Templating with Variables

Use Liquid.js to create dynamic prompts with variables, conditionals, and loops

Prompt files use Liquid.js for dynamic content. Instead of hardcoding values, you insert variables that get filled in when the prompt runs.

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---
provider: anthropic
model: claude-sonnet-4-20250514
temperature: 0.3
---

<system>
You summarize company information for sales teams.
</system>

<user>
Write a {{ max_paragraphs }}-paragraph summary of this company:

{{ company_data }}
</user>

When you call generateText({ prompt: 'generate_summary@v1', variables: { max_paragraphs: 2, company_data: '...' } }), Liquid replaces {{ max_paragraphs }} with 2 and {{ company_data }} with your content before the prompt is sent to the LLM.

Templates render before YAML parsing, so you can template the frontmatter too — useful for dynamic model selection or configuration.

Variables

The double-brace syntax {{ variableName }} renders values from the variables object you pass to the generate function.

<user>
Write a company summary for {{ company_name }} in the {{ industry }} industry.
</user>

await generateText({
  prompt: 'generate_summary@v1',
  variables: {
    company_name: 'Acme Corp',
    industry: 'SaaS'
  }
});

For nested objects, use dot notation:

<user>
Write a summary for {{ company.name }} ({{ company.industry }}, {{ company.size }} employees).
</user>

variables: {
  company: { name: 'Acme Corp', industry: 'SaaS', size: 250 }
}

Undefined variables render as empty strings — no error is thrown, but your prompt may not make sense. Validate inputs in your step before calling the LLM.

Conditionals

Use {% if %} to include content only when a condition is true:

<system>
You are a sales research assistant.
{% if include_competitors %}
When relevant, mention how this company compares to competitors.
{% endif %}
</system>

<user>
Write a summary for {{ company_name }}.
</user>

With include_competitors: true, the competitor instruction is included. With false or undefined, it’s omitted entirely.

Add {% else %} for alternate content:

<user>
{% if detailed %}
Provide a comprehensive analysis with industry context, competitive positioning, and growth trajectory.
{% else %}
Give a brief overview in 2-3 paragraphs.
{% endif %}
</user>

Use {% unless %} for the inverse — it renders when the condition is falsy:

{% unless skip_pain_points %}
Include potential pain points we could address in a sales call.
{% endunless %}

Loops

Iterate over arrays with {% for %}:

<user>
Evaluate this company summary against these criteria:

{% for criterion in criteria %}
{{ forloop.index }}. {{ criterion }}
{% endfor %}

Summary: {{ summary }}
</user>

forloop.index gives you the 1-based position (1, 2, 3…). Use forloop.index0 for 0-based indexing.

For arrays of objects:

<user>
Compare these companies:

{% for company in companies %}
- {{ company.name }}: {{ company.industry }}, {{ company.size }} employees
{% endfor %}

Which is the strongest competitor?
</user>

Filters

Filters transform values using the pipe syntax {{ value | filter }}.

Text transformations:

{{ company_name | upcase }}        # "ACME CORP"
{{ company_name | downcase }}      # "acme corp"
{{ company_name | capitalize }}    # "Acme corp"

Arrays:

{{ industries | join: ", " }}    # "SaaS, FinTech, Healthcare"
{{ companies | first }}          # First element
{{ companies | last }}           # Last element
{{ companies | size }}           # Array length

Default values:

{{ tone | default: "professional" }}

If tone is undefined or empty, it renders as “professional”.

Chaining filters:

{{ company_name | downcase | replace: " ", "-" }}
# "Acme Corp" becomes "acme-corp"

For the full filter reference, see the Liquid.js documentation.

Templating Frontmatter

Variables work in the YAML frontmatter too. This lets you dynamically configure model settings:

---
provider: {{ provider }}
model: {{ model_name }}
temperature: {{ temperature | default: 0.7 }}
---

<user>
{{ prompt_text }}
</user>

await generateText({
  prompt: 'flexible@v1',
  variables: {
    provider: 'anthropic',
    model_name: 'claude-sonnet-4-20250514',
    temperature: 0.3,
    prompt_text: 'Summarize this company...'
  }
});

Use this pattern when you need to switch models based on task complexity or cost.

Common Patterns

Optional sections based on input:

<user>
Research {{ company_name }}.

{% if focus_areas %}
Focus specifically on:
{% for area in focus_areas %}
- {{ area }}
{% endfor %}
{% endif %}

{% if competitor_names %}
Compare against these competitors: {{ competitor_names | join: ", " }}
{% endif %}
</user>

Building structured instructions from an array of rules:

<system>
You are a {{ role }} assistant.

Your response must:
{% for rule in rules %}
{{ forloop.index }}. {{ rule }}
{% endfor %}
</system>

Conditional system behavior:

<system>
You help sales teams research companies.
{% if include_reasoning %}
Explain your reasoning before providing the final output.
{% endif %}
{% unless allow_creativity %}
Stick strictly to the facts provided. Do not add information.
{% endunless %}
</system>

Cheatsheet

Quick reference for the most common Liquid.js syntax.

Output

Syntax	Description	Example
{{ variable }}	Output a variable	{{ name }} → Acme Corp
{{ obj.property }}	Dot notation	{{ company.industry }} → SaaS
{{ array[0] }}	Array index	{{ items[0] }} → first item

Tags

Syntax	Description
{% if condition %}...{% endif %}	Conditional block
{% if condition %}...{% else %}...{% endif %}	If/else
{% elsif condition %}	Else if
{% unless condition %}...{% endunless %}	Negative conditional
{% for item in array %}...{% endfor %}	Loop over array
{% assign var = value %}	Create a variable
{% capture var %}...{% endcapture %}	Capture block into variable
{% comment %}...{% endcomment %}	Comment (not rendered)

Operators

Operator	Description	Example
==	Equals	{% if status == "active" %}
!=	Not equals	{% if type != "draft" %}
>, <, >=, <=	Comparison	{% if count > 0 %}
and	Logical and	{% if a and b %}
or	Logical or	{% if a or b %}
contains	String/array contains	{% if tags contains "saas" %}

Loop Variables

Variable	Description
forloop.index	Current iteration (1-based)
forloop.index0	Current iteration (0-based)
forloop.first	True if first iteration
forloop.last	True if last iteration
forloop.length	Total iterations

String Filters

Filter	Example	Result
upcase	{{ "hello" | upcase }}	HELLO
downcase	{{ "HELLO" | downcase }}	hello
capitalize	{{ "hello world" | capitalize }}	Hello world
strip	{{ " hi " | strip }}	hi
truncate: n	{{ "hello world" | truncate: 5 }}	he...
truncatewords: n	{{ "one two three" | truncatewords: 2 }}	one two...
replace: a, b	{{ "hello" | replace: "l", "x" }}	hexxo
remove: str	{{ "hello" | remove: "l" }}	heo
append: str	{{ "hello" | append: "!" }}	hello!
prepend: str	{{ "world" | prepend: "hello " }}	hello world
split: delim	{{ "a,b,c" | split: "," }}	["a","b","c"]

Array Filters

Filter	Example	Result
join: delim	{{ arr | join: ", " }}	a, b, c
first	{{ arr | first }}	First element
last	{{ arr | last }}	Last element
size	{{ arr | size }}	Array length
reverse	{{ arr | reverse }}	Reversed array
sort	{{ arr | sort }}	Sorted array
uniq	{{ arr | uniq }}	Unique elements
map: prop	{{ users | map: "name" }}	Array of names
where: prop, val	{{ users | where: "active", true }}	Filtered array

Other Filters

Filter	Example	Result
default: val	{{ x | default: "none" }}	none if x is nil/empty
date: format	{{ now | date: "%Y-%m-%d" }}	2024-01-15
json	{{ obj | json }}	JSON string
escape	{{ html | escape }}	HTML-escaped string
newline_to_br	{{ text | newline_to_br }}	\n → <br>

Number Filters

Filter	Example	Result
plus: n	{{ 5 | plus: 3 }}	8
minus: n	{{ 5 | minus: 3 }}	2
times: n	{{ 5 | times: 3 }}	15
divided_by: n	{{ 10 | divided_by: 3 }}	3
modulo: n	{{ 10 | modulo: 3 }}	1
round	{{ 4.6 | round }}	5
ceil	{{ 4.2 | ceil }}	5
floor	{{ 4.8 | floor }}	4