diff --git a/agent_cli/dev/terminals/tmux.py b/agent_cli/dev/terminals/tmux.py
index 68c0922d..faac99c5 100644
--- a/agent_cli/dev/terminals/tmux.py
+++ b/agent_cli/dev/terminals/tmux.py
@@ -122,13 +122,21 @@ def _create_session(
         session_name: str,
     ) -> TerminalHandle | None:
         """Create a detached tmux session and return its initial pane handle."""
-        return self._spawn_target(
+        handle = self._spawn_target(
             ["tmux", "new-session", "-d", "-s", session_name],
             path=path,
             command=command,
             tab_name=tab_name,
             session_name=session_name,
         )
+        if handle is None:
+            return None
+        subprocess.run(
+            ["tmux", "set-option", "-t", session_name, "renumber-windows", "off"],  # noqa: S607
+            capture_output=True,
+            check=False,
+        )
+        return handle
 
     def _spawn_target(
         self,
diff --git a/tests/dev/test_terminals.py b/tests/dev/test_terminals.py
index 6f27a2f3..e9a539aa 100644
--- a/tests/dev/test_terminals.py
+++ b/tests/dev/test_terminals.py
@@ -97,6 +97,35 @@ def test_open_in_session_creates_detached_session_when_missing(self) -> None:
             session_name="repo-session",
         )
 
+    def test_create_session_disables_renumber_windows(self) -> None:
+        """New tmux sessions disable window renumbering."""
+        terminal = Tmux()
+        mock_handle = MagicMock(handle="%42", session_name="repo-session")
+        with (
+            patch.object(terminal, "_spawn_target", return_value=mock_handle) as mock_spawn,
+            patch("subprocess.run") as mock_run,
+        ):
+            handle = terminal._create_session(
+                Path("/some/path"),
+                "echo hello",
+                "test",
+                session_name="repo-session",
+            )
+
+        assert handle is mock_handle
+        mock_spawn.assert_called_once_with(
+            ["tmux", "new-session", "-d", "-s", "repo-session"],
+            path=Path("/some/path"),
+            command="echo hello",
+            tab_name="test",
+            session_name="repo-session",
+        )
+        mock_run.assert_called_once_with(
+            ["tmux", "set-option", "-t", "repo-session", "renumber-windows", "off"],
+            capture_output=True,
+            check=False,
+        )
+
     def test_open_in_session_reuses_existing_session(self) -> None:
         """Outside tmux, a named session gets a new window when it already exists."""
         terminal = Tmux()