23 lines (19 with data), 631 Bytes

{% if messages[0]['role'] == 'system' %}
    {% set offset = 1 %}
{% else %}
    {% set offset = 0 %}
{% endif %}

{% for message in messages %}
    {% if (message['role'] == 'user') != (loop.index0 % 2 == offset) %}
        {{ raise_exception('Conversation roles must alternate user/bot/user/bot/...') }}
    {% endif %}

    {% if message['role'] == 'assistant' %}
        {{ set role = 'bot' }}
    {% else %}
        {{ set role = message['role'] }}
    {% endif %}

    {{ bos_token + role + '\n' + message['content'] | trim + eos_token }}
{% endfor %}

{% if add_generation_prompt %}
    {{ bos_token + 'bot\n' }}
{% endif %}