diff --git a/esphome/components/api/api.proto b/esphome/components/api/api.proto
index 6ea124d155..1942ff568b 100644
--- a/esphome/components/api/api.proto
+++ b/esphome/components/api/api.proto
@@ -232,6 +232,7 @@ enum SerialProxyPortType {
 message SerialProxyInfo {
   string name = 1;                    // Human-readable port name
   SerialProxyPortType port_type = 2;  // Port type (RS232, RS485)
+  uint32 configured_line_states = 3;  // Bitmask of SerialProxyLineStateFlags this instance can drive
 }
 
 // DeviceInfoResponse max_data_length values:
@@ -2626,6 +2627,22 @@ message ZWaveProxyRequest {
   bytes data = 2;
 }
 
+enum ZWaveProxyStatus {
+  ZWAVE_PROXY_STATUS_OK = 0;             // Request completed successfully
+  ZWAVE_PROXY_STATUS_IN_USE = 1;         // Denied: another client is already subscribed
+  ZWAVE_PROXY_STATUS_NOT_SUPPORTED = 2;  // Request type not supported
+}
+
+// Acknowledges a ZWaveProxyRequest (subscribe/unsubscribe). Sent since API 1.16.
+message ZWaveProxyRequestResponse {
+  option (id) = 151;
+  option (source) = SOURCE_SERVER;
+  option (ifdef) = "USE_ZWAVE_PROXY";
+
+  ZWaveProxyRequestType type = 1;  // Which request type this responds to
+  ZWaveProxyStatus status = 2;     // Result status
+}
+
 // ==================== INFRARED ====================
 // Note: Feature and capability flag enums are defined in
 // esphome/components/infrared/infrared.h
@@ -2769,12 +2786,18 @@ message SerialProxyGetModemPinsResponse {
 
   uint32 instance = 1;          // Instance index (0-based)
   uint32 line_states = 2;       // Bitmask of SerialProxyLineStateFlags
+  SerialProxyStatus status = 3; // INVALID_ARGUMENT if the instance index is out of range (since API 1.16)
 }
 
 enum SerialProxyRequestType {
   SERIAL_PROXY_REQUEST_TYPE_SUBSCRIBE = 0;   // Subscribe to receive data from this serial proxy instance
   SERIAL_PROXY_REQUEST_TYPE_UNSUBSCRIBE = 1; // Unsubscribe from this serial proxy instance
   SERIAL_PROXY_REQUEST_TYPE_FLUSH = 2;       // Flush the serial port (block until all TX data is sent)
+  // Values below are only valid in SerialProxyRequestResponse.type, identifying which
+  // operation is being acknowledged. Sending them in SerialProxyRequest.type is an
+  // error the device answers with INVALID_ARGUMENT.
+  SERIAL_PROXY_REQUEST_TYPE_CONFIGURE = 3;      // Acknowledges a SerialProxyConfigureRequest
+  SERIAL_PROXY_REQUEST_TYPE_SET_MODEM_PINS = 4; // Acknowledges a SerialProxySetModemPinsRequest
 }
 
 enum SerialProxyStatus {
@@ -2783,6 +2806,8 @@ enum SerialProxyStatus {
   SERIAL_PROXY_STATUS_ERROR = 2;           // Driver or hardware error
   SERIAL_PROXY_STATUS_TIMEOUT = 3;         // Timed out before TX completed
   SERIAL_PROXY_STATUS_NOT_SUPPORTED = 4;   // Request type not supported by this instance
+  SERIAL_PROXY_STATUS_PORT_IN_USE = 5;     // Denied: another client holds the port
+  SERIAL_PROXY_STATUS_INVALID_ARGUMENT = 6; // Invalid instance index or parameter value
 }
 
 // Generic request message for simple serial proxy operations
@@ -2795,7 +2820,9 @@ message SerialProxyRequest {
   SerialProxyRequestType type = 2;      // Request type
 }
 
-// Response to a SerialProxyRequest (e.g. flush completion or failure)
+// Acknowledges a serial proxy operation; the type field identifies which
+// operation is being acknowledged. Flush has been acknowledged since the
+// message was introduced; all other acknowledgements are sent since API 1.16.
 message SerialProxyRequestResponse {
   option (id) = 147;
   option (source) = SOURCE_SERVER;
diff --git a/esphome/components/api/api_connection.cpp b/esphome/components/api/api_connection.cpp
index bb6c1695dd..05abbf0b75 100644
--- a/esphome/components/api/api_connection.cpp
+++ b/esphome/components/api/api_connection.cpp
@@ -1381,7 +1381,12 @@ void APIConnection::on_z_wave_proxy_frame(const ZWaveProxyFrame &msg) {
 }
 
 void APIConnection::on_z_wave_proxy_request(const ZWaveProxyRequest &msg) {
-  zwave_proxy::global_zwave_proxy->zwave_proxy_request(this, msg.type);
+  ZWaveProxyRequestResponse resp{};
+  resp.type = msg.type;
+  resp.status = zwave_proxy::global_zwave_proxy->zwave_proxy_request(this, msg.type);
+  if (!this->send_message(resp)) {
+    API_LOG_MSG_DROPPED(TAG, "Z-Wave proxy response");
+  }
 }
 #endif
 
@@ -1550,15 +1555,50 @@ void APIConnection::send_infrared_rf_receive_event(const InfraredRFReceiveEvent
 #endif
 
 #ifdef USE_SERIAL_PROXY
+static enums::SerialProxyStatus serial_proxy_result_to_status(serial_proxy::SerialProxyResult result) {
+  switch (result) {
+    case serial_proxy::SerialProxyResult::SERIAL_PROXY_RESULT_OK:
+      return enums::SERIAL_PROXY_STATUS_OK;
+    case serial_proxy::SerialProxyResult::SERIAL_PROXY_RESULT_ASSUMED_SUCCESS:
+      return enums::SERIAL_PROXY_STATUS_ASSUMED_SUCCESS;
+    case serial_proxy::SerialProxyResult::SERIAL_PROXY_RESULT_PORT_IN_USE:
+      return enums::SERIAL_PROXY_STATUS_PORT_IN_USE;
+    case serial_proxy::SerialProxyResult::SERIAL_PROXY_RESULT_INVALID_ARGUMENT:
+      return enums::SERIAL_PROXY_STATUS_INVALID_ARGUMENT;
+    case serial_proxy::SerialProxyResult::SERIAL_PROXY_RESULT_TIMEOUT:
+      return enums::SERIAL_PROXY_STATUS_TIMEOUT;
+    case serial_proxy::SerialProxyResult::SERIAL_PROXY_RESULT_NOT_SUPPORTED:
+      return enums::SERIAL_PROXY_STATUS_NOT_SUPPORTED;
+    case serial_proxy::SerialProxyResult::SERIAL_PROXY_RESULT_ERROR:
+      return enums::SERIAL_PROXY_STATUS_ERROR;
+  }
+  return enums::SERIAL_PROXY_STATUS_ERROR;  // Unreachable; all enum values handled above
+}
+
+static void send_serial_proxy_ack(APIConnection *conn, uint32_t instance, enums::SerialProxyRequestType type,
+                                  enums::SerialProxyStatus status) {
+  SerialProxyRequestResponse resp{};
+  resp.instance = instance;
+  resp.type = type;
+  resp.status = status;
+  if (!conn->send_message(resp)) {
+    API_LOG_MSG_DROPPED(TAG, "Serial proxy response");
+  }
+}
+
 void APIConnection::on_serial_proxy_configure_request(const SerialProxyConfigureRequest &msg) {
   auto &proxies = App.get_serial_proxies();
   if (msg.instance >= proxies.size()) {
     ESP_LOGW(TAG, "Serial proxy instance %" PRIu32 " out of range (max %" PRIu32 ")", msg.instance,
              static_cast<uint32_t>(proxies.size()));
+    send_serial_proxy_ack(this, msg.instance, enums::SERIAL_PROXY_REQUEST_TYPE_CONFIGURE,
+                          enums::SERIAL_PROXY_STATUS_INVALID_ARGUMENT);
     return;
   }
-  proxies[msg.instance]->configure(this, msg.baudrate, msg.flow_control, static_cast<uint8_t>(msg.parity),
-                                   msg.stop_bits, msg.data_size);
+  serial_proxy::SerialProxyResult result = proxies[msg.instance]->configure(
+      this, msg.baudrate, msg.flow_control, static_cast<uint8_t>(msg.parity), msg.stop_bits, msg.data_size);
+  send_serial_proxy_ack(this, msg.instance, enums::SERIAL_PROXY_REQUEST_TYPE_CONFIGURE,
+                        serial_proxy_result_to_status(result));
 }
 
 void APIConnection::on_serial_proxy_write_request(const SerialProxyWriteRequest &msg) {
@@ -1574,20 +1614,30 @@ void APIConnection::on_serial_proxy_set_modem_pins_request(const SerialProxySetM
   auto &proxies = App.get_serial_proxies();
   if (msg.instance >= proxies.size()) {
     ESP_LOGW(TAG, "Serial proxy instance %" PRIu32 " out of range", msg.instance);
+    send_serial_proxy_ack(this, msg.instance, enums::SERIAL_PROXY_REQUEST_TYPE_SET_MODEM_PINS,
+                          enums::SERIAL_PROXY_STATUS_INVALID_ARGUMENT);
     return;
   }
-  proxies[msg.instance]->set_modem_pins(this, msg.line_states);
+  serial_proxy::SerialProxyResult result = proxies[msg.instance]->set_modem_pins(this, msg.line_states);
+  send_serial_proxy_ack(this, msg.instance, enums::SERIAL_PROXY_REQUEST_TYPE_SET_MODEM_PINS,
+                        serial_proxy_result_to_status(result));
 }
 
 void APIConnection::on_serial_proxy_get_modem_pins_request(const SerialProxyGetModemPinsRequest &msg) {
   auto &proxies = App.get_serial_proxies();
-  if (msg.instance >= proxies.size()) {
-    ESP_LOGW(TAG, "Serial proxy instance %" PRIu32 " out of range", msg.instance);
-    return;
-  }
   SerialProxyGetModemPinsResponse resp{};
   resp.instance = msg.instance;
-  resp.line_states = proxies[msg.instance]->get_modem_pins();
+  if (msg.instance >= proxies.size()) {
+    ESP_LOGW(TAG, "Serial proxy instance %" PRIu32 " out of range", msg.instance);
+    // Pre-1.16 clients do not read the status field and would take this error
+    // for a successful "both pins deasserted" answer; let them time out as before
+    if (!this->client_supports_api_version(1, 16)) {
+      return;
+    }
+    resp.status = enums::SERIAL_PROXY_STATUS_INVALID_ARGUMENT;
+  } else {
+    resp.line_states = proxies[msg.instance]->get_modem_pins();
+  }
   if (!this->send_message(resp)) {
     API_LOG_MSG_DROPPED(TAG, "Serial proxy response");
   }
@@ -1597,40 +1647,31 @@ void APIConnection::on_serial_proxy_request(const SerialProxyRequest &msg) {
   auto &proxies = App.get_serial_proxies();
   if (msg.instance >= proxies.size()) {
     ESP_LOGW(TAG, "Serial proxy instance %" PRIu32 " out of range", msg.instance);
+    send_serial_proxy_ack(this, msg.instance, msg.type, enums::SERIAL_PROXY_STATUS_INVALID_ARGUMENT);
     return;
   }
+  auto *proxy = proxies[msg.instance];
+  enums::SerialProxyStatus status;
   switch (msg.type) {
     case enums::SERIAL_PROXY_REQUEST_TYPE_SUBSCRIBE:
     case enums::SERIAL_PROXY_REQUEST_TYPE_UNSUBSCRIBE:
-      proxies[msg.instance]->serial_proxy_request(this, msg.type);
+      status = serial_proxy_result_to_status(proxy->serial_proxy_request(this, msg.type));
       break;
-    case enums::SERIAL_PROXY_REQUEST_TYPE_FLUSH: {
-      SerialProxyRequestResponse resp{};
-      resp.instance = msg.instance;
-      resp.type = enums::SERIAL_PROXY_REQUEST_TYPE_FLUSH;
-      switch (proxies[msg.instance]->flush_port()) {
-        case uart::UARTFlushResult::UART_FLUSH_RESULT_SUCCESS:
-          resp.status = enums::SERIAL_PROXY_STATUS_OK;
-          break;
-        case uart::UARTFlushResult::UART_FLUSH_RESULT_ASSUMED_SUCCESS:
-          resp.status = enums::SERIAL_PROXY_STATUS_ASSUMED_SUCCESS;
-          break;
-        case uart::UARTFlushResult::UART_FLUSH_RESULT_TIMEOUT:
-          resp.status = enums::SERIAL_PROXY_STATUS_TIMEOUT;
-          break;
-        case uart::UARTFlushResult::UART_FLUSH_RESULT_FAILED:
-          resp.status = enums::SERIAL_PROXY_STATUS_ERROR;
-          break;
-      }
-      if (!this->send_message(resp)) {
-        API_LOG_MSG_DROPPED(TAG, "Serial proxy response");
-      }
+    case enums::SERIAL_PROXY_REQUEST_TYPE_FLUSH:
+      status = serial_proxy_result_to_status(proxy->flush_port(this));
+      break;
+    case enums::SERIAL_PROXY_REQUEST_TYPE_CONFIGURE:
+    case enums::SERIAL_PROXY_REQUEST_TYPE_SET_MODEM_PINS:
+      // Response-only discriminators; never valid in a request
+      ESP_LOGW(TAG, "Response-only serial proxy request type: %" PRIu32, static_cast<uint32_t>(msg.type));
+      status = enums::SERIAL_PROXY_STATUS_INVALID_ARGUMENT;
       break;
-    }
     default:
       ESP_LOGW(TAG, "Unknown serial proxy request type: %" PRIu32, static_cast<uint32_t>(msg.type));
+      status = enums::SERIAL_PROXY_STATUS_NOT_SUPPORTED;
       break;
   }
+  send_serial_proxy_ack(this, msg.instance, msg.type, status);
 }
 
 void APIConnection::send_serial_proxy_data(const SerialProxyDataReceived &msg) {
@@ -1757,7 +1798,7 @@ bool APIConnection::send_hello_response_(const HelloRequest &msg) {
 
   HelloResponse resp;
   resp.api_version_major = 1;
-  resp.api_version_minor = 15;
+  resp.api_version_minor = 16;
   // Send only the version string - the client only logs this for debugging and doesn't use it otherwise
   resp.server_info = ESPHOME_VERSION_REF;
   resp.name = StringRef(App.get_name());
@@ -1891,6 +1932,7 @@ bool APIConnection::send_device_info_response_() {
     auto &info = resp.serial_proxies[serial_proxy_index++];
     info.name = StringRef(proxy->get_name());
     info.port_type = proxy->get_port_type();
+    info.configured_line_states = proxy->get_configured_modem_pins();
   }
 #endif
 #ifdef USE_API_NOISE
@@ -1951,6 +1993,7 @@ bool APIConnection::send_device_capabilities_response_() {
     auto &info = resp.serial_proxies[serial_proxy_index++];
     info.name = StringRef(proxy->get_name());
     info.port_type = proxy->get_port_type();
+    info.configured_line_states = proxy->get_configured_modem_pins();
   }
 #endif
   return this->send_message(resp);
diff --git a/esphome/components/api/api_pb2.cpp b/esphome/components/api/api_pb2.cpp
index 33611c5ee1..b5062f9e9f 100644
--- a/esphome/components/api/api_pb2.cpp
+++ b/esphome/components/api/api_pb2.cpp
@@ -102,12 +102,14 @@ uint8_t *SerialProxyInfo::encode(ProtoWriteBuffer &buffer PROTO_ENCODE_DEBUG_PAR
   uint8_t *__restrict__ pos = buffer.get_pos();
   ProtoEncode::encode_string(pos PROTO_ENCODE_DEBUG_ARG, 1, this->name);
   ProtoEncode::encode_uint32(pos PROTO_ENCODE_DEBUG_ARG, 2, static_cast<uint32_t>(this->port_type));
+  ProtoEncode::encode_uint32(pos PROTO_ENCODE_DEBUG_ARG, 3, this->configured_line_states);
   return pos;
 }
 uint32_t SerialProxyInfo::calculate_size() const {
   uint32_t size = 0;
   size += ProtoSize::calc_length(1, this->name.size());
   size += this->port_type ? 2 : 0;
+  size += ProtoSize::calc_uint32(1, this->configured_line_states);
   return size;
 }
 #endif
@@ -3942,6 +3944,18 @@ uint32_t ZWaveProxyRequest::calculate_size() const {
   size += ProtoSize::calc_length(1, this->data_len);
   return size;
 }
+uint8_t *ZWaveProxyRequestResponse::encode(ProtoWriteBuffer &buffer PROTO_ENCODE_DEBUG_PARAM) const {
+  uint8_t *__restrict__ pos = buffer.get_pos();
+  ProtoEncode::encode_uint32(pos PROTO_ENCODE_DEBUG_ARG, 1, static_cast<uint32_t>(this->type));
+  ProtoEncode::encode_uint32(pos PROTO_ENCODE_DEBUG_ARG, 2, static_cast<uint32_t>(this->status));
+  return pos;
+}
+uint32_t ZWaveProxyRequestResponse::calculate_size() const {
+  uint32_t size = 0;
+  size += this->type ? 2 : 0;
+  size += this->status ? 2 : 0;
+  return size;
+}
 #endif
 #ifdef USE_INFRARED
 uint8_t *ListEntitiesInfraredResponse::encode(ProtoWriteBuffer &buffer PROTO_ENCODE_DEBUG_PARAM) const {
@@ -4184,12 +4198,14 @@ uint8_t *SerialProxyGetModemPinsResponse::encode(ProtoWriteBuffer &buffer PROTO_
   uint8_t *__restrict__ pos = buffer.get_pos();
   ProtoEncode::encode_uint32(pos PROTO_ENCODE_DEBUG_ARG, 1, this->instance);
   ProtoEncode::encode_uint32(pos PROTO_ENCODE_DEBUG_ARG, 2, this->line_states);
+  ProtoEncode::encode_uint32(pos PROTO_ENCODE_DEBUG_ARG, 3, static_cast<uint32_t>(this->status));
   return pos;
 }
 uint32_t SerialProxyGetModemPinsResponse::calculate_size() const {
   uint32_t size = 0;
   size += ProtoSize::calc_uint32(1, this->instance);
   size += ProtoSize::calc_uint32(1, this->line_states);
+  size += this->status ? 2 : 0;
   return size;
 }
 bool SerialProxyRequest::decode_varint(uint32_t field_id, proto_varint_value_t value) {
diff --git a/esphome/components/api/api_pb2.h b/esphome/components/api/api_pb2.h
index 13db857467..cd2f32deaf 100644
--- a/esphome/components/api/api_pb2.h
+++ b/esphome/components/api/api_pb2.h
@@ -334,6 +334,11 @@ enum ZWaveProxyRequestType : uint32_t {
   ZWAVE_PROXY_REQUEST_TYPE_UNSUBSCRIBE = 1,
   ZWAVE_PROXY_REQUEST_TYPE_HOME_ID_CHANGE = 2,
 };
+enum ZWaveProxyStatus : uint32_t {
+  ZWAVE_PROXY_STATUS_OK = 0,
+  ZWAVE_PROXY_STATUS_IN_USE = 1,
+  ZWAVE_PROXY_STATUS_NOT_SUPPORTED = 2,
+};
 #endif
 #ifdef USE_SERIAL_PROXY
 enum SerialProxyParity : uint32_t {
@@ -345,6 +350,8 @@ enum SerialProxyRequestType : uint32_t {
   SERIAL_PROXY_REQUEST_TYPE_SUBSCRIBE = 0,
   SERIAL_PROXY_REQUEST_TYPE_UNSUBSCRIBE = 1,
   SERIAL_PROXY_REQUEST_TYPE_FLUSH = 2,
+  SERIAL_PROXY_REQUEST_TYPE_CONFIGURE = 3,
+  SERIAL_PROXY_REQUEST_TYPE_SET_MODEM_PINS = 4,
 };
 enum SerialProxyStatus : uint32_t {
   SERIAL_PROXY_STATUS_OK = 0,
@@ -352,6 +359,8 @@ enum SerialProxyStatus : uint32_t {
   SERIAL_PROXY_STATUS_ERROR = 2,
   SERIAL_PROXY_STATUS_TIMEOUT = 3,
   SERIAL_PROXY_STATUS_NOT_SUPPORTED = 4,
+  SERIAL_PROXY_STATUS_PORT_IN_USE = 5,
+  SERIAL_PROXY_STATUS_INVALID_ARGUMENT = 6,
 };
 #endif
 
@@ -523,6 +532,7 @@ class SerialProxyInfo final : public ProtoMessage {
  public:
   StringRef name{};
   enums::SerialProxyPortType port_type{};
+  uint32_t configured_line_states{0};
   uint8_t *encode(ProtoWriteBuffer &buffer PROTO_ENCODE_DEBUG_PARAM) const;
   uint32_t calculate_size() const;
 #ifdef HAS_PROTO_MESSAGE_DUMP
@@ -3130,6 +3140,23 @@ class ZWaveProxyRequest final : public ProtoDecodableMessage {
   bool decode_length(uint32_t field_id, ProtoLengthDelimited value) override;
   bool decode_varint(uint32_t field_id, proto_varint_value_t value) override;
 };
+class ZWaveProxyRequestResponse final : public ProtoMessage {
+ public:
+  static constexpr uint8_t MESSAGE_TYPE = 151;
+  static constexpr uint8_t ESTIMATED_SIZE = 4;
+#ifdef HAS_PROTO_MESSAGE_DUMP
+  const LogString *message_name() const override { return LOG_STR("z_wave_proxy_request_response"); }
+#endif
+  enums::ZWaveProxyRequestType type{};
+  enums::ZWaveProxyStatus status{};
+  uint8_t *encode(ProtoWriteBuffer &buffer PROTO_ENCODE_DEBUG_PARAM) const;
+  uint32_t calculate_size() const;
+#ifdef HAS_PROTO_MESSAGE_DUMP
+  const char *dump_to(DumpBuffer &out) const override;
+#endif
+
+ protected:
+};
 #endif
 #ifdef USE_INFRARED
 class ListEntitiesInfraredResponse final : public InfoResponseProtoMessage {
@@ -3314,12 +3341,13 @@ class SerialProxyGetModemPinsRequest final : public ProtoDecodableMessage {
 class SerialProxyGetModemPinsResponse final : public ProtoMessage {
  public:
   static constexpr uint8_t MESSAGE_TYPE = 143;
-  static constexpr uint8_t ESTIMATED_SIZE = 8;
+  static constexpr uint8_t ESTIMATED_SIZE = 10;
 #ifdef HAS_PROTO_MESSAGE_DUMP
   const LogString *message_name() const override { return LOG_STR("serial_proxy_get_modem_pins_response"); }
 #endif
   uint32_t instance{0};
   uint32_t line_states{0};
+  enums::SerialProxyStatus status{};
   uint8_t *encode(ProtoWriteBuffer &buffer PROTO_ENCODE_DEBUG_PARAM) const;
   uint32_t calculate_size() const;
 #ifdef HAS_PROTO_MESSAGE_DUMP
diff --git a/esphome/components/api/api_pb2_dump.cpp b/esphome/components/api/api_pb2_dump.cpp
index d54215ba2e..9f53438531 100644
--- a/esphome/components/api/api_pb2_dump.cpp
+++ b/esphome/components/api/api_pb2_dump.cpp
@@ -816,6 +816,18 @@ template<> const char *proto_enum_to_string<enums::ZWaveProxyRequestType>(enums:
       return ESPHOME_PSTR("UNKNOWN");
   }
 }
+template<> const char *proto_enum_to_string<enums::ZWaveProxyStatus>(enums::ZWaveProxyStatus value) {
+  switch (value) {
+    case enums::ZWAVE_PROXY_STATUS_OK:
+      return ESPHOME_PSTR("ZWAVE_PROXY_STATUS_OK");
+    case enums::ZWAVE_PROXY_STATUS_IN_USE:
+      return ESPHOME_PSTR("ZWAVE_PROXY_STATUS_IN_USE");
+    case enums::ZWAVE_PROXY_STATUS_NOT_SUPPORTED:
+      return ESPHOME_PSTR("ZWAVE_PROXY_STATUS_NOT_SUPPORTED");
+    default:
+      return ESPHOME_PSTR("UNKNOWN");
+  }
+}
 #endif
 #ifdef USE_SERIAL_PROXY
 template<> const char *proto_enum_to_string<enums::SerialProxyParity>(enums::SerialProxyParity value) {
@@ -838,6 +850,10 @@ template<> const char *proto_enum_to_string<enums::SerialProxyRequestType>(enums
       return ESPHOME_PSTR("SERIAL_PROXY_REQUEST_TYPE_UNSUBSCRIBE");
     case enums::SERIAL_PROXY_REQUEST_TYPE_FLUSH:
       return ESPHOME_PSTR("SERIAL_PROXY_REQUEST_TYPE_FLUSH");
+    case enums::SERIAL_PROXY_REQUEST_TYPE_CONFIGURE:
+      return ESPHOME_PSTR("SERIAL_PROXY_REQUEST_TYPE_CONFIGURE");
+    case enums::SERIAL_PROXY_REQUEST_TYPE_SET_MODEM_PINS:
+      return ESPHOME_PSTR("SERIAL_PROXY_REQUEST_TYPE_SET_MODEM_PINS");
     default:
       return ESPHOME_PSTR("UNKNOWN");
   }
@@ -854,6 +870,10 @@ template<> const char *proto_enum_to_string<enums::SerialProxyStatus>(enums::Ser
       return ESPHOME_PSTR("SERIAL_PROXY_STATUS_TIMEOUT");
     case enums::SERIAL_PROXY_STATUS_NOT_SUPPORTED:
       return ESPHOME_PSTR("SERIAL_PROXY_STATUS_NOT_SUPPORTED");
+    case enums::SERIAL_PROXY_STATUS_PORT_IN_USE:
+      return ESPHOME_PSTR("SERIAL_PROXY_STATUS_PORT_IN_USE");
+    case enums::SERIAL_PROXY_STATUS_INVALID_ARGUMENT:
+      return ESPHOME_PSTR("SERIAL_PROXY_STATUS_INVALID_ARGUMENT");
     default:
       return ESPHOME_PSTR("UNKNOWN");
   }
@@ -914,6 +934,7 @@ const char *SerialProxyInfo::dump_to(DumpBuffer &out) const {
   MessageDumpHelper helper(out, ESPHOME_PSTR("SerialProxyInfo"));
   dump_field(out, ESPHOME_PSTR("name"), this->name);
   dump_field(out, ESPHOME_PSTR("port_type"), static_cast<enums::SerialProxyPortType>(this->port_type));
+  dump_field(out, ESPHOME_PSTR("configured_line_states"), this->configured_line_states);
   return out.c_str();
 }
 #endif
@@ -2644,6 +2665,12 @@ const char *ZWaveProxyRequest::dump_to(DumpBuffer &out) const {
   dump_bytes_field(out, ESPHOME_PSTR("data"), this->data, this->data_len);
   return out.c_str();
 }
+const char *ZWaveProxyRequestResponse::dump_to(DumpBuffer &out) const {
+  MessageDumpHelper helper(out, ESPHOME_PSTR("ZWaveProxyRequestResponse"));
+  dump_field(out, ESPHOME_PSTR("type"), static_cast<enums::ZWaveProxyRequestType>(this->type));
+  dump_field(out, ESPHOME_PSTR("status"), static_cast<enums::ZWaveProxyStatus>(this->status));
+  return out.c_str();
+}
 #endif
 #ifdef USE_INFRARED
 const char *ListEntitiesInfraredResponse::dump_to(DumpBuffer &out) const {
@@ -2753,6 +2780,7 @@ const char *SerialProxyGetModemPinsResponse::dump_to(DumpBuffer &out) const {
   MessageDumpHelper helper(out, ESPHOME_PSTR("SerialProxyGetModemPinsResponse"));
   dump_field(out, ESPHOME_PSTR("instance"), this->instance);
   dump_field(out, ESPHOME_PSTR("line_states"), this->line_states);
+  dump_field(out, ESPHOME_PSTR("status"), static_cast<enums::SerialProxyStatus>(this->status));
   return out.c_str();
 }
 const char *SerialProxyRequest::dump_to(DumpBuffer &out) const {
diff --git a/esphome/components/serial_proxy/serial_proxy.cpp b/esphome/components/serial_proxy/serial_proxy.cpp
index 4b3a907416..94cefc8700 100644
--- a/esphome/components/serial_proxy/serial_proxy.cpp
+++ b/esphome/components/serial_proxy/serial_proxy.cpp
@@ -89,12 +89,12 @@ void SerialProxy::dump_config() {
                 this->dtr_pin_ != nullptr ? "configured" : "not configured");
 }
 
-void SerialProxy::configure(api::APIConnection *api_connection, uint32_t baudrate, bool flow_control, uint8_t parity,
-                            uint8_t stop_bits, uint8_t data_size) {
+SerialProxyResult SerialProxy::configure(api::APIConnection *api_connection, uint32_t baudrate, bool flow_control,
+                                         uint8_t parity, uint8_t stop_bits, uint8_t data_size) {
 #ifdef USE_API
   if (this->port_claimed_by_other_(api_connection)) {
     ESP_LOGW(TAG, "Ignoring configure request from client without port access [%" PRIu32 "]", this->instance_index_);
-    return;
+    return SerialProxyResult::SERIAL_PROXY_RESULT_PORT_IN_USE;
   }
 #endif
   ESP_LOGD(TAG,
@@ -105,25 +105,29 @@ void SerialProxy::configure(api::APIConnection *api_connection, uint32_t baudrat
   auto *uart_comp = this->parent_;
   if (uart_comp == nullptr) {
     ESP_LOGE(TAG, "UART component not available");
-    return;
+    return SerialProxyResult::SERIAL_PROXY_RESULT_ERROR;
   }
 
   // Validate all parameters before applying any (values come from a remote client)
   if (baudrate == 0) {
     ESP_LOGW(TAG, "Invalid baud rate: 0");
-    return;
+    return SerialProxyResult::SERIAL_PROXY_RESULT_INVALID_ARGUMENT;
   }
   if (stop_bits < 1 || stop_bits > 2) {
     ESP_LOGW(TAG, "Invalid stop bits: %u (must be 1 or 2)", stop_bits);
-    return;
+    return SerialProxyResult::SERIAL_PROXY_RESULT_INVALID_ARGUMENT;
   }
   if (data_size < 5 || data_size > 8) {
     ESP_LOGW(TAG, "Invalid data bits: %u (must be 5-8)", data_size);
-    return;
+    return SerialProxyResult::SERIAL_PROXY_RESULT_INVALID_ARGUMENT;
   }
   if (parity > 2) {
     ESP_LOGW(TAG, "Invalid parity: %u (must be 0-2)", parity);
-    return;
+    return SerialProxyResult::SERIAL_PROXY_RESULT_INVALID_ARGUMENT;
+  }
+  if (flow_control) {
+    ESP_LOGW(TAG, "Hardware flow control requested but is not yet supported");
+    return SerialProxyResult::SERIAL_PROXY_RESULT_NOT_SUPPORTED;
   }
 
   // Apply validated parameters
@@ -143,10 +147,7 @@ void SerialProxy::configure(api::APIConnection *api_connection, uint32_t baudrat
 #if defined(USE_ESP8266) || defined(USE_ESP32)
   uart_comp->load_settings(true);
 #endif
-
-  if (flow_control) {
-    ESP_LOGW(TAG, "Hardware flow control requested but is not yet supported");
-  }
+  return SerialProxyResult::SERIAL_PROXY_RESULT_OK;
 }
 
 void SerialProxy::write_from_client(api::APIConnection *api_connection, const uint8_t *data, size_t len) {
@@ -163,13 +164,20 @@ void SerialProxy::write_from_client(api::APIConnection *api_connection, const ui
   this->write_array(data, len);
 }
 
-void SerialProxy::set_modem_pins(api::APIConnection *api_connection, uint32_t line_states) {
+SerialProxyResult SerialProxy::set_modem_pins(api::APIConnection *api_connection, uint32_t line_states) {
 #ifdef USE_API
   if (this->port_claimed_by_other_(api_connection)) {
     ESP_LOGW(TAG, "Ignoring modem pin request from client without port access [%" PRIu32 "]", this->instance_index_);
-    return;
+    return SerialProxyResult::SERIAL_PROXY_RESULT_PORT_IN_USE;
   }
 #endif
+  // Asserting a pin that is not configured must fail so the client learns the signal never
+  // reached the wire; deasserting an absent pin is harmless and stays allowed. Clients can
+  // avoid this by masking against SerialProxyInfo.configured_line_states.
+  if ((line_states & ~this->get_configured_modem_pins()) != 0) {
+    ESP_LOGW(TAG, "Requested modem pin not configured on serial proxy [%" PRIu32 "]", this->instance_index_);
+    return SerialProxyResult::SERIAL_PROXY_RESULT_NOT_SUPPORTED;
+  }
   const bool rts = (line_states & SERIAL_PROXY_LINE_STATE_FLAG_RTS) != 0;
   const bool dtr = (line_states & SERIAL_PROXY_LINE_STATE_FLAG_DTR) != 0;
   ESP_LOGV(TAG, "Setting modem pins [%" PRIu32 "]: RTS=%s, DTR=%s", this->instance_index_, ONOFF(rts), ONOFF(dtr));
@@ -182,6 +190,7 @@ void SerialProxy::set_modem_pins(api::APIConnection *api_connection, uint32_t li
     this->dtr_state_ = dtr;
     this->dtr_pin_->digital_write(dtr);
   }
+  return SerialProxyResult::SERIAL_PROXY_RESULT_OK;
 }
 
 uint32_t SerialProxy::get_modem_pins() const {
@@ -189,9 +198,26 @@ uint32_t SerialProxy::get_modem_pins() const {
          (this->dtr_state_ ? static_cast<uint32_t>(SERIAL_PROXY_LINE_STATE_FLAG_DTR) : 0u);
 }
 
-uart::UARTFlushResult SerialProxy::flush_port() {
+SerialProxyResult SerialProxy::flush_port(api::APIConnection *api_connection) {
+#ifdef USE_API
+  // Flushing stalls the port, so it gets the same ownership check as writes
+  if (this->port_claimed_by_other_(api_connection)) {
+    ESP_LOGW(TAG, "Ignoring flush from client without port access [%" PRIu32 "]", this->instance_index_);
+    return SerialProxyResult::SERIAL_PROXY_RESULT_PORT_IN_USE;
+  }
+#endif
   ESP_LOGV(TAG, "Flushing serial proxy [%" PRIu32 "]", this->instance_index_);
-  return this->flush();
+  switch (this->flush()) {
+    case uart::UARTFlushResult::UART_FLUSH_RESULT_SUCCESS:
+      return SerialProxyResult::SERIAL_PROXY_RESULT_OK;
+    case uart::UARTFlushResult::UART_FLUSH_RESULT_ASSUMED_SUCCESS:
+      return SerialProxyResult::SERIAL_PROXY_RESULT_ASSUMED_SUCCESS;
+    case uart::UARTFlushResult::UART_FLUSH_RESULT_TIMEOUT:
+      return SerialProxyResult::SERIAL_PROXY_RESULT_TIMEOUT;
+    case uart::UARTFlushResult::UART_FLUSH_RESULT_FAILED:
+      return SerialProxyResult::SERIAL_PROXY_RESULT_ERROR;
+  }
+  return SerialProxyResult::SERIAL_PROXY_RESULT_ERROR;  // Unreachable; all enum values handled above
 }
 
 #ifdef USE_API
@@ -200,12 +226,13 @@ bool SerialProxy::port_claimed_by_other_(api::APIConnection *api_connection) con
          this->api_connection_->is_connection_setup();
 }
 
-void SerialProxy::serial_proxy_request(api::APIConnection *api_connection, api::enums::SerialProxyRequestType type) {
+SerialProxyResult SerialProxy::serial_proxy_request(api::APIConnection *api_connection,
+                                                    api::enums::SerialProxyRequestType type) {
   switch (type) {
     case api::enums::SERIAL_PROXY_REQUEST_TYPE_SUBSCRIBE:
       if (this->api_connection_ == api_connection) {
         ESP_LOGV(TAG, "API connection is already subscribed to serial proxy [%" PRIu32 "]", this->instance_index_);
-        return;
+        return SerialProxyResult::SERIAL_PROXY_RESULT_OK;
       }
       if (this->api_connection_ != nullptr) {
         // A living subscriber keeps exclusive access. Its connection may be dead without
@@ -213,26 +240,27 @@ void SerialProxy::serial_proxy_request(api::APIConnection *api_connection, api::
         // in that case let the new client take over instead of locking it out.
         if (this->api_connection_->is_connection_setup()) {
           ESP_LOGE(TAG, "Only one API subscription is allowed at a time");
-          return;
+          return SerialProxyResult::SERIAL_PROXY_RESULT_PORT_IN_USE;
         }
         ESP_LOGW(TAG, "Previous subscriber disconnected; taking over subscription");
       }
       this->api_connection_ = api_connection;
       this->enable_loop();
       ESP_LOGV(TAG, "API connection subscribed to serial proxy [%" PRIu32 "]", this->instance_index_);
-      break;
+      return SerialProxyResult::SERIAL_PROXY_RESULT_OK;
     case api::enums::SERIAL_PROXY_REQUEST_TYPE_UNSUBSCRIBE:
+      // Unsubscribe is idempotent: not being subscribed is not an error
       if (this->api_connection_ != api_connection) {
         ESP_LOGV(TAG, "API connection is not subscribed to serial proxy [%" PRIu32 "]", this->instance_index_);
-        return;
+        return SerialProxyResult::SERIAL_PROXY_RESULT_OK;
       }
       this->api_connection_ = nullptr;
       this->disable_loop();
       ESP_LOGV(TAG, "API connection unsubscribed from serial proxy [%" PRIu32 "]", this->instance_index_);
-      break;
+      return SerialProxyResult::SERIAL_PROXY_RESULT_OK;
     default:
       ESP_LOGW(TAG, "Unknown serial proxy request type: %" PRIu32, static_cast<uint32_t>(type));
-      break;
+      return SerialProxyResult::SERIAL_PROXY_RESULT_NOT_SUPPORTED;
   }
 }
 #endif
diff --git a/esphome/components/serial_proxy/serial_proxy.h b/esphome/components/serial_proxy/serial_proxy.h
index 268c1b52be..a0e47ee686 100644
--- a/esphome/components/serial_proxy/serial_proxy.h
+++ b/esphome/components/serial_proxy/serial_proxy.h
@@ -38,6 +38,17 @@ enum SerialProxyLineStateFlag : uint32_t {
   SERIAL_PROXY_LINE_STATE_FLAG_DTR = 1 << 1,  ///< DTR (Data Terminal Ready)
 };
 
+/// Result of a client-initiated operation; mapped to api::enums::SerialProxyStatus by the API layer
+enum class SerialProxyResult : uint8_t {
+  SERIAL_PROXY_RESULT_OK,                ///< Operation completed or request accepted
+  SERIAL_PROXY_RESULT_ASSUMED_SUCCESS,   ///< Platform cannot confirm TX drain; success assumed
+  SERIAL_PROXY_RESULT_PORT_IN_USE,       ///< Denied: another live client holds the port
+  SERIAL_PROXY_RESULT_INVALID_ARGUMENT,  ///< A parameter value is out of range
+  SERIAL_PROXY_RESULT_ERROR,             ///< Driver or hardware error
+  SERIAL_PROXY_RESULT_TIMEOUT,           ///< Timed out before TX completed
+  SERIAL_PROXY_RESULT_NOT_SUPPORTED,     ///< Requested feature is not available on this instance
+};
+
 /// Maximum bytes to read from UART in a single loop iteration
 inline constexpr size_t SERIAL_PROXY_MAX_READ_SIZE = 256;
 
@@ -73,14 +84,14 @@ class SerialProxy final : public uart::UARTDevice, public Component {
   /// @param parity Parity setting (0=none, 1=even, 2=odd)
   /// @param stop_bits Number of stop bits (1 or 2)
   /// @param data_size Number of data bits (5-8)
-  void configure(api::APIConnection *api_connection, uint32_t baudrate, bool flow_control, uint8_t parity,
-                 uint8_t stop_bits, uint8_t data_size);
+  SerialProxyResult configure(api::APIConnection *api_connection, uint32_t baudrate, bool flow_control, uint8_t parity,
+                              uint8_t stop_bits, uint8_t data_size);
 
   /// Get the currently subscribed API connection (nullptr if none)
   api::APIConnection *get_api_connection() { return this->api_connection_; }
 
   /// Handle a subscribe/unsubscribe request from an API client
-  void serial_proxy_request(api::APIConnection *api_connection, api::enums::SerialProxyRequestType type);
+  SerialProxyResult serial_proxy_request(api::APIConnection *api_connection, api::enums::SerialProxyRequestType type);
 
   /// Write data received from an API client to the serial device
   /// @param api_connection The API connection sending the data
@@ -89,13 +100,20 @@ class SerialProxy final : public uart::UARTDevice, public Component {
   void write_from_client(api::APIConnection *api_connection, const uint8_t *data, size_t len);
 
   /// Set modem pin states from a bitmask of SerialProxyLineStateFlag values
-  void set_modem_pins(api::APIConnection *api_connection, uint32_t line_states);
+  SerialProxyResult set_modem_pins(api::APIConnection *api_connection, uint32_t line_states);
 
   /// Get current modem pin states as a bitmask of SerialProxyLineStateFlag values
   uint32_t get_modem_pins() const;
 
+  /// Get the modem pins this instance can drive as a bitmask of SerialProxyLineStateFlag values
+  uint32_t get_configured_modem_pins() const {
+    return (this->rts_pin_ != nullptr ? static_cast<uint32_t>(SERIAL_PROXY_LINE_STATE_FLAG_RTS) : 0u) |
+           (this->dtr_pin_ != nullptr ? static_cast<uint32_t>(SERIAL_PROXY_LINE_STATE_FLAG_DTR) : 0u);
+  }
+
   /// Flush the serial port (block until all TX data is sent)
-  uart::UARTFlushResult flush_port();
+  /// @param api_connection The API connection requesting the flush
+  SerialProxyResult flush_port(api::APIConnection *api_connection);
 
   /// Set the RTS GPIO pin (from YAML configuration)
   void set_rts_pin(GPIOPin *pin) { this->rts_pin_ = pin; }
diff --git a/esphome/components/zwave_proxy/zwave_proxy.cpp b/esphome/components/zwave_proxy/zwave_proxy.cpp
index 6e3f109ca1..68750295e1 100644
--- a/esphome/components/zwave_proxy/zwave_proxy.cpp
+++ b/esphome/components/zwave_proxy/zwave_proxy.cpp
@@ -194,12 +194,13 @@ void ZWaveProxy::api_connection_authenticated(api::APIConnection *conn) {
   }
 }
 
-void ZWaveProxy::zwave_proxy_request(api::APIConnection *api_connection, api::enums::ZWaveProxyRequestType type) {
+api::enums::ZWaveProxyStatus ZWaveProxy::zwave_proxy_request(api::APIConnection *api_connection,
+                                                             api::enums::ZWaveProxyRequestType type) {
   switch (type) {
     case api::enums::ZWAVE_PROXY_REQUEST_TYPE_SUBSCRIBE:
       if (this->api_connection_ == api_connection) {
         ESP_LOGV(TAG, "API connection is already subscribed");
-        return;
+        return api::enums::ZWAVE_PROXY_STATUS_OK;
       }
       if (this->api_connection_ != nullptr) {
         // A living subscriber keeps exclusive access. Its connection may be dead without
@@ -207,25 +208,26 @@ void ZWaveProxy::zwave_proxy_request(api::APIConnection *api_connection, api::en
         // in that case let the new client take over instead of locking it out.
         if (this->api_connection_->is_connection_setup()) {
           ESP_LOGE(TAG, "Only one API subscription is allowed at a time");
-          return;
+          return api::enums::ZWAVE_PROXY_STATUS_IN_USE;
         }
         ESP_LOGW(TAG, "Previous subscriber disconnected; taking over subscription");
       }
       this->api_connection_ = api_connection;
       ESP_LOGV(TAG, "API connection is now subscribed");
-      break;
+      return api::enums::ZWAVE_PROXY_STATUS_OK;
 
     case api::enums::ZWAVE_PROXY_REQUEST_TYPE_UNSUBSCRIBE:
+      // Unsubscribe is idempotent: not being subscribed is not an error
       if (this->api_connection_ != api_connection) {
         ESP_LOGV(TAG, "API connection is not subscribed");
-        return;
+        return api::enums::ZWAVE_PROXY_STATUS_OK;
       }
       this->api_connection_ = nullptr;
-      break;
+      return api::enums::ZWAVE_PROXY_STATUS_OK;
 
     default:
       ESP_LOGW(TAG, "Unknown request type: %" PRIu32, static_cast<uint32_t>(type));
-      break;
+      return api::enums::ZWAVE_PROXY_STATUS_NOT_SUPPORTED;
   }
 }
 
diff --git a/esphome/components/zwave_proxy/zwave_proxy.h b/esphome/components/zwave_proxy/zwave_proxy.h
index cb60139ef8..75225d84a8 100644
--- a/esphome/components/zwave_proxy/zwave_proxy.h
+++ b/esphome/components/zwave_proxy/zwave_proxy.h
@@ -60,7 +60,8 @@ class ZWaveProxy final : public uart::UARTDevice, public Component {
   bool can_proceed() override;
 
   void api_connection_authenticated(api::APIConnection *conn);
-  void zwave_proxy_request(api::APIConnection *api_connection, api::enums::ZWaveProxyRequestType type);
+  api::enums::ZWaveProxyStatus zwave_proxy_request(api::APIConnection *api_connection,
+                                                   api::enums::ZWaveProxyRequestType type);
   api::APIConnection *get_api_connection() { return this->api_connection_; }
 
   uint32_t get_feature_flags() const { return ZWaveProxyFeature::FEATURE_ZWAVE_PROXY_ENABLED; }
diff --git a/tests/benchmarks/stubs/esphome/components/serial_proxy/serial_proxy.h b/tests/benchmarks/stubs/esphome/components/serial_proxy/serial_proxy.h
index d8b068fb36..6fc20f3350 100644
--- a/tests/benchmarks/stubs/esphome/components/serial_proxy/serial_proxy.h
+++ b/tests/benchmarks/stubs/esphome/components/serial_proxy/serial_proxy.h
@@ -13,17 +13,18 @@ namespace api {
 class APIConnection;
 }  // namespace api
 
-namespace uart {
-enum class UARTFlushResult : uint8_t {
-  UART_FLUSH_RESULT_SUCCESS,
-  UART_FLUSH_RESULT_ASSUMED_SUCCESS,
-  UART_FLUSH_RESULT_TIMEOUT,
-  UART_FLUSH_RESULT_FAILED,
-};
-}  // namespace uart
-
 namespace serial_proxy {
 
+enum class SerialProxyResult : uint8_t {
+  SERIAL_PROXY_RESULT_OK,
+  SERIAL_PROXY_RESULT_ASSUMED_SUCCESS,
+  SERIAL_PROXY_RESULT_PORT_IN_USE,
+  SERIAL_PROXY_RESULT_INVALID_ARGUMENT,
+  SERIAL_PROXY_RESULT_ERROR,
+  SERIAL_PROXY_RESULT_TIMEOUT,
+  SERIAL_PROXY_RESULT_NOT_SUPPORTED,
+};
+
 class SerialProxy {
  public:
   void set_instance_index(uint32_t index) { this->instance_index_ = index; }
@@ -31,13 +32,20 @@ class SerialProxy {
   const char *get_name() const { return ""; }
   api::enums::SerialProxyPortType get_port_type() const { return {}; }
   api::APIConnection *get_api_connection() { return nullptr; }
-  void serial_proxy_request(api::APIConnection *conn, api::enums::SerialProxyRequestType type) {}
-  void configure(api::APIConnection *api_connection, uint32_t baudrate, bool flow_control, uint8_t parity,
-                 uint32_t stop_bits, uint32_t data_size) {}
+  SerialProxyResult serial_proxy_request(api::APIConnection *conn, api::enums::SerialProxyRequestType type) {
+    return SerialProxyResult::SERIAL_PROXY_RESULT_OK;
+  }
+  SerialProxyResult configure(api::APIConnection *api_connection, uint32_t baudrate, bool flow_control, uint8_t parity,
+                              uint8_t stop_bits, uint8_t data_size) {
+    return SerialProxyResult::SERIAL_PROXY_RESULT_OK;
+  }
   void write_from_client(api::APIConnection *api_connection, const uint8_t *data, size_t len) {}
-  void set_modem_pins(api::APIConnection *api_connection, uint32_t line_states) {}
+  SerialProxyResult set_modem_pins(api::APIConnection *api_connection, uint32_t line_states) {
+    return SerialProxyResult::SERIAL_PROXY_RESULT_OK;
+  }
   uint32_t get_modem_pins() const { return 0; }
-  uart::UARTFlushResult flush_port() { return uart::UARTFlushResult::UART_FLUSH_RESULT_SUCCESS; }
+  uint32_t get_configured_modem_pins() const { return 0; }
+  SerialProxyResult flush_port(api::APIConnection *api_connection) { return SerialProxyResult::SERIAL_PROXY_RESULT_OK; }
 
  protected:
   uint32_t instance_index_{0};
diff --git a/tests/benchmarks/stubs/esphome/components/zwave_proxy/zwave_proxy.h b/tests/benchmarks/stubs/esphome/components/zwave_proxy/zwave_proxy.h
index b4ccd8fd00..3abbf7d257 100644
--- a/tests/benchmarks/stubs/esphome/components/zwave_proxy/zwave_proxy.h
+++ b/tests/benchmarks/stubs/esphome/components/zwave_proxy/zwave_proxy.h
@@ -15,7 +15,9 @@ namespace zwave_proxy {
 class ZWaveProxy {
  public:
   api::APIConnection *get_api_connection() { return nullptr; }
-  void zwave_proxy_request(api::APIConnection *conn, api::enums::ZWaveProxyRequestType type) {}
+  api::enums::ZWaveProxyStatus zwave_proxy_request(api::APIConnection *conn, api::enums::ZWaveProxyRequestType type) {
+    return api::enums::ZWAVE_PROXY_STATUS_OK;
+  }
   void send_frame(api::APIConnection *api_connection, const uint8_t *data, size_t length) {}
   void api_connection_authenticated(api::APIConnection *conn) {}
   uint32_t get_feature_flags() const { return 0; }
diff --git a/tests/unit_tests/components/api/test_api_proto.py b/tests/unit_tests/components/api/test_api_proto.py
index 35aa5ff529..31297911f5 100644
--- a/tests/unit_tests/components/api/test_api_proto.py
+++ b/tests/unit_tests/components/api/test_api_proto.py
@@ -263,6 +263,17 @@ def test_device_capabilities_response_has_id_150() -> None:
     )
 
 
+def test_z_wave_proxy_request_response_has_id_151() -> None:
+    body = _extract_proto_message(PROTO_TEXT, "ZWaveProxyRequestResponse")
+    match = re.search(r"option \(id\) = (\d+);", body)
+    assert match is not None, "ZWaveProxyRequestResponse is missing `option (id)`"
+    assert int(match.group(1)) == 151, (
+        f"ZWaveProxyRequestResponse has id {match.group(1)}, expected 151. "
+        "Message ids are part of the wire protocol and must not change once "
+        "assigned."
+    )
+
+
 def test_superseded_fields_are_not_marked_deprecated_in_proto() -> None:
     """The six superseded fields must not carry `[deprecated = true]` in
     api.proto, or the generator drops them and old clients stop receiving