diff --git a/libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/kotlin/org/jetbrains/kotlin/gradle/native/NativeDownloadAndPlatformLibsIT.kt b/libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/kotlin/org/jetbrains/kotlin/gradle/native/NativeDownloadAndPlatformLibsIT.kt
index ecb0490f9903a..5b7fcf3a2f5d9 100644
--- a/libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/kotlin/org/jetbrains/kotlin/gradle/native/NativeDownloadAndPlatformLibsIT.kt
+++ b/libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/kotlin/org/jetbrains/kotlin/gradle/native/NativeDownloadAndPlatformLibsIT.kt
@@ -127,6 +127,33 @@ class NativeDownloadAndPlatformLibsIT : KGPBaseTest() {
         }
     }
 
+    @DisplayName("Toolchain leaves a distribution installed by the legacy downloader alone (KT-86251)")
+    @GradleTest
+    fun testToolchainDoesNotReinstallOverLegacyInstallation(gradleVersion: GradleVersion) {
+        platformLibrariesProject("linuxX64", gradleVersion = gradleVersion) {
+            val konanDataDir = workingDir.resolve(".konan")
+
+            // this path has to write 'provisioned.ok' as well, that is the marker the toolchain looks for
+            build("tasks", buildOptions = defaultBuildOptions.copy(konanDataDir = konanDataDir)) {
+                assertOutputContains("Unpack Kotlin/Native compiler to ")
+            }
+
+            // the toolchain has to leave that installation alone. Unpacking on top of it happens in place, so
+            // anything reading the distribution at the same time sees half-written files.
+            build(
+                "downloadKotlinNativeDistribution",
+                buildOptions = defaultBuildOptions.copy(
+                    konanDataDir = konanDataDir,
+                    freeArgs = listOf("-Pkotlin.native.toolchain.enabled=true"),
+                ),
+            ) {
+                assertTasksExecuted(":downloadKotlinNativeDistribution")
+                assertOutputDoesNotContain("Native bundle files will be overwritten")
+                assertOutputDoesNotContain("Moving Kotlin/Native bundle from")
+            }
+        }
+    }
+
     @DisplayName("K/N distribution with platform libraries generation")
     @GradleTest
     fun testLibrariesGeneration(gradleVersion: GradleVersion) {
diff --git a/libraries/tools/kotlin-gradle-plugin/src/common/kotlin/org/jetbrains/kotlin/gradle/targets/native/NativeCompilerDownloader.kt b/libraries/tools/kotlin-gradle-plugin/src/common/kotlin/org/jetbrains/kotlin/gradle/targets/native/NativeCompilerDownloader.kt
index 2f76a32f19f62..f3524744553b9 100644
--- a/libraries/tools/kotlin-gradle-plugin/src/common/kotlin/org/jetbrains/kotlin/gradle/targets/native/NativeCompilerDownloader.kt
+++ b/libraries/tools/kotlin-gradle-plugin/src/common/kotlin/org/jetbrains/kotlin/gradle/targets/native/NativeCompilerDownloader.kt
@@ -24,6 +24,7 @@ import org.jetbrains.kotlin.gradle.report.GradleBuildMetricsReporter
 import org.jetbrains.kotlin.gradle.targets.native.internal.NativeDistributionTypeProvider
 import org.jetbrains.kotlin.gradle.targets.native.internal.PlatformLibrariesGenerator
 import org.jetbrains.kotlin.gradle.targets.native.konanPropertiesBuildService
+import org.jetbrains.kotlin.gradle.targets.native.toolchain.NativeVersionValueSource
 import org.jetbrains.kotlin.internal.compilerRunner.native.nativeCompilerClasspath
 import org.jetbrains.kotlin.konan.target.HostManager
 import org.jetbrains.kotlin.konan.target.KonanTarget
@@ -202,13 +203,24 @@ class NativeCompilerDownloader(
                     it.into(tmpDir)
                 }
                 val compilerTmp = tmpDir.resolve(dependencyNameWithOsAndVersion)
-                if (!compilerTmp.renameTo(compilerDirectory)) {
-                    project.copy {
-                        it.from(compilerTmp)
-                        it.into(compilerDirectory)
+                val installed = when {
+                    compilerTmp.renameTo(compilerDirectory) -> true
+                    !compilerDirectory.exists() -> {
+                        project.copy {
+                            it.from(compilerTmp)
+                            it.into(compilerDirectory)
+                        }
+                        true
                     }
+                    // another build got there first, copying on top would rewrite files it is reading (KT-86251)
+                    else -> false
                 }
                 logger.debug("Moved Kotlin/Native compiler from $tmpDir to $compilerDirectory")
+                if (installed) {
+                    // without the marker a toolchain build sharing this konan data dir unpacks the archive over the
+                    // distribution again (KT-86251). Keep it the last thing written.
+                    compilerDirectory.resolve(NativeVersionValueSource.MARKER_FILE).createNewFile()
+                }
             } finally {
                 tmpDir.deleteRecursively()
             }