diff --git a/assets/locales/pending/validation-once-per-feature.json b/assets/locales/pending/validation-once-per-feature.json
new file mode 100644
index 0000000000..849b18ff3a
--- /dev/null
+++ b/assets/locales/pending/validation-once-per-feature.json
@@ -0,0 +1,65 @@
+{
+  "ar": {
+    "cmd.diagnostics_validation": "التحقق",
+    "prefs.validation": "التحقق من الأجسام",
+    "prefs.validation.each_feature": "مرة لكل ميزة",
+    "prefs.validation.each_step": "بعد كل خطوة",
+    "prefs.validation.label": "فحص SMLib الكامل",
+    "prefs.validation.off": "متوقف (تجريبي)",
+    "prefs.validation_note": "مرة لكل ميزة: تُفحص الأجسام التي تنتجها الميزة مرة واحدة قبل الاحتفاظ بها، وتفشل الميزة إذا رُفض أحدها. بعد كل خطوة: يُفحص ناتج كل عملية سحب ودمج وتقويس، كما في السابق. متوقف: لا فحص، فقد يُحتفظ بجسم معيب."
+  },
+  "de": {
+    "cmd.diagnostics_validation": "Prüfung",
+    "prefs.validation": "Körperprüfung",
+    "prefs.validation.each_feature": "Einmal je Feature",
+    "prefs.validation.each_step": "Nach jedem Schritt",
+    "prefs.validation.label": "Vollständige SMLib-Prüfung",
+    "prefs.validation.off": "Aus (experimentell)",
+    "prefs.validation_note": "Einmal je Feature: Die Körper, die ein Feature erzeugt, werden einmal geprüft, bevor sie übernommen werden; wird einer abgelehnt, schlägt das Feature fehl. Nach jedem Schritt: Das Ergebnis jedes Sweeps, jeder Boolesche Operation und jeder Rundung wird geprüft, wie bisher. Aus: keine Prüfung, ein fehlerhafter Körper kann bleiben."
+  },
+  "en": {
+    "cmd.diagnostics_validation": "Validation",
+    "prefs.validation": "Body check",
+    "prefs.validation.each_feature": "Once per feature",
+    "prefs.validation.each_step": "After every step",
+    "prefs.validation.label": "SMLib's full check",
+    "prefs.validation.off": "Off (experimental)",
+    "prefs.validation_note": "Once per feature: the bodies a feature makes are checked once before they are kept, and the feature fails if one is refused. After every step: the result of every sweep, boolean and blend is checked, as before. Off: no check, so a faulty body can be kept."
+  },
+  "he": {
+    "cmd.diagnostics_validation": "אימות",
+    "prefs.validation": "בדיקת גופים",
+    "prefs.validation.each_feature": "פעם אחת לכל תכונה",
+    "prefs.validation.each_step": "אחרי כל שלב",
+    "prefs.validation.label": "הבדיקה המלאה של SMLib",
+    "prefs.validation.off": "כבוי (ניסיוני)",
+    "prefs.validation_note": "פעם אחת לכל תכונה: הגופים שתכונה יוצרת נבדקים פעם אחת לפני שהם נשמרים, והתכונה נכשלת אם אחד מהם נדחה. אחרי כל שלב: התוצאה של כל סחיפה, פעולה בוליאנית ועיגול נבדקת, כמו קודם. כבוי: אין בדיקה, ולכן גוף פגום עלול להישמר."
+  },
+  "ja": {
+    "cmd.diagnostics_validation": "検証",
+    "prefs.validation": "ボディのチェック",
+    "prefs.validation.each_feature": "フィーチャーごとに 1 回",
+    "prefs.validation.each_step": "各ステップの後",
+    "prefs.validation.label": "SMLib の完全チェック",
+    "prefs.validation.off": "オフ (試験的)",
+    "prefs.validation_note": "フィーチャーごとに 1 回: フィーチャーが作るボディを保持する前に 1 回チェックし、拒否されたものがあればフィーチャーは失敗します。各ステップの後: これまでどおり、スイープ、ブーリアン、ブレンドのたびに結果をチェックします。オフ: チェックしないため、不正なボディが残ることがあります。"
+  },
+  "ko": {
+    "cmd.diagnostics_validation": "검증",
+    "prefs.validation": "바디 검사",
+    "prefs.validation.each_feature": "피처마다 한 번",
+    "prefs.validation.each_step": "모든 단계 후",
+    "prefs.validation.label": "SMLib 전체 검사",
+    "prefs.validation.off": "끄기 (실험적)",
+    "prefs.validation_note": "피처마다 한 번: 피처가 만든 바디를 유지하기 전에 한 번 검사하며, 하나라도 거부되면 피처가 실패합니다. 모든 단계 후: 이전처럼 스윕, 불리언, 블렌드마다 결과를 검사합니다. 끄기: 검사하지 않으므로 결함 있는 바디가 남을 수 있습니다."
+  },
+  "zh": {
+    "cmd.diagnostics_validation": "验证",
+    "prefs.validation": "实体检查",
+    "prefs.validation.each_feature": "每个特征一次",
+    "prefs.validation.each_step": "每一步之后",
+    "prefs.validation.label": "SMLib 完整检查",
+    "prefs.validation.off": "关闭（实验性）",
+    "prefs.validation_note": "每个特征一次：特征生成的实体在保留前检查一次，若有实体被拒绝，该特征失败。每一步之后：与以前一样，每次扫掠、布尔运算和倒圆后都检查结果。关闭：不检查，因此可能保留有缺陷的实体。"
+  }
+}
diff --git a/crates/ui-egui/src/documents.rs b/crates/ui-egui/src/documents.rs
index 8e859fb55e..8b9c351b5c 100644
--- a/crates/ui-egui/src/documents.rs
+++ b/crates/ui-egui/src/documents.rs
@@ -160,6 +160,7 @@ fn swap_in(app: &mut SolveApp, mut slot: DocSlot) -> DocSlot {
     app.session.auto_project = out.session.auto_project;
     app.session.metric_tap_drill_major_less_pitch = out.session.metric_tap_drill_major_less_pitch;
     app.session.surface_extrude_symmetric_per_side = out.session.surface_extrude_symmetric_per_side;
+    app.session.set_validation(out.session.validation());
     // So does the host's pre-command check: every tab is held to it.
     app.session.set_command_check(out.session.command_check());
     // Views cache by revision: the incoming design must not look unchanged.
diff --git a/crates/ui-egui/src/file_load.rs b/crates/ui-egui/src/file_load.rs
index 8bb48e18e4..bcf5fa58f7 100644
--- a/crates/ui-egui/src/file_load.rs
+++ b/crates/ui-egui/src/file_load.rs
@@ -408,8 +408,10 @@ fn start(app: &mut SolveApp) {
     let before = if insert { crate::open_meshing::session_world(&app.session).bodies.iter().map(|b| b.name.clone()).collect() } else { Vec::new() };
     let update = request.kind == Kind::UpdateLinks;
     let mut scratch = if insert || update || request.kind == Kind::ExportDxf { app.session.scratch() } else { Session::default() };
-    // A design opened or recovered here is run through the host's pre-command check too.
+    // A design opened or recovered here is run through the host's pre-command check too, and
+    // evaluated under the user's validation mode.
     scratch.set_command_check(app.session.command_check());
+    scratch.set_validation(app.preferences.validation);
     if insert {
         scratch.selection = app.session.selection.clone();
     }
@@ -537,6 +539,7 @@ fn start_staged(app: &mut SolveApp) {
     let before = if insert { crate::open_meshing::session_world(&app.session).bodies.iter().map(|b| b.name.clone()).collect() } else { Vec::new() };
     let mut session = if insert { app.session.scratch() } else { Session::default() };
     session.set_command_check(app.session.command_check());
+    session.set_validation(app.preferences.validation);
     if insert {
         session.selection = app.session.selection.clone();
     }
diff --git a/crates/ui-egui/src/lib.rs b/crates/ui-egui/src/lib.rs
index 8b0819186c..e467206210 100644
--- a/crates/ui-egui/src/lib.rs
+++ b/crates/ui-egui/src/lib.rs
@@ -982,6 +982,7 @@ impl SolveApp {
         // next frame applies them.
         self.session.metric_tap_drill_major_less_pitch = self.preferences.metric_tap_drill_major_less_pitch;
         self.session.surface_extrude_symmetric_per_side = self.preferences.surface_extrude_symmetric_per_side;
+        self.session.set_validation(self.preferences.validation);
         let t0 = now_ms();
         let r = self.session.execute(id, &params).map_err(|e| e.to_string());
         self.big_operation |= now_ms() - t0 >= recovery_ui::BIG_OPERATION_MS;
@@ -1010,6 +1011,8 @@ impl SolveApp {
                         self.ui.visual_style = index;
                     }
                 }
+                // The command and the preference are one setting.
+                "diagnostics.validation" => self.preferences.validation = self.session.validation(),
                 "view.grid" => self.ui.show_grid = v.get("on").and_then(Value::as_bool).unwrap_or(!self.ui.show_grid),
                 "view.ground_shadow" => self.ui.ground_shadow = v.get("on").and_then(Value::as_bool).unwrap_or(!self.ui.ground_shadow),
                 "view.navigation_bar" => {
diff --git a/crates/ui-egui/src/prefs.rs b/crates/ui-egui/src/prefs.rs
index 21286834f1..89fd87ba33 100644
--- a/crates/ui-egui/src/prefs.rs
+++ b/crates/ui-egui/src/prefs.rs
@@ -641,6 +641,10 @@ preference_settings! {
     /// extrudes; off by default: the distance is the total, split evenly around the start plane.
     /// Saved designs keep the reading they stored either way.
     surface_extrude_symmetric_per_side: bool = false, PAGE_DESIGN => check_id("prefs.surface_extrude_symmetric_per_side");
+    /// When SMLib's full check runs while features are computed (`diagnostics.validation`):
+    /// `each_feature` (the default) checks the bodies each feature keeps once; `each_step`, as
+    /// before, checks after every sweep, boolean and blend; `off` (experimental) never does.
+    validation: solidcraft_engine::kernel::Validation = solidcraft_engine::kernel::Validation::EachFeature, PAGE_DESIGN;
     /// Reopen the saved designs that were open when the app was last closed (#5542).
     reopen_designs: bool = true, PAGE_GENERAL;
     /// Those designs' files, in tab order, as the last quit left them.
@@ -1130,6 +1134,8 @@ pub fn apply(app: &mut SolveApp, ctx: &egui::Context) {
     // symmetric reading.
     app.session.metric_tap_drill_major_less_pitch = app.preferences.metric_tap_drill_major_less_pitch;
     app.session.surface_extrude_symmetric_per_side = app.preferences.surface_extrude_symmetric_per_side;
+    // The features computed from now on are checked as the preference says.
+    app.session.set_validation(app.preferences.validation);
     // The profiler records while the preference is on (switched when it changes; at start-up only
     // when on, so SOLIDCRAFT_PROFILE=1 is not undone by the default).
     let profile = app.preferences.profile_operations;
@@ -2341,6 +2347,11 @@ pub fn set(app: &mut SolveApp, p: &serde_json::Value) -> Result<serde_json::Valu
     {
         return Err("`viewport_theme` must be theme, light or dark".into());
     }
+    if let Some(v) = p.get("validation")
+        && v.as_str().and_then(solidcraft_engine::kernel::Validation::parse).is_none()
+    {
+        return Err("`validation` must be each_step, each_feature or off".into());
+    }
     let new: Prefs = migrate(serde_json::from_value(cur).map_err(|e| e.to_string())?);
     if !UNITS.contains(&new.default_units.as_str()) {
         return Err(format!("units must be one of {}", UNITS.join(", ")));
@@ -3843,6 +3854,30 @@ mod tests {
         assert_eq!(app.preferences.display_quality, "normal");
     }
 
+    /// The validation mode is once per feature by default; set by name or by the command, the
+    /// two stay one setting, the session computes under it, and Restore Defaults on Design brings
+    /// each_feature back. A saved unknown mode reads as the default.
+    #[test]
+    fn validation_defaults_to_each_feature_and_follows_the_command() {
+        use serde_json::json;
+        use solidcraft_engine::kernel::Validation;
+        let mut app = crate::SolveApp::new(solidcraft_engine::Session::default(), crate::Services::default());
+        assert_eq!(app.preferences.validation, Validation::EachFeature);
+        assert_eq!(super::lenient(&json!({"validation": "sometimes"})).validation, Validation::EachFeature);
+        assert!(super::set(&mut app, &json!({"validation": "never"})).unwrap_err().contains("each_step, each_feature or off"));
+        let out = super::set(&mut app, &json!({"validation": "each_step"})).unwrap();
+        assert_eq!(out["validation"], "each_step");
+        assert_eq!(super::lenient(&out).validation, Validation::EachStep);
+        // A command runs under the preference.
+        app.run("diagnostics.validation", json!({})).unwrap();
+        assert_eq!(app.session.validation(), Validation::EachStep);
+        // The command sets the preference too.
+        app.run("diagnostics.validation", json!({"mode": "off"})).unwrap();
+        assert_eq!((app.preferences.validation, app.session.validation()), (Validation::Off, Validation::Off));
+        super::defaults(&mut app, super::PAGE_DESIGN);
+        assert_eq!(app.preferences.validation, Validation::EachFeature);
+    }
+
     #[test]
     fn sketch_halo_is_off_by_default_saved_and_restored() {
         use serde_json::json;
diff --git a/crates/ui-egui/src/prefs/design.rs b/crates/ui-egui/src/prefs/design.rs
index 5b7b902c98..6dbaea3b4a 100644
--- a/crates/ui-egui/src/prefs/design.rs
+++ b/crates/ui-egui/src/prefs/design.rs
@@ -44,6 +44,25 @@ pub(super) fn show(app: &mut SolveApp, ui: &mut egui::Ui) {
         let check = setting::surface_extrude_symmetric_per_side(app, ui);
         crate::scenario::publish_handle("prefs:surface_extrude_symmetric_per_side", check.rect.center());
     });
+    // When SMLib's full check runs while features are computed (`diagnostics.validation`).
+    ui_kit::settings_section(ui, "validation", app.tr("prefs.validation"), |ui| {
+        use solidcraft_engine::kernel::Validation;
+        let label = |v: &Validation| match v {
+            Validation::EachStep => "prefs.validation.each_step",
+            Validation::EachFeature => "prefs.validation.each_feature",
+            Validation::Off => "prefs.validation.off",
+        };
+        let labels: Vec<String> = Validation::ALL.iter().map(|v| app.tr(label(v)).to_string()).collect();
+        let options: Vec<&str> = labels.iter().map(String::as_str).collect();
+        let mut selected = Validation::ALL.iter().position(|v| *v == app.preferences.validation).unwrap_or(1);
+        ui_kit::row(ui, app.tr("prefs.validation.label"), |ui| {
+            ui_kit::segmented(ui, &mut selected, &options, ui_kit::Segments { intrinsic: true, ..Default::default() });
+        });
+        if let Some(v) = Validation::ALL.get(selected) {
+            app.preferences.validation = *v;
+        }
+        ui_kit::text(ui, app.tr("prefs.validation_note"), ui_kit::Text::Hint("validation"));
+    });
     ui_kit::settings_section(ui, "orientation", app.tr("prefs.orientation"), |ui| {
         ui.horizontal_wrapped(|ui| {
             for (id, label, y_up) in [("z", "Z up", false), ("y", "Y up", true)] {