FetchPrompt Team • 25 Jan 2026

Variable Interpolation in AI Prompts: Dynamic Content Made Easy

Most AI prompts aren't static. They need to include dynamic information: a user's name, a product description, a conversation history, or a date. Without a proper system for handling this, developers end up building prompts with string concatenation — a fragile and error-prone approach.

Variable interpolation solves this by letting you define placeholders in your prompt templates that get replaced with actual values at runtime.

What is Variable Interpolation?

Variable interpolation is the process of replacing placeholder tokens in a prompt template with actual values. Instead of hardcoding dynamic content, you define a template:

You are helping {{user_name}} with their {{product_name}} subscription.
They signed up on {{signup_date}} and are on the {{plan_name}} plan.
Please assist them with: {{user_question}}

At fetch time, you pass the actual values, and the system returns the fully interpolated prompt:

You are helping Alice with their FetchPrompt subscription.
They signed up on 2025-03-15 and are on the Pro plan.
Please assist them with: How do I create an API key?

Why Not Just Use String Concatenation?

You could build prompts in code with template literals or string concatenation:

const prompt = `You are helping ${userName} with their
${productName} subscription...`;

This works, but it has significant drawbacks:

The Template is Hidden in Code

When the prompt template is mixed with code, non-engineers can't see or edit it. The actual prompt structure is obscured by code syntax.

Changes Require Deploys

If you need to rephrase the prompt around the variables, you need to edit code, create a PR, and deploy. Variable interpolation in an external system lets you change the template without touching code.

No Validation

String concatenation gives you no visibility into which variables a prompt expects. External variable interpolation can detect variables automatically and validate that all required values are passed.

No Reusability

When the template lives in code, reusing it across services means duplicating the string. External templates are fetched via API and are inherently shared.

How Variable Interpolation Works

1. Define Your Template

Write your prompt using {{variable_name}} syntax for any dynamic content:

Translate the following text from {{source_language}} to
{{target_language}}. Maintain the original tone and formatting.

Text to translate:
{{text}}

2. Pass Variables at Fetch Time

When your application fetches the prompt via the API, pass the variable values as query parameters or in the request body:

const response = await fetch(
  "https://api.fetchprompt.com/v1/prompts/translator" +
  "?source_language=English&target_language=Spanish" +
  "&text=Hello, how are you?",
  { headers: { Authorization: "Bearer fp_prod_xxx" } }
);

3. Receive the Interpolated Prompt

The API returns the prompt with all variables replaced:

Translate the following text from English to Spanish.
Maintain the original tone and formatting.

Text to translate:
Hello, how are you?

Best Practices for Prompt Variables

Use Descriptive Variable Names

Bad:  {{x}}, {{val1}}, {{input}}
Good: {{user_name}}, {{product_category}}, {{support_ticket_id}}

Descriptive names make templates self-documenting and reduce errors when passing values.

Keep Variables Focused

Each variable should represent one piece of information. Avoid passing large blocks of unstructured text as a single variable when it could be broken into semantic pieces:

Bad:  {{user_info}}
Good: {{user_name}}, {{user_email}}, {{user_plan}}

Handle Missing Variables Gracefully

Decide how your system should behave when a variable isn't passed:

  • Leave the placeholder: {{variable_name}} remains in the text (useful for debugging)
  • Replace with empty string: The placeholder is removed
  • Return an error: The API rejects the request

FetchPrompt leaves unreplaced variables as-is, making it easy to spot missing values during testing.

Document Expected Variables

For each prompt, document what variables it expects, what values are valid, and what the default behavior should be. This is especially important when multiple teams or services use the same prompt.

Real-World Use Cases

Customer Support

You are a {{company_name}} support agent. The customer ({{customer_name}})
has a {{priority}} priority issue about {{issue_category}}.
Their account status is {{account_status}}.

Content Generation

Write a {{content_type}} about {{topic}} for {{target_audience}}.
Tone: {{tone}}. Length: approximately {{word_count}} words.
Include a call to action for {{cta_action}}.

Data Processing

Extract the following fields from the text below:
{{fields_to_extract}}

Format the output as {{output_format}}.

Text:
{{input_text}}

Variables in FetchPrompt

FetchPrompt automatically detects {{variable}} syntax in your prompts and displays them in the editor. When fetching via API, pass variable values as query parameters or in the JSON body — the server handles interpolation and returns the complete prompt.

This keeps your prompt templates clean, your code focused on logic, and your team able to iterate on prompt content independently.

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