diff --git a/crates/ruff_server/src/edit/replacement.rs b/crates/ruff_server/src/edit/replacement.rs
index 24a58ec3f15d6..ed934cffbe88b 100644
--- a/crates/ruff_server/src/edit/replacement.rs
+++ b/crates/ruff_server/src/edit/replacement.rs
@@ -1,5 +1,6 @@
 use ruff_text_size::{TextLen, TextRange, TextSize};
 
+#[derive(Debug)]
 pub(crate) struct Replacement {
     pub(crate) source_range: TextRange,
     pub(crate) modified_range: TextRange,
@@ -15,41 +16,46 @@ impl Replacement {
         modified_line_starts: &[TextSize],
     ) -> Self {
         let mut source_start = TextSize::default();
-        let mut replaced_start = TextSize::default();
-        let mut source_end = source.text_len();
-        let mut replaced_end = modified.text_len();
-        let mut line_iter = source_line_starts
+        let mut modified_start = TextSize::default();
+
+        for (source_line_start, modified_line_start) in source_line_starts
             .iter()
             .copied()
-            .zip(modified_line_starts.iter().copied());
-        for (source_line_start, modified_line_start) in line_iter.by_ref() {
-            if source_line_start != modified_line_start
-                || source[TextRange::new(source_start, source_line_start)]
-                    != modified[TextRange::new(replaced_start, modified_line_start)]
+            .zip(modified_line_starts.iter().copied())
+            .skip(1)
+        {
+            if source[TextRange::new(source_start, source_line_start)]
+                != modified[TextRange::new(modified_start, modified_line_start)]
             {
                 break;
             }
             source_start = source_line_start;
-            replaced_start = modified_line_start;
+            modified_start = modified_line_start;
         }
 
-        let mut line_iter = line_iter.rev();
+        let mut source_end = source.text_len();
+        let mut modified_end = modified.text_len();
 
-        for (old_line_start, new_line_start) in line_iter.by_ref() {
-            if old_line_start <= source_start
-                || new_line_start <= replaced_start
-                || source[TextRange::new(old_line_start, source_end)]
-                    != modified[TextRange::new(new_line_start, replaced_end)]
+        for (source_line_start, modified_line_start) in source_line_starts
+            .iter()
+            .rev()
+            .copied()
+            .zip(modified_line_starts.iter().rev().copied())
+        {
+            if source_line_start < source_start
+                || modified_line_start < modified_start
+                || source[TextRange::new(source_line_start, source_end)]
+                    != modified[TextRange::new(modified_line_start, modified_end)]
             {
                 break;
             }
-            source_end = old_line_start;
-            replaced_end = new_line_start;
+            source_end = source_line_start;
+            modified_end = modified_line_start;
         }
 
         Replacement {
             source_range: TextRange::new(source_start, source_end),
-            modified_range: TextRange::new(replaced_start, replaced_end),
+            modified_range: TextRange::new(modified_start, modified_end),
         }
     }
 }
@@ -57,42 +63,166 @@ impl Replacement {
 #[cfg(test)]
 mod tests {
     use ruff_source_file::LineIndex;
+    use ruff_text_size::TextRange;
 
     use super::Replacement;
 
-    #[test]
-    fn find_replacement_range_works() {
-        let original = r#"
-         aaaa
-         bbbb
-         cccc
-         dddd
-         eeee
-         "#;
-        let original_index = LineIndex::from_source_text(original);
-        let new = r#"
-         bb
-         cccc
-         dd
-         "#;
-        let new_index = LineIndex::from_source_text(new);
-        let expected = r#"
-         bb
-         cccc
-         dd
-         "#;
+    fn compute_replacement(source: &str, modified: &str) -> (Replacement, String) {
+        let source_index = LineIndex::from_source_text(source);
+        let modified_index = LineIndex::from_source_text(modified);
         let replacement = Replacement::between(
-            original,
-            original_index.line_starts(),
-            new,
-            new_index.line_starts(),
+            source,
+            source_index.line_starts(),
+            modified,
+            modified_index.line_starts(),
         );
-        let mut test = original.to_string();
-        test.replace_range(
+        let mut expected = source.to_string();
+        expected.replace_range(
             replacement.source_range.start().to_usize()..replacement.source_range.end().to_usize(),
-            &new[replacement.modified_range],
+            &modified[replacement.modified_range],
+        );
+        (replacement, expected)
+    }
+
+    #[test]
+    fn delete_first_line() {
+        let source = "aaaa
+bbbb
+cccc
+";
+        let modified = "bbbb
+cccc
+";
+        let (replacement, expected) = compute_replacement(source, modified);
+        assert_eq!(replacement.source_range, TextRange::new(0.into(), 5.into()));
+        assert_eq!(replacement.modified_range, TextRange::empty(0.into()));
+        assert_eq!(modified, &expected);
+    }
+
+    #[test]
+    fn delete_middle_line() {
+        let source = "aaaa
+bbbb
+cccc
+dddd
+";
+        let modified = "aaaa
+bbbb
+dddd
+";
+        let (replacement, expected) = compute_replacement(source, modified);
+        assert_eq!(
+            replacement.source_range,
+            TextRange::new(10.into(), 15.into())
         );
+        assert_eq!(replacement.modified_range, TextRange::empty(10.into()));
+        assert_eq!(modified, &expected);
+    }
 
-        assert_eq!(expected, &test);
+    #[test]
+    fn delete_multiple_lines() {
+        let source = "aaaa
+bbbb
+cccc
+dddd
+eeee
+ffff
+";
+        let modified = "aaaa
+cccc
+dddd
+ffff
+";
+        let (replacement, expected) = compute_replacement(source, modified);
+        assert_eq!(
+            replacement.source_range,
+            TextRange::new(5.into(), 25.into())
+        );
+        assert_eq!(
+            replacement.modified_range,
+            TextRange::new(5.into(), 15.into())
+        );
+        assert_eq!(modified, &expected);
+    }
+
+    #[test]
+    fn insert_first_line() {
+        let source = "bbbb
+cccc
+";
+        let modified = "aaaa
+bbbb
+cccc
+";
+        let (replacement, expected) = compute_replacement(source, modified);
+        assert_eq!(replacement.source_range, TextRange::empty(0.into()));
+        assert_eq!(
+            replacement.modified_range,
+            TextRange::new(0.into(), 5.into())
+        );
+        assert_eq!(modified, &expected);
+    }
+
+    #[test]
+    fn insert_middle_line() {
+        let source = "aaaa
+cccc
+";
+        let modified = "aaaa
+bbbb
+cccc
+";
+        let (replacement, expected) = compute_replacement(source, modified);
+        assert_eq!(replacement.source_range, TextRange::empty(5.into()));
+        assert_eq!(
+            replacement.modified_range,
+            TextRange::new(5.into(), 10.into())
+        );
+        assert_eq!(modified, &expected);
+    }
+
+    #[test]
+    fn insert_multiple_lines() {
+        let source = "aaaa
+cccc
+eeee
+";
+        let modified = "aaaa
+bbbb
+cccc
+dddd
+";
+        let (replacement, expected) = compute_replacement(source, modified);
+        assert_eq!(
+            replacement.source_range,
+            TextRange::new(5.into(), 15.into())
+        );
+        assert_eq!(
+            replacement.modified_range,
+            TextRange::new(5.into(), 20.into())
+        );
+        assert_eq!(modified, &expected);
+    }
+
+    #[test]
+    fn replace_lines() {
+        let source = "aaaa
+bbbb
+cccc
+";
+        let modified = "aaaa
+bbcb
+cccc
+";
+        let (replacement, expected) = compute_replacement(source, modified);
+        assert_eq!(
+            replacement.source_range,
+            TextRange::new(5.into(), 10.into())
+        );
+        assert_eq!(
+            replacement.modified_range,
+            TextRange::new(5.into(), 10.into())
+        );
+        assert_eq!(modified, &expected);
     }
 }