diff --git a/crates/femm-doc/src/edit.rs b/crates/femm-doc/src/edit.rs
index 59174a8..a0c00dd 100644
--- a/crates/femm-doc/src/edit.rs
+++ b/crates/femm-doc/src/edit.rs
@@ -1,7 +1,15 @@
 //! geometry editing primitives on [`FemmDoc`]: add, delete, closest-point queries.
 
+use crate::geom_math::{
+    line_arc_intersection, line_line_intersection, shortest_distance_from_segment,
+};
 use crate::{ArcSegment, BlockLabel, FemmDoc, Node, Segment};
 
+/// fraction of the node-bbox diagonal used as auto-tolerance for intersection-node coalescing.
+const BBOX_TOLERANCE_FRAC: f64 = 1.0e-6;
+/// fraction of a segment's length within which an off-endpoint node triggers a recursive split.
+const ON_LINE_FRAC: f64 = 1.0e-5;
+
 impl FemmDoc {
     /// adds a node at (x, y), returning the index of an existing node within `tol` distance when present.
     pub fn add_node(&mut self, x: f64, y: f64, tol: f64) -> usize {
@@ -50,23 +58,103 @@ impl FemmDoc {
         self.block_labels.len() - 1
     }
 
-    /// adds a segment between two node indices, or returns the index of an existing duplicate.
-    pub fn add_segment(&mut self, n0: i32, n1: i32) -> Option<usize> {
-        if n0 == n1 { return None; }
+    /// adds a segment between two node indices, splitting at every crossing and through any on-line node.
+    pub fn add_segment(&mut self, n0: i32, n1: i32) -> bool {
+        self.add_segment_with_marker(n0, n1, "")
+    }
+
+    /// PSLG-aware variant propagating a boundary-marker name onto every resulting segment piece.
+    pub fn add_segment_with_marker(&mut self, n0: i32, n1: i32, marker: &str) -> bool {
+        if n0 == n1 { return false; }
+        let nn = self.nodes.len() as i32;
+        if n0 < 0 || n1 < 0 || n0 >= nn || n1 >= nn { return false; }
+
+        // duplicate either-orientation.
         let (a, b) = if n0 < n1 { (n0, n1) } else { (n1, n0) };
-        for (i, s) in self.segments.iter().enumerate() {
+        for s in &self.segments {
             let (sa, sb) = if s.n0 < s.n1 { (s.n0, s.n1) } else { (s.n1, s.n0) };
-            if sa == a && sb == b { return Some(i); }
+            if sa == a && sb == b { return false; }
         }
+
+        let n0p = (self.nodes[n0 as usize].x, self.nodes[n0 as usize].y);
+        let n1p = (self.nodes[n1 as usize].x, self.nodes[n1 as usize].y);
+
+        // crossings with existing segments and arcs.
+        let mut new_points: Vec<(f64, f64)> = Vec::new();
+        for s in &self.segments {
+            let sp0 = (self.nodes[s.n0 as usize].x, self.nodes[s.n0 as usize].y);
+            let sp1 = (self.nodes[s.n1 as usize].x, self.nodes[s.n1 as usize].y);
+            if let Some(hit) = line_line_intersection(n0p, n1p, sp0, sp1) {
+                new_points.push(hit);
+            }
+        }
+        for arc in &self.arcs {
+            let ap0 = (self.nodes[arc.n0 as usize].x, self.nodes[arc.n0 as usize].y);
+            let ap1 = (self.nodes[arc.n1 as usize].x, self.nodes[arc.n1 as usize].y);
+            for hit in line_arc_intersection(n0p, n1p, ap0, ap1, arc.arc_length) {
+                new_points.push(hit);
+            }
+        }
+
+        let tol = self.bbox_tolerance();
+        for (x, y) in new_points {
+            self.add_node(x, y, tol);
+        }
+
         self.segments.push(Segment {
             n0, n1,
             max_side_length: -1.0,
-            boundary_marker: String::new(),
+            boundary_marker: marker.to_string(),
             hidden: false,
             in_group: 0,
             selected: false,
         });
-        Some(self.segments.len() - 1)
+
+        // first non-endpoint node on the new segment's interior, if any.
+        let length = ((n1p.0 - n0p.0).powi(2) + (n1p.1 - n0p.1).powi(2)).sqrt();
+        let dmin = length * ON_LINE_FRAC;
+
+        let mut split_at: Option<i32> = None;
+        for (i, node) in self.nodes.iter().enumerate() {
+            let idx = i as i32;
+            if idx == n0 || idx == n1 { continue; }
+            let np = (node.x, node.y);
+            let de0 = ((np.0 - n0p.0).powi(2) + (np.1 - n0p.1).powi(2)).sqrt();
+            let de1 = ((np.0 - n1p.0).powi(2) + (np.1 - n1p.1).powi(2)).sqrt();
+            if de0 < dmin || de1 < dmin { continue; }
+            let d = shortest_distance_from_segment(np, n0p, n1p);
+            if d < dmin {
+                split_at = Some(idx);
+                break;
+            }
+        }
+
+        if let Some(mid) = split_at {
+            self.segments.pop();
+            let a = self.add_segment_with_marker(n0, mid, marker);
+            let b = self.add_segment_with_marker(mid, n1, marker);
+            a || b
+        } else {
+            true
+        }
+    }
+
+    /// auto-tolerance for intersection-node coalescing, derived from the node bounding box.
+    fn bbox_tolerance(&self) -> f64 {
+        if self.nodes.len() < 2 {
+            return 1.0e-8;
+        }
+        let mut xmin = f64::INFINITY;
+        let mut xmax = f64::NEG_INFINITY;
+        let mut ymin = f64::INFINITY;
+        let mut ymax = f64::NEG_INFINITY;
+        for n in &self.nodes {
+            xmin = xmin.min(n.x); xmax = xmax.max(n.x);
+            ymin = ymin.min(n.y); ymax = ymax.max(n.y);
+        }
+        let dx = xmax - xmin;
+        let dy = ymax - ymin;
+        (dx * dx + dy * dy).sqrt() * BBOX_TOLERANCE_FRAC
     }
 
     /// adds an arc segment between two node indices, sweeping `arc_length_deg` degrees.
@@ -210,3 +298,63 @@ fn point_to_segment_distance(px: f64, py: f64, ax: f64, ay: f64, bx: f64, by: f6
     let cy = ay + t * dy;
     (px - cx).hypot(py - cy)
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn doc_with_corners() -> FemmDoc {
+        let mut d = FemmDoc::default();
+        d.add_node(-1.0, 0.0, 0.0);
+        d.add_node( 1.0, 0.0, 0.0);
+        d.add_node( 0.0,-1.0, 0.0);
+        d.add_node( 0.0, 1.0, 0.0);
+        d
+    }
+
+    #[test]
+    fn duplicate_segment_rejected() {
+        let mut d = FemmDoc::default();
+        d.add_node(0.0, 0.0, 0.0);
+        d.add_node(1.0, 0.0, 0.0);
+        assert!(d.add_segment(0, 1));
+        assert!(!d.add_segment(0, 1));
+        assert!(!d.add_segment(1, 0));
+        assert_eq!(d.segments.len(), 1);
+    }
+
+    #[test]
+    fn segment_passing_through_existing_node_splits() {
+        // nodes at (-1, 0), (1, 0), and (0, 0); adding a segment between the outer two
+        // must split at the midpoint node, yielding two segments.
+        let mut d = FemmDoc::default();
+        d.add_node(-1.0, 0.0, 0.0);
+        d.add_node( 1.0, 0.0, 0.0);
+        d.add_node( 0.0, 0.0, 0.0);
+        assert!(d.add_segment(0, 1));
+        assert_eq!(d.segments.len(), 2);
+        let touches_mid = d.segments.iter().filter(|s| s.n0 == 2 || s.n1 == 2).count();
+        assert_eq!(touches_mid, 2);
+    }
+
+    #[test]
+    fn crossing_second_segment_splits_at_intersection() {
+        // horizontal (-1,0)->(1,0) added first stays whole.
+        // the vertical (0,-1)->(0,1) is added second; it intersects at origin,
+        // so a new node lands at (0,0) and the vertical splits into two pieces.
+        let mut d = doc_with_corners();
+        assert!(d.add_segment(0, 1));
+        assert!(d.add_segment(2, 3));
+
+        assert_eq!(d.nodes.len(), 5, "intersection node added");
+        let new_idx = 4;
+        assert!((d.nodes[new_idx].x).abs() < 1e-9);
+        assert!((d.nodes[new_idx].y).abs() < 1e-9);
+
+        // exactly one piece touches node 0 (-1,0) and node 4 (0,0): the original horizontal stays whole here.
+        // expected pieces: 0->1 (horizontal whole), 2->4, 4->3 (vertical halves).
+        assert_eq!(d.segments.len(), 3);
+        let touches_new = d.segments.iter().filter(|s| s.n0 == new_idx as i32 || s.n1 == new_idx as i32).count();
+        assert_eq!(touches_new, 2);
+    }
+}