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Input and output files


Prompts from raw text

By default, the config will accept raw text as prompts:

- 'Translate the following text to French: "{{name}}: {{text}}"'
- 'Translate the following text to German: "{{name}}: {{text}}"'

YAML supports multiline strings too:

- |-
Hi there LLM,
Please translate the following text to French:
"{{name}}: {{text}}"
- |-
Translate the following text to German:
"{{name}}: {{text}}"

Use Nunjucks templating syntax to include variables in your prompts, which will be replaced with actual values from your test cases during evaluation.

Prompts from file

Your prompts may be complicated enough that it's difficult to maintain them inline. In that case, reference a file. Filepaths are relative to the configuration file directory:

- file://path/to/prompt1.txt
- file://path/to/prompt2.txt
- file://path/to/prompt.json
- file://path/to/prompt.yaml
# Globs are supported
- file://prompts/*.txt
- file://path/**/*
# Prompt functions
- file:///root/path/to/prompt.js
- file://./path/to/

Example of multiple prompt files:

Translate the following text to French: "{{name}}: {{text}}"
Translate the following text to German: "{{name}}: {{text}}"

Prompts can be JSON too. Use this to configure multi-shot prompt formats:

"role": "system",
"content": "You are a translator can converts input to {{ language }}."
"role": "user",
"content": "{{ text }}"

Multiple prompts in a single file

If you have only one file, you can include multiple prompts in the file, separated by the delimiter ---. If you have multiple files, each prompt should be in a separate file.

Example of a single prompt file with multiple prompts (prompts.txt):

Translate the following text to French: "{{name}}: {{text}}"
Translate the following text to German: "{{name}}: {{text}}"

Prompt functions

Prompt functions allow you to incorporate custom logic in your prompts. These functions are written in JavaScript or Python and are included in the prompt files with .js or .py extensions.

To specify a prompt function in promptfooconfig.yaml, reference the file directly. For example:

prompts: ['prompt.js', '']

In the prompt function, you can access the test case variables through the vars object. The function should return a string or an object that represents the prompt.


A Javascript prompt function, prompt.js:

module.exports = async function ({ vars }) {
return [
role: 'system',
content: `You're an angry pirate. Be concise and stay in character.`,
role: 'user',
content: `Tell me about ${vars.topic}`,

To reference a specific function in your prompt file, use the following syntax: filename.js:functionName:

module.exports.prompt1 = async function ({ vars }) {
return [
role: 'system',
content: `You're an angry pirate. Be concise and stay in character.`,
role: 'user',
content: `Tell me about ${vars.topic}`,

A Python prompt function,
import json
import sys

def generate_prompt(context: dict) -> str:
return (
f"Describe {context['vars']['topic']} concisely, comparing it to the Python"
" programming language."

if __name__ == "__main__":

To verify that your function is producing the correct prompt:

  1. Run promptfoo view
  2. Check that the table header contains your function code.
  3. Hover over a particular output that you want to investigate and click the Magnifying Glass (🔎) to view the final prompt in the details pane.

By default, promptfoo runs the python executable in your shell.

To override the Python executable, set the PROMPTFOO_PYTHON environment variable to an executable (e.g. /usr/bin/python3.11 or python3.11).

Nunjucks filters

Nunjucks is a templating language with many built-in filters that can be applied to variables. For example: {{ varName | capitalize }}.

Nunjucks custom filters are Javascript functions that can be applied to variables in your templates.

To define a Nunjucks filter, create a JavaScript file that exports a function. This function will be used as the filter and it should take the input value as an argument and return the transformed value.

Here's an example of a custom Nunjucks filter that transforms a string to uppercase (allcaps.js):

module.exports = function (str) {
return str.toUpperCase();

To use a custom Nunjucks filter in PromptFoo, add it to your configuration file (promptfooconfig.yaml). The nunjucksFilters field should contain a mapping of filter names to the paths of the JavaScript files that define them:

prompts: [prompts.txt]
providers: [openai:gpt-3.5-turbo]
allcaps: ./allcaps.js
# ...

Then, use the filter in prompts by appending it to a variable or expression with the pipe (|) symbol:

Translate this to {{language}}: {{body | allcaps}}

In this example, the body variable is passed through the allcaps filter before it's used in the prompt. This means that the text will be transformed to uppercase.

Tests File

The tests file is an optional CSV file that can be used to define test cases separately from the promptfooconfig configuration file.

The first row of the CSV file should contain the variable names, and each subsequent row should contain the corresponding values for each test case.

Vars are substituted by Nunjucks templating syntax into prompts. The first row is the variable names. All other rows are variable values.

Example of a tests file (tests.csv):

German,"Hello, world!"
Spanish,Where is the library?

The tests file optionally supports several special columns:

  • __expected: A column that includes test assertions. This column lets you automatically mark output according to quality expectations.
  • __prefix: This string is prepended to each prompt before it's sent to the API
  • __suffix: This string is appended to each prompt before it's sent to the API

Output File

The results of the evaluation are written to this file. Each record in the output file corresponds to a test case and includes the original prompt, the output generated by the LLM, and the values of the variables used in the test case.

For example outputs, see the examples/ directory.

The output file is specified by the outputPath key in the promptfoo configuration.

Permuting inputs and assertions

A vanilla prompts.txt/promptfooconfig.yaml pair supports each test combining one set of variables with one set of assertions. Trying to combine many sets of variables with many sets of assertions can lead to exponentially more config entries.

Scenarios enables one to use all possible combinations of 1+ sets of variables and 1+ sets of assertions within one config entry.