Your first prompt goes here
---
Next prompt goes here. You can substitute variables like this: {{var1}} {{var2}} {{var3}}
---
This is the next prompt.

These prompts are nunjucks templates, so you can use logic like this:
{% if var1 %}
  {{ var1 }}
{% endif %}
---
[
  {"role": "system", "content": "This is another prompt. JSON is supported."},
  {"role": "user", "content": "Using this format, you may construct multi-shot OpenAI prompts"}
  {"role": "user", "content": "Variable substitution still works: {{ var3 }}"}
]
---
If you prefer, you can break prompts into multiple files (make sure to edit promptfooconfig.yaml accordingly)