diff --git a/crates/engine/src/cmd/constraints.rs b/crates/engine/src/cmd/constraints.rs
index 368c778c..1873818c 100644
--- a/crates/engine/src/cmd/constraints.rs
+++ b/crates/engine/src/cmd/constraints.rs
@@ -806,7 +806,20 @@ fn run_delconstraint(s: &mut Session, p: &Value) -> Result<Value> {
     };
     let n = s.doc()?.constraints.iter().filter(|c| doomed(c)).count();
     if n > 0 {
-        s.doc_mut()?.constraints.retain(|c| !doomed(c));
+        let mut d = s.doc()?.clone();
+        let broken_before: Vec<String> = cs::parameter_table(&d).into_iter().filter(|r| r.error.is_some()).map(|r| r.name).collect();
+        let removed: Vec<String> = d.constraints.iter().filter(|c| doomed(c)).map(|c| c.name.clone()).collect();
+        d.constraints.retain(|c| !doomed(c));
+        // Reject when a surviving expression would lose a dimension it refers to (same guard as `parameters delete`).
+        if let Some(row) = cs::parameter_table(&d).into_iter().find(|r| r.error.is_some() && !broken_before.contains(&r.name)) {
+            let mut refs = Vec::new();
+            if let Ok(e) = cs::expr::parse(&row.expr) {
+                cs::expr::names(&e, &mut refs);
+            }
+            let used = removed.iter().find(|n| refs.contains(*n)).or(removed.first()).cloned().unwrap_or_default();
+            return Err(EngineError::Other(format!("`{used}` is used by `{}`", row.name)));
+        }
+        *s.doc_mut()? = d;
     }
     Ok(json!({ "removed": n, "message": format!("{n} constraint(s) removed") }))
 }
diff --git a/crates/engine/src/cmd/constraints_tests.rs b/crates/engine/src/cmd/constraints_tests.rs
index 46e418d3..a07deb54 100644
--- a/crates/engine/src/cmd/constraints_tests.rs
+++ b/crates/engine/src/cmd/constraints_tests.rs
@@ -190,3 +190,23 @@ fn hostile_constraint_params_never_panic() {
         assert!(s.running.is_none());
     }
 }
+
+#[test]
+fn delconstraint_rejects_referenced_dimension() {
+    let mut s = Session::new();
+    let a = line(&mut s, [0.0, 0.0], [10.0, 0.0]);
+    let b = line(&mut s, [0.0, 5.0], [4.0, 5.0]);
+    s.execute("dimconstraint", &json!({ "type": "horizontal", "h1": a, "expr": "8" })).unwrap();
+    s.execute("dimconstraint", &json!({ "type": "horizontal", "h1": b, "expr": "d1/2" })).unwrap();
+    let before = s.doc().unwrap().clone();
+    let first = s.doc().unwrap().constraints[0].id;
+    let e = s.execute("delconstraint", &json!({ "ids": [first] })).unwrap_err().to_string();
+    assert!(e.contains("`d1` is used by `d2`"), "{e}");
+    assert_eq!(*s.doc().unwrap(), before);
+    assert!(s.execute("delconstraint", &json!({ "handles": [a] })).is_err());
+    assert_eq!(*s.doc().unwrap(), before);
+    // Selecting both dimensions removes both; the dependent one may go with its source.
+    let ids: Vec<u32> = s.doc().unwrap().constraints.iter().map(|c| c.id).collect();
+    s.execute("delconstraint", &json!({ "ids": ids })).unwrap();
+    assert!(s.doc().unwrap().constraints.is_empty());
+}