diff --git a/src/app/commands/display.rs b/src/app/commands/display.rs
index 2f00501d6..553ed01ef 100644
--- a/src/app/commands/display.rs
+++ b/src/app/commands/display.rs
@@ -782,7 +782,7 @@ impl OpenCADStudio {
                 use crate::modules::insert::solid3d_cmds::PresspullCommand;
                 let color = self.tabs[i].scene.layer_color(&self.tabs[i].active_layer);
                 let mut command = PresspullCommand::new(color);
-                command.set_isolines(self.tabs[i].scene.document.header.isolines.max(0) as usize);
+                command.set_isolines(crate::entities::solid3d::clamp_header_isolines(self.tabs[i].scene.document.header.isolines));
                 command.set_preselection(self.presspull_preselection());
                 self.command_line.push_info(&command.prompt());
                 self.tabs[i].active_cmd = Some(Box::new(command));
@@ -793,7 +793,7 @@ impl OpenCADStudio {
                 use crate::modules::insert::solid3d_cmds::RevolveCommand;
                 let selected: Vec<_> = self.tabs[i].scene.selected_entities().into_iter().collect();
                 let color = self.tabs[i].scene.layer_color(&self.tabs[i].active_layer);
-                let isolines = self.tabs[i].scene.document.header.isolines.max(0) as usize;
+                let isolines = crate::entities::solid3d::clamp_header_isolines(self.tabs[i].scene.document.header.isolines);
                 let mut cmd = RevolveCommand::new(color, isolines);
                 if !selected.is_empty() {
                     cmd.set_preselection(
@@ -811,7 +811,7 @@ impl OpenCADStudio {
             "SWEEP" => {
                 use crate::modules::insert::solid3d_cmds::SweepCommand;
                 let color = self.tabs[i].scene.layer_color(&self.tabs[i].active_layer);
-                let isolines = self.tabs[i].scene.document.header.isolines.max(0) as usize;
+                let isolines = crate::entities::solid3d::clamp_header_isolines(self.tabs[i].scene.document.header.isolines);
                 let mut cmd = SweepCommand::new(color, isolines);
                 let selected = self.tabs[i].scene.selected_handles_in_order()
                     .into_iter()
@@ -829,7 +829,7 @@ impl OpenCADStudio {
             "LOFT" => {
                 use crate::modules::insert::solid3d_cmds::LoftCommand;
                 let color = self.tabs[i].scene.layer_color(&self.tabs[i].active_layer);
-                let isolines = self.tabs[i].scene.document.header.isolines.max(0) as usize;
+                let isolines = crate::entities::solid3d::clamp_header_isolines(self.tabs[i].scene.document.header.isolines);
                 let selected = self.tabs[i].scene.selected_handles_in_order().into_iter()
                     .filter_map(|handle| self.tabs[i].scene.document.get_entity(handle)
                         .cloned().map(|entity| (handle, entity))).collect();
diff --git a/src/app/commands/styleprops.rs b/src/app/commands/styleprops.rs
index 929f6fde7..c54d29b57 100644
--- a/src/app/commands/styleprops.rs
+++ b/src/app/commands/styleprops.rs
@@ -2223,6 +2223,14 @@ impl OpenCADStudio {
                                 Some(v) => v
                                     .parse::<i16>()
                                     .map(|x| {
+                                        // Stored values stay inside the range
+                                        // the Options slider can ask for; the
+                                        // consumption sites clamp again for
+                                        // file-sourced headers.
+                                        let x = x.clamp(
+                                            0,
+                                            crate::entities::solid3d::MAX_HEADER_ISOLINES,
+                                        );
                                         h.isolines = x;
                                         (format!("ISOLINES = {x}"), true)
                                     })
@@ -3416,6 +3424,27 @@ mod tests {
         app
     }
 
+    /// `SETVAR ISOLINES` stored any parseable `i16` verbatim. The header
+    /// value is the per-face isolate count handed to the tessellation
+    /// kernel (which loops `0..count`), so `ISOLINES 32767` made every
+    /// rebuild and per-cursor preview allocate without bound. The stored
+    /// value must stay inside the range the app's own UI can ask for
+    /// (the Options slider is 0..=64).
+    #[test]
+    fn setvar_isolines_stays_within_the_renderable_range() {
+        let mut app = fresh_app();
+        let i = app.active_tab;
+
+        let _ = app.run_command_line("SETVAR ISOLINES 32767");
+        assert_eq!(app.tabs[i].scene.document.header.isolines, 64);
+
+        let _ = app.run_command_line("SETVAR ISOLINES 8");
+        assert_eq!(app.tabs[i].scene.document.header.isolines, 8);
+
+        let _ = app.run_command_line("SETVAR ISOLINES -3");
+        assert_eq!(app.tabs[i].scene.document.header.isolines, 0);
+    }
+
     #[test]
     fn selection_commands_gather_instead_of_using_the_whole_drawing() {
         for command in ["SETBYLAYER", "OVERKILL"] {
diff --git a/src/app/control/mod.rs b/src/app/control/mod.rs
index 9fb65a887..e2e64735a 100644
--- a/src/app/control/mod.rs
+++ b/src/app/control/mod.rs
@@ -1240,7 +1240,7 @@ impl OpenCADStudio {
                         [1.; 4],
                         h.facet_resolution,
                         crate::entities::solid3d::display_deflection(h, h.facet_resolution),
-                        h.isolines.max(0) as usize,
+                        crate::entities::solid3d::clamp_header_isolines(h.isolines),
                     )
                 });
             let curve = crate::entities::curve::entity_curve(entity).map(|planar| {
diff --git a/src/entities/solid3d.rs b/src/entities/solid3d.rs
index d24b9a703..b8721f0fe 100644
--- a/src/entities/solid3d.rs
+++ b/src/entities/solid3d.rs
@@ -110,10 +110,25 @@ fn write_surface_property_state(surface: &mut Surface, state: SurfacePropertySta
     surface.common.extended_data.upsert_record(record);
 }
 
+/// Upper bound for the header `$ISOLINES` value handed to tessellation —
+/// the Options slider's own ceiling ("Isolines per surface", 0..=64).
+pub(crate) const MAX_HEADER_ISOLINES: i16 = 64;
+
+/// Upper bound for per-surface `u/v_isolines` — the plugin editor's
+/// writable range for those fields (0..=200).
+pub(crate) const MAX_SURFACE_ISOLINES: i16 = 200;
+
+/// Header `$ISOLINES` (file- or command-controlled `i16`) → the bounded
+/// per-face isolate count handed to the kernel, which draws `0..count`
+/// isolines per face with no ceiling of its own.
+pub(crate) fn clamp_header_isolines(raw: i16) -> usize {
+    raw.clamp(0, MAX_HEADER_ISOLINES) as usize
+}
+
 pub(crate) fn surface_isoline_counts(surface: &Surface) -> [usize; 2] {
     [
-        surface.u_isolines.max(0) as usize,
-        surface.v_isolines.max(0) as usize,
+        surface.u_isolines.clamp(0, MAX_SURFACE_ISOLINES) as usize,
+        surface.v_isolines.clamp(0, MAX_SURFACE_ISOLINES) as usize,
     ]
 }
 
@@ -939,4 +954,27 @@ mod tests {
         assert_eq!(min, [0.0, 0.0, 0.0]);
         assert_eq!(max, [10.0, 10.0, 0.0]);
     }
+
+    /// `Surface.u/v_isolines` are raw `i16` — a file can carry 32767 and the
+    /// native property editor only floors at 0 — and the counts go straight
+    /// into the kernel, which draws `0..count` isolines per face. The
+    /// bounded answer the rest of the product agrees on is the plugin
+    /// editor's writable range, 0..=200.
+    #[test]
+    fn surface_isolate_counts_stay_within_the_plugin_editor_range() {
+        use codec::entities::{Surface, SurfaceKind};
+
+        let mut surface = Surface::new(SurfaceKind::Extruded);
+        surface.u_isolines = i16::MAX;
+        surface.v_isolines = -1;
+        assert_eq!(
+            surface_isoline_counts(&surface),
+            [200, 0],
+            "a hostile surface must not hand the kernel 32767 isolines"
+        );
+
+        surface.u_isolines = 8;
+        surface.v_isolines = 6;
+        assert_eq!(surface_isoline_counts(&surface), [8, 6]);
+    }
 }
diff --git a/src/scene/entity.rs b/src/scene/entity.rs
index d16269d3a..067b4cbf4 100644
--- a/src/scene/entity.rs
+++ b/src/scene/entity.rs
@@ -319,7 +319,7 @@ impl Scene {
         let facet_res = self.document.header.facet_resolution;
         let chordal_deflection =
             crate::entities::solid3d::display_deflection(&self.document.header, facet_res);
-        let isolines = self.document.header.isolines.max(0) as usize;
+        let isolines = crate::entities::solid3d::clamp_header_isolines(self.document.header.isolines);
         let mesh_seed = if matches!(
             &entity,
             EntityType::Solid3D(_)
@@ -603,7 +603,7 @@ impl Scene {
         let facet_res = self.document.header.facet_resolution;
         let chordal_deflection =
             crate::entities::solid3d::display_deflection(&self.document.header, facet_res);
-        let isolines = self.document.header.isolines.max(0) as usize;
+        let isolines = crate::entities::solid3d::clamp_header_isolines(self.document.header.isolines);
         let mesh_seed = if matches!(
             &entity,
             EntityType::Solid3D(_)
@@ -832,7 +832,7 @@ impl Scene {
         let facet_res = self.document.header.facet_resolution;
         let chordal_deflection =
             crate::entities::solid3d::display_deflection(&self.document.header, facet_res);
-        let isolines = self.document.header.isolines.max(0) as usize;
+        let isolines = crate::entities::solid3d::clamp_header_isolines(self.document.header.isolines);
         use crate::par::prelude::*;
         let built: Vec<(Handle, MeshLodSet, bool)> = entries
             .into_par_iter()
@@ -2578,7 +2578,7 @@ impl Scene {
         let facet_res = self.document.header.facet_resolution;
         let chordal_deflection =
             crate::entities::solid3d::display_deflection(&self.document.header, facet_res);
-        let isolines = self.document.header.isolines.max(0) as usize;
+        let isolines = crate::entities::solid3d::clamp_header_isolines(self.document.header.isolines);
         // Top-level solids: offset into the render frame, drawn flat.
         // Block-definition solids: keep block-local coords for per-INSERT
         // instancing (no offset applied here).
diff --git a/src/scene/mod.rs b/src/scene/mod.rs
index 84987dcda..af1598ac3 100644
--- a/src/scene/mod.rs
+++ b/src/scene/mod.rs
@@ -1294,7 +1294,7 @@ fn build_derived_caches_impl(
     // definition solids keep block-local coords for per-INSERT instancing. (#123)
     let facet_res = doc.header.facet_resolution;
     let chordal_deflection = crate::entities::solid3d::display_deflection(&doc.header, facet_res);
-    let isolines = doc.header.isolines.max(0) as usize;
+    let isolines = crate::entities::solid3d::clamp_header_isolines(doc.header.isolines);
     // Real layout blocks come from the Layout objects' block_record handles —
     // `BlockRecord::is_layout()` is unreliable here (it flags ordinary blocks).
     let layout_blocks: std::collections::HashSet<Handle> = doc
@@ -3818,7 +3818,7 @@ impl Scene {
                 crate::entities::solid3d::surface_isoline_counts(surface),
                 crate::entities::solid3d::surface_property_state(surface).isolines,
             ),
-            _ => ([self.document.header.isolines.max(0) as usize; 2], false),
+            _ => ([crate::entities::solid3d::clamp_header_isolines(self.document.header.isolines); 2], false),
         };
         let (mut set, wires, center) = crate::scene::model::solid_model::display_from_solid(
             solid,
diff --git a/src/scene/model/solid_model.rs b/src/scene/model/solid_model.rs
index a236241e2..031c15c04 100644
--- a/src/scene/model/solid_model.rs
+++ b/src/scene/model/solid_model.rs
@@ -640,6 +640,41 @@ mod tests {
         assert_eq!(display([2, 2], false), boundaries);
     }
 
+    /// A file-sourced header `$ISOLINES` bypasses `SETVAR` entirely, so the
+    /// consumption side must clamp too: `prepare_solid_model_display` hands
+    /// the header count to the kernel for every curved face, and 32767
+    /// isolines on a cylinder is a ~100k-vertex preview rebuilt per cursor
+    /// event.
+    #[test]
+    fn header_isolines_are_clamped_before_display_tessellation() {
+        let measure = |value: i16| {
+            let mut scene = crate::scene::Scene::new();
+            scene.document.header.isolines = value;
+            let body = cylinder_solid([0.0; 3], 5.0, 12.0).unwrap();
+            scene
+                .prepare_solid_model_display(codec::Handle::new(0xDEAD), &body)
+                .expect("cylinder display")
+                .0
+                .edge_verts
+                .len()
+        };
+
+        // A hostile file-sourced header must render exactly like the
+        // largest value the Options slider can request — and a negative
+        // one exactly like zero.
+        let hostile = measure(i16::MAX);
+        assert_eq!(
+            hostile,
+            measure(crate::entities::solid3d::MAX_HEADER_ISOLINES),
+            "header.isolines must be clamped before tessellation"
+        );
+        assert_eq!(measure(-5), measure(0));
+        assert!(
+            hostile < 20_000,
+            "even the ceiling must stay a bounded preview: {hostile} vertices"
+        );
+    }
+
     #[test]
     fn box_face_centers_are_six() {
         let body = box_solid([0.0, 0.0, 0.0], 10.0, 10.0, 10.0).unwrap();