diff --git a/src/re_agent/core/session.py b/src/re_agent/core/session.py
index 1bb795c..41f9b5b 100644
--- a/src/re_agent/core/session.py
+++ b/src/re_agent/core/session.py
@@ -29,7 +29,7 @@ def save(self) -> None:
         self.path.parent.mkdir(parents=True, exist_ok=True)
         tmp = self.path.with_suffix(".tmp")
         tmp.write_text(json.dumps(self._data, indent=2), encoding="utf-8")
-        tmp.rename(self.path)
+        tmp.replace(self.path)
 
     def record_result(self, result: ReversalResult) -> None:
         addr = normalize_address(result.target.address)
diff --git a/tests/test_core/test_session.py b/tests/test_core/test_session.py
new file mode 100644
index 0000000..d99ee07
--- /dev/null
+++ b/tests/test_core/test_session.py
@@ -0,0 +1,17 @@
+from pathlib import Path
+
+from re_agent.core.session import Session
+
+
+def test_session_save_replaces_existing_file(tmp_path: Path) -> None:
+    session_file = tmp_path / "progress.json"
+
+    session = Session(session_file)
+    session.save()
+
+    assert session_file.exists()
+
+    session._data["runs"].append({"test": 1})
+    session.save()
+
+    assert session_file.exists()
\ No newline at end of file