State what an engine has to do, once, for every engine

build.gradle.kts

@@ -15,11 +15,12 @@ subprojects {
     version = "1.2.0"
 }
 
-// The runnable example and the benchmark harness are not published libraries — exclude them from
-// public-API tracking.
+// The runnable example, the benchmark harness and the shared test suite are not published libraries —
+// exclude them from public-API tracking.
 apiValidation {
     ignoredProjects.add("example-rag")
     ignoredProjects.add("benchmarks")
+    ignoredProjects.add("kdrant-testkit")
     // The gRPC engine's protobuf and stub classes are generated from Qdrant's own .proto files, so
     // their surface is Qdrant's to change, not ours to promise. Tracking them would bury the module's
     // real API — the transport factory — under thousands of generated lines. Everything hand-written
@@ -40,6 +41,7 @@ configure(
         project(":kdrant-micrometer"),
         project(":kdrant-koog"),
         project(":kdrant-transport-grpc"),
+        project(":kdrant-testkit"),
         project(":example-rag"),
     ),
 ) {

config/detekt/detekt.yml

@@ -6,6 +6,12 @@ naming:
   FunctionNaming:
     ignoreAnnotated: []
     functionPattern: '[a-zA-Z][a-zA-Z0-9]*'
+    # kdrant-testkit holds the shared client contract, which is a test suite that happens to live in a
+    # main source set because two other modules' test source sets consume it. Its test names are
+    # backtick sentences like every other test in the repository, so it gets the exclusion detekt
+    # already gives src/test.
+    excludes: ['**/test/**', '**/androidTest/**', '**/commonTest/**', '**/jvmTest/**', '**/*.Test.kt',
+               '**/*.Spec.kt', '**/*.Spek.kt', '**/kdrant-testkit/**']
   # DSL helpers are grouped by concept into *Dsl.kt files (e.g. PayloadDsl.kt holds PayloadBuilder + payloadOf).
   MatchingDeclarationName:
     active: false

kdrant-testkit/build.gradle.kts

@@ -0,0 +1,31 @@
+import org.gradle.api.tasks.testing.logging.TestExceptionFormat
+
+plugins {
+    alias(libs.plugins.kotlin.jvm)
+}
+
+// Not published. This module exists so both engines can be held to the same behaviour, and its only
+// consumers are the two transport modules' test source sets. It carries no `mavenPublishing` block and
+// is excluded from public-API tracking for that reason.
+
+kotlin {
+    jvmToolchain(17)
+}
+
+dependencies {
+    // `api` throughout: everything here is part of what a subclass of the contract sees.
+    api(project(":kdrant-core"))
+    api(platform(libs.junit.bom))
+    api(libs.junit.jupiter)
+    api(platform(libs.testcontainers.bom))
+    api(libs.testcontainers.qdrant)
+    api(libs.kotlinx.coroutines.core)
+}
+
+tasks.test {
+    useJUnitPlatform()
+    testLogging {
+        events("passed", "skipped", "failed")
+        exceptionFormat = TestExceptionFormat.FULL
+    }
+}