{
    "task_id": "performance_medium_2",
    "task_name": "Inefficient String Concatenation",
    "description": "Find the performance issue with string concatenation in a loop",
    "difficulty": "medium",
    "code_diff": "def build_string(items):\n    result = \"\"\n    for item in items:\n        result = result + item + \",\"\n    return result[:-1]",
    "expected_issues": [
        {
            "line": 4,
            "type": "performance",
            "severity": "medium",
            "description": "Inefficient string concatenation in loop"
        }
    ]
}