diff --git a/amber/src/test/scala/org/apache/texera/amber/engine/architecture/scheduling/ControllerRpcProbeSpec.scala b/amber/src/test/scala/org/apache/texera/amber/engine/architecture/scheduling/ControllerRpcProbeSpec.scala
new file mode 100644
index 00000000000..46828fc158c
--- /dev/null
+++ b/amber/src/test/scala/org/apache/texera/amber/engine/architecture/scheduling/ControllerRpcProbeSpec.scala
@@ -0,0 +1,103 @@
+/*
+ * 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.engine.architecture.scheduling
+
+import org.apache.texera.amber.engine.architecture.rpc.controlcommands.{
+  AsyncRPCContext,
+  ControlInvocation,
+  EmptyRequest
+}
+import org.apache.texera.amber.engine.architecture.scheduling.RegionCoordinatorTestSupport._
+import org.apache.texera.amber.engine.common.virtualidentity.util.CONTROLLER
+import org.scalatest.flatspec.AnyFlatSpec
+
+import java.util.concurrent.CountDownLatch
+import java.util.concurrent.atomic.AtomicReference
+
+/**
+  * Concurrency regression test for `ControllerRpcProbe`.
+  *
+  * The probe's `calls` buffer is appended to on a scheduler thread while the test thread reads it.
+  * Before the fix, a read racing an append tripped Scala 2.13's `MutationTracker` and threw a
+  * `ConcurrentModificationException`. This test forces that race so it fails deterministically
+  * rather than as a CI flake.
+  */
+class ControllerRpcProbeSpec extends AnyFlatSpec {
+
+  "ControllerRpcProbe" should "tolerate reads racing with concurrent appends" in {
+    // Hold every endWorker pending so appends have no fulfill side effects.
+    val probe = new ControllerRpcProbe(_ => None)
+    val appends = 20000
+    val failure = new AtomicReference[Throwable]()
+    // Release both threads at once so the reader polls while the writer is still appending;
+    // otherwise the writer could finish before the reader starts and miss the race entirely.
+    val startGate = new CountDownLatch(1)
+
+    // Writer: the actor side. Each sendTo drives handleOutput -> calls += call.
+    val writer = new Thread(() => {
+      try {
+        startGate.await()
+        var i = 0
+        while (i < appends) {
+          probe.outputGateway.sendTo(
+            CONTROLLER,
+            ControlInvocation(
+              EndWorker,
+              EmptyRequest(),
+              AsyncRPCContext(CONTROLLER, CONTROLLER),
+              i.toLong
+            )
+          )
+          i += 1
+        }
+      } catch {
+        case t: Throwable => failure.compareAndSet(null, t)
+      }
+    })
+
+    // Reader: the test side. Read through every helper while appends are in flight.
+    val reader = new Thread(() => {
+      try {
+        startGate.await()
+        while (writer.isAlive) {
+          probe.endWorkerCalls
+          probe.methodTrace
+          probe.initializedWorkers
+          probe.startedWorkers
+        }
+      } catch {
+        case t: Throwable => failure.compareAndSet(null, t)
+      }
+    })
+
+    writer.start()
+    reader.start()
+    startGate.countDown()
+    writer.join(testTimeout.inMilliseconds)
+    reader.join(testTimeout.inMilliseconds)
+
+    assert(!writer.isAlive && !reader.isAlive, "stress threads did not finish within the deadline")
+    assert(
+      failure.get() == null,
+      s"concurrent access to the probe threw ${failure.get()}"
+    )
+    assert(probe.endWorkerCalls.size == appends)
+  }
+}
diff --git a/amber/src/test/scala/org/apache/texera/amber/engine/architecture/scheduling/RegionCoordinatorTestSupport.scala b/amber/src/test/scala/org/apache/texera/amber/engine/architecture/scheduling/RegionCoordinatorTestSupport.scala
index 5673c02691f..e311349077e 100644
--- a/amber/src/test/scala/org/apache/texera/amber/engine/architecture/scheduling/RegionCoordinatorTestSupport.scala
+++ b/amber/src/test/scala/org/apache/texera/amber/engine/architecture/scheduling/RegionCoordinatorTestSupport.scala
@@ -91,25 +91,34 @@ object RegionCoordinatorTestSupport {
     * hold termination pending, fail an attempt, or allow it to succeed.
     */
   class ControllerRpcProbe(endWorkerResponse: WorkerRpcCall => Option[ControlReturn]) {
-    val calls: mutable.ArrayBuffer[WorkerRpcCall] = mutable.ArrayBuffer()
+    // `calls` is appended to on the actor/scheduler thread (via the output-gateway callback) while
+    // test assertions read it from the test thread. Guard every access with `callsLock`, and hand
+    // readers an immutable snapshot so iteration never races a concurrent append — which Scala
+    // 2.13's MutationTracker would otherwise surface as a ConcurrentModificationException.
+    private val callsLock = new Object
+    private val calls: mutable.ArrayBuffer[WorkerRpcCall] = mutable.ArrayBuffer()
+
     val inputGateway = new NetworkInputGateway(CONTROLLER)
     val outputGateway = new NetworkOutputGateway(CONTROLLER, handleOutput)
     val asyncRPCClient = new AsyncRPCClient(inputGateway, outputGateway, CONTROLLER)
 
-    def methodTrace: Seq[String] = calls.map(_.methodName).toSeq
+    private def callsSnapshot: Seq[WorkerRpcCall] = callsLock.synchronized(calls.toSeq)
+
+    def methodTrace: Seq[String] = callsSnapshot.map(_.methodName)
 
     def initializedWorkers: Seq[ActorVirtualIdentity] =
-      calls.filter(_.methodName == InitializeExecutor).map(_.receiver).toSeq
+      callsSnapshot.filter(_.methodName == InitializeExecutor).map(_.receiver)
 
     def startedWorkers: Seq[ActorVirtualIdentity] =
-      calls.filter(_.methodName == StartWorker).map(_.receiver).toSeq
+      callsSnapshot.filter(_.methodName == StartWorker).map(_.receiver)
 
     def endWorkerCalls: Seq[WorkerRpcCall] =
-      calls.filter(_.methodName == EndWorker).toSeq
+      callsSnapshot.filter(_.methodName == EndWorker)
 
     def onlyEndWorkerCall: WorkerRpcCall = {
-      assert(endWorkerCalls.size == 1)
-      endWorkerCalls.head
+      val ends = endWorkerCalls
+      assert(ends.size == 1)
+      ends.head
     }
 
     def fulfill(call: WorkerRpcCall, returnValue: ControlReturn): Unit = {
@@ -131,7 +140,7 @@ object RegionCoordinatorTestSupport {
         receiver = invocation.context.receiver,
         commandId = invocation.commandId
       )
-      calls += call
+      callsLock.synchronized(calls += call)
       immediateReturn(call).foreach(fulfill(call, _))
     }