Only first line of ValidationError message is displayed.

When the error message has multiple lines, only the first is displayed.

from prompt_toolkit.validation import Validator, ValidationError
from prompt_toolkit import prompt

class NumberValidator(Validator):
    def validate(self, document):
        text = document.text

        if text and not text.isdigit():
            i = 0

            # Get index of fist non numeric character.
            # We want to move the cursor here.
            for i, c in enumerate(text):
                if not c.isdigit():
                    break

            raise ValidationError(message='This input contains non-numeric characters.\nTHIS IS NOT DISPLAYED.',
                                  cursor_position=i)


number = int(prompt('Give a number: ', validator=NumberValidator()))
print('You said: %i' % number)

Start by running the provided Validator and prompt example, then trace how ValidationError messages are rendered by the prompt_toolkit validation flow. Done means the complete multi-line message, including the second line, is displayed while preserving the reported cursor position.