diff --git a/src/copilot_usage/cli.py b/src/copilot_usage/cli.py
index 858eb06..290f048 100644
--- a/src/copilot_usage/cli.py
+++ b/src/copilot_usage/cli.py
@@ -215,13 +215,15 @@ def _read_line_nonblocking(timeout: float = 0.5) -> str | None:
     """Return a line from stdin if available within *timeout*, else ``None``.
 
     Uses ``select.select`` to poll stdin.  Raises :class:`ValueError` or
-    :class:`OSError` when stdin is not selectable (e.g. Windows, or a
-    detached stdin buffer in tests); callers should fall back to a threaded
-    reader in that case.
+    :class:`OSError` when stdin is not selectable (e.g. Windows, detached
+    stdin buffer in tests, or ``sys.stdin is None``); callers should fall
+    back to a threaded reader in that case.
 
     Raises :class:`EOFError` when stdin is closed (``readline()`` returns
     an empty string), preventing an infinite polling loop.
     """
+    if sys.stdin is None:
+        raise ValueError("stdin is None")
     ready, _, _ = select.select([sys.stdin], [], [], timeout)
     if ready:
         line = sys.stdin.readline()
diff --git a/src/copilot_usage/docs/implementation.md b/src/copilot_usage/docs/implementation.md
index 8ec2642..7acd6d1 100644
--- a/src/copilot_usage/docs/implementation.md
+++ b/src/copilot_usage/docs/implementation.md
@@ -250,7 +250,7 @@ This is **Unix only** — `select()` on stdin doesn't work on Windows. The 500ms
 
 ### Fallback to threaded `_start_input_reader_thread()`
 
-If `select()` raises `ValueError` or `OSError` (e.g. stdin is not selectable, notably on Windows, or stdin is detached during testing), the loop starts a daemon thread via `_start_input_reader_thread()` (in `cli.py`) that feeds lines into a `queue.SimpleQueue`:
+If `select()` raises `ValueError` or `OSError` (e.g. stdin is not selectable, notably on Windows, stdin is detached during testing, or `sys.stdin is None`), the loop starts a daemon thread via `_start_input_reader_thread()` (in `cli.py`) that feeds lines into a `queue.SimpleQueue`:
 
 ```python
 except (ValueError, OSError):
diff --git a/tests/copilot_usage/test_cli.py b/tests/copilot_usage/test_cli.py
index 81a3abc..215d5e0 100644
--- a/tests/copilot_usage/test_cli.py
+++ b/tests/copilot_usage/test_cli.py
@@ -2168,12 +2168,34 @@ def test_returns_stripped_line(self) -> None:
             r_file.close()
             os.close(w_fd)
 
+    def test_none_stdin_raises_value_error(self) -> None:
+        """When sys.stdin is None, _read_line_nonblocking raises ValueError."""
+        with patch("copilot_usage.cli.sys.stdin", None):
+            with pytest.raises(ValueError, match="stdin is None"):
+                _read_line_nonblocking(timeout=0.05)
+
 
 # ---------------------------------------------------------------------------
 # Gap 3 — _interactive_loop stdin fallback (issue #258)
 # ---------------------------------------------------------------------------
 
 
+def test_interactive_loop_none_stdin_falls_back_to_input(
+    tmp_path: Path, monkeypatch: Any
+) -> None:
+    """When sys.stdin is None, _interactive_loop must fall back to threaded input()."""
+    _write_session(tmp_path, "fb_none0-0000-0000-0000-000000000000", name="NoneStdin")
+
+    import copilot_usage.cli as cli_mod
+
+    monkeypatch.setattr(cli_mod.sys, "stdin", None)
+    monkeypatch.setattr("builtins.input", lambda *_: "q")
+
+    runner = CliRunner()
+    result = runner.invoke(main, ["--path", str(tmp_path)])
+    assert result.exit_code == 0
+
+
 def test_interactive_loop_select_value_error_falls_back_to_input(
     tmp_path: Path, monkeypatch: Any
 ) -> None: