diff --git a/nanobot/cli/commands.py b/nanobot/cli/commands.py
index 0f69bf792..9e1501955 100644
--- a/nanobot/cli/commands.py
+++ b/nanobot/cli/commands.py
@@ -326,11 +326,19 @@ def _build_cli_key_bindings() -> KeyBindings:
     # the case prompt_toolkit itself calls out) send for a plain Enter
     # keypress. Aliasing to ControlJ made Enter stop submitting there, since
     # our handler shadowed prompt_toolkit's own "treat \n as \r" fallback.
+    #
+    # "\x1b[27;2;13~" (the older xterm modifyOtherKeys / rxvt encoding of
+    # Shift+Enter) is already registered by prompt_toolkit by default -- as
+    # Keys.ControlM, i.e. plain Enter/submit. A plain setdefault() would be a
+    # no-op against that existing entry, silently leaving this Shift+Enter
+    # variant behaving like a submit instead of inserting a newline. Assign
+    # directly to override it, since inserting a newline is the whole point
+    # of a Shift+Enter binding here.
     with suppress(Exception):
         from prompt_toolkit.input import ansi_escape_sequences as _aes
 
         for _seq in ("\x1b[13;2u", "\x1b[27;2;13~"):
-            _aes.ANSI_SEQUENCES.setdefault(_seq, Keys.ControlF3)
+            _aes.ANSI_SEQUENCES[_seq] = Keys.ControlF3
 
     kb = KeyBindings()
 
diff --git a/tests/cli/test_cli_input.py b/tests/cli/test_cli_input.py
index b38cd20f8..824409f2a 100644
--- a/tests/cli/test_cli_input.py
+++ b/tests/cli/test_cli_input.py
@@ -118,6 +118,28 @@ async def test_raw_lf_enter_still_submits_like_wsl_terminals():
     assert result == "hello"
 
 
+@pytest.mark.asyncio
+async def test_xterm_modifyotherkeys_shift_enter_inserts_newline():
+    """"\\x1b[27;2;13~" (the older xterm modifyOtherKeys/rxvt encoding of
+    Shift+Enter) is registered by prompt_toolkit *by default* as plain
+    Enter/submit, so a `setdefault()`-based override would silently no-op
+    against it. This must actually insert a newline like the kitty CSI-u
+    encoding does, which only a real PromptSession/parser can confirm.
+    """
+    from prompt_toolkit.application import create_app_session
+    from prompt_toolkit.input import create_pipe_input
+    from prompt_toolkit.output import DummyOutput
+
+    with create_pipe_input() as pipe_input:
+        with create_app_session(input=pipe_input, output=DummyOutput()):
+            commands._init_prompt_session()
+            session = commands._PROMPT_SESSION
+            pipe_input.send_text("foo\x1b[27;2;13~bar\r")
+            result = await session.prompt_async("> ")
+
+    assert result == "foo\nbar"
+
+
 def test_thinking_spinner_pause_stops_and_restarts():
     """Pause should stop the active spinner and restart it afterward."""
     spinner = MagicMock()