diff --git a/include/secsgem/hsms/connection.hpp b/include/secsgem/hsms/connection.hpp
index 511d42b..38edc4a 100644
--- a/include/secsgem/hsms/connection.hpp
+++ b/include/secsgem/hsms/connection.hpp
@@ -108,8 +108,12 @@ class Connection : public std::enable_shared_from_this<Connection> {
 
   uint32_t next_system_bytes_ = 1;
 
-  // outstanding data request transactions, keyed by system bytes
+  // outstanding data request transactions, keyed by system bytes.
+  // We track the EXPECTED reply stream/function so we can match exactly
+  // instead of inferring "this is a reply" from function parity.
   struct PendingRequest {
+    uint8_t expected_stream;
+    uint8_t expected_function;
     ReplyHandler cb;
     std::shared_ptr<asio::steady_timer> t3;
   };
diff --git a/src/hsms/connection.cpp b/src/hsms/connection.cpp
index daba753..2aae8b2 100644
--- a/src/hsms/connection.cpp
+++ b/src/hsms/connection.cpp
@@ -113,10 +113,14 @@ void Connection::handle_frame(Frame frame) {
 void Connection::handle_data(const Frame& frame) {
   const Header& h = frame.header;
 
-  // Reply correlation: an even function with matching system bytes completes an
-  // outstanding request.
+  // Reply correlation: match on (system_bytes, stream, function) exactly.
+  // SECS-II replies use the same system_bytes as their request and have
+  // stream==request.stream, function==request.function+1; we recorded both
+  // when send_request stored the pending transaction.
   auto it = pending_requests_.find(h.system_bytes);
-  if ((h.function() % 2 == 0) && it != pending_requests_.end()) {
+  if (it != pending_requests_.end() &&
+      it->second.expected_stream == h.stream() &&
+      it->second.expected_function == h.function()) {
     secs2::Message reply;
     try {
       reply = secs2::Message::from_body(h.stream(), h.function(), h.w_bit(), frame.body);
@@ -132,6 +136,18 @@ void Connection::handle_data(const Frame& frame) {
     return;
   }
 
+  // System bytes match but stream/function disagree: protocol violation
+  // (peer replied with the wrong SxFy). Surface this as a real diagnostic
+  // rather than silently falling through to the primary handler.
+  if (it != pending_requests_.end()) {
+    log("!! unexpected " + h.describe() + " for pending sys=" +
+        std::to_string(h.system_bytes) + " (expected S" +
+        std::to_string(it->second.expected_stream) + "F" +
+        std::to_string(it->second.expected_function) + ")");
+    // Treat the data as a primary message anyway so the application
+    // handler can deal with it; the pending request stays open until T3.
+  }
+
   // Primary message; only valid while SELECTED.
   if (state_ != State::Selected) {
     send_frame(Frame(Header::control(SType::RejectReq, h.system_bytes, kControlSessionId,
@@ -267,7 +283,11 @@ void Connection::send_request(secs2::Message msg, ReplyHandler cb) {
 
   auto t3 = std::make_shared<asio::steady_timer>(socket_.get_executor());
   t3->expires_after(timers_.t3);
-  pending_requests_.emplace(sys, PendingRequest{std::move(cb), t3});
+  // SECS-II reply convention: same stream, function+1.
+  pending_requests_.emplace(sys, PendingRequest{
+      /*expected_stream=*/msg.stream,
+      /*expected_function=*/static_cast<uint8_t>(msg.function + 1),
+      std::move(cb), t3});
 
   auto self = shared_from_this();
   t3->async_wait([this, self, sys](std::error_code ec) {