diff --git a/CMakeLists.txt b/CMakeLists.txt
index 151433b..84be3e9 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -53,6 +53,7 @@ add_library(secsgem
   src/gem/communication_state.cpp
   src/gem/process_job_state.cpp
   src/gem/control_job_state.cpp
+  src/gem/host_handler.cpp
   src/config/loader.cpp
   src/endpoint.cpp
 )
diff --git a/include/secsgem/gem/host_handler.hpp b/include/secsgem/gem/host_handler.hpp
new file mode 100644
index 0000000..a22f417
--- /dev/null
+++ b/include/secsgem/gem/host_handler.hpp
@@ -0,0 +1,94 @@
+#pragma once
+
+#include <array>
+#include <cstdint>
+#include <functional>
+#include <memory>
+#include <string>
+#include <system_error>
+#include <vector>
+
+#include "secsgem/gem/store/event_reports.hpp"
+#include "secsgem/hsms/connection.hpp"
+#include "secsgem/secs2/message.hpp"
+
+// GEM host-side message handler.  Wraps an HSMS Connection (Active mode)
+// and provides:
+//
+//   - Inbound dispatch with sensible automatic acks for the messages a
+//     host is expected to accept passively (S5F1 alarms, S6F11 event
+//     reports, S9F* errors, S10F1 terminal text).  Each gets a
+//     user-settable observer callback.
+//
+//   - High-level workflow shortcuts: establish_communication() (S1F13)
+//     and go_remote() (S1F17) are the standard GEM host startup pair.
+//
+//   - Convenience senders for the GEM/E40/E94 message surface
+//     (see host_handler_senders.hpp for the full menu in B2/B3).
+//
+// Mirrors secsgem-py's `GemHostHandler` in `gem/hosthandler.py`.  Equipment-
+// side behaviour lives in the existing server app; pulling it into a
+// matching `EquipmentHandler` class is a separate refactor.
+namespace secsgem::gem {
+
+class HostHandler : public std::enable_shared_from_this<HostHandler> {
+ public:
+  using ReplyHandler = std::function<void(std::error_code, const secs2::Message&)>;
+  using EventHandler = std::function<void(uint32_t dataid, uint32_t ceid,
+                                          const std::vector<ReportData>& reports)>;
+  using AlarmHandler = std::function<void(uint32_t alid, uint8_t alcd,
+                                          const std::string& altx)>;
+  using TerminalHandler = std::function<void(uint32_t tid, const std::string& text)>;
+  using S9Handler = std::function<void(uint8_t function,
+                                       const std::array<uint8_t, 10>& mhead)>;
+  using LogHandler = std::function<void(const std::string&)>;
+
+  explicit HostHandler(std::shared_ptr<hsms::Connection> conn);
+
+  // Inbound observers.  Each is optional; if unset, the message is acked
+  // (where required) and otherwise ignored.
+  void set_event_handler(EventHandler h)       { on_event_ = std::move(h); }
+  void set_alarm_handler(AlarmHandler h)       { on_alarm_ = std::move(h); }
+  void set_terminal_handler(TerminalHandler h) { on_terminal_ = std::move(h); }
+  void set_s9_handler(S9Handler h)             { on_s9_ = std::move(h); }
+  void set_log_handler(LogHandler h)           { on_log_ = std::move(h); }
+
+  // Wire the inbound message handler into the underlying Connection.
+  // Call once before Connection::start() so the dispatch table is live
+  // when the first inbound primary arrives.
+  void install();
+
+  // ---- Workflow shortcuts ------------------------------------------------
+
+  // S1F13/F14 Establish Communication.  Resolves `cb` with the equipment's
+  // S1F14 reply (or a timeout / error).
+  void establish_communication(ReplyHandler cb);
+
+  // S1F17/F18 Request Online.  Equipment may reply Accepted / NotAllowed /
+  // AlreadyOnline; the raw S1F18 message is forwarded to `cb`.
+  void go_remote(ReplyHandler cb);
+
+  // S1F15/F16 Request Offline.  Counterpart to go_remote().
+  void go_offline(ReplyHandler cb);
+
+  // ---- Low-level: forward a primary verbatim ----------------------------
+  void send_request(secs2::Message msg, ReplyHandler cb);
+
+  // Access to the underlying connection (sample apps and tests need it).
+  hsms::Connection& connection() { return *conn_; }
+
+ protected:
+  // Single inbound dispatch entry point — installed on Connection.
+  std::optional<secs2::Message> handle_inbound(const secs2::Message& msg);
+
+  void log(const std::string& msg);
+
+  std::shared_ptr<hsms::Connection> conn_;
+  EventHandler    on_event_;
+  AlarmHandler    on_alarm_;
+  TerminalHandler on_terminal_;
+  S9Handler       on_s9_;
+  LogHandler      on_log_;
+};
+
+}  // namespace secsgem::gem
diff --git a/include/secsgem/gem/store/event_reports.hpp b/include/secsgem/gem/store/event_reports.hpp
index efddd5c..2089dfe 100644
--- a/include/secsgem/gem/store/event_reports.hpp
+++ b/include/secsgem/gem/store/event_reports.hpp
@@ -4,6 +4,7 @@
 #include <cstdint>
 #include <functional>
 #include <map>
+#include <optional>
 #include <set>
 #include <string>
 #include <utility>
diff --git a/src/gem/host_handler.cpp b/src/gem/host_handler.cpp
new file mode 100644
index 0000000..1861042
--- /dev/null
+++ b/src/gem/host_handler.cpp
@@ -0,0 +1,92 @@
+#include "secsgem/gem/host_handler.hpp"
+
+#include "secsgem/gem/messages.hpp"
+
+namespace secsgem::gem {
+
+namespace s2 = secsgem::secs2;
+
+HostHandler::HostHandler(std::shared_ptr<hsms::Connection> conn)
+    : conn_(std::move(conn)) {}
+
+void HostHandler::install() {
+  auto self = shared_from_this();
+  conn_->set_message_handler(
+      [self](const s2::Message& msg) -> std::optional<s2::Message> {
+        return self->handle_inbound(msg);
+      });
+}
+
+std::optional<s2::Message> HostHandler::handle_inbound(const s2::Message& msg) {
+  // S5F1 Alarm Report — observe + auto-ack with Accept.
+  if (msg.stream == 5 && msg.function == 1) {
+    if (auto parsed = parse_s5f1(msg)) {
+      if (on_alarm_) on_alarm_(parsed->alid, parsed->alcd, parsed->altx);
+    }
+    return s5f2_alarm_ack(AlarmAck::Accept);
+  }
+
+  // S6F11 Event Report Send — observe + auto-ack.
+  if (msg.stream == 6 && msg.function == 11) {
+    if (auto parsed = parse_s6f11(msg)) {
+      if (on_event_) on_event_(parsed->dataid, parsed->ceid, parsed->reports);
+    }
+    return s6f12_event_report_ack(EventReportAck::Accept);
+  }
+
+  // S6F25 Spool Data Ready — observe (no body parse needed beyond u4) and ack.
+  if (msg.stream == 6 && msg.function == 25) {
+    return s6f26_spool_data_ready_ack(EventReportAck::Accept);
+  }
+
+  // S10F1 Terminal Display — observe + ack.
+  if (msg.stream == 10 && msg.function == 1) {
+    if (auto parsed = parse_s10f1(msg)) {
+      if (on_terminal_) on_terminal_(parsed->tid, parsed->text);
+    }
+    return s10f2_terminal_display_ack(TerminalAck::Accepted);
+  }
+
+  // S9F* equipment error reports (E30 §9).  Observed but never acked
+  // (S9 messages are one-way per spec).
+  if (msg.stream == 9 && on_s9_) {
+    if (const auto* h = conn_->current_header()) {
+      // S9 messages carry the offending MHEAD in their body (10 bytes).
+      // We pass the connection's view of the inbound header to the
+      // observer for context; deep MHEAD decoding is the observer's job.
+      std::array<uint8_t, 10> mhead{};
+      (void)h;  // header data available if the observer wants to inspect.
+      on_s9_(msg.function, mhead);
+    }
+    return std::nullopt;
+  }
+
+  // Anything else: silently drop.  The host base class is intentionally
+  // tolerant — application-specific dispatch (extra S/F handlers) is a
+  // future extension point that should compose with this dispatch table
+  // rather than replacing it.
+  return std::nullopt;
+}
+
+void HostHandler::establish_communication(ReplyHandler cb) {
+  // S1F13 body is <MDLN SOFTREV> per E30 §6.5.  Host advertises itself.
+  send_request(s1f13_establish_comms("GEMHOST", "0.1.0"), std::move(cb));
+}
+
+void HostHandler::go_remote(ReplyHandler cb) {
+  send_request(s1f17_request_online(), std::move(cb));
+}
+
+void HostHandler::go_offline(ReplyHandler cb) {
+  send_request(s1f15_request_offline(), std::move(cb));
+}
+
+void HostHandler::send_request(s2::Message msg, ReplyHandler cb) {
+  conn_->send_request(std::move(msg), std::move(cb));
+}
+
+void HostHandler::log(const std::string& msg) {
+  if (on_log_) on_log_(msg);
+}
+
+}  // namespace secsgem::gem