Prompt Template

Overview

The prompt_template node takes a string template with {{variable}} placeholders and produces a fully rendered string by substituting values from upstream workflow data or static defaults. It is most commonly placed immediately before a lens_execute or code node to construct the exact text sent to a model. The interpolation engine supports simple variable substitution and dot-path access (e.g. {{user.name}}); it does not execute logic — use code for conditional rendering.

Configuration

Field	Type	Required	Default	Description
template	string	Yes	—	The template string. Wrap variable names in double curly braces: {{variable_name}}. Supports dot-path notation: {{result.score}}.
variables	object	No	{}	Static default values for template variables. These are overridden by any matching key in the input port's data.
strict	boolean	No	false	When true, the node fails if any placeholder has no resolved value (neither from input nor from variables). When false, unresolved placeholders are replaced with an empty string.
trim_output	boolean	No	true	Strip leading and trailing whitespace from the rendered string before emitting.

Inputs

Port	Type	Description
input	object	Key-value pairs that override or supplement variables during rendering. Nested objects are accessible via dot notation in the template.

Outputs

Port	Type	Description
output	string	The fully rendered template string.
error	object	Present only when strict is true and one or more placeholders could not be resolved; contains message and the list of unresolved keys in missing_keys.

Example

{
  "nodeType": "prompt_template",
  "config": {
::: v-pre
    "template": "You are a {{role}} assistant. Summarise the following in {{language}}:\n\n{{content}}",
:::
    "variables": {
      "role": "helpful",
      "language": "English"
    },
    "strict": true,
    "trim_output": true
  }
}

Given input = { "content": "The battle ended in a draw.", "language": "Turkish" }, the rendered output would be:

You are a helpful assistant. Summarise the following in Turkish:

The battle ended in a draw.

Notes

• Template variables are resolved by merging variables (lower priority) with the input object (higher priority), so upstream data always wins over static defaults.

• Dot-path access works for any depth: {{a.b.c}} resolves the nested key c inside b inside a. Array indexing ({{items.0}}) is also supported.

• For templates longer than a few hundred characters, consider storing the template text in a workflow variable or kv_store node and reading it in dynamically — this makes it easier to update without editing the workflow graph.
• This node does not call any AI model; it is purely string interpolation and adds negligible latency.