LangWatch prompts use Liquid as their template language. Liquid gives you variables, conditionals, loops, and filters so you can build dynamic prompts without custom code.
Templates are rendered when you call compile() in the Python or TypeScript SDK, or when the platform executes a prompt on your behalf.
Variables
Use double curly braces to insert dynamic values into your prompt.
Hello, {{ user_name }}! You have {{ message_count }} new messages.
{ user_name: "Alice", message_count: 3 }, this produces:
Hello, Alice! You have 3 new messages.
Accessing nested properties
Dot notation lets you reach into objects:
Customer: {{ user.name }} ({{ user.email }})
Available variables
The variables available depend on how you invoke the prompt:
| Context | Variables |
|---|
SDK compile() | Any key-value pairs you pass in |
| Prompt Playground | Values set in the Variables tab |
| Optimization Studio | Node inputs defined in the workflow |
| Scenario adapters | input, messages, and adapter-specific fields |
Conditionals
Control which parts of a prompt appear based on runtime values.
if / elsif / else
{% if tone == "formal" %}Dear {{ user_name }},{% elsif tone == "friendly" %}Hey {{ user_name }}!{% else %}Hello,{% endif %}
How can I help you today?
{ tone: "formal", user_name: "Dr. Smith" }:
Dear Dr. Smith,
How can I help you today?
unless
unless is the inverse of if — the block renders when the condition is false.
{% unless context %}Answer the question using only your general knowledge.{% endunless %}
Operators
| Operator | Meaning |
|---|
== | Equal |
!= | Not equal |
> | Greater than |
< | Less than |
>= | Greater than or equal |
<= | Less than or equal |
contains | String contains substring, or array contains element |
and | Both conditions true |
or | Either condition true |
{% if input contains "refund" %}You handle refunds.{% else %}You are a general assistant.{% endif %}
Loops
Iterate over arrays with for.
Consider these topics:
{% for topic in topics %}- {{ topic }}
{% endfor %}
{ topics: ["AI safety", "Alignment", "Governance"] }:
Consider these topics:
- AI safety
- Alignment
- Governance
Comma-separated lists
Use forloop.last to avoid a trailing separator:
Topics: {% for item in topics %}{{ item }}{% unless forloop.last %}, {% endunless %}{% endfor %}
Topics: AI, ML, NLP
Loop variables
Inside a for block, these variables are available:
| Variable | Description |
|---|
forloop.index | Current iteration (1-based) |
forloop.index0 | Current iteration (0-based) |
forloop.first | true on the first iteration |
forloop.last | true on the last iteration |
forloop.length | Total number of items |
Nested loops and conditionals
Loops and conditionals compose freely:
{% for user in users %}{% if user.active %}{{ user.name }}: {{ user.role }}
{% endif %}{% endfor %}
Filters
Filters transform a value. Chain them with the pipe character (|).
{{ name | upcase }}
{{ description | truncate: 50 }}
{{ tags | join: ", " }}
String filters
| Filter | Example | Result |
|---|
upcase | {{ "alice" | upcase }} | ALICE |
downcase | {{ "HELLO" | downcase }} | hello |
capitalize | {{ "hello world" | capitalize }} | Hello world |
truncate | {{ "A long sentence" | truncate: 10 }} | A long ... |
strip | {{ " hi " | strip }} | hi |
replace | {{ "Hello" | replace: "Hello", "Hi" }} | Hi |
append | {{ "hello" | append: " world" }} | hello world |
prepend | {{ "world" | prepend: "hello " }} | hello world |
split | {{ "a,b,c" | split: "," }} | array ["a","b","c"] |
Array filters
| Filter | Example | Result |
|---|
join | {{ tags | join: ", " }} | ai, ml |
first | {{ items | first }} | First element |
last | {{ items | last }} | Last element |
size | {{ items | size }} | Number of elements |
sort | {{ items | sort }} | Sorted array |
reverse | {{ items | reverse }} | Reversed array |
Other filters
| Filter | Example | Result |
|---|
default | {{ name | default: "User" }} | User when name is empty |
json | {{ data | json }} | JSON-encoded string |
Truncate behavior differs between SDKs. In the TypeScript SDK (LiquidJS), truncate: N outputs N characters of content then appends "...". In the Python SDK (python-liquid), truncate: N means N total characters including the "..." suffix. Keep this in mind if you rely on exact output length.
Assignment
Create local variables within the template using assign.
{% assign greeting = "Hello" %}{% assign formatted_name = name | capitalize %}{{ greeting }}, {{ formatted_name }}!
{ name: "alice" }:
Assigned variables are local to the template — they do not need to be passed in and will not appear as required input variables.
Use comment tags for notes that should not appear in the rendered output.
{% comment %}
This section is only included for premium users.
TODO: add tier-based routing.
{% endcomment %}
{% if is_premium %}You have access to advanced features.{% endif %}
Practical examples
Conditional system prompt
Adapt the system message based on user input:
{% if input contains "refund" %}
You are a refund specialist. Follow the company refund policy strictly.
{% elsif input contains "billing" %}
You are a billing assistant. You can look up invoices and payment history.
{% else %}
You are a general customer support agent. Be helpful and concise.
{% endif %}
The customer said: {{ input }}
Few-shot examples from a list
Build few-shot examples dynamically:
You are a sentiment classifier. Classify the sentiment as positive, negative, or neutral.
{% for example in examples %}
Input: {{ example.text }}
Sentiment: {{ example.label }}
{% endfor %}
Input: {{ input }}
Sentiment:
Context-aware RAG prompt
Include retrieved context only when it exists:
{% if context %}Use the following context to answer the question:
{% for doc in context %}[{{ forloop.index }}] {{ doc }}
{% endfor %}
{% endif %}Question: {{ question }}
Multi-language greeting
{% if lang == "en" %}Hello{% elsif lang == "es" %}Hola{% elsif lang == "fr" %}Bonjour{% else %}Hi{% endif %}, {{ user_name }}!
Strict vs. lenient compilation
The SDKs offer two compilation modes:
compile() — Lenient. Undefined variables resolve to empty strings and missing conditions evaluate to false. Useful for optional fields.compileStrict() (TypeScript) / compile_strict() (Python) — Strict. Throws a PromptCompilationError if any variable used in the template is not provided. Use this to catch typos and missing data early.
# Python - strict mode catches missing variables
prompt.compile_strict(name="Alice") # raises PromptCompilationError if template uses {{ email }}
// TypeScript - strict mode catches missing variables
prompt.compileStrict({ name: "Alice" }); // throws PromptCompilationError if template uses {{ email }}
Further reading
