23 lines (20 with data), 768 Bytes

{% if messages[0]['role'] == 'system' %}
    {% set system_message = messages[0]['content'] | trim + '\n\n' %}
    {% set messages = messages[1:] %}
{% else %}
    {% set system_message = '' %}
{% endif %}

{{ bos_token + system_message }}
{% for message in messages %}
    {% if (message['role'] == 'user') != (loop.index0 % 2 == 0) %}
        {{ raise_exception('Conversation roles must alternate user/assistant/user/assistant/...') }}
    {% endif %}

    {% if message['role'] == 'user' %}
        {{ 'USER: ' + message['content'] | trim + '\n' }}
    {% elif message['role'] == 'assistant' %}
        {{ 'ASSISTANT: ' + message['content'] | trim + eos_token + '\n' }}
    {% endif %}
{% endfor %}

{% if add_generation_prompt %}
    {{ 'ASSISTANT:' }}
{% endif %}