diff --git a/testing/SimHarness.cpp b/testing/SimHarness.cpp
index 7f716d67f3..1817469294 100644
--- a/testing/SimHarness.cpp
+++ b/testing/SimHarness.cpp
@@ -75,6 +75,63 @@ uint8_t SimHarness::CycleTime() const {
   return millis_per_cycle_;
 }
 
+// Serial support implementation
+void SimHarness::ProcessSerialInput() {
+  // This will be called by RunCycle() to process any pending serial input
+  // The actual processing is handled by the virtual HardwareSerial implementation
+}
+
+void SimHarness::SendSerialData(const uint8_t *data, size_t length) {
+  // Get direct access to the virtual device's serial port
+  auto &serial = Serial;
+  serial.injectInput(data, length);
+}
+
+std::vector<uint8_t> SimHarness::GetSerialOutput() {
+  // Get direct access to the virtual device's serial port
+  auto &serial = Serial;
+  return serial.getOutputBuffer();
+}
+
+// Focus protocol helper implementations
+std::string SimHarness::SendFocusCommand(const std::string &command) {
+  // Ensure command ends with newline
+  std::string cmd = command;
+  if (cmd.empty() || cmd.back() != '\n') {
+    cmd += '\n';
+  }
+
+  // Send command
+  SendString(cmd);
+
+  // Run cycles until we get a complete response
+  // (ends with \r\n.\r\n)
+  std::string response;
+  size_t max_cycles = 100;  // Prevent infinite loops
+  size_t cycles     = 0;
+
+  while (cycles++ < max_cycles) {
+    RunCycle();
+    response = GetSerialOutputAsString();
+    if (IsFocusResponse(response)) break;
+  }
+
+  return StripFocusTerminator(response);
+}
+
+bool SimHarness::IsFocusResponse(const std::string &response) {
+  static const std::string terminator = "\r\n.\r\n";
+  return response.length() >= terminator.length() &&
+         response.substr(response.length() - terminator.length()) == terminator;
+}
+
+std::string SimHarness::StripFocusTerminator(const std::string &response) {
+  static const std::string terminator = "\r\n.\r\n";
+  if (IsFocusResponse(response)) {
+    return response.substr(0, response.length() - terminator.length());
+  }
+  return response;
+}
 
 }  // namespace testing
 }  // namespace kaleidoscope
diff --git a/testing/SimHarness.h b/testing/SimHarness.h
index 424a43cab1..f070b19102 100644
--- a/testing/SimHarness.h
+++ b/testing/SimHarness.h
@@ -18,6 +18,8 @@
 
 #include <cstddef>  // for size_t
 #include <cstdint>  // for uint8_t
+#include <vector>
+#include <string>
 
 #include "kaleidoscope/KeyAddr.h"  // for KeyAddr
 #include "testing/gtest.h"         // IWYU pragma: keep
@@ -37,6 +39,23 @@ class SimHarness {
   void SetCycleTime(uint8_t millis);
   uint8_t CycleTime() const;
 
+  // Serial support
+  void ProcessSerialInput();
+  void SendString(const std::string &str) {
+    SendSerialData(reinterpret_cast<const uint8_t *>(str.c_str()), str.length());
+  }
+  void SendSerialData(const uint8_t *data, size_t length);
+  std::vector<uint8_t> GetSerialOutput();
+  std::string GetSerialOutputAsString() {
+    auto output = GetSerialOutput();
+    return std::string(output.begin(), output.end());
+  }
+
+  // Focus protocol helpers
+  std::string SendFocusCommand(const std::string &command);
+  static bool IsFocusResponse(const std::string &response);
+  static std::string StripFocusTerminator(const std::string &response);
+
  private:
   uint8_t millis_per_cycle_ = 1;
 };