diff --git a/docs/csg-performance-fixtures.md b/docs/csg-performance-fixtures.md
index e6c49df..5d201a4 100644
--- a/docs/csg-performance-fixtures.md
+++ b/docs/csg-performance-fixtures.md
@@ -115,6 +115,13 @@ Dense-kernel regression guardrail: `rotated_cut_stack_64` is pinned by tests at
 This protects against "optimizations" that alter ordered CSG semantics while
 looking faster. Output-contract changes need to be explicit.
 
+Realtime editing seam: `Assembler` can now mutate brush primitives and
+operations by `BrushId`, invalidating cached output and incrementing generation.
+`DirtyDemandFrontier` computes the conservative ordered suffix after the first
+dirty brush. The prefix before that index is the cacheable region; the suffix is
+the live region because ordered CSG decisions propagate forward. This is still a
+planning surface, not an incremental mesh rebuild yet.
+
 ## Latest Local Baseline
 
 Captured on 2026-05-07 with `.\tools\run_csg_perf.ps1 -UseRealtimeCsgCpp`:
diff --git a/src/assembler.rs b/src/assembler.rs
index 8d948c6..865e8df 100644
--- a/src/assembler.rs
+++ b/src/assembler.rs
@@ -3,8 +3,9 @@ use std::cell::RefCell;
 use bevy_math::Vec3;
 
 use crate::{
-    Aabb, Brush, BrushId, BrushOp, ConvexPolygon, ConvexSolid, MaterialId, PolygonRouteScratch,
-    Primitive, TriangleMesh, append_cylinder_z, append_dome_cap_z, append_floret_arm,
+    Aabb, Brush, BrushId, BrushOp, ConvexPolygon, ConvexSolid, DirtyDemandFrontier, MaterialId,
+    PolygonRouteScratch, Primitive, TriangleMesh, append_cylinder_z, append_dome_cap_z,
+    append_floret_arm,
     primitives::{DomeCapZSpec, FloretArmSpec},
 };
 
@@ -90,6 +91,28 @@ impl Assembler {
         self.generation
     }
 
+    pub fn brush_index(&self, id: BrushId) -> Option<usize> {
+        self.brushes.iter().position(|brush| brush.id == id)
+    }
+
+    pub fn dirty_frontier_for_indices(
+        &self,
+        dirty_indices: impl IntoIterator<Item = usize>,
+    ) -> DirtyDemandFrontier {
+        DirtyDemandFrontier::from_dirty_indices(&self.brushes, dirty_indices)
+    }
+
+    pub fn dirty_frontier_for_brushes(
+        &self,
+        dirty_brushes: impl IntoIterator<Item = BrushId>,
+    ) -> DirtyDemandFrontier {
+        let indices = dirty_brushes
+            .into_iter()
+            .filter_map(|id| self.brush_index(id))
+            .collect::<Vec<_>>();
+        self.dirty_frontier_for_indices(indices)
+    }
+
     pub fn add_brush(
         &mut self,
         name: impl Into<String>,
@@ -107,6 +130,30 @@ impl Assembler {
         id
     }
 
+    pub fn set_brush_primitive(&mut self, id: BrushId, primitive: Primitive) -> bool {
+        let Some(index) = self.brush_index(id) else {
+            return false;
+        };
+        self.brushes[index].primitive = primitive;
+        self.brushes[index].set_dirty();
+        self.compiled[index] = CompiledBrush::from_brush(&self.brushes[index]);
+        self.generation = self.generation.wrapping_add(1);
+        self.cache.borrow_mut().take();
+        true
+    }
+
+    pub fn set_brush_op(&mut self, id: BrushId, op: BrushOp) -> bool {
+        let Some(index) = self.brush_index(id) else {
+            return false;
+        };
+        self.brushes[index].op = op;
+        self.brushes[index].set_dirty();
+        self.compiled[index] = CompiledBrush::from_brush(&self.brushes[index]);
+        self.generation = self.generation.wrapping_add(1);
+        self.cache.borrow_mut().take();
+        true
+    }
+
     pub fn solid_box(
         &mut self,
         name: impl Into<String>,
@@ -909,6 +956,59 @@ mod tests {
         assert_eq!(routed.mesh.indices, stable.mesh.indices);
     }
 
+    #[test]
+    fn mutating_brush_primitive_invalidates_cache_and_exposes_dirty_frontier() {
+        let mut asm = Assembler::new();
+        let source = asm.solid_box(
+            "source",
+            Aabb::from_center_size(Vec3::ZERO, Vec3::splat(4.0)),
+            MaterialId(1),
+        );
+        let cutter = asm.cut_box("void", Aabb::from_center_size(Vec3::ZERO, Vec3::splat(1.0)));
+        let before = asm.build();
+
+        assert!(asm.set_brush_primitive(
+            cutter,
+            Primitive::Box {
+                bounds: Aabb::from_center_size(Vec3::ZERO, Vec3::splat(2.0)),
+            },
+        ));
+        let frontier = asm.dirty_frontier_for_brushes([cutter]);
+        let after = asm.build();
+
+        assert_eq!(frontier.first_dirty_index, Some(1));
+        assert_eq!(frontier.operator_brushes, 1);
+        assert_eq!(frontier.candidate_pairs(), 1);
+        assert_eq!(frontier.rejected_pairs, 0);
+        assert!(after.generation > before.generation);
+        assert_ne!(after.mesh.positions, before.mesh.positions);
+        assert_eq!(asm.brush_index(source), Some(0));
+        assert_eq!(asm.brush_index(cutter), Some(1));
+    }
+
+    #[test]
+    fn mutating_brush_operation_changes_ordered_output() {
+        let mut asm = Assembler::new();
+        let _source = asm.solid_box(
+            "source",
+            Aabb::from_center_size(Vec3::ZERO, Vec3::splat(4.0)),
+            MaterialId(1),
+        );
+        let other = asm.solid_box(
+            "other",
+            Aabb::from_center_size(Vec3::new(0.5, 0.0, 0.0), Vec3::splat(3.0)),
+            MaterialId(1),
+        );
+        let before = asm.rebuild();
+
+        assert!(asm.set_brush_op(other, BrushOp::Intersect));
+        let after = asm.rebuild();
+
+        assert_eq!(after.report.operator_brushes, 1);
+        assert_eq!(after.report.candidate_pairs, 1);
+        assert!(after.mesh.triangle_count() < before.mesh.triangle_count());
+    }
+
     #[test]
     fn rotated_cut_stack_regression_preserves_ordered_output_shape() {
         let mut asm = Assembler::new();
diff --git a/src/frontier.rs b/src/frontier.rs
index 09d51f1..aff6e18 100644
--- a/src/frontier.rs
+++ b/src/frontier.rs
@@ -13,6 +13,15 @@ pub struct DemandFrontier {
     pub operator_brushes: usize,
 }
 
+#[derive(Clone, Debug, Default, PartialEq, Eq)]
+pub struct DirtyDemandFrontier {
+    pub first_dirty_index: Option<usize>,
+    pub pairs: Vec<DemandPair>,
+    pub rejected_pairs: usize,
+    pub operator_brushes: usize,
+    pub skipped_prefix_pairs: usize,
+}
+
 impl DemandFrontier {
     pub fn from_ordered_brushes(brushes: &[Brush]) -> Self {
         let mut frontier = Self::default();
@@ -65,6 +74,89 @@ impl DemandFrontier {
     }
 }
 
+impl DirtyDemandFrontier {
+    pub fn from_dirty_indices(
+        brushes: &[Brush],
+        dirty_indices: impl IntoIterator<Item = usize>,
+    ) -> Self {
+        let first_dirty_index = dirty_indices
+            .into_iter()
+            .filter(|index| *index < brushes.len())
+            .min();
+        let Some(first_dirty_index) = first_dirty_index else {
+            return Self::default();
+        };
+
+        let mut frontier = Self {
+            first_dirty_index: Some(first_dirty_index),
+            ..Self::default()
+        };
+        let mut active_sources = Vec::<usize>::new();
+
+        for (operator_index, brush) in brushes.iter().enumerate() {
+            match brush.op {
+                BrushOp::Add => active_sources.push(operator_index),
+                BrushOp::Subtract => {
+                    frontier.operator_brushes += usize::from(operator_index >= first_dirty_index);
+                    frontier.collect_pairs(
+                        brushes,
+                        &active_sources,
+                        operator_index,
+                        first_dirty_index,
+                    );
+                }
+                BrushOp::Intersect => {
+                    frontier.operator_brushes += usize::from(operator_index >= first_dirty_index);
+                    let retained = frontier.collect_pairs(
+                        brushes,
+                        &active_sources,
+                        operator_index,
+                        first_dirty_index,
+                    );
+                    active_sources = retained;
+                }
+            }
+        }
+
+        frontier
+    }
+
+    pub fn candidate_pairs(&self) -> usize {
+        self.pairs.len()
+    }
+
+    fn collect_pairs(
+        &mut self,
+        brushes: &[Brush],
+        active_sources: &[usize],
+        operator_index: usize,
+        first_dirty_index: usize,
+    ) -> Vec<usize> {
+        let operator_bounds = brushes[operator_index].bounds();
+        let mut retained = Vec::new();
+
+        for &source_index in active_sources {
+            if brushes[source_index].bounds().intersects(operator_bounds) {
+                if operator_index >= first_dirty_index {
+                    self.pairs.push(DemandPair {
+                        source_index,
+                        operator_index,
+                    });
+                } else {
+                    self.skipped_prefix_pairs += 1;
+                }
+                retained.push(source_index);
+            } else if operator_index >= first_dirty_index {
+                self.rejected_pairs += 1;
+            } else {
+                self.skipped_prefix_pairs += 1;
+            }
+        }
+
+        retained
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use bevy_math::{Quat, Vec3};
@@ -164,4 +256,65 @@ mod tests {
             ]
         );
     }
+
+    #[test]
+    fn dirty_frontier_preserves_prefix_before_first_dirty_brush() {
+        let brushes = vec![
+            brush(
+                "source",
+                BrushOp::Add,
+                Aabb::from_center_size(Vec3::ZERO, Vec3::splat(8.0)),
+            ),
+            brush(
+                "early",
+                BrushOp::Subtract,
+                Aabb::from_center_size(Vec3::new(-2.0, 0.0, 0.0), Vec3::splat(2.0)),
+            ),
+            brush(
+                "dirty",
+                BrushOp::Subtract,
+                Aabb::from_center_size(Vec3::ZERO, Vec3::splat(2.0)),
+            ),
+            brush(
+                "later",
+                BrushOp::Subtract,
+                Aabb::from_center_size(Vec3::new(2.0, 0.0, 0.0), Vec3::splat(2.0)),
+            ),
+        ];
+
+        let frontier = DirtyDemandFrontier::from_dirty_indices(&brushes, [2]);
+
+        assert_eq!(frontier.first_dirty_index, Some(2));
+        assert_eq!(frontier.operator_brushes, 2);
+        assert_eq!(frontier.skipped_prefix_pairs, 1);
+        assert_eq!(frontier.candidate_pairs(), 2);
+        assert_eq!(
+            frontier.pairs,
+            vec![
+                DemandPair {
+                    source_index: 0,
+                    operator_index: 2,
+                },
+                DemandPair {
+                    source_index: 0,
+                    operator_index: 3,
+                },
+            ]
+        );
+    }
+
+    #[test]
+    fn dirty_frontier_ignores_out_of_range_dirty_indices() {
+        let brushes = vec![brush(
+            "source",
+            BrushOp::Add,
+            Aabb::from_center_size(Vec3::ZERO, Vec3::splat(2.0)),
+        )];
+
+        let frontier = DirtyDemandFrontier::from_dirty_indices(&brushes, [99]);
+
+        assert_eq!(frontier.first_dirty_index, None);
+        assert_eq!(frontier.candidate_pairs(), 0);
+        assert_eq!(frontier.operator_brushes, 0);
+    }
 }
diff --git a/src/lib.rs b/src/lib.rs
index 1eef8e7..82fae68 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -19,7 +19,7 @@ pub use assembler::{Assembler, BuildOutput, BuildReport, BuildWarning};
 pub use brush::{Aabb, Brush, BrushId, BrushOp, MaterialId, PolygonCategory, Primitive};
 pub use convex::{CategorizedPolygons, ConvexPolygon, ConvexSolid, Plane, PolygonRouteScratch};
 pub use dsl::LevelDsl;
-pub use frontier::{DemandFrontier, DemandPair};
+pub use frontier::{DemandFrontier, DemandPair, DirtyDemandFrontier};
 pub use mesh::TriangleMesh;
 pub use primitives::{
     DomeCapZSpec, FloretArmSpec, append_cylinder_z, append_dome_cap_z, append_floret_arm,