Both files are written to `/workspace/.repos/yardstick/runs/bench-opus48-max/outbox/CODE-0006/`. The solution passes all tests. Per the instructions, my final message is the raw content of the requested output file `solution.py`.

"""Evaluate an arithmetic expression given as a string.

A small, self-contained recursive-descent parser. No ``eval``/``exec`` and no
expression-parsing library is used; only the Python standard library.

Grammar (whitespace between tokens is ignored)::

    expr   := term   (('+' | '-') term)*
    term   := factor (('*' | '/') factor)*
    factor := ('+' | '-') factor | '(' expr ')' | number
    number := digits | digits '.' [digits] | '.' digits

``+`` and ``-`` are lower precedence than ``*`` and ``/``; all four binary
operators are left associative; unary ``+``/``-`` bind tighter than the binary
operators and may stack (``--5`` == ``5``). ``/`` is true division. A result that
is an exact integer value is returned as ``int``; otherwise as ``float``.
"""

# Token kinds.
_NUM = "NUM"
_PLUS = "+"
_MINUS = "-"
_MUL = "*"
_DIV = "/"
_LPAR = "("
_RPAR = ")"

_OPERATOR_CHARS = {
    "+": _PLUS,
    "-": _MINUS,
    "*": _MUL,
    "/": _DIV,
    "(": _LPAR,
    ")": _RPAR,
}

_NUMBER_CHARS = "0123456789."


def _tokenize(s):
    """Split ``s`` into a list of ``(kind, value)`` tokens.

    Whitespace is skipped. Numbers are contiguous runs of digits/dots and are
    validated (at least one digit, at most one dot). Any other character is an
    unknown token. Raises ``ValueError`` on a malformed number or unknown
    character.
    """
    tokens = []
    i = 0
    n = len(s)
    while i < n:
        ch = s[i]

        # Ignore any whitespace between tokens.
        if ch.isspace():
            i += 1
            continue

        # Single-character operators and parentheses.
        kind = _OPERATOR_CHARS.get(ch)
        if kind is not None:
            tokens.append((kind, ch))
            i += 1
            continue

        # Number literal: greedily consume digits/dots, then validate so that a
        # lone '.' or something like '1.2.3' is rejected as malformed.
        if ch in _NUMBER_CHARS:
            start = i
            while i < n and s[i] in _NUMBER_CHARS:
                i += 1
            literal = s[start:i]
            tokens.append((_NUM, _parse_number(literal)))
            continue

        raise ValueError("unknown token: %r" % ch)

    return tokens


def _parse_number(literal):
    """Turn a number literal string into an ``int`` (no dot) or ``float``.

    Validates structure: at least one digit and at most one decimal point.
    """
    if literal.count(".") > 1 or not any(c.isdigit() for c in literal):
        raise ValueError("malformed number: %r" % literal)
    if "." in literal:
        return float(literal)
    return int(literal)


class _Parser:
    """Recursive-descent parser over a token list with a position cursor."""

    def __init__(self, tokens):
        self._tokens = tokens
        self._pos = 0

    def _peek(self):
        if self._pos < len(self._tokens):
            return self._tokens[self._pos]
        return None

    def _advance(self):
        token = self._tokens[self._pos]
        self._pos += 1
        return token

    def parse(self):
        """Parse a complete expression and ensure all input is consumed."""
        value = self._parse_expr()
        if self._pos != len(self._tokens):
            # Leftover tokens: e.g. two adjacent numbers, a stray ')', or
            # trailing junk.
            raise ValueError("unexpected trailing input")
        return value

    def _parse_expr(self):
        value = self._parse_term()
        while True:
            token = self._peek()
            if token is None or token[0] not in (_PLUS, _MINUS):
                return value
            op = self._advance()[0]
            right = self._parse_term()
            if op == _PLUS:
                value = value + right
            else:
                value = value - right

    def _parse_term(self):
        value = self._parse_factor()
        while True:
            token = self._peek()
            if token is None or token[0] not in (_MUL, _DIV):
                return value
            op = self._advance()[0]
            right = self._parse_factor()
            if op == _MUL:
                value = value * right
            else:
                if right == 0:
                    raise ValueError("division by zero")
                value = value / right

    def _parse_factor(self):
        token = self._peek()
        if token is None:
            # Operand expected but input ran out: empty input or a dangling
            # operator.
            raise ValueError("expected an operand")

        kind = token[0]

        # Unary plus / minus: bind tighter than the binary operators and may
        # stack (the recursion makes them right associative).
        if kind == _PLUS:
            self._advance()
            return +self._parse_factor()
        if kind == _MINUS:
            self._advance()
            return -self._parse_factor()

        # Parenthesised sub-expression.
        if kind == _LPAR:
            self._advance()
            value = self._parse_expr()
            closing = self._peek()
            if closing is None or closing[0] != _RPAR:
                raise ValueError("missing closing parenthesis")
            self._advance()
            return value

        # Number literal.
        if kind == _NUM:
            return self._advance()[1]

        # Anything else here (e.g. a ')' or a binary operator with no left
        # operand) is malformed.
        raise ValueError("unexpected token: %r" % (token[1],))


def _normalize(value):
    """Return ``value`` as an ``int`` when it is an exact integer, else float."""
    if isinstance(value, float):
        if value.is_integer():
            return int(value)
        return value
    return value


def eval_expr(s):
    """Evaluate the arithmetic expression ``s`` and return its numeric value.

    Supports ``+ - * /`` with standard precedence and left associativity,
    parentheses, unary ``+``/``-``, and integer/decimal literals. ``/`` is true
    division. Returns an ``int`` for an exact integer result, otherwise a
    ``float``. Raises ``ValueError`` for a malformed expression (empty input, a
    dangling operator, mismatched parentheses, two adjacent numbers, an unknown
    token) or for division by zero.
    """
    if not isinstance(s, str):
        raise ValueError("expression must be a string")
    tokens = _tokenize(s)
    value = _Parser(tokens).parse()
    return _normalize(value)