MCP: a parameter nested too deeply is refused as bad parameters

A value that nests past a command schema's 16-level limit was reported
as an internal error ("please report this bug"). It is now a parameter
error that names the value's path, like the schema's other refusals; a
schema that is wrong in itself is still an internal error.

The schema check now also refuses a list of ids or a point inside 16
lists, since every value of those types is a list and the value check
refuses a list at that depth. A color there still takes a string.

Docs: docs/mcp.md, docs/target-app-parity.md.

crates/apischema/src/lib.rs

@@ -250,7 +250,8 @@ impl ParamSchema {
     /// What is wrong with the schema itself, or `None`: a parameter name that is empty, repeated
     /// or holds `.`, `[` or `]`; a required parameter with a default; a default that isn't JSON or
     /// doesn't pass its parameter's type; an enum with no names or a repeated name; a range with
-    /// an end that isn't finite or that no value can be in; nesting past [`MAX_DEPTH`].
+    /// an end that isn't finite or that no value can be in; nesting past [`MAX_DEPTH`], or a
+    /// [`Type::Ids`] or [`Type::Point`], whose values are all lists, at the nesting limit.
     pub fn definition_problem(&self) -> Option<String> {
         object_problem("", self.params, 0)
     }
@@ -319,7 +320,9 @@ pub enum Problem {
     Missing,
     /// The value is not of the parameter's type: `expected` says what it must be.
     WrongType { expected: String, got: String },
-    /// The schema nests deeper than [`MAX_DEPTH`] here ([`ParamSchema::definition_problem`]).
+    /// The value nests deeper than [`MAX_DEPTH`] here: a list given for a [`Type::Color`] at the
+    /// nesting limit, or any value under a schema that nests too deeply itself
+    /// ([`ParamSchema::definition_problem`]).
     TooDeep,
 }
 
@@ -779,6 +782,8 @@ fn type_problem(at: &str, ty: &Type, depth: usize) -> Option<String> {
         Type::Object(params) => object_problem(at, params, depth + 1),
         Type::Array(_) if depth >= MAX_DEPTH => Some(format!("`{at}` nests deeper than {MAX_DEPTH} levels")),
         Type::Array(item) => type_problem(&format!("{at}[]"), item, depth + 1),
+        // Every value of these is a list, and a list at the nesting limit is refused.
+        Type::Ids | Type::Point if depth >= MAX_DEPTH => Some(format!("`{at}` nests deeper than {MAX_DEPTH} levels")),
         _ => None,
     }
 }

crates/apischema/src/tests.rs

@@ -156,6 +156,36 @@ fn id_lists_points_and_color_lists_count_toward_the_nesting_limit() {
     assert!(check(MAX_DEPTH, &Type::Color, &json!("#ff0000")).is_ok());
 }
 
+/// `ty` inside `levels` lists.
+fn in_lists(levels: usize, ty: Type) -> Type {
+    (0..levels).fold(ty, |inner, _| Type::Array(Box::leak(Box::new(inner))))
+}
+
+/// `v` inside `levels` lists.
+fn wrapped(levels: usize, v: Value) -> Value {
+    (0..levels).fold(v, |inner, _| Value::Array(vec![inner]))
+}
+
+/// The schema check and the value check agree on the fixed-shape lists at the nesting limit: an id
+/// list or a point there could take no value, so the schema is wrong in itself, and a color there
+/// takes a string, while a color list is refused as nested too deeply, at its path.
+#[test]
+fn fixed_shape_lists_at_the_nesting_limit_follow_the_schema_check() {
+    for ty in [Type::Ids, Type::Point, Type::Color] {
+        assert_eq!(one(in_lists(MAX_DEPTH - 1, ty)).definition_problem(), None, "{ty:?}");
+    }
+    for ty in [Type::Ids, Type::Point] {
+        let problem = one(in_lists(MAX_DEPTH, ty)).definition_problem();
+        assert!(problem.as_deref().is_some_and(|p| p.contains("nests deeper")), "{ty:?}: {problem:?}");
+    }
+    let color = one(in_lists(MAX_DEPTH, Type::Color));
+    assert_eq!(color.definition_problem(), None);
+    assert_eq!(color.validate(&json!({"x": wrapped(MAX_DEPTH, json!("#ff0000"))})).unwrap(), None);
+    let e = color.validate(&json!({"x": wrapped(MAX_DEPTH, json!([1, 0, 0]))})).unwrap_err();
+    assert_eq!(e.problem, Problem::TooDeep);
+    assert_eq!(e.path, format!("x{}", "[0]".repeat(MAX_DEPTH)));
+}
+
 #[test]
 fn a_saved_retired_key_past_the_nesting_limit_is_refused() {
     let params = holding("older", nested_lists(500_000));

crates/engine/src/schemas.rs

@@ -69,7 +69,7 @@ fn param_error(id: &str, e: &ParamError) -> EngineError {
         Problem::Unknown { keys } => bad(id, format!("unknown parameter `{path}`; it takes: {keys}", keys = keys.join(", "))),
         Problem::Missing => bad(id, format!("missing `{path}`")),
         Problem::WrongType { expected, got } => bad(id, format!("`{path}` must be {expected}, not {got}")),
-        Problem::TooDeep => EngineError::Internal { cmd: id.into(), msg: e.to_string() },
+        Problem::TooDeep => bad(id, format!("`{path}` is nested too deeply")),
     }
 }
 
@@ -246,6 +246,30 @@ mod tests {
         assert!(s.doc().unwrap().history.undo.is_empty());
     }
 
+    /// A value nested past the limit of a sound schema is a parameter error that names it, not an
+    /// internal error: a color list at the nesting limit, where a color string still passes.
+    #[test]
+    fn a_value_nested_too_deeply_is_a_parameter_error() {
+        let levels = vectorcraft_apischema::MAX_DEPTH;
+        let ty = (0..levels).fold(Type::Color, |inner, _| Type::Array(Box::leak(Box::new(inner))));
+        let params: &'static [Param] = Box::leak(Box::new([Param::new("fills", ty)]));
+        let schema = ParamSchema::new(params, "");
+        assert_eq!(schema.definition_problem(), None);
+        let wrapped = |v: Value| (0..levels).fold(v, |inner, _| Value::Array(vec![inner]));
+        assert!(schema.validate(&json!({"fills": wrapped(json!("#ff0000"))})).is_ok());
+        let e = schema.validate(&json!({"fills": wrapped(json!([1, 0, 0]))})).unwrap_err();
+        let path = format!("fills{}", "[0]".repeat(levels));
+        match param_error("view.synthetic", &e) {
+            EngineError::BadParams { cmd, msg } => {
+                assert_eq!(cmd, "view.synthetic");
+                assert_eq!(msg, format!("`{path}` is nested too deeply"));
+            }
+            other => panic!("not a parameter error: {other}"),
+        }
+        let shown = param_error("view.synthetic", &e).to_string();
+        assert!(shown.starts_with("invalid parameters for `view.synthetic`") && !shown.contains("report this bug"), "{shown}");
+    }
+
     /// Each command's JSON schema follows the 2020-12 rules, and a validator of those rules, which
     /// shares no code with the writer, gives the same answer as the schema check. None of these
     /// schemas has a lenient parameter.

crates/ui-egui/src/i18n/ca.tsv

@@ -6103,6 +6103,7 @@ digits	Malayalam	Malaiàlam
 @msg	unknown parameter `{path}`; it takes: {keys}	paràmetre desconegut `{path}`; admet: {keys}
 @msg	missing `{path}`	falta `{path}`
 @msg	`{path}` must be {expected}, not {got}	`{path}` ha de ser {expected}, no {got}
+@msg	`{path}` is nested too deeply	`{path}` està imbricat massa profundament
 @msg	unknown preference `{key}`	preferència desconeguda `{key}`
 @msg	Preferences imported.	S’han importat les preferències.
 @msg	not a preferences file: {e}	no és un fitxer de preferències: {e}

crates/ui-egui/src/i18n/cs.tsv

@@ -5794,6 +5794,7 @@ mark-style	Japanese	Japonské
 @msg	unknown parameter `{path}`; it takes: {keys}	neznámý parametr `{path}`; přijímá: {keys}
 @msg	missing `{path}`	chybí `{path}`
 @msg	`{path}` must be {expected}, not {got}	`{path}` musí být {expected}, nikoli {got}
+@msg	`{path}` is nested too deeply	`{path}` je vnořený příliš hluboko
 	Load EPS into Clipboard	Načíst EPS do schránky
 @msg	an image of {w} × {h} pixels is too large to draw	obraz o velikosti {w} × {h} obrazových bodů je příliš velký na vykreslení
 @msg	{path}: not a PNG, JPEG, WebP, TIFF or BMP image	{path}: není obraz PNG, JPEG, WebP, TIFF ani BMP

crates/ui-egui/src/i18n/de.tsv

@@ -5369,6 +5369,7 @@ lighting	Light	Licht
 @msg	unknown parameter `{path}`; it takes: {keys}	unbekannter Parameter `{path}`; akzeptiert werden: {keys}
 @msg	missing `{path}`	`{path}` fehlt
 @msg	`{path}` must be {expected}, not {got}	`{path}` muss {expected} sein, nicht {got}
+@msg	`{path}` is nested too deeply	`{path}` ist zu tief verschachtelt
 	Load EPS into Clipboard	EPS in die Zwischenablage laden
 @msg	an image of {w} × {h} pixels is too large to draw	ein Bild mit {w} × {h} Pixeln ist zu groß, um gezeichnet zu werden
 @msg	{path}: not a PNG, JPEG, WebP, TIFF or BMP image	{path}: kein PNG-, JPEG-, WebP-, TIFF- oder BMP-Bild

crates/ui-egui/src/i18n/el.tsv

@@ -5153,6 +5153,7 @@ digits	Malayalam	Μαλαγιάλαμ
 @msg	unknown parameter `{path}`; it takes: {keys}	άγνωστη παράμετρος `{path}`· δέχεται: {keys}
 @msg	missing `{path}`	λείπει το `{path}`
 @msg	`{path}` must be {expected}, not {got}	Το `{path}` πρέπει να είναι {expected}, όχι {got}
+@msg	`{path}` is nested too deeply	Το `{path}` έχει υπερβολικά βαθιά ένθεση
 	1 Layer	1 επίπεδο
 	1 bit	1 bit
 	16 Colors	16 χρώματα

crates/ui-egui/src/i18n/es.tsv

@@ -5735,6 +5735,7 @@ mark-style	Japanese	Japonesas
 @msg	unknown parameter `{path}`; it takes: {keys}	parámetro desconocido `{path}`; acepta: {keys}
 @msg	missing `{path}`	falta `{path}`
 @msg	`{path}` must be {expected}, not {got}	`{path}` debe ser {expected}, no {got}
+@msg	`{path}` is nested too deeply	`{path}` está anidado a demasiada profundidad
 	Load EPS into Clipboard	Cargar EPS en el portapapeles
 @msg	an image of {w} × {h} pixels is too large to draw	una imagen de {w} × {h} píxeles es demasiado grande para dibujarse
 @msg	{path}: not a PNG, JPEG, WebP, TIFF or BMP image	{path}: no es una imagen PNG, JPEG, WebP, TIFF ni BMP

crates/ui-egui/src/i18n/fi.tsv

@@ -5676,6 +5676,7 @@ mark-style	Japanese	Japanilainen
 @msg	unknown parameter `{path}`; it takes: {keys}	tuntematon parametri `{path}`; parametrit: {keys}
 @msg	missing `{path}`	`{path}` puuttuu
 @msg	`{path}` must be {expected}, not {got}	`{path}` on oltava {expected}, ei {got}
+@msg	`{path}` is nested too deeply	`{path}` on liian syvällä sisäkkäin
 	Load EPS into Clipboard	Lataa EPS leikepöydälle
 @msg	an image of {w} × {h} pixels is too large to draw	kuva, jonka koko on {w} × {h} pikseliä, on liian suuri piirrettäväksi
 @msg	{path}: not a PNG, JPEG, WebP, TIFF or BMP image	{path}: ei ole PNG-, JPEG-, WebP-, TIFF- tai BMP-kuva

crates/ui-egui/src/i18n/fr.tsv

@@ -5364,6 +5364,7 @@ lighting	Light	Lumière
 @msg	unknown parameter `{path}`; it takes: {keys}	paramètre inconnu `{path}` ; il accepte : {keys}
 @msg	missing `{path}`	`{path}` manquant
 @msg	`{path}` must be {expected}, not {got}	`{path}` doit être {expected}, pas {got}
+@msg	`{path}` is nested too deeply	`{path}` est imbriqué trop profondément
 	1 Layer	1 calque
 	1 bit	1 bit
 	16 Colors	16 couleurs

crates/ui-egui/src/i18n/hi.tsv

@@ -8144,6 +8144,7 @@ antiAlias	Sharp	शार्प
 @msg	unknown parameter `{path}`; it takes: {keys}	अज्ञात पैरामीटर `{path}`; यह लेता है: {keys}
 @msg	missing `{path}`	लापता `{path}`
 @msg	`{path}` must be {expected}, not {got}	`{path}` {expected} होना चाहिए, {got} नहीं
+@msg	`{path}` is nested too deeply	`{path}` बहुत गहराई तक नेस्ट किया गया है
 @msg	unknown PDF preset `{name}` (presets: {_1})	अज्ञात PDF प्रीसेट `{name}` (presets: {_1})
 @msg	unknown pieLegend `{v}` (none, standard or wedges)	अज्ञात pieLegend `{v}` (one, मानक या wedges)
 @msg	unknown piePosition `{v}` (even, ratio or stacked)	अज्ञात piePosition `{v}` (यहां तक कि अनुपात या स्टैक्ड)

crates/ui-egui/src/i18n/hu.tsv

@@ -5777,6 +5777,7 @@ imageTrace	Outline	Körvonal
 @msg	unknown parameter `{path}`; it takes: {keys}	ismeretlen paraméter: `{path}`; ezeket fogadja el: {keys}
 @msg	missing `{path}`	hiányzik: `{path}`
 @msg	`{path}` must be {expected}, not {got}	`{path}`: {expected} kell, nem ez: {got}
+@msg	`{path}` is nested too deeply	`{path}` túl mélyen van egymásba ágyazva
 	Load EPS into Clipboard	EPS betöltése a vágólapra
 @msg	an image of {w} × {h} pixels is too large to draw	egy {w} × {h} pixeles kép túl nagy a kirajzoláshoz
 @msg	{path}: not a PNG, JPEG, WebP, TIFF or BMP image	{path}: nem PNG, JPEG, WebP, TIFF vagy BMP kép

crates/ui-egui/src/i18n/it.tsv

@@ -5411,6 +5411,7 @@ lighting	Light	Luce
 @msg	unknown parameter `{path}`; it takes: {keys}	parametro sconosciuto `{path}`; accetta: {keys}
 @msg	missing `{path}`	manca `{path}`
 @msg	`{path}` must be {expected}, not {got}	`{path}` deve essere {expected}, non {got}
+@msg	`{path}` is nested too deeply	`{path}` è annidato troppo in profondità
 	Load EPS into Clipboard	Carica EPS negli appunti
 @msg	an image of {w} × {h} pixels is too large to draw	un'immagine di {w} × {h} pixel è troppo grande per essere disegnata
 @msg	{path}: not a PNG, JPEG, WebP, TIFF or BMP image	{path}: non è un'immagine PNG, JPEG, WebP, TIFF o BMP

crates/ui-egui/src/i18n/ja.tsv

@@ -5453,6 +5453,7 @@ lighting	Light	ライト
 @msg	unknown parameter `{path}`; it takes: {keys}	`{path}` は不明なパラメーターです。使えるパラメーター: {keys}
 @msg	missing `{path}`	`{path}` が指定されていません
 @msg	`{path}` must be {expected}, not {got}	`{path}` は {expected} にしてください ({got} は使えません)
+@msg	`{path}` is nested too deeply	`{path}` の入れ子が深すぎます
 	Load EPS into Clipboard	EPS をクリップボードに読み込む
 @msg	an image of {w} × {h} pixels is too large to draw	{w} × {h} ピクセルの画像は大きすぎて描画できません
 @msg	{path}: not a PNG, JPEG, WebP, TIFF or BMP image	{path}: PNG、JPEG、WebP、TIFF、BMP のいずれの画像でもありません

crates/ui-egui/src/i18n/ka.tsv

@@ -5161,6 +5161,7 @@ font-technology	Variable	ცვლადი
 @msg	unknown parameter `{path}`; it takes: {keys}	უცნობი პარამეტრი `{path}`; იღებს: {keys}
 @msg	missing `{path}`	აკლია `{path}`
 @msg	`{path}` must be {expected}, not {got}	`{path}` უნდა იყოს {expected} და არა {got}
+@msg	`{path}` is nested too deeply	`{path}` ზედმეტად ღრმადაა ჩადგმული
 file-filter	All readable files	ყველა წაკითხვადი ფაილი
 file-filter	All placeable files	ყველა განთავსებადი ფაილი
 file-filter	Swatch libraries	ფერის ნიმუშების ბიბლიოთეკები

crates/ui-egui/src/i18n/ko.tsv

@@ -5372,6 +5372,7 @@ lighting	Light	광원
 @msg	unknown parameter `{path}`; it takes: {keys}	알 수 없는 매개변수 `{path}`: 받는 매개변수: {keys}
 @msg	missing `{path}`	`{path}`이(가) 없습니다
 @msg	`{path}` must be {expected}, not {got}	`{path}`은(는) {expected}이어야 합니다({got} 사용 불가)
+@msg	`{path}` is nested too deeply	`{path}`이(가) 너무 깊게 중첩되어 있습니다
 	Load EPS into Clipboard	EPS를 클립보드로 불러오기
 @msg	an image of {w} × {h} pixels is too large to draw	{w} × {h}픽셀 이미지는 너무 커서 그릴 수 없습니다
 @msg	{path}: not a PNG, JPEG, WebP, TIFF or BMP image	{path}: PNG, JPEG, WebP, TIFF 또는 BMP 이미지가 아닙니다

crates/ui-egui/src/i18n/pl.tsv

@@ -5633,6 +5633,7 @@ mark-style	Japanese	Japońskie
 @msg	unknown parameter `{path}`; it takes: {keys}	nieznany parametr `{path}`; przyjmuje: {keys}
 @msg	missing `{path}`	brak `{path}`
 @msg	`{path}` must be {expected}, not {got}	`{path}` musi być {expected}, a nie {got}
+@msg	`{path}` is nested too deeply	`{path}` jest zagnieżdżone zbyt głęboko
 	Load EPS into Clipboard	Wczytaj EPS do schowka
 @msg	an image of {w} × {h} pixels is too large to draw	obraz o wymiarach {w} × {h} pikseli jest zbyt duży, aby go narysować
 @msg	{path}: not a PNG, JPEG, WebP, TIFF or BMP image	{path}: to nie jest obraz PNG, JPEG, WebP, TIFF ani BMP

crates/ui-egui/src/i18n/pt-br.tsv

@@ -4667,6 +4667,7 @@ axis	Both	Ambos
 @msg	unknown parameter `{path}`; it takes: {keys}	parâmetro desconhecido `{path}`; aceita: {keys}
 @msg	missing `{path}`	falta `{path}`
 @msg	`{path}` must be {expected}, not {got}	`{path}` deve ser {expected}, não {got}
+@msg	`{path}` is nested too deeply	`{path}` está aninhado com profundidade excessiva
 	Load EPS into Clipboard	Carregar EPS na Área de Transferência
 	Drawer menu	Menu da gaveta
 	Toolbar: Show Controls	Barra de ferramentas: Mostrar controles

crates/ui-egui/src/i18n/ru.tsv

@@ -5397,6 +5397,7 @@ lighting	Light	Свет
 @msg	unknown parameter `{path}`; it takes: {keys}	неизвестный параметр `{path}`; принимает: {keys}
 @msg	missing `{path}`	отсутствует `{path}`
 @msg	`{path}` must be {expected}, not {got}	`{path}` должно быть {expected}, а не {got}
+@msg	`{path}` is nested too deeply	`{path}` вложено слишком глубоко
 	Load EPS into Clipboard	Загрузить EPS в буфер обмена
 @msg	an image of {w} × {h} pixels is too large to draw	изображение размером {w} × {h} пикселей слишком велико для отрисовки
 @msg	{path}: not a PNG, JPEG, WebP, TIFF or BMP image	{path}: это не изображение PNG, JPEG, WebP, TIFF или BMP

crates/ui-egui/src/i18n/sv.tsv

@@ -5265,6 +5265,7 @@ lighting	Light	Ljus
 @msg	unknown parameter `{path}`; it takes: {keys}	okänd parameter `{path}`; den tar: {keys}
 @msg	missing `{path}`	`{path}` saknas
 @msg	`{path}` must be {expected}, not {got}	`{path}` måste vara {expected}, inte {got}
+@msg	`{path}` is nested too deeply	`{path}` är för djupt kapslat
 	1 Layer	1 lager
 	1 bit	1 bit
 	16 Colors	16 färger

crates/ui-egui/src/i18n/tr.tsv

@@ -5060,6 +5060,7 @@ lighting	Light	Işık
 @msg	unknown parameter `{path}`; it takes: {keys}	bilinmeyen parametre `{path}`; şunları alır: {keys}
 @msg	missing `{path}`	`{path}` eksik
 @msg	`{path}` must be {expected}, not {got}	`{path}` {expected} olmalıdır, {got} değil
+@msg	`{path}` is nested too deeply	`{path}` çok derin iç içe geçmiş
 	1 Layer	1 Katman
 	1 bit	1 bit
 	16 Colors	16 Renk

crates/ui-egui/src/i18n/uk.tsv

@@ -5391,6 +5391,7 @@ lighting	Light	Світло
 @msg	unknown parameter `{path}`; it takes: {keys}	невідомий параметр `{path}`; приймає: {keys}
 @msg	missing `{path}`	відсутнє `{path}`
 @msg	`{path}` must be {expected}, not {got}	`{path}` має бути {expected}, а не {got}
+@msg	`{path}` is nested too deeply	`{path}` вкладено надто глибоко
 	Load EPS into Clipboard	Завантажити EPS у буфер обміну
 @msg	an image of {w} × {h} pixels is too large to draw	зображення розміром {w} × {h} пікселів завелике для відображення
 @msg	{path}: not a PNG, JPEG, WebP, TIFF or BMP image	{path}: це не зображення PNG, JPEG, WebP, TIFF чи BMP

crates/ui-egui/src/i18n/vi.tsv

@@ -5305,6 +5305,7 @@ theme	Appearance	Giao diện
 @msg	unknown parameter `{path}`; it takes: {keys}	tham số không xác định `{path}`; nhận: {keys}
 @msg	missing `{path}`	thiếu `{path}`
 @msg	`{path}` must be {expected}, not {got}	`{path}` phải là {expected}, không phải {got}
+@msg	`{path}` is nested too deeply	`{path}` lồng nhau quá sâu
 @msg	unknown preference `{key}`	tùy chọn không xác định `{key}`
 @msg	Preferences imported.	Đã nhập tùy chọn.
 @msg	not a preferences file: {e}	không phải tệp tùy chọn: {e}

crates/ui-egui/src/i18n/zh-hans.tsv

@@ -5667,6 +5667,7 @@ lighting	Light	光源
 @msg	unknown parameter `{path}`; it takes: {keys}	未知参数 `{path}`；它接受：{keys}
 @msg	missing `{path}`	缺少 `{path}`
 @msg	`{path}` must be {expected}, not {got}	`{path}` 必须是 {expected}，而不是 {got}
+@msg	`{path}` is nested too deeply	`{path}` 嵌套过深
 	Load EPS into Clipboard	将 EPS 载入剪贴板
 @msg	an image of {w} × {h} pixels is too large to draw	{w} × {h} 像素的图像过大，无法绘制
 @msg	the JPEG XL file is cut short	JPEG XL 文件不完整

crates/ui-egui/src/i18n/zh-hant.tsv

@@ -5499,6 +5499,7 @@ lighting	Light	光源
 @msg	unknown parameter `{path}`; it takes: {keys}	未知的參數 `{path}`；它接受：{keys}
 @msg	missing `{path}`	未提供 `{path}`
 @msg	`{path}` must be {expected}, not {got}	`{path}` 必須是 {expected}，而不是 {got}
+@msg	`{path}` is nested too deeply	`{path}` 巢狀層級過深
 	Load EPS into Clipboard	將 EPS 載入剪貼簿
 @msg	an image of {w} × {h} pixels is too large to draw	{w} × {h} 像素的影像過大，無法繪製
 @msg	{path}: not a PNG, JPEG, WebP, TIFF or BMP image	{path}：不是 PNG、JPEG、WebP、TIFF 或 BMP 影像

docs/mcp.md

@@ -1784,6 +1784,11 @@ path to a value inside a list or an object, and the document and its undo histor
 → invalid parameters for `view.saved.edit`: `edits[0].delete` must be true or false, not 1
 ```
 
+A schema nests at most 16 lists and objects. A value that would go past that depth is refused with the same kind of
+parameter error, ``invalid parameters for `<id>`: `<path>` is nested too deeply``, where `<path>` is the path to the
+value. A schema that is wrong in itself, such as one that nests past 16 levels or puts a list of ids or a point
+inside 16 lists, is an internal error in the command, and the command does not run.
+
 An omitted value with a `default` takes it (`rotation` 0 in `view.saved.new`); the journal keeps the params as they
 were given. Saved steps played from a journal or an action keep a key the schema does not name, unless its value nests
 more than 16 levels deep, and the values of the parameters it names are checked as in a new call. The schemas of built-in commands belong to API level 3

docs/target-app-parity.md

@@ -719,7 +719,7 @@ Moved from the ROADMAP's "Shipped so far" on 2026-10-10. Update it when a featur
 - **Linux file associations:** the desktop entry lists the MIME type of every format File › Open reads but PSD, PSB, `.ps` PostScript, Targa, PCX, Pixar, JPEG 2000, HEIC/HEIF, JPEG XL and HP-GL, `.ai`, `.ait` and EPS included, and the MIME package declares the types shared-mime-info lacks (VectorCraft documents and templates, `.ait`, Affinity), so file managers list VectorCraft under Open With for them, as Finder does on macOS. `the_linux_desktop_entry_opens_every_readable_format` in `apps/vectorcraft/src/tests.rs` checks both files against `OPEN_EXTS`. On Debian 13, Ubuntu 24.04 and Fedora 42, `desktop-file-validate` accepts the desktop entry, `update-mime-database` accepts the MIME package, and `gio` registers the app for every one of these types.
 
 - **Architecture:** 19+ crates with enforced layering (`cargo xtask layers`). Every action is a command (~400 engine + ~50 UI). Undo is unlimited via structural sharing. `command.batch` runs several commands as one transaction.
-- **Parameter schemas:** a command can carry a machine-readable parameter schema (the new `vectorcraft-apischema` crate): each parameter's type, range, unit, whether it is required, its default, whether null counts as left out, and the API levels it was added and deprecated at. `engine.commands` lists the schema in the shape of a JSON Schema 2020-12 object schema, a test keeps the command's `params` text equal to the text the schema writes, and a new call has the params checked against it before the command runs (batch steps before the first step), with an error naming the parameter and no change to the document; saved steps keep a key the schema does not name, and their values are checked. The View commands `view.capsLock`, `view.saved.new`, `view.saved.edit`, `view.saved.list`, `view.transparencyGrid` and `view.snapToGlyph.options` have schemas. The schemas belong to API level 1 (`vectorcraft_engine::schemas::API_VERSION`), and a test fails when a schema changes without a new level. Tests: `crates/apischema/src/tests.rs` (with `conformance.rs`, a separate check of the JSON form against the JSON Schema 2020-12 rules), `schemas::tests` and `cmd::views::tests::each_view_command_refuses_a_bad_value_naming_the_parameter` in `vectorcraft-engine`.
+- **Parameter schemas:** a command can carry a machine-readable parameter schema (the new `vectorcraft-apischema` crate): each parameter's type, range, unit, whether it is required, its default, whether null counts as left out, and the API levels it was added and deprecated at. `engine.commands` lists the schema in the shape of a JSON Schema 2020-12 object schema, a test keeps the command's `params` text equal to the text the schema writes, and a new call has the params checked against it before the command runs (batch steps before the first step), with an error naming the parameter and no change to the document (a value nested past the schema's 16 levels is such a parameter error, and only a schema that is wrong in itself is an internal error); saved steps keep a key the schema does not name, and their values are checked. The View commands `view.capsLock`, `view.saved.new`, `view.saved.edit`, `view.saved.list`, `view.transparencyGrid` and `view.snapToGlyph.options` have schemas. The schemas belong to API level 1 (`vectorcraft_engine::schemas::API_VERSION`), and a test fails when a schema changes without a new level. Tests: `crates/apischema/src/tests.rs` (with `conformance.rs`, a separate check of the JSON form against the JSON Schema 2020-12 rules), `schemas::tests` and `cmd::views::tests::each_view_command_refuses_a_bad_value_naming_the_parameter` in `vectorcraft-engine`.
 - **Input schemas in the command listings:** `engine.commands` and the MCP tools `command_list` and `list_commands` give each command with a parameter schema an `inputSchema` next to `schema`: the same schema with the root `type` `"object"`, the form an MCP tool's input schema takes ([MCP schema reference](https://modelcontextprotocol.io/specification/2026-07-28/schema)). A command without a schema lists neither, and `brief` leaves out both schemas with the `params` text. Tests: `schemas::tests::engine_commands_list_an_input_schema_with_an_object_root` and `schemas::tests::an_input_schema_keeps_deprecation` in `vectorcraft-engine`, and `tests::conventions::command_listings_carry_input_schemas` in `vectorcraft-mcp`, which check every listed input schema against the JSON Schema 2020-12 rules.
 - **Command API reference:** `engine.apiReference` returns a Markdown reference written from the parameter schemas when it is asked for, and MCP serves the same text as the resource `vectorcraft://commands/reference` (`text/markdown; charset=utf-8`). Each command that has a schema gets a section in registry order with its label, menu path, API levels, what it writes, summary and return, and a table of its parameters (type, range with its ends marked included or not, unit, default, required, levels and description; members of objects as dotted rows). Commands without a schema, plug-in commands included, are left out. Schema text is escaped so it stays plain text in its table cell, the same build always returns the same bytes, and a reference past 1 MiB is an error rather than a cut-short text. The command's own schema raised the API level to 2. This is a VectorCraft documentation feature; no score changes. Tests: `cmd::apiref::tests` and `schemas::tests` in `vectorcraft-engine`, `resources::tests` in `vectorcraft-mcp`.
 - **Automation:**