diff --git a/codex-cli/tests/text-buffer-boundary.test.ts b/codex-cli/tests/text-buffer-boundary.test.ts
new file mode 100644
index 000000000..be63e9d35
--- /dev/null
+++ b/codex-cli/tests/text-buffer-boundary.test.ts
@@ -0,0 +1,45 @@
+import TextBuffer from "../src/text-buffer";
+import { describe, it, expect } from "vitest";
+
+describe("TextBuffer – boundary tests", () => {
+  describe("extremely long lines", () => {
+    it("should handle very long single line", () => {
+      const longLine = "a".repeat(10000);
+      const buf = new TextBuffer(longLine);
+      
+      // Test cursor movement in long line
+      buf.move("end");
+      expect(buf.getCursor()).toEqual([0, 10000]);
+      
+      // Test insert in middle of long line
+      buf.move("left");
+      buf.insert("b");
+      expect(buf.getText().length).toBe(10001);
+      
+      // Test delete in long line
+      buf.del();
+      expect(buf.getText().length).toBe(10000);
+    });
+  });
+
+  describe("large number of lines", () => {
+    it("should handle buffer with many lines", () => {
+      const manyLines = Array(1000).fill("test").join("\n");
+      const buf = new TextBuffer(manyLines);
+      
+      // Test cursor movement through many lines
+      buf.move("end");
+      expect(buf.getCursor()[0]).toBe(999);
+      
+      // Test insert at end of many lines
+      buf.insert("x");
+      expect(buf.getLines().length).toBe(1000);
+      
+      // Test newline in middle of many lines
+      buf.move("up");
+      buf.move("end");
+      buf.insert("\n");
+      expect(buf.getLines().length).toBe(1001);
+    });
+  });
+}); 
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