diff --git a/crates/tinymist-query/src/upstream/complete.rs b/crates/tinymist-query/src/upstream/complete.rs
index 91b5c500..11561900 100644
--- a/crates/tinymist-query/src/upstream/complete.rs
+++ b/crates/tinymist-query/src/upstream/complete.rs
@@ -995,8 +995,11 @@ impl<'a, 'w> CompletionContext<'a, 'w> {
 
     /// A small window of context before the cursor.
     fn before_window(&self, size: usize) -> &str {
-        // todo: bad slicing
-        &self.before[self.cursor.saturating_sub(size)..]
+        safe_str_slice(
+            self.before,
+            self.cursor.saturating_sub(size),
+            self.before.len(),
+        )
     }
 
     /// Add a prefix and suffix to all applications.
@@ -1265,3 +1268,20 @@ impl<'a, 'w> CompletionContext<'a, 'w> {
         }
     }
 }
+
+fn safe_str_slice(s: &str, mut start: usize, mut end: usize) -> &str {
+    // todo: bad slicing
+    // &self.before[self.cursor.saturating_sub(size)..]
+    while start < s.len() && !s.is_char_boundary(start) {
+        start += 1;
+    }
+    while end > 0 && !s.is_char_boundary(end) {
+        end -= 1;
+    }
+
+    if end >= start {
+        &s[start..end]
+    } else {
+        ""
+    }
+}
diff --git a/crates/tinymist-query/src/upstream/complete/ext.rs b/crates/tinymist-query/src/upstream/complete/ext.rs
index a2e99af2..7a007207 100644
--- a/crates/tinymist-query/src/upstream/complete/ext.rs
+++ b/crates/tinymist-query/src/upstream/complete/ext.rs
@@ -814,3 +814,19 @@ pub fn complete_path(
             .collect_vec(),
     )
 }
+
+#[cfg(test)]
+
+mod tests {
+    use crate::upstream::complete::safe_str_slice;
+
+    #[test]
+    fn test_before() {
+        const TEST_UTF8_STR: &str = "我们";
+        for i in 0..=TEST_UTF8_STR.len() {
+            for j in 0..=TEST_UTF8_STR.len() {
+                let _s = std::hint::black_box(safe_str_slice(TEST_UTF8_STR, i, j));
+            }
+        }
+    }
+}