diff --git a/crates/ui-egui/src/lib.rs b/crates/ui-egui/src/lib.rs
index aa759b47..4293e258 100644
--- a/crates/ui-egui/src/lib.rs
+++ b/crates/ui-egui/src/lib.rs
@@ -531,6 +531,27 @@ impl SolveApp {
             self.last_command = Some((spec.id.to_string(), spec.label.to_string()));
         }
         if let Some(mut t) = tools::Tool::for_command(id) {
+            // Curve-pair constraints can consume an exact preselection. Leave ambiguous
+            // selections to the ordinary pick tool; never silently constrain a subset.
+            if matches!(
+                id,
+                "sketch.constraint.parallel"
+                    | "sketch.constraint.perpendicular"
+                    | "sketch.constraint.equal"
+                    | "sketch.constraint.tangent"
+                    | "sketch.constraint.concentric"
+                    | "sketch.constraint.collinear"
+            ) && let [solvecraft_engine::Sel::SketchCurve { id: a }, solvecraft_engine::Sel::SketchCurve { id: b }] =
+                self.session.selection.as_slice()
+                && a != b
+            {
+                let params = json!({ "a": a, "b": b });
+                if self.run(id, params).is_ok() {
+                    let _ = self.session.execute("select.clear", &json!({}));
+                    self.set_status("Constraint applied", false);
+                }
+                return;
+            }
             if id == "sketch.dimension" {
                 t.picks = self
                     .session
diff --git a/crates/ui-egui/tests/constraint_preselection.rs b/crates/ui-egui/tests/constraint_preselection.rs
new file mode 100644
index 00000000..6787aee7
--- /dev/null
+++ b/crates/ui-egui/tests/constraint_preselection.rs
@@ -0,0 +1,148 @@
+//! Constraint tools accept a selected curve pair as well as tool-first clicks.
+
+use serde_json::json;
+use solvecraft_engine::{Sel, sketch::ConstraintKind};
+use solvecraft_ui_egui::scenario::Harness;
+
+fn lines() -> Harness {
+    let mut h = Harness::new();
+    h.run(&json!([
+        {"call":"engine.execute","params":{"command":"sketch.create","params":{"plane":"XY"}}},
+        {"call":"engine.execute","params":{"command":"sketch.line","params":{"points":[[10,10],[40,10]],"infer":false}}},
+        {"call":"engine.execute","params":{"command":"sketch.line","params":{"points":[[10,30],[40,35]],"infer":false}}},
+        {"call":"ui.view","params":{"view":"top"}}
+    ]))
+    .unwrap();
+    h
+}
+
+fn select_lines(h: &mut Harness) {
+    h.run(&json!([
+        {"click":{"sketch":[25,10]}},
+        {"click":{"sketch":[25,32.5]},"shift":true}
+    ]))
+    .unwrap();
+    assert_eq!(h.app.session.selection, vec![Sel::SketchCurve { id: "l1".into() }, Sel::SketchCurve { id: "l2".into() }]);
+}
+
+fn assert_line_geometry(h: &Harness, command: &str) {
+    let sk = &h.app.session.model.state().sketches[0].sketch;
+    let point = |name| sk.point(sk.resolve_point(name).unwrap()).unwrap();
+    let a = point("l1.start");
+    let b = point("l2.start");
+    let u = point("l1.end") - a;
+    let v = point("l2.end") - b;
+    // Normalization also rejects a collapsed line; zero-length vectors must not
+    // accidentally satisfy the parallel/perpendicular checks.
+    let du = u.normalized().unwrap();
+    let dv = v.normalized().unwrap();
+    match command {
+        "parallel" => assert!(du.cross(dv).abs() < 1e-6),
+        "perpendicular" => assert!(du.dot(dv).abs() < 1e-6),
+        "equal" => assert!((u.len() - v.len()).abs() < 1e-6),
+        "collinear" => {
+            assert!(du.cross(dv).abs() < 1e-6);
+            assert!((b - a).cross(du).abs() < 1e-6);
+        }
+        _ => panic!("unexpected line constraint: {command}"),
+    }
+}
+
+#[test]
+fn selected_lines_apply_constraints_with_one_undo_step() {
+    for command in ["parallel", "perpendicular", "equal", "collinear"] {
+        let mut h = lines();
+        select_lines(&mut h);
+        let before = h.app.session.doc.clone();
+        let undo = h.app.session.undo.len();
+        h.run(&json!([{"start":format!("sketch.constraint.{command}")}])).unwrap();
+        assert!(!h.app.status.as_ref().is_some_and(|(_, _, error)| *error), "{command}");
+        let sk = &h.app.session.model.state().sketches[0].sketch;
+        assert!(
+            sk.constraints.iter().any(|c| match command {
+                "parallel" => matches!(c.kind, ConstraintKind::Parallel { .. }),
+                "perpendicular" => matches!(c.kind, ConstraintKind::Perpendicular { .. }),
+                "equal" => matches!(c.kind, ConstraintKind::Equal { .. }),
+                _ => matches!(c.kind, ConstraintKind::Collinear { .. }),
+            }),
+            "{command}"
+        );
+        assert_line_geometry(&h, command);
+        let after = h.app.session.doc.clone();
+        assert_eq!(h.app.session.undo.len(), undo + 1);
+        assert!(h.app.session.selection.is_empty());
+        assert!(h.app.tool.is_none());
+        h.app.session.execute("edit.undo", &json!({})).unwrap();
+        assert_eq!(serde_json::to_value(&h.app.session.doc).unwrap(), serde_json::to_value(before).unwrap());
+        h.app.session.execute("edit.redo", &json!({})).unwrap();
+        assert_eq!(serde_json::to_value(&h.app.session.doc).unwrap(), serde_json::to_value(after).unwrap());
+        assert_line_geometry(&h, command);
+    }
+}
+
+#[test]
+fn selected_circles_apply_constraints() {
+    for command in ["tangent", "concentric", "equal"] {
+        let mut h = Harness::new();
+        h.run(&json!([
+            {"call":"engine.execute","params":{"command":"sketch.create","params":{"plane":"XY"}}},
+            {"call":"engine.execute","params":{"command":"sketch.circle.center","params":{"center":[10,10],"radius":5}}},
+            {"call":"engine.execute","params":{"command":"sketch.circle.center","params":{"center":[25,12],"radius":7}}},
+            {"call":"ui.view","params":{"view":"top"}},
+            {"click":{"sketch":[10,15]}},
+            {"click":{"sketch":[25,19]},"shift":true}
+        ]))
+        .unwrap();
+        assert_eq!(h.app.session.selection.len(), 2);
+        let undo = h.app.session.undo.len();
+        h.run(&json!([{"start":format!("sketch.constraint.{command}")}])).unwrap();
+        assert!(!h.app.status.as_ref().is_some_and(|(_, _, error)| *error), "{command}");
+        assert_eq!(h.app.session.undo.len(), undo + 1, "{command}");
+        let sk = &h.app.session.model.state().sketches[0].sketch;
+        assert!(sk.constraints.iter().any(|c| match command {
+            "tangent" => matches!(c.kind, ConstraintKind::Tangent { .. }),
+            "concentric" => matches!(c.kind, ConstraintKind::Concentric { .. }),
+            _ => matches!(c.kind, ConstraintKind::Equal { .. }),
+        }));
+        assert!(h.app.session.selection.is_empty());
+    }
+}
+
+#[test]
+fn ambiguous_selection_preserves_tool_first_workflow() {
+    for selection in [
+        vec![],
+        vec![Sel::SketchCurve { id: "l1".into() }],
+        vec![Sel::SketchCurve { id: "l1".into() }, Sel::SketchCurve { id: "l2".into() }, Sel::SketchPoint { id: "origin".into() }],
+    ] {
+        let mut h = lines();
+        h.app.session.selection = selection;
+        let undo = h.app.session.undo.len();
+        h.run(&json!([{"start":"sketch.constraint.parallel"}])).unwrap();
+        assert_eq!(h.app.session.undo.len(), undo);
+        assert!(h.app.tool.as_ref().unwrap().picks.is_empty());
+        h.run(&json!([{"click":{"sketch":[25,10]}}, {"click":{"sketch":[25,32.5]}}])).unwrap();
+        assert_eq!(h.app.session.undo.len(), undo + 1);
+        assert!(!h.app.status.as_ref().is_some_and(|(_, _, error)| *error));
+    }
+}
+
+#[test]
+fn rejected_preselection_preserves_document_and_error() {
+    let mut h = lines();
+    h.app.session.execute("sketch.constraint.fix", &json!({"entities":["l1","l2"]})).unwrap();
+    select_lines(&mut h);
+    let before = serde_json::to_value(&h.app.session.doc).unwrap();
+    let undo = h.app.session.undo.len();
+    let selection = h.app.session.selection.clone();
+    h.run(&json!([{"start":"sketch.constraint.parallel"}])).unwrap();
+    assert!(h.app.status.as_ref().is_some_and(|(_, _, error)| *error));
+    assert_eq!(serde_json::to_value(&h.app.session.doc).unwrap(), before);
+    assert_eq!(h.app.session.undo.len(), undo);
+    assert_eq!(h.app.session.selection, selection);
+    // Correct the conflict and retry without reselecting; the old error must disappear.
+    h.app.session.execute("sketch.constraint.fix", &json!({"entities":["l1","l2"]})).unwrap();
+    h.run(&json!([{"start":"sketch.constraint.parallel"}])).unwrap();
+    assert!(!h.app.status.as_ref().is_some_and(|(_, _, error)| *error));
+    assert!(h.app.session.selection.is_empty());
+}
diff --git a/docs/book/src/guide/sketching.md b/docs/book/src/guide/sketching.md
index de24f0d4..7bb00f1b 100644
--- a/docs/book/src/guide/sketching.md
+++ b/docs/book/src/guide/sketching.md
@@ -60,6 +60,11 @@ midpoint, symmetry, fix, curvature (G2) and polygon. **Sketch Dimension** (D) ad
 aligned, horizontal, vertical, radius, diameter and angle dimensions. Each is a parameter
 (`d1`, `d2`, …), so it can hold an expression.
 
+For Parallel, Perpendicular, Equal, Tangent, Concentric and Collinear, select exactly two
+sketch curves (Shift-click to add the second) and choose the constraint to apply it immediately.
+You can also start the constraint first and then pick its curves. An incomplete or mixed
+selection starts that pick workflow; a rejected constraint keeps the selection for correction.
+
 Under-constrained geometry is blue and fully constrained geometry turns dark, and the status
 shows the remaining degrees of freedom. A constraint or dimension that would over-constrain the
 sketch is refused rather than applied. Drag a point to move it; the solver keeps every