diff --git a/esphome/components/api/api.proto b/esphome/components/api/api.proto
index f57579fd27..8d6737fe81 100644
--- a/esphome/components/api/api.proto
+++ b/esphome/components/api/api.proto
@@ -2733,7 +2733,8 @@ enum SerialProxyParity {
   SERIAL_PROXY_PARITY_ODD = 2;
 }
 
-// Configure UART parameters for a serial proxy instance
+// Configure UART parameters for a serial proxy instance. Only the subscribed client may
+// configure the port; others are refused with PORT_IN_USE (since API 1.17).
 message SerialProxyConfigureRequest {
   option (id) = 138;
   option (source) = SOURCE_CLIENT;
@@ -2759,7 +2760,8 @@ message SerialProxyDataReceived {
   bytes data = 2;               // Raw data received from the serial device
 }
 
-// Write data to a serial device
+// Write data to a serial device. Only the subscribed client may write; writes from
+// others are ignored (since API 1.17).
 message SerialProxyWriteRequest {
   option (id) = 140;
   option (source) = SOURCE_CLIENT;
@@ -2770,7 +2772,8 @@ message SerialProxyWriteRequest {
   bytes data = 2;               // Raw data to write to the serial device
 }
 
-// Set modem control pin states (RTS and DTR)
+// Set modem control pin states (RTS and DTR). Only the subscribed client may set them;
+// others are refused with PORT_IN_USE (since API 1.17).
 message SerialProxySetModemPinsRequest {
   option (id) = 141;
   option (source) = SOURCE_CLIENT;
@@ -2809,6 +2812,7 @@ enum SerialProxyRequestType {
   // 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
+  SERIAL_PROXY_REQUEST_TYPE_SET_MODE = 5;       // Acknowledges a SerialProxySetModeRequest (since API 1.17)
 }
 
 enum SerialProxyStatus {
@@ -2821,7 +2825,8 @@ enum SerialProxyStatus {
   SERIAL_PROXY_STATUS_INVALID_ARGUMENT = 6; // Invalid instance index or parameter value
 }
 
-// Generic request message for simple serial proxy operations
+// Generic request message for simple serial proxy operations. FLUSH requires an active
+// subscription; it is refused with PORT_IN_USE otherwise (since API 1.17).
 message SerialProxyRequest {
   option (id) = 144;
   option (source) = SOURCE_CLIENT;
@@ -2856,8 +2861,11 @@ enum SerialProxyMode {
   SERIAL_PROXY_MODE_PROTOCOL = 1;
 }
 
+// Only the subscribed client may change the mode; any other caller -- including one that
+// never subscribed -- is refused with PORT_IN_USE. PROTOCOL is refused with NOT_SUPPORTED
+// when the port has no protocol-aware tap configured.
 message SerialProxySetModeRequest {
-  option (id) = 151;
+  option (id) = 152;
   option (source) = SOURCE_CLIENT;
   option (ifdef) = "USE_SERIAL_PROXY";
 
@@ -2894,7 +2902,7 @@ enum ZigbeeProxyRequestType {
 }
 
 message ZigbeeProxyRequest {
-  option (id) = 150;
+  option (id) = 153;
   option (source) = SOURCE_BOTH;
   option (ifdef) = "USE_ZIGBEE_PROXY";
 
diff --git a/esphome/components/api/api_connection.cpp b/esphome/components/api/api_connection.cpp
index 3fc46e5c19..59aa93e126 100644
--- a/esphome/components/api/api_connection.cpp
+++ b/esphome/components/api/api_connection.cpp
@@ -1670,6 +1670,7 @@ void APIConnection::on_serial_proxy_request(const SerialProxyRequest &msg) {
       break;
     case enums::SERIAL_PROXY_REQUEST_TYPE_CONFIGURE:
     case enums::SERIAL_PROXY_REQUEST_TYPE_SET_MODEM_PINS:
+    case enums::SERIAL_PROXY_REQUEST_TYPE_SET_MODE:
       // 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;
@@ -1686,9 +1687,13 @@ void APIConnection::on_serial_proxy_set_mode_request(const SerialProxySetModeReq
   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_MODE,
+                          enums::SERIAL_PROXY_STATUS_INVALID_ARGUMENT);
     return;
   }
-  proxies[msg.instance]->set_mode(this, msg.mode);
+  serial_proxy::SerialProxyResult result = proxies[msg.instance]->set_mode_from_client(this, msg.mode);
+  send_serial_proxy_ack(this, msg.instance, enums::SERIAL_PROXY_REQUEST_TYPE_SET_MODE,
+                        serial_proxy_result_to_status(result));
 }
 
 void APIConnection::send_serial_proxy_data(const SerialProxyDataReceived &msg) {
@@ -1822,7 +1827,7 @@ bool APIConnection::send_hello_response_(const HelloRequest &msg) {
 
   HelloResponse resp;
   resp.api_version_major = 1;
-  resp.api_version_minor = 16;
+  resp.api_version_minor = 17;
   // 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());
diff --git a/esphome/components/api/api_pb2.h b/esphome/components/api/api_pb2.h
index 26958e8a50..7755d82966 100644
--- a/esphome/components/api/api_pb2.h
+++ b/esphome/components/api/api_pb2.h
@@ -356,6 +356,7 @@ enum SerialProxyRequestType : uint32_t {
   SERIAL_PROXY_REQUEST_TYPE_FLUSH = 2,
   SERIAL_PROXY_REQUEST_TYPE_CONFIGURE = 3,
   SERIAL_PROXY_REQUEST_TYPE_SET_MODEM_PINS = 4,
+  SERIAL_PROXY_REQUEST_TYPE_SET_MODE = 5,
 };
 enum SerialProxyStatus : uint32_t {
   SERIAL_PROXY_STATUS_OK = 0,
@@ -3420,7 +3421,7 @@ class SerialProxyRequestResponse final : public ProtoMessage {
 };
 class SerialProxySetModeRequest final : public ProtoDecodableMessage {
  public:
-  static constexpr uint16_t MESSAGE_TYPE = 151;
+  static constexpr uint16_t MESSAGE_TYPE = 152;
   static constexpr uint8_t ESTIMATED_SIZE = 6;
 #ifdef HAS_PROTO_MESSAGE_DUMP
   const LogString *message_name() const override { return LOG_STR("serial_proxy_set_mode_request"); }
@@ -3476,7 +3477,7 @@ class BluetoothSetConnectionParamsResponse final : public ProtoMessage {
 #ifdef USE_ZIGBEE_PROXY
 class ZigbeeProxyRequest final : public ProtoDecodableMessage {
  public:
-  static constexpr uint16_t MESSAGE_TYPE = 150;
+  static constexpr uint16_t MESSAGE_TYPE = 153;
   static constexpr uint8_t ESTIMATED_SIZE = 21;
 #ifdef HAS_PROTO_MESSAGE_DUMP
   const LogString *message_name() const override { return LOG_STR("zigbee_proxy_request"); }
diff --git a/esphome/components/api/api_pb2_dump.cpp b/esphome/components/api/api_pb2_dump.cpp
index 828e06f96d..98dbae5bcc 100644
--- a/esphome/components/api/api_pb2_dump.cpp
+++ b/esphome/components/api/api_pb2_dump.cpp
@@ -854,6 +854,8 @@ template<> const char *proto_enum_to_string<enums::SerialProxyRequestType>(enums
       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");
+    case enums::SERIAL_PROXY_REQUEST_TYPE_SET_MODE:
+      return ESPHOME_PSTR("SERIAL_PROXY_REQUEST_TYPE_SET_MODE");
     default:
       return ESPHOME_PSTR("UNKNOWN");
   }
diff --git a/esphome/components/api/api_pb2_service.cpp b/esphome/components/api/api_pb2_service.cpp
index 1b6ed7e228..edf460c4b6 100644
--- a/esphome/components/api/api_pb2_service.cpp
+++ b/esphome/components/api/api_pb2_service.cpp
@@ -712,17 +712,6 @@ void APIConnection::read_message_(uint32_t msg_size, uint32_t msg_type, const ui
       this->on_device_capabilities_request();
       break;
     }
-#ifdef USE_ZIGBEE_PROXY
-    case ZigbeeProxyRequest::MESSAGE_TYPE: {
-      ZigbeeProxyRequest msg;
-      msg.decode(msg_data, msg_size);
-#ifdef HAS_PROTO_MESSAGE_DUMP
-      this->log_receive_message_(LOG_STR("on_zigbee_proxy_request"), msg);
-#endif
-      this->on_zigbee_proxy_request(msg);
-      break;
-    }
-#endif
 #ifdef USE_SERIAL_PROXY
     case SerialProxySetModeRequest::MESSAGE_TYPE: {
       SerialProxySetModeRequest msg;
@@ -733,6 +722,17 @@ void APIConnection::read_message_(uint32_t msg_size, uint32_t msg_type, const ui
       this->on_serial_proxy_set_mode_request(msg);
       break;
     }
+#endif
+#ifdef USE_ZIGBEE_PROXY
+    case ZigbeeProxyRequest::MESSAGE_TYPE: {
+      ZigbeeProxyRequest msg;
+      msg.decode(msg_data, msg_size);
+#ifdef HAS_PROTO_MESSAGE_DUMP
+      this->log_receive_message_(LOG_STR("on_zigbee_proxy_request"), msg);
+#endif
+      this->on_zigbee_proxy_request(msg);
+      break;
+    }
 #endif
     default:
       break;
diff --git a/esphome/components/serial_proxy/__init__.py b/esphome/components/serial_proxy/__init__.py
index 158c9609e4..b6e780fabd 100644
--- a/esphome/components/serial_proxy/__init__.py
+++ b/esphome/components/serial_proxy/__init__.py
@@ -18,7 +18,7 @@ from esphome import pins
 import esphome.codegen as cg
 from esphome.components import uart
 import esphome.config_validation as cv
-from esphome.const import CONF_ID, CONF_MODE, CONF_NAME
+from esphome.const import CONF_ID, CONF_NAME
 from esphome.core import CORE, coroutine_with_priority
 from esphome.coroutine import CoroPriority
 from esphome.types import ConfigType
@@ -40,16 +40,6 @@ SERIAL_PROXY_PORT_TYPES = {
     "RS485": SerialProxyPortType.SERIAL_PROXY_PORT_TYPE_RS485,
 }
 
-SerialProxyMode = api_enums_ns.enum("SerialProxyMode")
-# The mode a port starts in. `raw` is a plain byte pipe; `protocol` activates the
-# port's tap (if one is configured), letting it observe traffic and inject protocol
-# bytes such as acknowledgements. Clients may change it at runtime, so this only
-# decides what the device boots into.
-SERIAL_PROXY_MODES = {
-    "RAW": SerialProxyMode.SERIAL_PROXY_MODE_RAW,
-    "PROTOCOL": SerialProxyMode.SERIAL_PROXY_MODE_PROTOCOL,
-}
-
 CONF_DTR_PIN = "dtr_pin"
 CONF_PORT_TYPE = "port_type"
 CONF_RTS_PIN = "rts_pin"
@@ -74,9 +64,6 @@ CONFIG_SCHEMA = (
             cv.GenerateID(): cv.declare_id(SerialProxy),
             cv.Required(CONF_NAME): cv.string_strict,
             cv.Required(CONF_PORT_TYPE): cv.enum(SERIAL_PROXY_PORT_TYPES, upper=True),
-            cv.Optional(CONF_MODE, default="RAW"): cv.enum(
-                SERIAL_PROXY_MODES, upper=True
-            ),
             cv.Optional(CONF_RTS_PIN): pins.gpio_output_pin_schema,
             cv.Optional(CONF_DTR_PIN): pins.gpio_output_pin_schema,
         }
@@ -101,7 +88,6 @@ async def to_code(config: ConfigType) -> None:
     cg.add(cg.App.register_serial_proxy(var))
     cg.add(var.set_name(config[CONF_NAME]))
     cg.add(var.set_port_type(config[CONF_PORT_TYPE]))
-    cg.add(var.set_mode(config[CONF_MODE]))
     cg.add_define("USE_SERIAL_PROXY")
 
     # Track instance count for the FINAL priority define
diff --git a/esphome/components/serial_proxy/serial_proxy.cpp b/esphome/components/serial_proxy/serial_proxy.cpp
index 5d0e9cbbd7..129745c1c9 100644
--- a/esphome/components/serial_proxy/serial_proxy.cpp
+++ b/esphome/components/serial_proxy/serial_proxy.cpp
@@ -43,6 +43,7 @@ void SerialProxy::setup() {
   this->disable_loop();
 }
 
+#ifdef USE_SERIAL_PROXY_TAP
 void SerialProxy::reset_mode_() {
   // The mode belongs to a session, not to the port. Carrying a departed client's choice
   // over to the next one would inject protocol bytes into a stream that never asked for
@@ -55,6 +56,7 @@ void SerialProxy::reset_mode_() {
   ESP_LOGD(TAG, "Session ended, returning serial proxy [%" PRIu32 "] to RAW mode", this->instance_index_);
   this->mode_ = api::enums::SERIAL_PROXY_MODE_RAW;
 }
+#endif
 
 void SerialProxy::loop() {
 #ifdef USE_API
@@ -120,11 +122,10 @@ bool SerialProxy::tap_observing_() const {
   if (this->tap_ == nullptr) {
     return false;
   }
-  // A tap that needs the port is mid-protocol-work of its own -- the boot-time handshake
-  // with the device, which runs before any client has connected and so before anyone could
-  // have chosen a mode. Withholding bytes from it there would strand it, so it is served
-  // regardless of mode.
-  if (this->tap_->tap_needs_port()) {
+  // With no subscriber, a tap doing its own protocol work (the boot-time handshake with
+  // the device, say) is served regardless of mode -- nobody has chosen one yet. Once a
+  // subscriber holds the port, the mode alone decides, so RAW stays inert.
+  if (this->api_connection_ == nullptr && this->tap_->tap_needs_port()) {
     return true;
   }
   // Otherwise the mode decides. RAW must be inert: a client that flips to RAW before
@@ -135,6 +136,10 @@ bool SerialProxy::tap_observing_() const {
 
 void SerialProxy::tap_pump() {
 #ifdef USE_API
+  // Nothing would consume the bytes; leave them in the FIFO
+  if (!this->tap_observing_() && this->api_connection_ == nullptr) {
+    return;
+  }
   const size_t available = this->available();
   if (available > 0) {
     this->read_and_send_(available);
@@ -144,28 +149,26 @@ void SerialProxy::tap_pump() {
 #endif
 
 void SerialProxy::dump_config() {
-  ESP_LOGCONFIG(
-      TAG,
-      "Serial Proxy [%" PRIu32 "]:\n"
-      "  Name: %s\n"
-      "  Port Type: %s\n"
-      "  Mode: %s\n"
-      "  RTS Pin: %s\n"
-      "  DTR Pin: %s",
-      this->instance_index_, this->name_ != nullptr ? this->name_ : "",
-      this->port_type_ == api::enums::SERIAL_PROXY_PORT_TYPE_RS485   ? LOG_STR_LITERAL("RS485")
-      : this->port_type_ == api::enums::SERIAL_PROXY_PORT_TYPE_RS232 ? LOG_STR_LITERAL("RS232")
-                                                                     : LOG_STR_LITERAL("TTL"),
-      this->mode_ == api::enums::SERIAL_PROXY_MODE_PROTOCOL ? LOG_STR_LITERAL("PROTOCOL") : LOG_STR_LITERAL("RAW"),
-      this->rts_pin_ != nullptr ? LOG_STR_LITERAL("configured") : LOG_STR_LITERAL("not configured"),
-      this->dtr_pin_ != nullptr ? LOG_STR_LITERAL("configured") : LOG_STR_LITERAL("not configured"));
+  ESP_LOGCONFIG(TAG,
+                "Serial Proxy [%" PRIu32 "]:\n"
+                "  Name: %s\n"
+                "  Port Type: %s\n"
+                "  RTS Pin: %s\n"
+                "  DTR Pin: %s",
+                this->instance_index_, this->name_ != nullptr ? this->name_ : "",
+                this->port_type_ == api::enums::SERIAL_PROXY_PORT_TYPE_RS485   ? LOG_STR_LITERAL("RS485")
+                : this->port_type_ == api::enums::SERIAL_PROXY_PORT_TYPE_RS232 ? LOG_STR_LITERAL("RS232")
+                                                                               : LOG_STR_LITERAL("TTL"),
+                this->rts_pin_ != nullptr ? LOG_STR_LITERAL("configured") : LOG_STR_LITERAL("not configured"),
+                this->dtr_pin_ != nullptr ? LOG_STR_LITERAL("configured") : LOG_STR_LITERAL("not configured"));
 }
 
 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_);
+  if (!this->is_subscriber_(api_connection)) {
+    ESP_LOGW(TAG, "Ignoring configure request from client without port subscription [%" PRIu32 "]",
+             this->instance_index_);
     return SerialProxyResult::SERIAL_PROXY_RESULT_PORT_IN_USE;
   }
 #endif
@@ -231,20 +234,37 @@ SerialProxyResult SerialProxy::configure(api::APIConnection *api_connection, uin
   return SerialProxyResult::SERIAL_PROXY_RESULT_OK;
 }
 
-void SerialProxy::set_mode(api::APIConnection *api_connection, api::enums::SerialProxyMode mode) {
+SerialProxyResult SerialProxy::set_mode_from_client(api::APIConnection *api_connection,
+                                                    api::enums::SerialProxyMode mode) {
 #ifdef USE_API
-  if (this->port_claimed_by_other_(api_connection)) {
-    ESP_LOGW(TAG, "Ignoring mode request from client without port access [%" PRIu32 "]", this->instance_index_);
-    return;
+  // Only the live subscriber may change the mode, so the mode cannot outlive a session
+  if (!this->is_subscriber_(api_connection)) {
+    ESP_LOGW(TAG, "Ignoring mode request from client without port subscription [%" PRIu32 "]", this->instance_index_);
+    return SerialProxyResult::SERIAL_PROXY_RESULT_PORT_IN_USE;
   }
 #endif
+  // Values come from a remote client
+  if (mode != api::enums::SERIAL_PROXY_MODE_RAW && mode != api::enums::SERIAL_PROXY_MODE_PROTOCOL) {
+    ESP_LOGW(TAG, "Invalid mode: %" PRIu32, static_cast<uint32_t>(mode));
+    return SerialProxyResult::SERIAL_PROXY_RESULT_INVALID_ARGUMENT;
+  }
+  // PROTOCOL on a port with no tap would be a silent no-op; refuse so the client knows
+#ifdef USE_SERIAL_PROXY_TAP
+  const bool has_tap = this->tap_ != nullptr;
+#else
+  const bool has_tap = false;
+#endif
+  if (mode == api::enums::SERIAL_PROXY_MODE_PROTOCOL && !has_tap) {
+    ESP_LOGW(TAG, "No tap on serial proxy [%" PRIu32 "]; PROTOCOL mode unavailable", this->instance_index_);
+    return SerialProxyResult::SERIAL_PROXY_RESULT_NOT_SUPPORTED;
+  }
   ESP_LOGD(TAG, "Serial proxy [%" PRIu32 "] mode set to %s", this->instance_index_,
-           mode == api::enums::SERIAL_PROXY_MODE_PROTOCOL ? "PROTOCOL" : "RAW");
+           mode == api::enums::SERIAL_PROXY_MODE_PROTOCOL ? LOG_STR_LITERAL("PROTOCOL") : LOG_STR_LITERAL("RAW"));
+#ifdef USE_SERIAL_PROXY_TAP
   const bool leaving_protocol_mode =
       this->mode_ != api::enums::SERIAL_PROXY_MODE_RAW && mode == api::enums::SERIAL_PROXY_MODE_RAW;
   this->mode_ = mode;
 
-#ifdef USE_SERIAL_PROXY_TAP
   // Only for an explicit client request, not for reset_mode_() at the end of a session:
   // an ordinary disconnect says nothing about the device, whereas a client deliberately
   // asking for raw bytes usually precedes changing what the device is.
@@ -252,14 +272,22 @@ void SerialProxy::set_mode(api::APIConnection *api_connection, api::enums::Seria
     this->tap_->on_protocol_disabled();
   }
 #endif
+  return SerialProxyResult::SERIAL_PROXY_RESULT_OK;
 }
 
 void SerialProxy::write_from_client(api::APIConnection *api_connection, const uint8_t *data, size_t len) {
 #ifdef USE_API
-  // Bytes from a client other than the live subscriber would interleave with the
-  // subscriber's traffic on the wire
-  if (this->port_claimed_by_other_(api_connection)) {
-    ESP_LOGW(TAG, "Ignoring write from client without port access [%" PRIu32 "]", this->instance_index_);
+  // Bytes from anyone but the live subscriber would interleave with the subscriber's
+  // traffic -- or with an active tap's -- on the wire
+  if (!this->is_subscriber_(api_connection)) {
+    if (this->api_connection_ != nullptr) {
+      ESP_LOGW(TAG, "Ignoring write from client that does not hold serial proxy [%" PRIu32 "]", this->instance_index_);
+    } else {
+      // A legacy client streaming writes without subscribing would flood WARN, one per
+      // request; writes are the only high-rate, unacknowledged operation, so keep this
+      // visible without drowning the log
+      ESP_LOGV(TAG, "Ignoring write from client without port subscription [%" PRIu32 "]", this->instance_index_);
+    }
     return;
   }
 #endif
@@ -277,8 +305,9 @@ void SerialProxy::write_from_client(api::APIConnection *api_connection, const ui
 
 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_);
+  if (!this->is_subscriber_(api_connection)) {
+    ESP_LOGW(TAG, "Ignoring modem pin request from client without port subscription [%" PRIu32 "]",
+             this->instance_index_);
     return SerialProxyResult::SERIAL_PROXY_RESULT_PORT_IN_USE;
   }
 #endif
@@ -312,8 +341,8 @@ uint32_t SerialProxy::get_modem_pins() const {
 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_);
+  if (!this->is_subscriber_(api_connection)) {
+    ESP_LOGW(TAG, "Ignoring flush from client without port subscription [%" PRIu32 "]", this->instance_index_);
     return SerialProxyResult::SERIAL_PROXY_RESULT_PORT_IN_USE;
   }
 #endif
@@ -332,11 +361,6 @@ SerialProxyResult SerialProxy::flush_port(api::APIConnection *api_connection) {
 }
 
 #ifdef USE_API
-bool SerialProxy::port_claimed_by_other_(api::APIConnection *api_connection) const {
-  return this->api_connection_ != nullptr && this->api_connection_ != api_connection &&
-         this->api_connection_->is_connection_setup();
-}
-
 SerialProxyResult SerialProxy::serial_proxy_request(api::APIConnection *api_connection,
                                                     api::enums::SerialProxyRequestType type) {
   switch (type) {
@@ -354,6 +378,10 @@ SerialProxyResult SerialProxy::serial_proxy_request(api::APIConnection *api_conn
           return SerialProxyResult::SERIAL_PROXY_RESULT_PORT_IN_USE;
         }
         ESP_LOGW(TAG, "Previous subscriber disconnected; taking over subscription");
+        // End the dead client's session before starting the new one, so its mode
+        // cannot leak into a session that never asked for it
+        this->api_connection_ = nullptr;
+        this->reset_mode_();
       }
       this->api_connection_ = api_connection;
       this->enable_loop();
@@ -367,7 +395,14 @@ SerialProxyResult SerialProxy::serial_proxy_request(api::APIConnection *api_conn
       }
       this->api_connection_ = nullptr;
       this->reset_mode_();
+#ifdef USE_SERIAL_PROXY_TAP
+      // Keep the loop alive for a tap that still needs the port (mirrors loop())
+      if (this->tap_ == nullptr || !this->tap_->tap_needs_port()) {
+        this->disable_loop();
+      }
+#else
       this->disable_loop();
+#endif
       ESP_LOGV(TAG, "API connection unsubscribed from serial proxy [%" PRIu32 "]", this->instance_index_);
       return SerialProxyResult::SERIAL_PROXY_RESULT_OK;
     default:
diff --git a/esphome/components/serial_proxy/serial_proxy.h b/esphome/components/serial_proxy/serial_proxy.h
index 19a3a4f063..e3f4264cfa 100644
--- a/esphome/components/serial_proxy/serial_proxy.h
+++ b/esphome/components/serial_proxy/serial_proxy.h
@@ -71,7 +71,8 @@ class SerialProxyTap {
   virtual void on_client_tx(const uint8_t *data, size_t len) = 0;
 
   /// True when the port must keep reading even with no subscriber attached, so a tap can
-  /// do its own protocol work while nobody is listening.
+  /// do its own protocol work while nobody is listening. Honoured only while no
+  /// subscriber holds the port; with one attached, the port mode alone decides.
   virtual bool tap_needs_port() const = 0;
 
   /// A client explicitly turned protocol handling off for this port. Distinct from the
@@ -107,14 +108,8 @@ class SerialProxy final : public uart::UARTDevice, public Component {
   /// Get the port type
   api::enums::SerialProxyPortType get_port_type() const { return this->port_type_; }
 
-  /// Set the initial mode (from YAML configuration)
-  void set_mode(api::enums::SerialProxyMode mode) { this->mode_ = mode; }
-
-  /// Get the current mode
-  api::enums::SerialProxyMode get_mode() const { return this->mode_; }
-
   /// Handle a mode change requested by an API client
-  void set_mode(api::APIConnection *api_connection, api::enums::SerialProxyMode mode);
+  SerialProxyResult set_mode_from_client(api::APIConnection *api_connection, api::enums::SerialProxyMode mode);
 
   /// Configure UART parameters and apply them
   /// @param api_connection The API connection requesting the change
@@ -165,12 +160,25 @@ class SerialProxy final : public uart::UARTDevice, public Component {
   void set_tap(SerialProxyTap *tap) { this->tap_ = tap; }
 
   /// Write bytes originating from the tap rather than from a client. Bypasses the
-  /// subscriber ownership check, since the tap is part of the device, not a client of it.
-  void write_from_tap(const uint8_t *data, size_t len) { this->write_array(data, len); }
+  /// subscriber ownership check, but only while the tap is being served bytes -- so a
+  /// port in RAW mode with a subscriber attached stays inert. Returns false when the
+  /// bytes were dropped for that reason.
+  bool write_from_tap(const uint8_t *data, size_t len) {
+    if (!this->tap_observing_()) {
+      return false;
+    }
+    this->write_array(data, len);
+    return true;
+  }
+
+  /// Whether the tap is currently being served bytes. Can flip false with no callback
+  /// (a subscriber attaching in RAW mode, say), so a tap should check before starting
+  /// protocol work and when a reply seems overdue.
+  bool tap_is_observed() const { return this->tap_observing_(); }
 
   /// Resume reading after a tap's needs change. loop() disables itself when there is
   /// neither a subscriber nor a tap that wants the port, so a tap starting fresh work
-  /// must ask for it back.
+  /// must ask for it back. Must be called from the main loop.
   void tap_request_port() { this->enable_loop(); }
 
   /// Whether the underlying device is present. On a USB UART this tracks enumeration, so
@@ -179,7 +187,8 @@ class SerialProxy final : public uart::UARTDevice, public Component {
 
   /// Run one read-and-dispatch cycle immediately. Lets a tap make progress before the
   /// main loop is running -- during setup, for instance, while a component is still
-  /// blocking on can_proceed().
+  /// blocking on can_proceed(). Must not be called from on_device_rx() or
+  /// on_client_tx(): each nested cycle costs a 256-byte stack frame.
   void tap_pump();
 #endif
 
@@ -189,13 +198,20 @@ class SerialProxy final : public uart::UARTDevice, public Component {
   /// (slow path with a 256-byte stack buffer)
   void read_and_send_(size_t available);
 
-  /// True when a live subscriber other than the given connection holds the port
-  bool port_claimed_by_other_(api::APIConnection *api_connection) const;
+  /// True when the given connection is the live subscriber. Every port operation
+  /// (write, configure, modem pins, flush, mode) requires this, so an unsubscribed
+  /// client can never share the wire with the subscriber or an active tap.
+  bool is_subscriber_(api::APIConnection *api_connection) const { return this->api_connection_ == api_connection; }
 #endif
 
-  /// Return the port to RAW when a subscriber goes away, so the mode never outlives it.
-  /// Not tap-gated: the mode is a client-visible property whether or not a tap acts on it.
+#ifdef USE_SERIAL_PROXY_TAP
+  /// Return the port to RAW when a subscriber goes away, so the mode never outlives it
   void reset_mode_();
+#else
+  /// Without a tap, PROTOCOL is refused, so the mode is fixed at RAW and there is
+  /// nothing to reset
+  void reset_mode_() {}
+#endif
 
 #ifdef USE_SERIAL_PROXY_TAP
   /// True when the tap should be shown the traffic passing through this port
@@ -219,8 +235,10 @@ class SerialProxy final : public uart::UARTDevice, public Component {
   /// Port type
   api::enums::SerialProxyPortType port_type_{};
 
+#ifdef USE_SERIAL_PROXY_TAP
   /// How the bytes passing through are treated; zero is SERIAL_PROXY_MODE_RAW
   api::enums::SerialProxyMode mode_{};
+#endif
 
   /// Optional GPIO pins for modem control
   GPIOPin *rts_pin_{nullptr};
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 6fc20f3350..7da6fff017 100644
--- a/tests/benchmarks/stubs/esphome/components/serial_proxy/serial_proxy.h
+++ b/tests/benchmarks/stubs/esphome/components/serial_proxy/serial_proxy.h
@@ -40,6 +40,9 @@ class SerialProxy {
     return SerialProxyResult::SERIAL_PROXY_RESULT_OK;
   }
   void write_from_client(api::APIConnection *api_connection, const uint8_t *data, size_t len) {}
+  SerialProxyResult set_mode_from_client(api::APIConnection *api_connection, api::enums::SerialProxyMode mode) {
+    return SerialProxyResult::SERIAL_PROXY_RESULT_OK;
+  }
   SerialProxyResult set_modem_pins(api::APIConnection *api_connection, uint32_t line_states) {
     return SerialProxyResult::SERIAL_PROXY_RESULT_OK;
   }
diff --git a/tests/components/serial_proxy/test-tap.esp32-idf.yaml b/tests/components/serial_proxy/test-tap.esp32-idf.yaml
new file mode 100644
index 0000000000..5522e53c47
--- /dev/null
+++ b/tests/components/serial_proxy/test-tap.esp32-idf.yaml
@@ -0,0 +1,14 @@
+substitutions:
+  tx_pin: GPIO4
+  rx_pin: GPIO5
+
+# Compile the tap code paths; no tap is attached, so this exercises the
+# null-tap branches that a normal build never defines.
+esphome:
+  platformio_options:
+    build_flags:
+      - "-DUSE_SERIAL_PROXY_TAP"
+
+packages:
+  uart: !include ../../test_build_components/common/uart/esp32-idf.yaml
+  serial_proxy: !include common.yaml
diff --git a/tests/components/zigbee_proxy/.gitignore b/tests/components/zigbee_proxy/.gitignore
new file mode 100644
index 0000000000..d8b4157aef
--- /dev/null
+++ b/tests/components/zigbee_proxy/.gitignore
@@ -0,0 +1,5 @@
+# Gitignore settings for ESPHome
+# This is an example and may include too much for your use-case.
+# You can modify this file to suit your needs.
+/.esphome/
+/secrets.yaml
diff --git a/tests/components/zigbee_proxy/common.yaml b/tests/components/zigbee_proxy/common.yaml
index 91e27efd7d..b2b8cf1f57 100644
--- a/tests/components/zigbee_proxy/common.yaml
+++ b/tests/components/zigbee_proxy/common.yaml
@@ -20,4 +20,3 @@ serial_proxy:
     uart_id: zigbee_uart
     name: Zigbee
     port_type: TTL
-    mode: protocol
diff --git a/tests/components/zigbee_proxy/test-usb.esp32-s3-idf.yaml b/tests/components/zigbee_proxy/test-usb.esp32-s3-idf.yaml
index 93b7707d16..ed9ad9f24a 100644
--- a/tests/components/zigbee_proxy/test-usb.esp32-s3-idf.yaml
+++ b/tests/components/zigbee_proxy/test-usb.esp32-s3-idf.yaml
@@ -22,7 +22,6 @@ serial_proxy:
     uart_id: zigbee_usb_channel
     name: Zigbee
     port_type: TTL
-    mode: protocol
 
 zigbee_proxy:
   serial_proxy_id: zigbee_usb_serial