diff --git a/Framework/Core/include/Framework/MessageSet.h b/Framework/Core/include/Framework/MessageSet.h
index 166934238d647..323c0ad4608af 100644
--- a/Framework/Core/include/Framework/MessageSet.h
+++ b/Framework/Core/include/Framework/MessageSet.h
@@ -136,30 +136,6 @@ struct MessageSet {
     }
   }
 
-  fair::mq::MessagePtr& header(size_t partIndex)
-  {
-    return messages[messageMap[partIndex].position];
-  }
-
-  fair::mq::MessagePtr& payload(size_t partIndex, size_t payloadIndex = 0)
-  {
-    assert(partIndex < messageMap.size());
-    assert(messageMap[partIndex].position + payloadIndex + 1 < messages.size());
-    return messages[messageMap[partIndex].position + payloadIndex + 1];
-  }
-
-  fair::mq::MessagePtr const& header(size_t partIndex) const
-  {
-    return messages[messageMap[partIndex].position];
-  }
-
-  fair::mq::MessagePtr const& payload(size_t partIndex, size_t payloadIndex = 0) const
-  {
-    assert(partIndex < messageMap.size());
-    assert(messageMap[partIndex].position + payloadIndex + 1 < messages.size());
-    return messages[messageMap[partIndex].position + payloadIndex + 1];
-  }
-
   fair::mq::MessagePtr const& associatedHeader(size_t pos) const
   {
     return messages[messageMap[pairMap[pos].partIndex].position];
diff --git a/Framework/Core/src/DataProcessingDevice.cxx b/Framework/Core/src/DataProcessingDevice.cxx
index 0fa70947bf18c..bb6502758a95a 100644
--- a/Framework/Core/src/DataProcessingDevice.cxx
+++ b/Framework/Core/src/DataProcessingDevice.cxx
@@ -2140,8 +2140,9 @@ bool DataProcessingDevice::tryDispatchComputation(ServiceRegistryRef ref, std::v
         //   sequence is the header message
         // - each part has one or more payload messages
         // - InputRecord provides all payloads as header-payload pair
-        auto const& headerMsg = currentSetOfInputs[i].associatedHeader(partindex);
-        auto const& payloadMsg = currentSetOfInputs[i].associatedPayload(partindex);
+        auto const indices = currentSetOfInputs[i].messages | get_pair{partindex};
+        auto const& headerMsg = currentSetOfInputs[i].messages[indices.headerIdx];
+        auto const& payloadMsg = currentSetOfInputs[i].messages[indices.payloadIdx];
         headerptr = static_cast<char const*>(headerMsg->GetData());
         payloadptr = payloadMsg ? static_cast<char const*>(payloadMsg->GetData()) : nullptr;
         payloadSize = payloadMsg ? payloadMsg->GetSize() : 0;
diff --git a/Framework/Core/test/test_MessageSet.cxx b/Framework/Core/test/test_MessageSet.cxx
index aa7b49c1d1d3c..8c9ed4a7cbf1c 100644
--- a/Framework/Core/test/test_MessageSet.cxx
+++ b/Framework/Core/test/test_MessageSet.cxx
@@ -45,12 +45,16 @@ TEST_CASE("MessageSet")
   REQUIRE((msgSet.messages | get_pair{0}).headerIdx == 0);
   REQUIRE((msgSet.messages | get_pair{0}).payloadIdx == 1);
   CHECK_THROWS((msgSet.messages | get_pair{1}));
-  // Validate pipe operators match old API
-  REQUIRE(&(msgSet.messages | get_header{0}) == &msgSet.header(0));
-  REQUIRE(&(msgSet.messages | get_payload{0, 0}) == &msgSet.payload(0));
-  REQUIRE((msgSet.messages | get_num_payloads{0}) == msgSet.messageMap[0].size);
-  REQUIRE((msgSet.messages | count_parts{}) == msgSet.messageMap.size());
-  REQUIRE((msgSet.messages | count_payloads{}) == msgSet.pairMap.size());
+  REQUIRE((msgSet.messages | get_num_payloads{0}) == 1);
+  REQUIRE((msgSet.messages | count_parts{}) == 1);
+  // messages: [hdr, pl] — one pair
+  REQUIRE((msgSet.messages | get_pair{0}).headerIdx == 0);
+  REQUIRE((msgSet.messages | get_pair{0}).payloadIdx == 1);
+  for (size_t i = 0; i < msgSet.pairMap.size(); ++i) {
+    auto indices = msgSet.messages | get_pair{i};
+    REQUIRE(&msgSet.messages[indices.headerIdx] == &msgSet.associatedHeader(i));
+    REQUIRE(&msgSet.messages[indices.payloadIdx] == &msgSet.associatedPayload(i));
+  }
 }
 
 TEST_CASE("MessageSetWithFunction")
@@ -76,11 +80,13 @@ TEST_CASE("MessageSetWithFunction")
   REQUIRE((msgSet.messages | get_pair{0}).headerIdx == 0);
   REQUIRE((msgSet.messages | get_pair{0}).payloadIdx == 1);
   CHECK_THROWS((msgSet.messages | get_pair{1}));
-  REQUIRE(&(msgSet.messages | get_header{0}) == &msgSet.header(0));
-  REQUIRE(&(msgSet.messages | get_payload{0, 0}) == &msgSet.payload(0));
-  REQUIRE((msgSet.messages | get_num_payloads{0}) == msgSet.messageMap[0].size);
-  REQUIRE((msgSet.messages | count_parts{}) == msgSet.messageMap.size());
-  REQUIRE((msgSet.messages | count_payloads{}) == msgSet.pairMap.size());
+  REQUIRE((msgSet.messages | get_num_payloads{0}) == 1);
+  REQUIRE((msgSet.messages | count_parts{}) == 1);
+  for (size_t i = 0; i < msgSet.pairMap.size(); ++i) {
+    auto indices = msgSet.messages | get_pair{i};
+    REQUIRE(&msgSet.messages[indices.headerIdx] == &msgSet.associatedHeader(i));
+    REQUIRE(&msgSet.messages[indices.payloadIdx] == &msgSet.associatedPayload(i));
+  }
 }
 
 TEST_CASE("MessageSetWithMultipart")
@@ -112,13 +118,18 @@ TEST_CASE("MessageSetWithMultipart")
   REQUIRE((msgSet.messages | get_pair{1}).headerIdx == 0);
   REQUIRE((msgSet.messages | get_pair{1}).payloadIdx == 2);
   CHECK_THROWS((msgSet.messages | get_pair{2}));
-  // Validate pipe operators match old API for multi-payload
-  REQUIRE(&(msgSet.messages | get_header{0}) == &msgSet.header(0));
-  REQUIRE(&(msgSet.messages | get_payload{0, 0}) == &msgSet.payload(0, 0));
-  REQUIRE(&(msgSet.messages | get_payload{0, 1}) == &msgSet.payload(0, 1));
-  REQUIRE((msgSet.messages | get_num_payloads{0}) == msgSet.messageMap[0].size);
-  REQUIRE((msgSet.messages | count_parts{}) == msgSet.messageMap.size());
-  REQUIRE((msgSet.messages | count_payloads{}) == msgSet.pairMap.size());
+  REQUIRE((msgSet.messages | get_num_payloads{0}) == 2);
+  REQUIRE((msgSet.messages | count_parts{}) == 1);
+  // messages: [hdr, pl0, pl1] — one header, two payloads
+  REQUIRE((msgSet.messages | get_pair{0}).headerIdx == 0);
+  REQUIRE((msgSet.messages | get_pair{0}).payloadIdx == 1);
+  REQUIRE((msgSet.messages | get_pair{1}).headerIdx == 0);
+  REQUIRE((msgSet.messages | get_pair{1}).payloadIdx == 2);
+  for (size_t i = 0; i < msgSet.pairMap.size(); ++i) {
+    auto indices = msgSet.messages | get_pair{i};
+    REQUIRE(&msgSet.messages[indices.headerIdx] == &msgSet.associatedHeader(i));
+    REQUIRE(&msgSet.messages[indices.payloadIdx] == &msgSet.associatedPayload(i));
+  }
 }
 
 TEST_CASE("MessageSetAddPartRef")
@@ -190,18 +201,16 @@ TEST_CASE("MessageSetAddMultiple")
   REQUIRE((msgSet.messages | get_pair{2}).payloadIdx == 5);
   REQUIRE((msgSet.messages | get_pair{3}).headerIdx == 4);
   REQUIRE((msgSet.messages | get_pair{3}).payloadIdx == 6);
-  // Validate pipe operators match old API for mixed modes
-  for (size_t i = 0; i < 3; ++i) {
-    REQUIRE(&(msgSet.messages | get_header{i}) == &msgSet.header(i));
-    REQUIRE(&(msgSet.messages | get_payload{i, 0}) == &msgSet.payload(i, 0));
-  }
-  // Part 2 has a second payload (multi-payload with splitPayloadParts=2, splitPayloadIndex=2)
-  REQUIRE(&(msgSet.messages | get_payload{2, 1}) == &msgSet.payload(2, 1));
-  for (size_t i = 0; i < 3; ++i) {
-    REQUIRE((msgSet.messages | get_num_payloads{i}) == msgSet.messageMap[i].size);
+  REQUIRE((msgSet.messages | get_num_payloads{0}) == 1);
+  REQUIRE((msgSet.messages | get_num_payloads{1}) == 1);
+  REQUIRE((msgSet.messages | get_num_payloads{2}) == 2);
+  REQUIRE((msgSet.messages | count_parts{}) == 3);
+  REQUIRE((msgSet.messages | count_payloads{}) == 4);
+  for (size_t i = 0; i < msgSet.pairMap.size(); ++i) {
+    auto indices = msgSet.messages | get_pair{i};
+    REQUIRE(&msgSet.messages[indices.headerIdx] == &msgSet.associatedHeader(i));
+    REQUIRE(&msgSet.messages[indices.payloadIdx] == &msgSet.associatedPayload(i));
   }
-  REQUIRE((msgSet.messages | count_parts{}) == msgSet.messageMap.size());
-  REQUIRE((msgSet.messages | count_payloads{}) == msgSet.pairMap.size());
 }
 
 TEST_CASE("GetHeaderPayloadOperators")
@@ -251,13 +260,18 @@ TEST_CASE("GetHeaderPayloadOperators")
   REQUIRE(pl1.get() != nullptr);
   REQUIRE(pl1->GetSize() == 200);
 
-  // Validate pipe operators match old API
-  for (size_t i = 0; i < 2; ++i) {
-    REQUIRE(&(msgSet.messages | get_header{i}) == &msgSet.header(i));
-    REQUIRE(&(msgSet.messages | get_payload{i, 0}) == &msgSet.payload(i, 0));
+  REQUIRE((msgSet.messages | count_parts{}) == 2);
+  REQUIRE((msgSet.messages | count_payloads{}) == 2);
+  // messages: [hdr0, pl0, hdr1, pl1] — two standard pairs
+  REQUIRE((msgSet.messages | get_pair{0}).headerIdx == 0);
+  REQUIRE((msgSet.messages | get_pair{0}).payloadIdx == 1);
+  REQUIRE((msgSet.messages | get_pair{1}).headerIdx == 2);
+  REQUIRE((msgSet.messages | get_pair{1}).payloadIdx == 3);
+  for (size_t i = 0; i < msgSet.pairMap.size(); ++i) {
+    auto indices = msgSet.messages | get_pair{i};
+    REQUIRE(&msgSet.messages[indices.headerIdx] == &msgSet.associatedHeader(i));
+    REQUIRE(&msgSet.messages[indices.payloadIdx] == &msgSet.associatedPayload(i));
   }
-  REQUIRE((msgSet.messages | count_parts{}) == msgSet.messageMap.size());
-  REQUIRE((msgSet.messages | count_payloads{}) == msgSet.pairMap.size());
 }
 
 TEST_CASE("GetHeaderPayloadMultiPayload")
@@ -343,18 +357,24 @@ TEST_CASE("GetHeaderPayloadMultiPayload")
   // get_num_payloads for part 1 should be 3
   REQUIRE((msgSet.messages | get_num_payloads{1}) == 3);
 
-  // Validate pipe operators match old API for multi-payload (header, pl, pl, pl)
-  REQUIRE(&(msgSet.messages | get_header{0}) == &msgSet.header(0));
-  REQUIRE(&(msgSet.messages | get_header{1}) == &msgSet.header(1));
-  REQUIRE(&(msgSet.messages | get_payload{0, 0}) == &msgSet.payload(0, 0));
-  REQUIRE(&(msgSet.messages | get_payload{1, 0}) == &msgSet.payload(1, 0));
-  REQUIRE(&(msgSet.messages | get_payload{1, 1}) == &msgSet.payload(1, 1));
-  REQUIRE(&(msgSet.messages | get_payload{1, 2}) == &msgSet.payload(1, 2));
-  for (size_t i = 0; i < 2; ++i) {
-    REQUIRE((msgSet.messages | get_num_payloads{i}) == msgSet.messageMap[i].size);
+  REQUIRE((msgSet.messages | get_num_payloads{0}) == 1);
+  REQUIRE((msgSet.messages | get_num_payloads{1}) == 3);
+  REQUIRE((msgSet.messages | count_parts{}) == 2);
+  REQUIRE((msgSet.messages | count_payloads{}) == 4);
+  // messages: [hdr0, pl0, hdr1, pl1_0, pl1_1, pl1_2]
+  REQUIRE((msgSet.messages | get_pair{0}).headerIdx == 0);
+  REQUIRE((msgSet.messages | get_pair{0}).payloadIdx == 1);
+  REQUIRE((msgSet.messages | get_pair{1}).headerIdx == 2);
+  REQUIRE((msgSet.messages | get_pair{1}).payloadIdx == 3);
+  REQUIRE((msgSet.messages | get_pair{2}).headerIdx == 2);
+  REQUIRE((msgSet.messages | get_pair{2}).payloadIdx == 4);
+  REQUIRE((msgSet.messages | get_pair{3}).headerIdx == 2);
+  REQUIRE((msgSet.messages | get_pair{3}).payloadIdx == 5);
+  for (size_t i = 0; i < msgSet.pairMap.size(); ++i) {
+    auto indices = msgSet.messages | get_pair{i};
+    REQUIRE(&msgSet.messages[indices.headerIdx] == &msgSet.associatedHeader(i));
+    REQUIRE(&msgSet.messages[indices.payloadIdx] == &msgSet.associatedPayload(i));
   }
-  REQUIRE((msgSet.messages | count_parts{}) == msgSet.messageMap.size());
-  REQUIRE((msgSet.messages | count_payloads{}) == msgSet.pairMap.size());
 }
 
 TEST_CASE("TraditionalSplitParts")
@@ -418,14 +438,20 @@ TEST_CASE("TraditionalSplitParts")
 
   // get_num_payloads: each traditional split pair has 1 payload
   for (size_t i = 0; i < 3; ++i) {
-    REQUIRE((msgSet.messages | get_num_payloads{i}) == msgSet.messageMap[i].size);
+    REQUIRE((msgSet.messages | get_num_payloads{i}) == 1);
   }
-
-  // Validate pipe operators match old MessageSet::header()/payload() API
-  for (size_t i = 0; i < 3; ++i) {
-    REQUIRE(&(msgSet.messages | get_header{i}) == &msgSet.header(i));
-    REQUIRE(&(msgSet.messages | get_payload{i, 0}) == &msgSet.payload(i));
+  REQUIRE((msgSet.messages | count_parts{}) == 3);
+  REQUIRE((msgSet.messages | count_payloads{}) == 3);
+  // messages: [hdr0, pl0, hdr1, pl1, hdr2, pl2] — three traditional split pairs
+  REQUIRE((msgSet.messages | get_pair{0}).headerIdx == 0);
+  REQUIRE((msgSet.messages | get_pair{0}).payloadIdx == 1);
+  REQUIRE((msgSet.messages | get_pair{1}).headerIdx == 2);
+  REQUIRE((msgSet.messages | get_pair{1}).payloadIdx == 3);
+  REQUIRE((msgSet.messages | get_pair{2}).headerIdx == 4);
+  REQUIRE((msgSet.messages | get_pair{2}).payloadIdx == 5);
+  for (size_t i = 0; i < msgSet.pairMap.size(); ++i) {
+    auto indices = msgSet.messages | get_pair{i};
+    REQUIRE(&msgSet.messages[indices.headerIdx] == &msgSet.associatedHeader(i));
+    REQUIRE(&msgSet.messages[indices.payloadIdx] == &msgSet.associatedPayload(i));
   }
-  REQUIRE((msgSet.messages | count_parts{}) == msgSet.messageMap.size());
-  REQUIRE((msgSet.messages | count_payloads{}) == msgSet.pairMap.size());
 }