Skip to main content

Assertions & metrics

Assertions are used to compare the LLM output against expected values or conditions. While assertions are not required to run an eval, they are a useful way to automate your analysis.

Different types of assertions can be used to validate the output in various ways, such as checking for equality, JSON structure, similarity, or custom functions.

In machine learning, "Accuracy" is a metric that measures the proportion of correct predictions made by a model out of the total number of predictions. With promptfoo, accuracy is defined as the proportion of prompts that produce the expected or desired output.

Using assertions

To use assertions in your test cases, add an assert property to the test case with an array of assertion objects. Each assertion object should have a type property indicating the assertion type and any additional properties required for that assertion type.


- description: 'Test if output is equal to the expected value'
example: 'Hello, World!'
- type: equals
value: 'Hello, World!'

Assertion properties

typestringYesType of assertion
valuestringNoThe expected value, if applicable
thresholdnumberNoThe threshold value, applicable only to certain types such as similar, cost, javascript, python
weightstringNoHow heavily to weigh the assertion. Defaults to 1.0
providerstringNoSome assertions (similarity, llm-rubric, model-graded-*) require an LLM provider
rubricPromptstringNoLLM rubric grading prompt

Assertion types

Deterministic eval metrics

These metrics are programmatic tests that are run on LLM output. See all details

Assertion TypeReturns true if...
contains-alloutput contains all list of substrings
contains-anyoutput contains any of the listed substrings
contains-jsonoutput contains valid json (optional json schema validation)
containsoutput contains substring
costInference cost is below a threshold
equalsoutput matches exactly
icontains-alloutput contains all list of substrings, case insensitive
icontains-anyoutput contains any of the listed substrings, case insensitive
icontainsoutput contains substring, case insensitive
is-jsonoutput is valid json (optional json schema validation)
is-valid-openai-function-callEnsure that the function call matches the function's JSON schema
is-valid-openai-tools-callEnsure all tool calls match the tools JSON schema
javascriptprovided Javascript function validates the output
latencyLatency is below a threshold (milliseconds)
levenshteinLevenshtein distance is below a threshold
perplexityPerplexity is below a threshold
perplexity-scoreNormalized perplexity
pythonprovided Python function validates the output
regexoutput matches regex
starts-withoutput starts with string
webhookprovided webhook returns {pass: true}
rouge-nRouge-N score is above a given threshold

Every test type can be negated by prepending not-. For example, not-equals or not-regex.

Model-assisted eval metrics

These metrics are model-assisted, and rely on LLMs or other machine learning models.

See Model-graded evals, classification, and similarity docs for more information.

Assertion TypeMethod
similarEmbeddings and cosine similarity are above a threshold
classifierRun LLM output through a classifier
llm-rubricLLM output matches a given rubric, using a Language Model to grade output
answer-relevanceEnsure that LLM output is related to original query
context-faithfulnessEnsure that LLM output uses the context
context-recallEnsure that ground truth appears in context
context-relevanceEnsure that context is relevant to original query
factualityLLM output adheres to the given facts, using Factuality method from OpenAI eval
model-graded-closedqaLLM output adheres to given criteria, using Closed QA method from OpenAI eval
select-bestCompare multiple outputs for a test case and pick the best one

Weighted assertions

In some cases, you might want to assign different weights to your assertions depending on their importance. The weight property is a number that determines the relative importance of the assertion. The default weight is 1.

The final score of the test case is calculated as the weighted average of the scores of all assertions, where the weights are the weight values of the assertions.

Here's an example:

- type: equals
value: 'Hello world'
weight: 2
- type: contains
value: 'world'
weight: 1

In this example, the equals assertion is twice as important as the contains assertion.

If the LLM output is Goodbye world, the equals assertion fails but the contains assertion passes, and the final score is 0.33 (1/3).

Setting a score requirement

Test cases support an optional threshold property. If set, the pass/fail status of a test case is determined by whether the combined weighted score of all assertions exceeds the threshold value.

For example:

threshold: 0.5
- type: equals
value: 'Hello world'
weight: 2
- type: contains
value: 'world'
weight: 1

If the LLM outputs Goodbye world, the equals assertion fails but the contains assertion passes and the final score is 0.33. Because this is below the 0.5 threshold, the test case fails. If the threshold were lowered to 0.2, the test case would succeed.

Load assertions from external file

Raw files

The value of an assertion can be loaded directly from a file using the file:// syntax:

- assert:
- type: contains
value: file://gettysburg_address.txt


If the file ends in .js, the Javascript is executed:

- assert:
- type: javascript
value: file://path/to/assert.js

The type definition is:

type AssertionResponse = string | boolean | number | GradingResult;
type AssertFunction = (output: string, context: { vars: Record<string, string> }) => AssertResponse;

See GradingResult definition.

Here's an example assert.js:

module.exports = (output, { vars }) => {
console.log(`Received ${output} using variables ${JSON.stringify(vars)}`);
return {
pass: true,
score: 0.5,
reason: 'Some custom reason',

You can also use Javascript files in non-javascript-type asserts. For example, using a Javascript file in a contains assertion will check that the output contains the string returned by Javascript.


If the file ends in .py, the Python is executed:

- assert:
- type: python
value: file://path/to/

The assertion expects an output that is bool, float, or a JSON GradingResult.

For example:

import sys
import json

output = sys.argv[1]
context = json.loads(sys.argv[2])

# Use `output` and `context['vars']` to determine result ...

'pass': False,
'score': 0.5,
'reason': 'Some custom reason',

Load assertions from CSV

The Tests file is an optional format that lets you specify test cases outside of the main config file.

To add an assertion to a test case in a vars file, use the special __expected column.

Here's an example tests.csv:

Hello, world!Bonjour le monde
Goodbye, everyone!fn:output.includes('Au revoir');
I am a pineapplegrade:doesn't reference any fruits besides pineapple

All assertion types can be used in __expected. The column supports exactly one assertion.

  • is-json and contains-json are supported directly, and do not require any value
  • fn indicates javascript type. For example: fn:output.includes('foo')
  • similar takes a threshold value. For example: similar(0.8):hello world
  • grade indicates llm-rubric. For example: grade: does not mention being an AI
  • By default, __expected will use type equals

When the __expected field is provided, the success and failure statistics in the evaluation summary will be based on whether the expected criteria are met.

To run multiple assertions, use column names __expected1, __expected2, __expected3, etc.

For more advanced test cases, we recommend using a testing framework like Jest or Mocha and using promptfoo as a library.

Reusing assertions with templates

If you have a set of common assertions that you want to apply to multiple test cases, you can create assertion templates and reuse them across your configuration.

type: javascript
value: output.toLowerCase().includes('mental health')

prompts: [prompt1.txt, prompt2.txt]
providers: [openai:gpt-3.5-turbo, localai:chat:vicuna]
- vars:
input: Tell me about the benefits of exercise.
- $ref: "#/assertionTemplates/containsMentalHealth"
- vars:
input: How can I improve my well-being?
- $ref: "#/assertionTemplates/containsMentalHealth"

In this example, the containsMentalHealth assertion template is defined at the top of the configuration file and then reused in two test cases. This approach helps maintain consistency and reduces duplication in your configuration.

Defining named metrics

Each assertion supports a metric field that allows you to tag the result however you like. Use this feature to combine related assertions into aggregate metrics.

For example, these asserts will aggregate results into two metrics, Tone and Consistency.

- assert:
- type: equals
value: Yarr
metric: Tone

- assert:
- type: icontains
value: grub
metric: Tone

- assert:
- type: is-json
metric: Consistency

- assert:
- type: python
value: max(0, len(output) - 300)
metric: Consistency

- type: similar
value: Ahoy, world
metric: Tone

- assert:
- type: llm-rubric
value: Is spoken like a pirate
metric: Tone

These metrics will be shown in the UI:

llm eval metrics

See named metrics example.

Running assertions directly on outputs

If you already have LLM outputs and want to run assertions on them, the eval command supports standalone assertion files.

Put your outputs in a JSON string array, like this output.json:

["Hello world", "Greetings, planet", "Salutations, Earth"]

And create a list of assertions (asserts.yaml):

- type: icontains
value: hello

- type: javascript
value: 1 / (output.length + 1) # prefer shorter outputs

- type: model-graded-closedqa
value: ensure that the output contains a greeting

Then run the eval command:

promptfoo eval --assertions asserts.yaml --model-outputs outputs.json

Tagging outputs

Promptfoo accepts a slightly more complex JSON structure that includes an output field for the model's output and a tags field for the associated tags. These tags are shown in the web UI as a comma-separated list. It's useful if you want to keep track of certain output attributes:

{ "output": "Hello world", "tags": ["foo", "bar"] },
{ "output": "Greetings, planet", "tags": ["baz", "abc"] },
{ "output": "Salutations, Earth", "tags": ["def", "ghi"] }

Processing and formatting outputs

If you need to do any processing/formatting of outputs, use a Javascript provider, Python provider, or custom script.