diff --git a/render-wasm/src/state/ui.rs b/render-wasm/src/state/ui.rs
index d9226f7ab0..ee1a129ca1 100644
--- a/render-wasm/src/state/ui.rs
+++ b/render-wasm/src/state/ui.rs
@@ -22,12 +22,57 @@ impl GuidePool {
             }
         }
 
-        // TODO: handle the unwraps here
         self.horizontal
             .sort_by(|a, b| a.position().total_cmp(&b.position()));
         self.vertical
             .sort_by(|a, b| a.position().total_cmp(&b.position()));
     }
+
+    pub fn find_at(&self, x: f32, y: f32, zoom: f32, tolerance: f32) -> Option<&Guide> {
+        if zoom <= 0.0 || tolerance < 0.0 {
+            return None;
+        }
+
+        let world_tolerance = tolerance / zoom;
+        let vertical = Self::find_closest_in_axis(&self.vertical, x, world_tolerance);
+        let horizontal = Self::find_closest_in_axis(&self.horizontal, y, world_tolerance);
+
+        match (vertical, horizontal) {
+            (Some(v), Some(h)) => {
+                let v_dist = (v.position() - x).abs();
+                let h_dist = (h.position() - y).abs();
+                if v_dist <= h_dist {
+                    Some(v)
+                } else {
+                    Some(h)
+                }
+            }
+            (v, h) => v.or(h),
+        }
+    }
+
+    fn find_closest_in_axis(guides: &[Guide], coord: f32, world_tolerance: f32) -> Option<&Guide> {
+        if guides.is_empty() {
+            return None;
+        }
+
+        let idx = guides.partition_point(|guide| guide.position() < coord);
+        let mut closest: Option<&Guide> = None;
+        let mut closest_dist = world_tolerance;
+
+        for candidate_idx in [idx.wrapping_sub(1), idx] {
+            if candidate_idx < guides.len() {
+                let guide = &guides[candidate_idx];
+                let dist = (guide.position() - coord).abs();
+                if dist <= world_tolerance && dist <= closest_dist {
+                    closest_dist = dist;
+                    closest = Some(guide);
+                }
+            }
+        }
+
+        closest
+    }
 }
 
 pub struct UIState {
@@ -52,7 +97,78 @@ impl UIState {
     }
 
     #[allow(dead_code)]
-    fn find_guide_at(&self, _x: f32, _y: f32, _zoom: f32) -> Option<&Guide> {
-        unimplemented!("TODO: Implement guide finding");
+    fn find_guide_at(&self, x: f32, y: f32, zoom: f32, tolerance: f32) -> Option<&Guide> {
+        self.guides.find_at(x, y, zoom, tolerance)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::shapes::Color;
+
+    fn vertical_guide(position: f32, index: usize) -> Guide {
+        Guide::new(GuideKind::Vertical(position), Color::BLACK, Some(index))
+    }
+
+    fn horizontal_guide(position: f32, index: usize) -> Guide {
+        Guide::new(GuideKind::Horizontal(position), Color::BLACK, Some(index))
+    }
+
+    fn pool_with(guides: Vec<Guide>) -> GuidePool {
+        let mut pool = GuidePool::new();
+        pool.set(guides);
+        pool
+    }
+
+    #[test]
+    fn find_at_returns_none_when_no_guides() {
+        let pool = GuidePool::new();
+        assert!(pool.find_at(100.0, 100.0, 1.0, 8.0).is_none());
+    }
+
+    #[test]
+    fn find_at_finds_vertical_guide_within_tolerance() {
+        let pool = pool_with(vec![vertical_guide(100.0, 0)]);
+        let guide = pool.find_at(102.0, 50.0, 1.0, 8.0).unwrap();
+        assert_eq!(guide.index, 0);
+        assert_eq!(guide.kind, GuideKind::Vertical(100.0));
+    }
+
+    #[test]
+    fn find_at_misses_vertical_guide_outside_tolerance() {
+        let pool = pool_with(vec![vertical_guide(100.0, 0)]);
+        assert!(pool.find_at(110.0, 50.0, 1.0, 8.0).is_none());
+    }
+
+    #[test]
+    fn find_at_finds_horizontal_guide_within_tolerance() {
+        let pool = pool_with(vec![horizontal_guide(200.0, 1)]);
+        let guide = pool.find_at(50.0, 203.0, 1.0, 8.0).unwrap();
+        assert_eq!(guide.index, 1);
+        assert_eq!(guide.kind, GuideKind::Horizontal(200.0));
+    }
+
+    #[test]
+    fn find_at_picks_closest_vertical_guide() {
+        let pool = pool_with(vec![vertical_guide(100.0, 0), vertical_guide(105.0, 1)]);
+        let guide = pool.find_at(103.0, 0.0, 1.0, 8.0).unwrap();
+        assert_eq!(guide.index, 1);
+    }
+
+    #[test]
+    fn find_at_prefers_closer_guide_at_intersection() {
+        let pool = pool_with(vec![vertical_guide(102.0, 0), horizontal_guide(100.0, 1)]);
+        let guide = pool.find_at(100.0, 100.0, 1.0, 8.0).unwrap();
+        assert_eq!(guide.index, 1);
+        assert_eq!(guide.kind, GuideKind::Horizontal(100.0));
+    }
+
+    #[test]
+    fn find_at_scales_tolerance_with_zoom() {
+        let pool = pool_with(vec![vertical_guide(100.0, 0)]);
+
+        assert!(pool.find_at(104.0, 0.0, 2.0, 8.0).is_some());
+        assert!(pool.find_at(105.0, 0.0, 2.0, 8.0).is_none());
     }
 }