diff --git a/crates/engine/src/tests.rs b/crates/engine/src/tests.rs
index e72c3a8c..e93f5eab 100644
--- a/crates/engine/src/tests.rs
+++ b/crates/engine/src/tests.rs
@@ -209,6 +209,42 @@ fn fillet_the_edges_between_two_overlapping_cylinders() {
     assert!(rel(v, expect) < 2e-4, "{v} vs {expect}");
 }
 
+/// The top outline of two overlapping cylinders whose concave edges were rounded: the walls of
+/// the rounds meet the cylinders along the rolling-ball contact lines, straight but not lines.
+#[test]
+fn fillet_and_chamfer_the_top_loop_of_rounded_overlapping_cylinders() {
+    use std::f64::consts::{PI, TAU};
+    let (big, round, h) = (25.0_f64, 5.0_f64, 10.0);
+    let mut s = Session::default();
+    run(&mut s, "sketch.create", json!({"plane": "XY"}));
+    run(&mut s, "sketch.circle.center", json!({"center": [0, 0], "radius": big}));
+    run(&mut s, "sketch.circle.center", json!({"center": [-big, 0], "radius": big}));
+    run(&mut s, "sketch.finish", json!({}));
+    run(&mut s, "solid.extrude", json!({"distance": h, "profiles": [0, 1, 2]}));
+    // Plan view: ball centres at x = −12.5, |y| = yb; each round's arc turns by th, each big arc by
+    // TAU − 2·a1, and the loop turns by TAU in all.
+    let half = big / 2.0;
+    let yb = ((big + round).powi(2) - half * half).sqrt();
+    run(&mut s, "solid.fillet", json!({"edges": [[-half, half * 3f64.sqrt(), 5], [-half, -half * 3f64.sqrt(), 5]], "radius": round}));
+    let (a1, th) = (yb.atan2(half), 2.0 * (half / (big + round)).asin());
+    let base = volume(&mut s);
+    let top = [[big, 0.0, h], [-2.0 * big, 0.0, h], [-half, yb - round, h], [-half, round - yb, h]];
+    // Removed volume by Pappus: the corner's section swept along the loop at its centroid's offset.
+    let removed = |area: f64, xb: f64| area * (2.0 * (TAU - 2.0 * a1) * (big - xb) + 2.0 * th * (round + xb));
+    run(&mut s, "solid.fillet", json!({"edges": top, "radius": 2}));
+    let r: f64 = 2.0;
+    let expect = base - removed(r * r * (1.0 - PI / 4.0), (10.0 - 3.0 * PI) / (3.0 * (4.0 - PI)) * r);
+    let v = volume(&mut s);
+    assert!((v - expect).abs() < 1.5, "{v} vs {expect}");
+    let m = run(&mut s, "inspect.measure", json!({}));
+    assert_eq!(m["bodies"][0]["kernel"]["faces"].as_u64(), Some(10), "{m}");
+    run(&mut s, "edit.undo", json!({}));
+    run(&mut s, "solid.chamfer", json!({"edges": top, "distance": 2}));
+    let expect = base - removed(2.0, 2.0 / 3.0);
+    let v = volume(&mut s);
+    assert!((v - expect).abs() < 1.5, "{v} vs {expect}");
+}
+
 /// Sketch a 40 x 30 rectangle with dimensions, extrude 20, fillet a vertical edge, cut a hole.
 #[test]
 fn box_fillet_cut_part() {
diff --git a/crates/kernel/src/loopblend.rs b/crates/kernel/src/loopblend.rs
index 31c6ce00..e3653a47 100644
--- a/crates/kernel/src/loopblend.rs
+++ b/crates/kernel/src/loopblend.rs
@@ -97,6 +97,22 @@ fn seg_of(e: &mt::Edge, tol: f64) -> Option<Seg> {
     Some(Seg::Arc { a, c: centre, axis, angle, mid: m })
 }
 
+/// Whether an edge is a straight segment: a line, or a curve whose points all lie on the chord
+/// (the contact lines of a rolling-ball blend between parallel walls are such curves).
+fn is_straight(e: &mt::Edge, tol: f64) -> bool {
+    use mt::{BoundedCurve, ParametricCurve};
+    if matches!(e.curve(), mt::Curve::Line(_)) {
+        return true;
+    }
+    let (a, b) = (from_p3(e.absolute_front().point()), from_p3(e.absolute_back().point()));
+    if a.dist(b) < tol {
+        return false;
+    }
+    let c = e.curve();
+    let (t0, t1) = c.range_tuple();
+    (0..=16).all(|k| from_p3(c.subs(t0 + (t1 - t0) * k as f64 / 16.0)).dist_to_segment(a, b) < tol)
+}
+
 /// `p` turned about the line (c, axis) by `ang`.
 fn turn(p: Vec3, c: Vec3, axis: Vec3, ang: f64) -> Vec3 {
     let k = axis.normalized().unwrap_or(Vec3::Z);
@@ -518,7 +534,7 @@ fn build(
                 }
                 let (fa, fb) = (e.absolute_front().clone(), e.absolute_back().clone());
                 let other = if &fa == v { &fb } else { &fa };
-                if !matches!(e.curve(), mt::Curve::Line(_)) {
+                if !is_straight(&e, (size * 1e-5).max(1e-6)) {
                     return Err(unsupported("the wall seams must be straight"));
                 }
                 let face_side = on_face(&e);
diff --git a/crates/ui-egui/src/canvas.rs b/crates/ui-egui/src/canvas.rs
index 8a72aba9..a0b2557d 100644
--- a/crates/ui-egui/src/canvas.rs
+++ b/crates/ui-egui/src/canvas.rs
@@ -69,9 +69,9 @@ pub fn anchor(app: &SolveApp, d: &Dialog) -> Option<(Vec3, Option<Vec3>)> {
             let offset = if start.trim().is_empty() { 0.0 } else { app.session.doc.eval(start, ValueKind::Length).ok()? };
             (at + n * (from + offset), Some(n))
         }
-        Kind::Fillet { .. } | Kind::Section { .. } | Kind::Hole { .. } | Kind::OffsetPlane { .. } | Kind::OffsetFaces { .. } => (at, normal),
-        // Shell thickness grows into the body.
-        Kind::Shell { .. } => (at, normal.map(|n| -n)),
+        Kind::Section { .. } | Kind::Hole { .. } | Kind::OffsetPlane { .. } | Kind::OffsetFaces { .. } => (at, normal),
+        // Fillet radii, chamfer distances and shell thickness grow into the body.
+        Kind::Fillet { .. } | Kind::Shell { .. } => (at, normal.map(|n| -n)),
         Kind::Sheet(_) => (at, crate::dialogs_sheet::arrow(app, d).or(normal)),
         Kind::Plastic(_) => (at, normal),
         Kind::PatternRect { .. } => (at, d.inputs.get(1).and_then(|i| i.items.first()).and_then(|x| crate::dialogs::axis_of(app, x)).map(|a| a.1)),
@@ -98,8 +98,8 @@ fn reference_height(app: &SolveApp, d: &Dialog, label: &str, plane: &solvecraft_
     Some(solvecraft_engine::doc::object_height(&app.session.world_state(), plane, centre, p))
 }
 
-/// The outward direction between the two faces at an edge (where a fillet's radius arrow
-/// points).
+/// The outward direction between the two faces at an edge (a fillet's radius arrow points the
+/// other way, into the body).
 fn edge_bisector(m: &Mesh, index: usize, p: Vec3) -> Option<Vec3> {
     let faces = m.edge_faces.get(index)?;
     let mut sum = Vec3::ZERO;
diff --git a/crates/ui-egui/src/preselect_tests.rs b/crates/ui-egui/src/preselect_tests.rs
index b0343591..4553fa61 100644
--- a/crates/ui-egui/src/preselect_tests.rs
+++ b/crates/ui-egui/src/preselect_tests.rs
@@ -470,6 +470,19 @@ fn a_hole_is_mirrored_as_a_feature() {
     assert_eq!(cmds[0].1["features"], json!(["Hole1"]));
 }
 
+/// The radius arrow points into the body: dragging toward the material grows the round.
+#[test]
+fn fillet_and_chamfer_arrows_point_into_the_body() {
+    for id in ["solid.fillet", "solid.chamfer"] {
+        let mut app = boxed();
+        let e = edge(&app, Vec3::new(20.0, 0.0, 20.0));
+        let d = start(&mut app, vec![e], id);
+        let (_, dir) = crate::canvas::anchor(&app, &d).unwrap();
+        let dir = dir.unwrap();
+        assert!(dir.y > 0.5 && dir.z < -0.5, "{id}: {dir:?}");
+    }
+}
+
 #[test]
 fn chamfer_types_send_their_values() {
     let mut app = boxed();