diff --git a/packages/app/src/utils/file-mention-autocomplete.test.ts b/packages/app/src/utils/file-mention-autocomplete.test.ts
index db975a760..e5e2ec506 100644
--- a/packages/app/src/utils/file-mention-autocomplete.test.ts
+++ b/packages/app/src/utils/file-mention-autocomplete.test.ts
@@ -36,6 +36,14 @@ describe("findActiveFileMention", () => {
     });
     expect(mention).toBeNull();
   });
+
+  it("returns null when @ at start is followed by a delimiter", () => {
+    const mention = findActiveFileMention({
+      text: "@ ",
+      cursorIndex: 2,
+    });
+    expect(mention).toBeNull();
+  });
 });
 
 describe("applyFileMentionReplacement", () => {
diff --git a/packages/app/src/utils/file-mention-autocomplete.ts b/packages/app/src/utils/file-mention-autocomplete.ts
index 21c99a9c5..748a13458 100644
--- a/packages/app/src/utils/file-mention-autocomplete.ts
+++ b/packages/app/src/utils/file-mention-autocomplete.ts
@@ -24,7 +24,7 @@ export function findActiveFileMention(input: FindActiveFileMentionInput): FileMe
   for (
     let atIndex = beforeCursor.lastIndexOf("@");
     atIndex >= 0;
-    atIndex = beforeCursor.lastIndexOf("@", atIndex - 1)
+    atIndex = atIndex === 0 ? -1 : beforeCursor.lastIndexOf("@", atIndex - 1)
   ) {
     const query = beforeCursor.slice(atIndex + 1);
     if (INVALID_MENTION_QUERY_CHARS.test(query)) {