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Version: 5.7

Grouping Tests with Tags

Sometimes you don't want to run all tests and Kotest provides tags to be able to determine which tests are executed at runtime. Tags are objects inheriting from io.kotest.core.Tag.

For example, to group tests by operating system you could define the following tags:

object Linux : Tag()
object Windows: Tag()

Alternatively, tags can be defined using the NamedTag class. When using this class, observe the following rules:

  • A tag must not be null or blank.
  • A tag must not contain whitespace.
  • A tag must not contain ISO control characters.
  • A tag must not contain any of the following characters:
    • !: exclamation mark
    • (: left paren
    • ): right paren
    • &: ampersand
    • |: pipe

For example:

val tag = NamedTag("Linux")

Marking Tests

Test cases can then be marked with tags using the config function:

import io.kotest.specs.StringSpec

class MyTest : StringSpec() {
init {
"should run on Windows".config(tags = setOf(Windows)) {
// ...

"should run on Linux".config(tags = setOf(Linux)) {
// ...

"should run on Windows and Linux".config(tags = setOf(Windows, Linux)) {
// ...

Running with Tags

Then by invoking the test runner with a system property of kotest.tags you can control which tests are run. The expression to be passed in is a simple boolean expression using boolean operators: &, |, !, with parenthesis for association.

For example, Tag1 & (Tag2 | Tag3)

Provide the simple names of tag object (without package) when you run the tests. Please pay attention to the use of upper case and lower case! If two tag objects have the same simple name (in different name spaces) they are treated as the same tag.

Example: To run only test tagged with Linux, but not tagged with Database, you would invoke Gradle like this:

gradle test -Dkotest.tags="Linux & !Database"

Tags can also be included/excluded in runtime (for example, if you're running a project configuration instead of properties) through the RuntimeTagExtension:

RuntimeTagExpressionExtension.expression = "Linux & !Database"

Tag Expression Operators

Operators (in descending order of precedence)

&andlinux & integration
|orwindows | microservice

Tagging All Tests

You can add a tag to all tests in a spec using the tags function in the spec itself. For example:

class MyTestClass : FunSpec({

tags(Linux, Mysql)

test("my test") { } // automatically marked with the above tags

When tagging tests in this way, the spec class will still need to be instantiated in order to examine the tags on each test, because the test itself may define further tags.


If no root tests are active at runtime, the beforeSpec and afterSpec callbacks will not be invoked.

Tagging a Spec

There are two annotations you can add to a spec class itself - @Tags and @RequiresTag - which accept one or more tag names as their arguments.

The first tag - @Tags - will be applied to all tests in the class, however this will only stop a spec from being instantiated if we can guarantee that no tests would be executed (because a tag is being explicitly excluded).

Consider the following example:

class MyTestClass : FunSpec({


beforeSpec { println("Before") }

test("A").config(tags = setOf(Mysql)) {}
test("B").config(tags = setOf(Postgres)) {}
test("C") {}
Runtime TagsSpec CreatedCallbacksOutcome
kotest.tags=LinuxyesyesA, B, C are executed because all tests inherit the Linux tag from the annotation
kotest.tags=Linux & MysqlyesyesA is executed only because all tests have the Linux tag, but only A has the Mysql tag
kotest.tags=!LinuxnonoNo tests are executed, and the MyTestClass is not instantiated because we can exclude it based on the tags annotation
kotest.tags=!UnitTestyesnoNo tests are executed because all tests inherit UnitTest from the tags function. MyTestClass is instantiated in order to retrieve the tags defined in the class. The beforeSpec callback is not executed because there are no active tests.
kotest.tags=MysqlyesyesA is executed only, because that is the only test marked with Mysql
kotest.tags=!MysqlyesyesB, C are executed only, because A is excluded by being marked with Mysql
kotest.tags=Linux & !MysqlyesyesB, C are executed only, because all tests inherit Linux from the annotation, but A is excluded by the Mysql tag

The second tag - @RequiresTag - only checks that all the referenced tags are present and if not, will skip the spec.

For example, the following spec would be skipped and not instantiated unless the Linux and Mysql tags were specified at runtime.

@RequiresTag("Linux", "Mysql")
class MyTestClass : FunSpec()

Note that when you use these annotations you pass the tag string name, not the tag itself. This is due to Kotlin annotations only allow "primitive" arguments

Inheriting tags

By default, the @Tags annotation will only be considered on the immediate Spec which it was applied to. However, a Spec can also inherit tags from superclasses and superinterfaces. To enable this, toggle tagInheritance = true in your project config


Special attention is needed in your gradle configuration

To use System Properties (-Dx=y), your gradle must be configured to propagate them to the test executors, and an extra configuration must be added to your tests:


test {
//... Other configurations ...
systemProperties =

Kotlin Gradle DSL:

val test by tasks.getting(Test::class) {
// ... Other configurations ...
systemProperties = System.getProperties().associate { it.key.toString() to it.value }

This will guarantee that the system property is correctly read by the JVM.