fix: escape percent signs in argparse help text

src/binarylane/console/parser/help_formatter.py

@@ -27,6 +27,14 @@ def _split_lines(self, text: str, width: int) -> typing.List[str]:
 
         return [text for text in text.splitlines() for text in textwrap.wrap(text, width)]
 
+    def _get_help_string(self, action: argparse.Action) -> str:
+        """Escape help text for argparse.
+
+        Argparse uses %-formatting for help strings, so literal % must be escaped as %%
+        while %(...)s needs to be left as-is.
+        """
+        return (super()._get_help_string(action) or "").replace("%", "%%").replace("%%(", "%(")
+
     def add_usage(
         self,
         usage: Optional[str],

tests/integration/test_help.py

@@ -37,3 +37,14 @@ def test_required_argument_help(app: App, capsys: CaptureFixture[str]) -> None:
     captured = capsys.readouterr()
     assert "\nArguments:\n" in captured.out
     assert "\nParameters:\n" in captured.out
+
+
+def test_help_with_percent_in_description(app: App, capsys: CaptureFixture[str]) -> None:
+    # Help text containing "100%" should not cause argparse format string errors
+    # The % character must be escaped as %% to prevent interpretation as format specifier
+    with pytest.raises(SystemExit):
+        app.run(["server", "alert", "get", "--help"])
+
+    captured = capsys.readouterr()
+    assert "usage: bl server alert get" in captured.out
+    assert "100%" in captured.out