diff --git a/jvm/example-kotest/build.gradle.kts b/jvm/example-kotest/build.gradle.kts
index 248917e0a..178f67c28 100644
--- a/jvm/example-kotest/build.gradle.kts
+++ b/jvm/example-kotest/build.gradle.kts
@@ -17,6 +17,7 @@ dependencies {
 }
 tasks.test {
   useJUnitPlatform()
+  systemProperty("kotest.framework.config.fqn", "com.example.kotest.KotestConfig")
   environment(properties.filter { it.key == "selfie" || it.key == "OPENAI_API_KEY" })
   inputs.files(fileTree("src/test") {
     include("**/*.ss")
diff --git a/jvm/example-kotest/src/test/kotlin/com/example/kotest/KotestConfig.kt b/jvm/example-kotest/src/test/kotlin/com/example/kotest/KotestConfig.kt
index c16584b60..901a2248b 100644
--- a/jvm/example-kotest/src/test/kotlin/com/example/kotest/KotestConfig.kt
+++ b/jvm/example-kotest/src/test/kotlin/com/example/kotest/KotestConfig.kt
@@ -3,5 +3,5 @@ package com.example.kotest
 import com.diffplug.selfie.kotest.SelfieExtension
 
 class KotestConfig : io.kotest.core.config.AbstractProjectConfig() {
-    override fun extensions() = listOf(SelfieExtension(this))
+    override val extensions = listOf(SelfieExtension(this))
 }
\ No newline at end of file
diff --git a/jvm/gradle.properties b/jvm/gradle.properties
index 83dfda6f3..557243c9a 100644
--- a/jvm/gradle.properties
+++ b/jvm/gradle.properties
@@ -9,7 +9,7 @@ ver_JUNIT_PIONEER=2.3.0
 ver_OKIO=3.16.0
 ver_KOTLIN_TEST=2.0.0
 ver_KOTLIN_SERIALIZATION=1.9.0
-# Kotest 5.6.0 is the oldest that we support
-ver_KOTEST=5.6.0
+# Kotest 6.0.0 is the oldest that we support
+ver_KOTEST=6.1.0
 
 ver_JVM_TARGET=11
\ No newline at end of file
diff --git a/jvm/selfie-runner-junit5/src/main/kotlin/com/diffplug/selfie/junit5/SelfieExtension.kt b/jvm/selfie-runner-junit5/src/main/kotlin/com/diffplug/selfie/junit5/SelfieExtension.kt
index aed9aee40..b15fb563e 100644
--- a/jvm/selfie-runner-junit5/src/main/kotlin/com/diffplug/selfie/junit5/SelfieExtension.kt
+++ b/jvm/selfie-runner-junit5/src/main/kotlin/com/diffplug/selfie/junit5/SelfieExtension.kt
@@ -1,5 +1,5 @@
 /*
- * Copyright (C) 2024 DiffPlug
+ * Copyright (C) 2024-2026 DiffPlug
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -24,7 +24,7 @@ import io.kotest.core.listeners.BeforeSpecListener
 import io.kotest.core.listeners.FinalizeSpecListener
 import io.kotest.core.spec.Spec
 import io.kotest.core.test.TestCase
-import io.kotest.core.test.TestResult
+import io.kotest.engine.test.TestResult
 import kotlin.reflect.KClass
 import kotlinx.coroutines.currentCoroutineContext
 import kotlinx.coroutines.withContext
@@ -43,11 +43,11 @@ class SelfieExtension(projectConfig: AbstractProjectConfig) :
       execute: suspend (TestCase) -> TestResult
   ): TestResult {
     val file = SnapshotSystemJUnit5.forClass(testCase.spec::class.java.name)
-    val coroutineLocal = CoroutineDiskStorage(DiskStorageJUnit5(file, testCase.name.testName))
+    val coroutineLocal = CoroutineDiskStorage(DiskStorageJUnit5(file, testCase.name.name))
     return withContext(currentCoroutineContext() + coroutineLocal) {
-      file.startTest(testCase.name.testName, false)
+      file.startTest(testCase.name.name, false)
       val result = execute(testCase)
-      file.finishedTestWithSuccess(testCase.name.testName, false, result.isSuccess)
+      file.finishedTestWithSuccess(testCase.name.name, false, result.isSuccess)
       result
     }
   }
@@ -58,8 +58,8 @@ class SelfieExtension(projectConfig: AbstractProjectConfig) :
     val file = SnapshotSystemJUnit5.forClass(kclass.java.name)
     results.entries.forEach {
       if (it.value.isIgnored) {
-        file.startTest(it.key.name.testName, false)
-        file.finishedTestWithSuccess(it.key.name.testName, false, false)
+        file.startTest(it.key.name.name, false)
+        file.finishedTestWithSuccess(it.key.name.name, false, false)
       }
     }
     SnapshotSystemJUnit5.forClass(kclass.java.name)
diff --git a/jvm/selfie-runner-kotest/src/commonMain/kotlin/com/diffplug/selfie/kotest/FSOkio.kt b/jvm/selfie-runner-kotest/src/commonMain/kotlin/com/diffplug/selfie/kotest/FSOkio.kt
index 366fc526a..5bdac324e 100644
--- a/jvm/selfie-runner-kotest/src/commonMain/kotlin/com/diffplug/selfie/kotest/FSOkio.kt
+++ b/jvm/selfie-runner-kotest/src/commonMain/kotlin/com/diffplug/selfie/kotest/FSOkio.kt
@@ -1,5 +1,5 @@
 /*
- * Copyright (C) 2024 DiffPlug
+ * Copyright (C) 2024-2026 DiffPlug
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -18,18 +18,19 @@ package com.diffplug.selfie.kotest
 import com.diffplug.selfie.guts.FS
 import com.diffplug.selfie.guts.TypedPath
 import io.kotest.assertions.Actual
-import io.kotest.assertions.Exceptions
+import io.kotest.assertions.AssertionErrorBuilder
 import io.kotest.assertions.Expected
 import io.kotest.assertions.print.Printed
 import okio.FileSystem
 import okio.Path.Companion.toPath
 
-expect internal val FS_SYSTEM: FileSystem
+internal expect val FS_SYSTEM: FileSystem
 internal fun TypedPath.toPath(): okio.Path = absolutePath.toPath()
 
 internal object FSOkio : FS {
   override fun fileExists(typedPath: TypedPath): Boolean =
       FS_SYSTEM.metadataOrNull(typedPath.toPath())?.isRegularFile ?: false
+
   /** Walks the files (not directories) which are children and grandchildren of the given path. */
   override fun <T> fileWalk(typedPath: TypedPath, walk: (Sequence<TypedPath>) -> T): T =
       walk(
@@ -40,9 +41,11 @@ internal object FSOkio : FS {
       FS_SYSTEM.read(typedPath.toPath()) { readByteArray() }
   override fun fileWriteBinary(typedPath: TypedPath, content: ByteArray): Unit =
       FS_SYSTEM.write(typedPath.toPath()) { write(content) }
+
   /** Creates an assertion failed exception to throw. */
   override fun assertFailed(message: String, expected: Any?, actual: Any?): Throwable =
-      if (expected == null && actual == null) Exceptions.createAssertionError(message, null)
+      if (expected == null && actual == null)
+          AssertionErrorBuilder.create().withMessage(message).build()
       else {
         val expectedStr = nullableToString(expected, "")
         val actualStr = nullableToString(actual, "")
@@ -55,10 +58,11 @@ internal object FSOkio : FS {
           comparisonAssertion(message, expectedStr, actualStr)
         }
       }
-  private fun nullableToString(any: Any?, onNull: String): String =
-      any?.let { it.toString() } ?: onNull
+  private fun nullableToString(any: Any?, onNull: String): String = any?.toString() ?: onNull
   private fun comparisonAssertion(message: String, expected: String, actual: String): Throwable {
-    return Exceptions.createAssertionError(
-        message, null, Expected(Printed((expected))), Actual(Printed((actual))))
+    return AssertionErrorBuilder.create()
+        .withMessage(message)
+        .withValues(Expected(Printed((expected))), Actual(Printed((actual))))
+        .build()
   }
 }
diff --git a/jvm/selfie-runner-kotest/src/commonMain/kotlin/com/diffplug/selfie/kotest/SelfieExtension.kt b/jvm/selfie-runner-kotest/src/commonMain/kotlin/com/diffplug/selfie/kotest/SelfieExtension.kt
index ffd5fb653..3ee9a74b5 100644
--- a/jvm/selfie-runner-kotest/src/commonMain/kotlin/com/diffplug/selfie/kotest/SelfieExtension.kt
+++ b/jvm/selfie-runner-kotest/src/commonMain/kotlin/com/diffplug/selfie/kotest/SelfieExtension.kt
@@ -1,5 +1,5 @@
 /*
- * Copyright (C) 2024 DiffPlug
+ * Copyright (C) 2024-2026 DiffPlug
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -26,7 +26,7 @@ import io.kotest.core.listeners.FinalizeSpecListener
 import io.kotest.core.source.SourceRef
 import io.kotest.core.spec.Spec
 import io.kotest.core.test.TestCase
-import io.kotest.core.test.TestResult
+import io.kotest.engine.test.TestResult
 import kotlin.jvm.JvmStatic
 import kotlin.reflect.KClass
 import kotlinx.coroutines.currentCoroutineContext
@@ -62,18 +62,19 @@ class SelfieExtension(
     val classOrFilename: String =
         when (val source = testCase.source) {
           is SourceRef.ClassSource -> source.fqn
-          is SourceRef.FileSource -> source.fileName
+          is SourceRef.ClassLineSource -> source.fqn
           is SourceRef.None -> TODO("Handle SourceRef.None")
         }
     return system.forClassOrFilename(classOrFilename)
   }
+
   /** Called for every test method. */
   override suspend fun intercept(
       testCase: TestCase,
       execute: suspend (TestCase) -> TestResult
   ): TestResult {
     val file = snapshotFileFor(testCase)
-    val testName = testCase.name.testName
+    val testName = testCase.name.name
     val coroutineLocal = CoroutineDiskStorage(DiskStorageKotest(file, testName))
     return withContext(currentCoroutineContext() + coroutineLocal) {
       file.startTest(testName)
@@ -89,8 +90,8 @@ class SelfieExtension(
     val file = results.keys.map { snapshotFileFor(it) }.firstOrNull() ?: return
     results.entries.forEach {
       if (it.value.isIgnored) {
-        file.startTest(it.key.name.testName)
-        file.finishedTestWithSuccess(it.key.name.testName, false)
+        file.startTest(it.key.name.name)
+        file.finishedTestWithSuccess(it.key.name.name, false)
       }
     }
     file.finishedClassWithSuccess(results.entries.all { it.value.isSuccess })
diff --git a/jvm/undertest-junit5-kotest/build.gradle b/jvm/undertest-junit5-kotest/build.gradle
index e45f5e401..43fc8f05d 100644
--- a/jvm/undertest-junit5-kotest/build.gradle
+++ b/jvm/undertest-junit5-kotest/build.gradle
@@ -39,4 +39,5 @@ test {
   // defaults to 'write'
   systemProperty 'selfie', findProperty('selfie')
   systemProperty 'selfie.settings', findProperty('selfie.settings')
+  systemProperty 'kotest.framework.config.fqn', 'undertest.junit5.JunitKotestProjectConfig'
 }
\ No newline at end of file
diff --git a/jvm/undertest-junit5-kotest/harness/gradle.properties b/jvm/undertest-junit5-kotest/harness/gradle.properties
index 520a835d1..b48cfaa78 100644
--- a/jvm/undertest-junit5-kotest/harness/gradle.properties
+++ b/jvm/undertest-junit5-kotest/harness/gradle.properties
@@ -9,4 +9,4 @@ ver_JUNIT_PIONEER=2.2.0
 ver_OKIO=3.7.0
 ver_KOTLIN_TEST=1.9.22
 ver_KOTLIN_SERIALIZATION=1.6.3
-ver_KOTEST=5.8.0
\ No newline at end of file
+ver_KOTEST=6.1.0
\ No newline at end of file
diff --git a/jvm/undertest-junit5-kotest/src/test/kotlin/undertest/junit5/JunitKotestProjectConfig.kt b/jvm/undertest-junit5-kotest/src/test/kotlin/undertest/junit5/JunitKotestProjectConfig.kt
index 574982605..6750745d9 100644
--- a/jvm/undertest-junit5-kotest/src/test/kotlin/undertest/junit5/JunitKotestProjectConfig.kt
+++ b/jvm/undertest-junit5-kotest/src/test/kotlin/undertest/junit5/JunitKotestProjectConfig.kt
@@ -4,5 +4,5 @@ import com.diffplug.selfie.junit5.SelfieExtension
 import io.kotest.core.config.AbstractProjectConfig
 
 class JunitKotestProjectConfig : AbstractProjectConfig() {
-  override fun extensions() = listOf(SelfieExtension(this))
+  override val extensions = listOf(SelfieExtension(this))
 }
diff --git a/jvm/undertest-junit5-kotest/src/test/kotlin/undertest/junit5/UT_KotestConcurrencyStressTest.kt b/jvm/undertest-junit5-kotest/src/test/kotlin/undertest/junit5/UT_KotestConcurrencyStressTest.kt
index bdabe2565..540b9ddaf 100644
--- a/jvm/undertest-junit5-kotest/src/test/kotlin/undertest/junit5/UT_KotestConcurrencyStressTest.kt
+++ b/jvm/undertest-junit5-kotest/src/test/kotlin/undertest/junit5/UT_KotestConcurrencyStressTest.kt
@@ -1,12 +1,15 @@
 package undertest.junit5
 
 import com.diffplug.selfie.coroutines.expectSelfie
+import io.kotest.common.ExperimentalKotest
 import io.kotest.core.spec.style.FunSpec
+import io.kotest.engine.concurrency.TestExecutionMode
 import kotlinx.coroutines.delay
 
+@OptIn(ExperimentalKotest::class)
 class UT_KotestConcurrencyStressTest :
     FunSpec({
-      concurrency = 100
+      testExecutionMode = TestExecutionMode.LimitedConcurrency(100)
       for (d in 1..1000) {
         val digit = d
         test(String.format("test %04d", digit)) {
diff --git a/jvm/undertest-kotest/src/commonTest/kotlin/undertest/kotest/UT_ConcurrencyStressTest.kt b/jvm/undertest-kotest/src/commonTest/kotlin/undertest/kotest/UT_ConcurrencyStressTest.kt
index 995718021..65760c78f 100644
--- a/jvm/undertest-kotest/src/commonTest/kotlin/undertest/kotest/UT_ConcurrencyStressTest.kt
+++ b/jvm/undertest-kotest/src/commonTest/kotlin/undertest/kotest/UT_ConcurrencyStressTest.kt
@@ -1,12 +1,15 @@
 package undertest.kotest
 
 import com.diffplug.selfie.coroutines.expectSelfie
+import io.kotest.common.ExperimentalKotest
 import io.kotest.core.spec.style.FunSpec
+import io.kotest.engine.concurrency.TestExecutionMode
 import kotlinx.coroutines.delay
 
+@OptIn(ExperimentalKotest::class)
 class UT_ConcurrencyStressTest :
     FunSpec({
-      concurrency = 100
+      testExecutionMode = TestExecutionMode.LimitedConcurrency(100)
       for (d in 1..1000) {
         val digit = d
         test(String.format("test %04d", digit)) {
diff --git a/jvm/undertest-kotest/src/jvmTest/kotlin/kotest/KotestProjectConfig.kt b/jvm/undertest-kotest/src/jvmTest/kotlin/kotest/KotestProjectConfig.kt
index c15b38e8b..9632653e2 100644
--- a/jvm/undertest-kotest/src/jvmTest/kotlin/kotest/KotestProjectConfig.kt
+++ b/jvm/undertest-kotest/src/jvmTest/kotlin/kotest/KotestProjectConfig.kt
@@ -4,5 +4,5 @@ import com.diffplug.selfie.kotest.SelfieExtension
 import io.kotest.core.config.AbstractProjectConfig
 
 object KotestProjectConfig : AbstractProjectConfig() {
-  override fun extensions() = listOf(SelfieExtension(this))
+  override val extensions = listOf(SelfieExtension(this))
 }
diff --git a/selfie.dev/src/pages/jvm/kotest.mdx b/selfie.dev/src/pages/jvm/kotest.mdx
index b748affb1..8dcc5c81f 100644
--- a/selfie.dev/src/pages/jvm/kotest.mdx
+++ b/selfie.dev/src/pages/jvm/kotest.mdx
@@ -40,6 +40,9 @@ class ProjectConfig : AbstractProjectConfig() {
 }
 ```
 
+Note that your `AbstractProjectConfig` must either have the fully qualified name of `io.kotest.provided.ProjectConfig` or the system property `kotest.framework.config.fqn` must be set.
+See [Kotest docs](https://kotest.io/docs/framework/project-config.html) for more info.
+
 ## Selfie and coroutines
 
 In a regular JUnit 5 test, you call `Selfie.expectSelfie(...)`, like so