diff --git a/crates/engine/src/cmd/edit.rs b/crates/engine/src/cmd/edit.rs
index ec2c5bec..5db99fe9 100644
--- a/crates/engine/src/cmd/edit.rs
+++ b/crates/engine/src/cmd/edit.rs
@@ -100,7 +100,7 @@ fn outside_text(text: &str) -> Result<String> {
 fn paste_text_frame(s: &mut Session, p: &Value, text: &str, slice: Option<&designcraft_doc::Story>) -> Result<Value> {
     use designcraft_geom::{Point, Rect, shapes};
     const LIMIT: f64 = 1.0e5;
-    let sr = super::spread_param(p, "spread");
+    let sr = super::spread_param(p, "edit.paste")?;
     let at = super::point_param(p, "at")
         .filter(|a| a.x.is_finite() && a.y.is_finite())
         .map(|a| Point::new(a.x.clamp(-LIMIT, LIMIT), a.y.clamp(-LIMIT, LIMIT)));
@@ -313,7 +313,7 @@ pub fn specs() -> Vec<CommandSpec> {
                     return Err(super::bad("edit.paste", "clipboard is empty"));
                 };
                 let off = if bool_or(p, "inPlace", false) { 0.0 } else { 12.0 };
-                let to = super::spread_param(p, "spread");
+                let to = super::spread_param(p, "edit.paste")?;
                 let from = s.clipboard_origin;
                 let ids: Vec<ItemId> = clip.spreads.first().map(|sp| sp.items.iter().map(|i| i.id).collect()).unwrap_or_default();
                 s.edit(|d, sel| {
diff --git a/crates/engine/src/cmd/file.rs b/crates/engine/src/cmd/file.rs
index b7bde70a..d401a22b 100644
--- a/crates/engine/src/cmd/file.rs
+++ b/crates/engine/src/cmd/file.rs
@@ -465,7 +465,7 @@ fn file_place(s: &mut Session, p: &Value) -> Result<Value> {
         let st = s.active()?;
         st.selection.items.iter().copied().find(|i| st.doc.item(*i).is_some_and(|it| matches!(it.content, Content::Unassigned | Content::Graphic(_))))
     });
-    let spread = super::spread_param(p, "spread");
+    let spread = super::spread_param(p, "file.place")?;
     let lid = s.doc()?.active_layer;
     let px = p.get("x").and_then(Value::as_f64);
     let py = p.get("y").and_then(Value::as_f64);
@@ -618,7 +618,7 @@ fn place_drop(s: &mut Session, p: &Value) -> Result<Value> {
         s.loaded = None;
         return Err(bad("place.drop", "the loaded graphic is no longer in the document"));
     }
-    let sr = super::spread_param(p, "spread");
+    let sr = super::spread_param(p, "place.drop")?;
     let lid = s.doc()?.active_layer;
     let frame = super::id_param(p, "frame");
     let rect = super::rect_param(p, "rect");
@@ -716,7 +716,7 @@ fn place_layout_page(s: &mut Session, p: &Value, name: &str, bytes: &[u8]) -> Re
     if ids.is_empty() {
         return Err(bad(ID, format!("page {page} has no objects")));
     }
-    let to = super::spread_param(p, "spread");
+    let to = super::spread_param(p, ID)?;
     let off = match (p.get("x").and_then(Value::as_f64), p.get("y").and_then(Value::as_f64)) {
         (Some(x), Some(y)) => designcraft_geom::Vec2::new(x - pr.x0, y - pr.y0),
         _ => designcraft_geom::Vec2::new(-pr.x0, -pr.y0),
@@ -752,7 +752,7 @@ pub(crate) fn snippet_place(s: &mut Session, p: &Value) -> Result<Value> {
     if ids.is_empty() {
         return Err(bad("snippet.place", "the snippet is empty"));
     }
-    let sr = super::spread_param(p, "spread");
+    let sr = super::spread_param(p, "snippet.place")?;
     let bounds = ids.iter().filter_map(|i| src.item(*i).map(|it| it.bounds())).reduce(|a, b| a.union(b)).unwrap_or(Rect::ZERO);
     let off = match (p.get("x").and_then(Value::as_f64), p.get("y").and_then(Value::as_f64)) {
         (Some(x), Some(y)) => designcraft_geom::Vec2::new(x - bounds.x0, y - bounds.y0),
diff --git a/crates/engine/src/cmd/guides.rs b/crates/engine/src/cmd/guides.rs
index b5750fd2..b2bc378a 100644
--- a/crates/engine/src/cmd/guides.rs
+++ b/crates/engine/src/cmd/guides.rs
@@ -28,7 +28,7 @@ pub fn specs() -> Vec<CommandSpec> {
         cmd!("guide.move", "Move Guide", [], None, "{spread?, page, index, position}", has_doc, move_guide),
         cmd!("guide.delete", "Delete Guide", [], None, "{spread?, page, index}", has_doc, delete),
         cmd!("guide.deleteAll", "Delete All Guides on Spread", ["View", "Grids & Guides"], None, "{spread?}", has_doc, |s, p| {
-            let r = guide_spread(p, "guide.deleteAll")?;
+            let r = super::spread_param(p, "guide.deleteAll")?;
             s.edit(|d, _| {
                 let sp = d.spread_mut(r).ok_or_else(|| bad("guide.deleteAll", "no such spread"))?;
                 let mut n = 0;
@@ -41,7 +41,7 @@ pub fn specs() -> Vec<CommandSpec> {
         }),
         cmd!(query "guide.list", "Guides", [], None, "{spread?} → [{page, index, orientation, position, spread}]", has_doc, |s, p| {
             let d = &s.doc()?.doc;
-            let sp = d.spread(guide_spread(p, "guide.list")?).ok_or_else(|| bad("guide.list", "no such spread"))?;
+            let sp = d.spread(super::spread_param(p, "guide.list")?).ok_or_else(|| bad("guide.list", "no such spread"))?;
             let mut out = Vec::new();
             for (pi, pg) in sp.pages.iter().enumerate() {
                 for (i, g) in pg.guides.iter().enumerate() {
@@ -71,23 +71,6 @@ fn optional_usize(p: &Value, key: &str, command: &str) -> Result<Option<usize>>
     p.get(key).map(|value| checked_usize(value, key, command)).transpose()
 }
 
-/// Numeric spread shorthand or the serialized `SpreadRef` form, without lossy/defaulting casts.
-fn guide_spread(p: &Value, command: &str) -> Result<SpreadRef> {
-    let Some(value) = p.get("spread") else { return Ok(SpreadRef::Doc(0)) };
-    if value.is_number() {
-        return Ok(SpreadRef::Doc(checked_usize(value, "spread", command)?));
-    }
-    let object = value.as_object().ok_or_else(|| bad(command, "`spread` must be a non-negative integer or a spread reference"))?;
-    let kind = object.get("kind").and_then(Value::as_str).ok_or_else(|| bad(command, "`spread.kind` must be `doc` or `parent`"))?;
-    let index = object.get("index").ok_or_else(|| bad(command, "missing `spread.index`"))?;
-    let index = checked_usize(index, "spread.index", command)?;
-    match kind {
-        "doc" => Ok(SpreadRef::Doc(index)),
-        "parent" => Ok(SpreadRef::Parent(index)),
-        _ => Err(bad(command, "`spread.kind` must be `doc` or `parent`")),
-    }
-}
-
 fn finite_number(p: &Value, key: &str, command: &str) -> Result<Option<f64>> {
     let Some(value) = p.get(key) else { return Ok(None) };
     let number = value.as_f64().ok_or_else(|| bad(command, format!("`{key}` must be a finite number")))?;
@@ -111,7 +94,7 @@ fn page_at(d: &Document, r: SpreadRef, x: f64) -> usize {
 }
 
 fn add(s: &mut Session, p: &Value) -> Result<Value> {
-    let r = guide_spread(p, "guide.add")?;
+    let r = super::spread_param(p, "guide.add")?;
     let spread_guide = p.get("spreadGuide").and_then(Value::as_bool).unwrap_or(false);
     let page = optional_usize(p, "page", "guide.add")?;
     // (orientation, position, x used to find the page)
@@ -162,7 +145,7 @@ fn add(s: &mut Session, p: &Value) -> Result<Value> {
 fn guide_ref(p: &Value, cmd: &str) -> Result<(SpreadRef, usize, usize)> {
     let pi = optional_usize(p, "page", cmd)?.ok_or_else(|| bad(cmd, "missing page"))?;
     let i = optional_usize(p, "index", cmd)?.ok_or_else(|| bad(cmd, "missing index"))?;
-    Ok((guide_spread(p, cmd)?, pi, i))
+    Ok((super::spread_param(p, cmd)?, pi, i))
 }
 
 /// Commands must use the same guide/layer protection as pointer editing.
@@ -260,7 +243,7 @@ fn create_guides(s: &mut Session, p: &Value) -> Result<Value> {
     let cg = guide_gutter(p, "columnGutter")?;
     let to_page = str_param(p, "fitTo") == Some("page");
     let remove = p.get("removeExisting").and_then(Value::as_bool).unwrap_or(false);
-    let r = guide_spread(p, "layout.createGuides")?;
+    let r = super::spread_param(p, "layout.createGuides")?;
     let only = optional_usize(p, "page", "layout.createGuides")?;
     let page_count = {
         let sp = s.doc()?.doc.spread(r).ok_or_else(|| bad("layout.createGuides", "no such spread"))?;
diff --git a/crates/engine/src/cmd/linked.rs b/crates/engine/src/cmd/linked.rs
index 85f4fbd5..ab53837b 100644
--- a/crates/engine/src/cmd/linked.rs
+++ b/crates/engine/src/cmd/linked.rs
@@ -28,7 +28,7 @@ pub fn specs() -> Vec<CommandSpec> {
             |s, p| {
                 let parent = StoryId(p.get("story").and_then(Value::as_u64).ok_or_else(|| bad("story.placeAndLink", "missing story"))?);
                 let rect = rect_param(p, "rect").ok_or_else(|| bad("story.placeAndLink", "missing rect"))?;
-                let sr = spread_param(p, "spread");
+                let sr = spread_param(p, "story.placeAndLink")?;
                 let layer = s.doc()?.active_layer;
                 s.edit(|d, sel| {
                     if d.story(parent).is_none() {
diff --git a/crates/engine/src/cmd/liquid.rs b/crates/engine/src/cmd/liquid.rs
index 604369df..879fdb36 100644
--- a/crates/engine/src/cmd/liquid.rs
+++ b/crates/engine/src/cmd/liquid.rs
@@ -168,7 +168,7 @@ pub fn specs() -> Vec<CommandSpec> {
             }
         ),
         cmd!("guide.liquid", "Convert to Liquid Guide", [], None, "{spread?, page, index, on?: bool (default true)}", has_doc, |s, p| {
-            let sr = super::spread_param(p, "spread");
+            let sr = super::spread_param(p, "guide.liquid")?;
             let pi = p.get("page").and_then(Value::as_u64).unwrap_or(0) as usize;
             let gi = p.get("index").and_then(Value::as_u64).ok_or_else(|| bad("guide.liquid", "`index` required"))? as usize;
             let on = p.get("on").and_then(Value::as_bool).unwrap_or(true);
diff --git a/crates/engine/src/cmd/mathexpr.rs b/crates/engine/src/cmd/mathexpr.rs
index ef840b21..accb3563 100644
--- a/crates/engine/src/cmd/mathexpr.rs
+++ b/crates/engine/src/cmd/mathexpr.rs
@@ -70,7 +70,7 @@ fn insert(s: &mut Session, p: &Value) -> Result<Value> {
         });
     }
     let text = s.doc()?.selection.text.filter(|t| t.cell.is_none());
-    let spread = super::spread_param(p, "spread");
+    let spread = super::spread_param(p, "math.insert")?;
     let (px, py) = (p.get("x").and_then(Value::as_f64), p.get("y").and_then(Value::as_f64));
     s.edit(|d, sel| {
         if let Some(t) = text {
diff --git a/crates/engine/src/cmd/media.rs b/crates/engine/src/cmd/media.rs
index 6f5f7244..f331c0f1 100644
--- a/crates/engine/src/cmd/media.rs
+++ b/crates/engine/src/cmd/media.rs
@@ -20,7 +20,7 @@ pub(crate) fn place_media(s: &mut Session, p: &Value, name: String, mime: &str,
         let st = s.active()?;
         st.selection.items.iter().copied().find(|i| st.doc.item(*i).is_some_and(|it| matches!(it.content, Content::Unassigned | Content::Graphic(_))))
     });
-    let spread = super::spread_param(p, "spread");
+    let spread = super::spread_param(p, "file.place")?;
     let lid = s.doc()?.active_layer;
     let (px, py) = (p.get("x").and_then(Value::as_f64), p.get("y").and_then(Value::as_f64));
     let mime = mime.to_string();
diff --git a/crates/engine/src/cmd/mod.rs b/crates/engine/src/cmd/mod.rs
index 0a7fb5d1..5c0cdacd 100644
--- a/crates/engine/src/cmd/mod.rs
+++ b/crates/engine/src/cmd/mod.rs
@@ -272,12 +272,23 @@ pub(crate) fn rect_param(p: &Value, key: &str) -> Option<Rect> {
     let f = |i: usize| a.get(i).and_then(Value::as_f64);
     Some(Rect::new(f(0)?, f(1)?, f(2)?, f(3)?))
 }
-/// `{"kind":"doc","index":0}`, `0` or absent (= spread 0).
-pub(crate) fn spread_param(p: &Value, key: &str) -> SpreadRef {
-    match p.get(key) {
-        Some(Value::Number(n)) => SpreadRef::Doc(n.as_u64().unwrap_or(0) as usize),
-        Some(v) => serde_json::from_value(v.clone()).unwrap_or(SpreadRef::Doc(0)),
-        None => SpreadRef::Doc(0),
+/// The `spread` param: a body spread index (`0`), a spread reference (`{"kind":"doc"|"parent","index":0}`)
+/// or absent (= spread 0). Anything else is an error: read as spread 0, a script aiming at a parent
+/// spread got its items on the first page without a word.
+pub(crate) fn spread_param(p: &Value, command: &str) -> Result<SpreadRef> {
+    let index = |v: &Value, key: &str| {
+        v.as_u64().and_then(|n| usize::try_from(n).ok()).ok_or_else(|| bad(command, format!("`{key}` must be a non-negative integer")))
+    };
+    let Some(value) = p.get("spread") else { return Ok(SpreadRef::Doc(0)) };
+    if value.is_number() {
+        return Ok(SpreadRef::Doc(index(value, "spread")?));
+    }
+    let object = value.as_object().ok_or_else(|| bad(command, "`spread` must be a non-negative integer or {kind: doc|parent, index}"))?;
+    let i = index(object.get("index").ok_or_else(|| bad(command, "missing `spread.index`"))?, "spread.index")?;
+    match object.get("kind").and_then(Value::as_str) {
+        Some("doc") => Ok(SpreadRef::Doc(i)),
+        Some("parent") => Ok(SpreadRef::Parent(i)),
+        _ => Err(bad(command, "`spread.kind` must be `doc` or `parent`")),
     }
 }
 
diff --git a/crates/engine/src/cmd/object.rs b/crates/engine/src/cmd/object.rs
index 86ab5835..59fc5e71 100644
--- a/crates/engine/src/cmd/object.rs
+++ b/crates/engine/src/cmd/object.rs
@@ -33,7 +33,7 @@ pub fn specs() -> Vec<CommandSpec> {
             "Create Frame",
             [],
             None,
-            "{spread?, rect: [x0,y0,x1,y1] (spread coords), shape?: rectangle|ellipse|polygon, content?: graphic|text|unassigned, sides?: 6, text?: string, caret?: bool, vertical?: bool (text: a new vertical story)}",
+            "{spread?: index | {kind: doc|parent, index} (a parent spread: index into `parents` of document.inspect), rect: [x0,y0,x1,y1] (spread coords), shape?: rectangle|ellipse|polygon, content?: graphic|text|unassigned, sides?: 6, text?: string, caret?: bool, vertical?: bool (text: a new vertical story)}",
             has_doc,
             frame_create
         ),
@@ -851,7 +851,7 @@ fn frame_create(s: &mut Session, p: &Value) -> Result<Value> {
     let rect = rect_param(p, "rect").ok_or_else(|| bad("frame.create", "missing rect"))?;
     let shape = str_param(p, "shape").unwrap_or("rectangle");
     let content = str_param(p, "content").unwrap_or("graphic");
-    let sr = spread_param(p, "spread");
+    let sr = spread_param(p, "frame.create")?;
     let lid = s.doc()?.active_layer;
     let text = super::text_param(p, "text");
     let caret = bool_or(p, "caret", content == "text");
@@ -914,7 +914,7 @@ pub(crate) fn polygon_in(r: Rect, sides: u32, inset: f64) -> designcraft_geom::P
 fn line_create(s: &mut Session, p: &Value) -> Result<Value> {
     let a = point_param(p, "a").ok_or_else(|| bad("line.create", "missing a"))?;
     let b = point_param(p, "b").ok_or_else(|| bad("line.create", "missing b"))?;
-    let sr = spread_param(p, "spread");
+    let sr = spread_param(p, "line.create")?;
     let lid = s.doc()?.active_layer;
     s.edit(|d, sel| {
         let id = ItemId(d.alloc());
@@ -2165,7 +2165,7 @@ fn gap_move(s: &mut Session, p: &Value) -> Result<Value> {
         .and_then(Value::as_array)
         .and_then(|a| Some(Point::new(a.first()?.as_f64()?, a.get(1)?.as_f64()?)))
         .ok_or_else(|| bad(ID, "missing at"))?;
-    let sr = spread_param(p, "spread");
+    let sr = spread_param(p, ID)?;
     let (vertical, a, z, before, after) = find_gap(&s.doc()?.doc, sr, at).ok_or_else(|| bad(ID, "no gap there"))?;
     // `delta` along the gap's axis, or a drag (dx, dy) of which the axis part counts.
     let delta =
@@ -2671,7 +2671,7 @@ fn path_create(s: &mut Session, p: &Value) -> Result<Value> {
         return Err(bad("path.create", "a path needs at least two anchors"));
     }
     let closed = bool_or(p, "closed", false);
-    let sr = spread_param(p, "spread");
+    let sr = spread_param(p, "path.create")?;
     let lid = s.doc()?.active_layer;
     s.edit(|d, sel| {
         let id = ItemId(d.alloc());
@@ -4187,3 +4187,29 @@ mod fit_text_frame_tests {
         assert_eq!([bounds(&s, a), bounds(&s, b)], before);
     }
 }
+
+#[cfg(test)]
+mod spread_target_tests {
+    use super::*;
+
+    #[test]
+    fn creating_on_a_parent_spread_and_rejecting_unknown_spreads() {
+        let mut s = Session::new();
+        s.execute("file.new", &json!({"pages": 1, "facingPages": false})).unwrap();
+        let parent = json!({"kind": "parent", "index": 0});
+        let r = s.execute("frame.create", &json!({"spread": parent, "rect": [0, 0, 100, 100]})).unwrap();
+        let id = ItemId(r["id"].as_u64().unwrap());
+        assert_eq!(s.doc().unwrap().doc.find(id).unwrap().spread, SpreadRef::Parent(0));
+        s.execute("line.create", &json!({"spread": parent, "a": [0, 0], "b": [50, 50]})).unwrap();
+        assert_eq!(s.doc().unwrap().doc.parents[0].items.len(), 2);
+
+        // Each of these used to land on the first body page.
+        for bad in
+            [json!("A-Parent"), json!("0"), json!(-1), json!(1.5), json!({"kind": "master", "index": 0}), json!({"kind": "parent"}), json!([0])]
+        {
+            assert!(s.execute("frame.create", &json!({"spread": bad, "rect": [0, 0, 100, 100]})).is_err(), "frame.create spread {bad}");
+            assert!(s.execute("line.create", &json!({"spread": bad, "a": [0, 0], "b": [50, 50]})).is_err(), "line.create spread {bad}");
+        }
+        assert!(s.doc().unwrap().doc.spreads[0].items.is_empty());
+    }
+}
diff --git a/crates/engine/src/cmd/paste_svg.rs b/crates/engine/src/cmd/paste_svg.rs
index c6ec91ac..54513be5 100644
--- a/crates/engine/src/cmd/paste_svg.rs
+++ b/crates/engine/src/cmd/paste_svg.rs
@@ -284,7 +284,7 @@ fn paste_svg(s: &mut Session, p: &Value) -> Result<Value> {
     let mut art = Vec::new();
     convert(tree.root(), 0, &mut notes, &mut art);
     let b = bounds(&art).ok_or_else(|| bad(ID, "the SVG on the clipboard has no paths to paste"))?;
-    let sr = spread_param(p, "spread");
+    let sr = spread_param(p, ID)?;
     let layer = s.doc()?.active_layer;
     let at = match super::point_param(p, "at") {
         Some(at) => at,
diff --git a/crates/engine/src/cmd/qr.rs b/crates/engine/src/cmd/qr.rs
index c201de4e..d68f5e69 100644
--- a/crates/engine/src/cmd/qr.rs
+++ b/crates/engine/src/cmd/qr.rs
@@ -58,7 +58,7 @@ fn qr_code(s: &mut Session, p: &Value) -> Result<Value> {
     let text = encode(p).map_err(|e| bad(ID, e))?;
     let color = str_param(p, "color").unwrap_or(designcraft_color::swatch::BLACK).to_string();
     let rect = rect_param(p, "rect");
-    let sr = spread_param(p, "spread");
+    let sr = spread_param(p, ID)?;
     let st = s.doc()?;
     let lid = st.active_layer;
     let sel = st.selection.items.clone();
diff --git a/crates/engine/src/cmd/style.rs b/crates/engine/src/cmd/style.rs
index dde85b88..28799366 100644
--- a/crates/engine/src/cmd/style.rs
+++ b/crates/engine/src/cmd/style.rs
@@ -1169,7 +1169,7 @@ fn apply_color(s: &mut Session, p: &Value) -> Result<Value> {
 fn sample_color(s: &mut Session, p: &Value) -> Result<Value> {
     const ID: &str = "color.sample";
     let at = super::point_param(p, "at").ok_or_else(|| bad(ID, "missing at"))?;
-    let sr = super::spread_param(p, "spread");
+    let sr = super::spread_param(p, ID)?;
     let st = s.doc()?;
     let Some((from, color)) = image_color_at(&st.doc, sr, at) else {
         // No opaque image pixel there (an empty part of an image frame, a transparent pixel): the
diff --git a/crates/engine/src/cmd/thread.rs b/crates/engine/src/cmd/thread.rs
index ac13cb2f..8de3326f 100644
--- a/crates/engine/src/cmd/thread.rs
+++ b/crates/engine/src/cmd/thread.rs
@@ -68,7 +68,7 @@ fn frame_rect(p: &Value) -> Result<Option<Rect>> {
 
 fn thread(s: &mut Session, p: &Value) -> Result<Value> {
     let (from, to, rect) = (id_param(p, "from"), id_param(p, "to"), frame_rect(p)?);
-    let sr = spread_param(p, "spread");
+    let sr = spread_param(p, "text.thread")?;
     let autoflow = bool_or(p, "autoflow", false);
     let layer = s.doc()?.active_layer;
     s.edit(|d, sel| {
diff --git a/docs/mcp.md b/docs/mcp.md
index 82fdc60b..eccc1250 100644
--- a/docs/mcp.md
+++ b/docs/mcp.md
@@ -58,7 +58,9 @@ so inspect the document before retrying. The next request re-establishes the con
 
 Points (1/72 in), y down, in **spread** space: on a single-page spread the page's top-left is (0, 0); on a facing
 spread the right page starts at x = page width. `inspect_document` lists page bounds per spread. Page indices in
-`render_page` / `export_png` are 0-based. `pointer` events in connected mode may use `"space": "screen"` (egui points,
+`render_page` / `export_png` are 0-based. Commands that create items take `spread`: a body spread index, or
+`{"kind": "parent", "index": 0}` for a parent spread (its index in `parents` of `inspect_document`); any other
+value is an error. `pointer` events in connected mode may use `"space": "screen"` (egui points,
 see `ui_inspect` → `canvasRect`).
 
 ## Text offsets