diff --git a/amber/src/test/scala/org/apache/texera/amber/clustering/ClusterListenerSpec.scala b/amber/src/test/scala/org/apache/texera/amber/clustering/ClusterListenerSpec.scala
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+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.apache.texera.amber.clustering
+
+import org.apache.pekko.actor.{ActorRef, ActorSystem, Address, Props}
+import org.apache.pekko.cluster.Cluster
+import org.apache.pekko.testkit.{ImplicitSender, TestActorRef, TestKit}
+import org.apache.texera.amber.engine.common.AmberRuntime
+import org.apache.texera.web.SessionState
+import org.apache.texera.amber.util.JSONUtils.objectMapper
+import org.scalamock.scalatest.MockFactory
+import org.scalatest.BeforeAndAfterAll
+import org.scalatest.flatspec.AnyFlatSpecLike
+import org.scalatest.matchers.should.Matchers
+
+import java.util.UUID
+import java.util.concurrent.{ConcurrentLinkedQueue, Future => JFuture}
+import javax.websocket.{RemoteEndpoint, Session}
+import scala.collection.mutable.ArrayBuffer
+import scala.jdk.CollectionConverters.CollectionHasAsScala
+import scala.concurrent.duration._
+
+/**
+  * Unit tests for the cluster membership listener.
+  *
+  * This is the last file in the engine that sat at 0%, and it is worth covering because the count
+  * it maintains is what the frontend's cluster badge renders: `updateClusterStatus` recomputes
+  * `numWorkerNodesInCluster` on every membership event and pushes a `ClusterStatusUpdateEvent` to
+  * every open session. A listener that stops subscribing, or stops fanning out, leaves every
+  * client showing a stale node count with nothing failing.
+  *
+  * The suite runs a real single-node cluster: `AmberRuntime.pekkoConfig` selects the cluster
+  * provider with artery bound on port 0, so joining the node to itself produces genuine
+  * `MemberUp` events. That matters because `Member` is `private[cluster]` and cannot be
+  * synthesized — a real join is the only way to exercise the event path at all.
+  *
+  * Not covered, and deliberately: the `MemberRemoved` recovery arm. It walks
+  * `WorkflowService.getAllWorkflowServices` and calls `client.notifyNodeFailure` or
+  * `forcefullyStop` on each live execution, so it needs real executions with real Amber clients —
+  * integration scope, not this.
+  */
+class ClusterListenerSpec
+    extends TestKit(ActorSystem("ClusterListenerSpec", AmberRuntime.pekkoConfig))
+    with ImplicitSender
+    with AnyFlatSpecLike
+    with Matchers
+    with MockFactory
+    with BeforeAndAfterAll {
+
+  private val registeredSessions = ArrayBuffer[String]()
+  private var previousNodeCount: Int = 0
+
+  override def beforeAll(): Unit = {
+    // `numWorkerNodesInCluster` is a JVM-global var other suites may read; put it back afterwards.
+    previousNodeCount = ClusterListener.numWorkerNodesInCluster
+    // A single node joining itself becomes the cluster leader and reaches Up on its own, with no
+    // seed nodes and no second JVM.
+    Cluster(system).join(Cluster(system).selfAddress)
+  }
+
+  override def afterAll(): Unit = {
+    registeredSessions.foreach(id => scala.util.Try(SessionState.removeState(id)))
+    ClusterListener.numWorkerNodesInCluster = previousNodeCount
+    TestKit.shutdownActorSystem(system)
+  }
+
+  /**
+    * Registers a session whose outbound frames are collected, runs `body`, then removes it.
+    *
+    * The removal has to happen inside the test: ScalaMock scopes expectations per test, and the
+    * SessionState registry is JVM-global, so a session left behind is called by a LATER test's
+    * listener against an expired mock ("Unexpected call: Session.getAsyncRemote"). Observed.
+    */
+  private def withSession[A](body: (() => Seq[String]) => A): A = {
+    val (id, sent) = mockSession()
+    // The body receives a snapshot function rather than the live queue, so every assertion reads a
+    // consistent point-in-time view instead of iterating a collection the actor thread is appending to.
+    try body(() => sent.asScala.toList)
+    finally scala.util.Try(SessionState.removeState(id))
+  }
+
+  /**
+    * A session whose outbound frames are collected, so the fan-out is observable.
+    *
+    * The queue is concurrent on purpose: `sendText` is invoked on the listener's actor thread while
+    * the assertions read it from the test thread inside `awaitAssert`. A plain ArrayBuffer would be
+    * an unsynchronised hand-off across those two threads - the very hazard this suite exists to
+    * document on the production side.
+    */
+  private def mockSession(): (String, ConcurrentLinkedQueue[String]) = {
+    val sent = new ConcurrentLinkedQueue[String]()
+    val async = mock[RemoteEndpoint.Async]
+    (async
+      .sendText(_: String))
+      .expects(*)
+      .onCall { (text: String) =>
+        sent.add(text)
+        null.asInstanceOf[JFuture[Void]]
+      }
+      .anyNumberOfTimes()
+
+    val session = mock[Session]
+    val id = UUID.randomUUID().toString
+    (() => session.getId).expects().returning(id).anyNumberOfTimes()
+    (() => session.getAsyncRemote).expects().returning(async).anyNumberOfTimes()
+    (() => session.getUserProperties)
+      .expects()
+      .returning(new java.util.HashMap[String, Object]())
+      .anyNumberOfTimes()
+
+    SessionState.setState(id, new SessionState(session))
+    registeredSessions += id
+    (id, sent)
+  }
+
+  private def selfAddress: Address = Cluster(system).selfAddress
+
+  /**
+    * Runs `body` with a live listener and stops it afterwards.
+    *
+    * Stopping matters: a listener left running stays subscribed and keeps iterating
+    * `SessionState.getAllSessionStates` on every membership event. That registry is a plain
+    * unsynchronised mutable.HashMap, so a leftover listener from an earlier case will throw
+    * ConcurrentModificationException the moment a later case registers a session. (Observed - an
+    * earlier draft of this suite failed exactly that way.)
+    */
+  private def withListener[A](body: ActorRef => A): A = {
+    val listener = system.actorOf(Props[ClusterListener]())
+    try body(listener)
+    finally {
+      watch(listener)
+      system.stop(listener)
+      expectTerminated(listener, 10.seconds)
+      unwatch(listener)
+    }
+  }
+
+  behavior of "ClusterListener"
+
+  it should "report the cluster's member addresses on request" in {
+    // The node joined itself in beforeAll, so once it reaches Up the listener must report exactly
+    // that one address. Asserting the address (not just a count) is what pins getAllAddress to
+    // `cluster.state.members.map(_.address)` rather than, say, the seed-node list.
+    withListener { listener =>
+      awaitAssert(
+        {
+          listener ! ClusterListener.GetAvailableNodeAddresses()
+          expectMsgType[Array[Address]](2.seconds).toSeq shouldBe Seq(selfAddress)
+        },
+        15.seconds,
+        500.millis
+      )
+    }
+  }
+
+  it should "recompute the node count and push it to every open session on a membership event" in {
+    // Register the session BEFORE the listener exists, so nothing mutates the registry while a
+    // listener is iterating it (see withListener's note), and remove it after the listener stops.
+    withSession { sentFrames =>
+      // Restore the sentinel in a local finally, not just in afterAll: amber runs suites
+      // concurrently in one JVM, so a sibling reading this global should not be able to observe -1
+      // for any longer than this case needs it.
+      val previous = ClusterListener.numWorkerNodesInCluster
+      ClusterListener.numWorkerNodesInCluster = -1
+      try {
+        // Creating the listener subscribes it with InitialStateAsEvents, so the already-Up member is
+        // replayed to it as a MemberUp. That is what drives updateClusterStatus here - no synthetic
+        // Member is needed, and none could be built (Member is private[cluster]).
+        withListener { _ =>
+          awaitAssert(
+            {
+              // The count is recomputed from live membership, not left at the sentinel.
+              ClusterListener.numWorkerNodesInCluster shouldBe 1
+
+              val counts = sentFrames()
+                .map(objectMapper.readTree)
+                .filter(_.get("type").asText() == "ClusterStatusUpdateEvent")
+                .map(_.get("numWorkers").asInt())
+              // Every open session is told, and told the recomputed number.
+              counts should not be empty
+              counts.last shouldBe 1
+            },
+            15.seconds,
+            500.millis
+          )
+        }
+      } finally ClusterListener.numWorkerNodesInCluster = previous
+    }
+  }
+
+  it should "swallow an unrecognised message rather than failing on it" in {
+    // `TestActorRef.receive` invokes the actor's receive directly and lets the exception escape to
+    // the caller, so "did not throw" genuinely means the catch-all arm handled the message.
+    //
+    // Neither `!` nor a plain ActorRef would discriminate here: an exception thrown from receive
+    // is taken by the supervisor, the actor is restarted, and a restarted listener answers the
+    // next request exactly like one that never failed. (Checked - a version of this test written
+    // with `!` stayed green with the catch-all replaced by a `throw`.)
+    val listener = TestActorRef[ClusterListener](Props[ClusterListener]())
+    try {
+      noException should be thrownBy listener.receive("not a cluster event")
+    } finally {
+      // Stopped for the same reason the others are: it subscribed in preStart, and a listener left
+      // running keeps iterating the shared SessionState registry on every membership event.
+      watch(listener)
+      system.stop(listener)
+      expectTerminated(listener, 10.seconds)
+      unwatch(listener)
+    }
+  }
+
+  // Not asserted separately: that postStop unsubscribes. Every case above stops its listener via
+  // `withListener` and the suite stays green across repeated runs, which is the observable
+  // consequence - a listener that stayed subscribed after stop would trip the registry race
+  // described there. A direct assertion would need to reach into Cluster's subscriber set.
+}