diff --git a/common/DiffractionExperiment.cpp b/common/DiffractionExperiment.cpp
index 88fd6e22..2a8c0150 100644
--- a/common/DiffractionExperiment.cpp
+++ b/common/DiffractionExperiment.cpp
@@ -1647,3 +1647,8 @@ int64_t DiffractionExperiment::GetDarkMaskNumberOfFrames() const {
 bool DiffractionExperiment::IsRotationIndexing() const {
     return GetGoniometer().has_value() && indexing.GetRotationIndexing();
 }
+
+DiffractionExperiment &DiffractionExperiment::RunNumber(uint64_t input) {
+    series_id = input;
+    return *this;
+}
diff --git a/common/DiffractionExperiment.h b/common/DiffractionExperiment.h
index ffd2149e..e1dda665 100644
--- a/common/DiffractionExperiment.h
+++ b/common/DiffractionExperiment.h
@@ -157,6 +157,7 @@ public:
     DiffractionExperiment& PoniRot3_rad(float input);
     DiffractionExperiment& FluorescenceSpectrum(const XrayFluorescenceSpectrum& input);
     DiffractionExperiment& DetectIceRings(bool input);
+    DiffractionExperiment& RunNumber(uint64_t input);
 
     IndexingSettings GetIndexingSettings() const;
     DiffractionExperiment& ImportIndexingSettings(const IndexingSettings &input);
diff --git a/image_pusher/TCPStreamPusher.cpp b/image_pusher/TCPStreamPusher.cpp
index 02c357eb..52c0f582 100644
--- a/image_pusher/TCPStreamPusher.cpp
+++ b/image_pusher/TCPStreamPusher.cpp
@@ -607,6 +607,14 @@ void TCPStreamPusher::PersistentAckThread(Connection* c) {
         const bool fatal = (h.flags & TCP_ACK_FLAG_FATAL) != 0;
         const auto code = static_cast<TCPAckCode>(h.ack_code);
 
+        // Validate run number: discard stale ACKs from a previous run on a persistent connection
+        if (h.run_number != run_number) {
+            logger.Warning("Discarding ACK with stale run_number " + std::to_string(h.run_number)
+                           + " (expected " + std::to_string(run_number) + ") on socket "
+                           + std::to_string(c->socket_number));
+            continue;
+        }
+
         {
             std::unique_lock ul(c->ack_mutex);
             c->last_ack_code = code;
diff --git a/tests/TCPImagePusherTest.cpp b/tests/TCPImagePusherTest.cpp
index 991d4b40..05586959 100644
--- a/tests/TCPImagePusherTest.cpp
+++ b/tests/TCPImagePusherTest.cpp
@@ -16,6 +16,7 @@ TEST_CASE("TCPImageCommTest_2Writers_WithAck", "[TCP]") {
     x.Raw();
     x.PedestalG0Frames(0).NumTriggers(1).UseInternalPacketGenerator(false).IncidentEnergy_keV(12.4)
             .ImagesPerTrigger(nframes).Compression(CompressionAlgorithm::NO_COMPRESSION);
+    x.RunNumber(567);
 
     std::mt19937 g1(1387);
     std::uniform_int_distribution<uint16_t> dist;
@@ -45,6 +46,7 @@ TEST_CASE("TCPImageCommTest_2Writers_WithAck", "[TCP]") {
 
         StartMessage start{
             .images_per_file = images_per_file,
+            .run_number = x.GetRunNumber(),
             .write_master_file = true
         };
         EndMessage end{};
@@ -100,6 +102,7 @@ TEST_CASE("TCPImageCommTest_2Writers_WithAck", "[TCP]") {
                     ack.image_number = 0;
                     ack.processed_images = 0;
                     ack.error_code = TCPAckCode::None;
+                    REQUIRE(ack.run_number == x.GetRunNumber());
                     REQUIRE(puller[w]->SendAck(ack));
                     seen_start = true;
                     continue;