diff --git a/ROADMAP.md b/ROADMAP.md
index 709880c2..0ada4897 100644
--- a/ROADMAP.md
+++ b/ROADMAP.md
@@ -61,6 +61,8 @@ PCB / Package, Render, Animation, Simulation, Manufacture/CAM, Drawings.
 
 ## Dimensioning and error reporting
 
+- Inscribed, circumscribed and edge polygon tools preview their actual outline before
+  placement, including orientation and the selected side count.
 - Construction (X) preserves the active sketch drawing tool, picked anchors and inline
   dimension values, so line type can change while a shape is in progress.
 - Planar sketch construction previews and hover feedback keep the same dashed pattern as
diff --git a/crates/ui-egui/src/sketch_display_preview_tests.rs b/crates/ui-egui/src/sketch_display_preview_tests.rs
index fef371d4..0fc780c2 100644
--- a/crates/ui-egui/src/sketch_display_preview_tests.rs
+++ b/crates/ui-egui/src/sketch_display_preview_tests.rs
@@ -1,4 +1,4 @@
-use super::{Tool, preview_curve};
+use super::{Tool, polygon_outline, preview_curve};
 use crate::scenario::Harness;
 use crate::viewport::Proj;
 use serde_json::{Value, json};
@@ -120,3 +120,33 @@ fn pixel_preview_rejects_degenerate_and_unbounded_input_without_changing_documen
     let points = vec![[0., 0.]; 500];
     assert!(preview_curve(&tool("sketch.spline.fit_point", &points), Vec2::X, None).is_none());
 }
+
+#[test]
+fn polygon_preview_squares_have_the_expected_rotation_and_size() {
+    let cases = [
+        ("sketch.polygon.inscribed", [10.0, 0.0], [[10.0, 0.0], [0.0, 10.0], [-10.0, 0.0], [0.0, -10.0], [10.0, 0.0]]),
+        ("sketch.polygon.circumscribed", [10.0, 0.0], [[10.0, 10.0], [-10.0, 10.0], [-10.0, -10.0], [10.0, -10.0], [10.0, 10.0]]),
+        ("sketch.polygon.edge", [20.0, 0.0], [[0.0, 0.0], [20.0, 0.0], [20.0, 20.0], [0.0, 20.0], [0.0, 0.0]]),
+    ];
+    for (command, hover, expected) in cases {
+        let t = tool(command, &[[0.0, 0.0]]);
+        let outline = polygon_outline(&t, Vec2::new(hover[0], hover[1]), 4.0).unwrap();
+        assert_eq!(outline.len(), expected.len());
+        for (actual, expected) in outline.iter().zip(expected) {
+            assert!(actual.dist(Vec2::new(expected[0], expected[1])) < 1e-9, "{command}: {actual:?}");
+        }
+    }
+}
+
+#[test]
+fn polygon_preview_rejects_invalid_or_unbounded_input() {
+    let t = tool("sketch.polygon.inscribed", &[[0.0, 0.0]]);
+    for sides in [0.0, 2.0, 3.5, 201.0, f64::INFINITY, f64::NAN] {
+        assert!(polygon_outline(&t, Vec2::X, sides).is_none());
+    }
+    assert!(polygon_outline(&t, Vec2::ZERO, 6.0).is_none());
+    assert!(polygon_outline(&t, Vec2::new(f64::NAN, 0.0), 6.0).is_none());
+    assert_eq!(polygon_outline(&t, Vec2::X, 200.0).unwrap().len(), 201);
+    let t = tool("sketch.polygon.inscribed", &[[f64::INFINITY, 0.0]]);
+    assert!(polygon_outline(&t, Vec2::X, 6.0).is_none());
+}
diff --git a/crates/ui-egui/src/tools.rs b/crates/ui-egui/src/tools.rs
index f49be818..90fc63a2 100644
--- a/crates/ui-egui/src/tools.rs
+++ b/crates/ui-egui/src/tools.rs
@@ -580,6 +580,37 @@ pub(crate) fn preview_curve(t: &Tool, hover: Vec2, sweep: Option<f64>) -> Option
     Some(sk)
 }
 
+/// The polygon's actual straight edges, in the same orientation as the eventual command.
+/// Keep preview work bounded even while a side-count expression is being edited.
+pub(crate) fn polygon_outline(t: &Tool, hover: Vec2, sides: f64) -> Option<Vec<Vec2>> {
+    if !hover.is_finite() || !(3.0..=200.0).contains(&sides) || sides.fract() != 0.0 || t.pts.len() != 1 {
+        return None;
+    }
+    let a = t.pts.first()?.0;
+    let n = sides as usize;
+    let half = std::f64::consts::PI / sides;
+    let distance = a.dist(hover);
+    if !a.is_finite() || !distance.is_finite() || distance <= 1e-9 {
+        return None;
+    }
+    let (centre, radius, start) = match t.cmd {
+        "sketch.polygon.inscribed" => (a, distance, (hover - a).angle()),
+        "sketch.polygon.circumscribed" => (a, distance / half.cos(), (hover - a).angle() + half),
+        "sketch.polygon.edge" => {
+            let radius = distance / (2.0 * half.sin());
+            let centre = (a + hover) * 0.5 + (hover - a).normalized()?.perp() * (radius * half.cos());
+            (centre, radius, (a - centre).angle())
+        }
+        _ => return None,
+    };
+    let mut corners: Vec<_> = (0..n).map(|i| centre + Vec2::from_angle(start + std::f64::consts::TAU * i as f64 / sides) * radius).collect();
+    if corners.iter().any(|p| !p.is_finite()) {
+        return None;
+    }
+    corners.push(*corners.first()?);
+    Some(corners)
+}
+
 pub fn preview(app: &SolveApp, t: &Tool, painter: &egui::Painter, proj: &Proj) {
     if crate::sketch3d::preview(app, t, painter, proj) {
         return;
@@ -607,6 +638,15 @@ pub fn preview(app: &SolveApp, t: &Tool, painter: &egui::Painter, proj: &Proj) {
     }
     let Some((h, _)) = t.hover.clone() else { return };
     let h = crate::sketch_dims::effective(app, t, h);
+    if let Some(corners) = polygon_outline(t, h, crate::sketch_dims::locked_value(app, t, "Sides").unwrap_or(6.0)) {
+        let outline: Vec<_> = corners.iter().map(|c| ss.plane.to_world(*c)).collect();
+        paint_sketch_path(&outline, crate::sketch_palette::mode(app), painter, proj, stroke);
+        crate::scenario::publish_count("polygon_preview_edges", (corners.len() - 1) as f64);
+        if t.cmd == "sketch.polygon.edge" {
+            return;
+        }
+    }
+
     if let Some(curve) = preview_curve(t, h, crate::sketch_dims::locked_value(app, t, "Sweep")) {
         paint_preview_curve(curve, &ss.plane, painter, proj, stroke, crate::sketch_palette::mode(app));
         return;
diff --git a/crates/ui-egui/tests/scenarios/polygon_preview.json b/crates/ui-egui/tests/scenarios/polygon_preview.json
new file mode 100644
index 00000000..da1eb031
--- /dev/null
+++ b/crates/ui-egui/tests/scenarios/polygon_preview.json
@@ -0,0 +1,25 @@
+[
+ {"note":"#520: all polygon drawing variants show their real edge outline before committing, including a typed side count."},
+ {"call":"engine.execute","params":{"command":"sketch.create","params":{"plane":"XY"}}},
+ {"call":"ui.view","params":{"view":"top"}},
+ {"start":"sketch.polygon.inscribed"},
+ {"click":{"sketch":[0,0]}},
+ {"move":{"sketch":[20,8]}},
+ {"expect":{"count":{"name":"polygon_preview_edges","value":6},"errors":0}},
+ {"text":"10"},
+ {"key":"Tab"},
+ {"text":"4"},
+ {"move":{"sketch":[20,10]}},
+ {"expect":{"count":{"name":"polygon_preview_edges","value":4},"errors":0}},
+ {"key":"Escape"},
+ {"start":"sketch.polygon.circumscribed"},
+ {"click":{"sketch":[0,0]}},
+ {"move":{"sketch":[20,8]}},
+ {"expect":{"count":{"name":"polygon_preview_edges","value":6},"errors":0}},
+ {"key":"Escape"},
+ {"start":"sketch.polygon.edge"},
+ {"click":{"sketch":[0,0]}},
+ {"move":{"sketch":[20,8]}},
+ {"expect":{"count":{"name":"polygon_preview_edges","value":6},"errors":0}},
+ {"key":"Escape"}
+]
diff --git a/docs/book/src/guide/sketching.md b/docs/book/src/guide/sketching.md
index 63e96322..eedd669d 100644
--- a/docs/book/src/guide/sketching.md
+++ b/docs/book/src/guide/sketching.md
@@ -31,6 +31,10 @@ fit-point and control-point splines, conics, points and text. **Construction** (
 into reference geometry that doesn't form profiles. Modify tools include trim (T), extend, break,
 fillet, chamfer, offset (O), mirror, patterns, move/copy and scale.
 
+Polygon tools show their edge outline while positioning the second point, so the orientation
+can be checked before placement. Inscribed and circumscribed polygons retain their support
+circle and update the outline when the inline **Sides** value changes.
+
 **Text** is set in the built-in font (Liberation Sans) unless it is given another: `sketch.text` and
 `sketch.edit_text` take a `font`, the family or full name of an installed font ("DejaVu Sans") or the path
 of a `.ttf` / `.otf` file, and the **Edit Text** dialog has a **Font** field for it. An unknown font is an