != "*" && s.model.state().body(b).is_none()) {
-        return Err(EngineError::Other("`participants` must list existing bodies".into()));
+        return Err(EngineError::Other(format!("`{key}` must list existing bodies")));
     }
     if matches!(kind, FeatureKind::ConstructionPlane { .. })
         && let Some(n) = name
diff --git a/crates/engine/src/tests.rs b/crates/engine/src/tests.rs
index 307ba7e9..d58a6d4d 100644
--- a/crates/engine/src/tests.rs
+++ b/crates/engine/src/tests.rs
@@ -2500,6 +2500,23 @@ fn special_files_are_refused_by_every_file_command() {
     let _ = std::fs::remove_dir_all(&dir);
 }
 
+/// A join with explicit `targets` merges into those bodies only; a touched non-target stays (#259).
+#[test]
+fn join_with_targets_leaves_other_touched_bodies_alone() {
+    let mut s = Session::default();
+    run(&mut s, "solid.box", json!({"length": 10, "width": 10, "height": 10, "operation": "new_body", "body_name": "A"}));
+    run(&mut s, "solid.box", json!({"length": 10, "width": 10, "height": 10, "corner": [10, 0, 0], "operation": "new_body", "body_name": "B"}));
+    // Straddles the face between A and B; only A is a target.
+    run(&mut s, "solid.box", json!({"length": 4, "width": 4, "height": 4, "corner": [8, 3, 3], "operation": "join", "targets": ["A"]}));
+    let names: Vec<String> = s.model.state().bodies.iter().map(|b| b.name.clone()).collect();
+    assert_eq!(names, ["A", "B"], "B is not a target and must stay a body");
+    let b = run(&mut s, "inspect.measure", json!({"bodies": ["B"]}))["total"]["volume_mm3"].as_f64().unwrap();
+    assert!(rel(b, 1000.0) < 1e-6, "B is unchanged: {b}");
+    // A gains the half of the tool outside it.
+    let a = run(&mut s, "inspect.measure", json!({"bodies": ["A"]}))["total"]["volume_mm3"].as_f64().unwrap();
+    assert!(rel(a, 1032.0) < 1e-6, "A: {a}");
+}
+
 /// Deleting a sketch takes its extrude with it; a later combine that names the extrude's body must
 /// not be left failing silently: it is deleted too, or the reply reports it (#267).
 #[test]