In Langtail, a test consists of a collection of cases and assertions. When run, each individual case is evaluated against all of the assertions.

A case is a user message and/or variable values that are added to the prompt before it is executed in a test run.

An assertion evaluates the prompt’s output. Langtail supports the following assertion types:

  • equals: Checks if the specified value exactly matches the output.
  • contains: Checks if the specified value is somewhere in the output.
  • icontains: Similar to contains except it is case insensitive.
  • javascript: The most powerful and flexible assertion. Allows you to define a custom JavaScript function to check the output.


Let’s explain it further with an example.

Here is a prompt that that analyzes the sentiment of some text and returns a JSON object containing that sentiment. Here’s what a basic set of tests could look like:

Screenshot of Langtail's test section

  1. Test 1 - Positive Sentiment

    • Case: “I am happy.”
    • Assertion: An equals assertion with the value {"sentiment": "positive"}
  2. Test 2 - Positive Sentiment

    • Case: “I am sad.”
    • Assertion: An equals assertion with the value {"sentiment": "negative"}
  3. Test 3 - Is Valid JSON

    • Case: “I saw a blue car.”

    • Assertion: A javascript assertion with the following code:

      test(async ({ message, variables }) => {
        try {
              return true; // Parsing succeeded, so it's valid JSON
          } catch (e) {
              return false; // Parsing failed, so it's not valid JSON

There’s a lot more that could be tested, but having even a basic test suite for each of your prompts can be a great way to safeguard against typos (like misspelling “positive”) or parameter changes (like turning off JSON mode).

If you have any questions regarding tests or best practices for your project, get in touch with us.