[serial_proxy] Add tap interface and port mode (#18955)

Co-authored-by: puddly <32534428+puddly@users.noreply.github.com>

esphome/components/api/api.proto

@@ -77,6 +77,7 @@ service APIConnection {
   rpc serial_proxy_set_modem_pins(SerialProxySetModemPinsRequest) returns (void) {}
   rpc serial_proxy_get_modem_pins(SerialProxyGetModemPinsRequest) returns (void) {}
   rpc serial_proxy_request(SerialProxyRequest) returns (void) {}
+  rpc serial_proxy_set_mode(SerialProxySetModeRequest) returns (void) {}
 }
 
 
@@ -2726,7 +2727,8 @@ 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;
@@ -2752,7 +2754,8 @@ 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;
@@ -2763,7 +2766,8 @@ 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;
@@ -2802,6 +2806,7 @@ 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 {
@@ -2814,7 +2819,8 @@ 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;
@@ -2838,6 +2844,29 @@ message SerialProxyRequestResponse {
   string error_message = 4;            // Additional detail on failure (optional)
 }
 
+// How a port treats the bytes passing through it. RAW is a plain byte pipe; PROTOCOL
+// activates the port's protocol-aware tap (if one is configured), letting it observe
+// traffic and inject protocol bytes such as acknowledgements. Which protocol the tap
+// speaks is a property of the device configuration, discoverable from the tap
+// component's own API surface. A client that is about to flash firmware selects RAW
+// first, which definitively disables that injection.
+enum SerialProxyMode {
+  SERIAL_PROXY_MODE_RAW = 0;
+  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) = 152;
+  option (source) = SOURCE_CLIENT;
+  option (ifdef) = "USE_SERIAL_PROXY";
+
+  uint32 instance = 1;
+  SerialProxyMode mode = 2;
+}
+
 // ==================== BLUETOOTH CONNECTION PARAMS ====================
 message BluetoothSetConnectionParamsRequest {
   option (id) = 145;

esphome/components/api/api_connection.cpp

@@ -1661,6 +1661,7 @@ 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;
@@ -1673,6 +1674,19 @@ void APIConnection::on_serial_proxy_request(const SerialProxyRequest &msg) {
   send_serial_proxy_ack(this, msg.instance, msg.type, status);
 }
 
+void APIConnection::on_serial_proxy_set_mode_request(const SerialProxySetModeRequest &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, enums::SERIAL_PROXY_REQUEST_TYPE_SET_MODE,
+                          enums::SERIAL_PROXY_STATUS_INVALID_ARGUMENT);
+    return;
+  }
+  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) {
   if (!this->send_message(msg)) {
     ESP_LOGV(TAG, "Serial proxy data dropped, TCP buffer full");
@@ -1799,7 +1813,7 @@ 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());

esphome/components/api/api_connection.h

@@ -244,6 +244,7 @@ class APIConnection final : public APIServerConnectionBase {
   void on_serial_proxy_set_modem_pins_request(const SerialProxySetModemPinsRequest &msg);
   void on_serial_proxy_get_modem_pins_request(const SerialProxyGetModemPinsRequest &msg);
   void on_serial_proxy_request(const SerialProxyRequest &msg);
+  void on_serial_proxy_set_mode_request(const SerialProxySetModeRequest &msg);
   void send_serial_proxy_data(const SerialProxyDataReceived &msg);
 #endif
 

esphome/components/api/api_pb2.cpp

@@ -4253,6 +4253,19 @@ uint32_t SerialProxyRequestResponse::calculate_size() const {
   size += ProtoSize::calc_length(1, this->error_message.size());
   return size;
 }
+bool SerialProxySetModeRequest::decode_varint(uint32_t field_id, proto_varint_value_t value) {
+  switch (field_id) {
+    case 1:
+      this->instance = value;
+      break;
+    case 2:
+      this->mode = static_cast<enums::SerialProxyMode>(value);
+      break;
+    default:
+      return false;
+  }
+  return true;
+}
 #endif
 #ifdef USE_BLUETOOTH_PROXY_CONNECTIONS
 bool BluetoothSetConnectionParamsRequest::decode_varint(uint32_t field_id, proto_varint_value_t value) {

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,
@@ -366,6 +367,10 @@ enum SerialProxyStatus : uint32_t {
   SERIAL_PROXY_STATUS_PORT_IN_USE = 5,
   SERIAL_PROXY_STATUS_INVALID_ARGUMENT = 6,
 };
+enum SerialProxyMode : uint32_t {
+  SERIAL_PROXY_MODE_RAW = 0,
+  SERIAL_PROXY_MODE_PROTOCOL = 1,
+};
 #endif
 
 }  // namespace enums
@@ -3403,6 +3408,22 @@ class SerialProxyRequestResponse final : public ProtoMessage {
 
  protected:
 };
+class SerialProxySetModeRequest final : public ProtoDecodableMessage {
+ public:
+  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"); }
+#endif
+  uint32_t instance{0};
+  enums::SerialProxyMode mode{};
+#ifdef HAS_PROTO_MESSAGE_DUMP
+  const char *dump_to(DumpBuffer &out) const override;
+#endif
+
+ protected:
+  bool decode_varint(uint32_t field_id, proto_varint_value_t value) override;
+};
 #endif
 #ifdef USE_BLUETOOTH_PROXY_CONNECTIONS
 class BluetoothSetConnectionParamsRequest final : public ProtoDecodableMessage {

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");
   }
@@ -878,6 +880,16 @@ template<> const char *proto_enum_to_string<enums::SerialProxyStatus>(enums::Ser
       return ESPHOME_PSTR("UNKNOWN");
   }
 }
+template<> const char *proto_enum_to_string<enums::SerialProxyMode>(enums::SerialProxyMode value) {
+  switch (value) {
+    case enums::SERIAL_PROXY_MODE_RAW:
+      return ESPHOME_PSTR("SERIAL_PROXY_MODE_RAW");
+    case enums::SERIAL_PROXY_MODE_PROTOCOL:
+      return ESPHOME_PSTR("SERIAL_PROXY_MODE_PROTOCOL");
+    default:
+      return ESPHOME_PSTR("UNKNOWN");
+  }
+}
 #endif
 
 const char *HelloRequest::dump_to(DumpBuffer &out) const {
@@ -2805,6 +2817,12 @@ const char *SerialProxyRequestResponse::dump_to(DumpBuffer &out) const {
   dump_field(out, ESPHOME_PSTR("error_message"), this->error_message);
   return out.c_str();
 }
+const char *SerialProxySetModeRequest::dump_to(DumpBuffer &out) const {
+  MessageDumpHelper helper(out, ESPHOME_PSTR("SerialProxySetModeRequest"));
+  dump_field(out, ESPHOME_PSTR("instance"), this->instance);
+  dump_field(out, ESPHOME_PSTR("mode"), static_cast<enums::SerialProxyMode>(this->mode));
+  return out.c_str();
+}
 #endif
 #ifdef USE_BLUETOOTH_PROXY_CONNECTIONS
 const char *BluetoothSetConnectionParamsRequest::dump_to(DumpBuffer &out) const {

esphome/components/api/api_pb2_service.cpp

@@ -712,6 +712,17 @@ void APIConnection::read_message_(uint32_t msg_size, uint32_t msg_type, const ui
       this->on_device_capabilities_request();
       break;
     }
+#ifdef USE_SERIAL_PROXY
+    case SerialProxySetModeRequest::MESSAGE_TYPE: {
+      SerialProxySetModeRequest msg;
+      msg.decode(msg_data, msg_size);
+#ifdef HAS_PROTO_MESSAGE_DUMP
+      this->log_receive_message_(LOG_STR("on_serial_proxy_set_mode_request"), msg);
+#endif
+      this->on_serial_proxy_set_mode_request(msg);
+      break;
+    }
+#endif
     default:
       break;
   }

esphome/components/api/api_pb2_service.h

@@ -235,6 +235,9 @@ class APIServerConnectionBase {
   void on_serial_proxy_request(const SerialProxyRequest &value){};
 #endif
 
+#ifdef USE_SERIAL_PROXY
+  void on_serial_proxy_set_mode_request(const SerialProxySetModeRequest &value){};
+#endif
 #ifdef USE_BLUETOOTH_PROXY_CONNECTIONS
   void on_bluetooth_set_connection_params_request(const BluetoothSetConnectionParamsRequest &value){};
 #endif

esphome/components/serial_proxy/__init__.py

@@ -30,6 +30,7 @@ MULTI_CONF = True
 
 serial_proxy_ns = cg.esphome_ns.namespace("serial_proxy")
 SerialProxy = serial_proxy_ns.class_("SerialProxy", cg.Component, uart.UARTDevice)
+SerialProxyTap = serial_proxy_ns.class_("SerialProxyTap")
 
 api_enums_ns = cg.esphome_ns.namespace("api").namespace("enums")
 SerialProxyPortType = api_enums_ns.enum("SerialProxyPortType")

esphome/components/serial_proxy/serial_proxy.cpp

@@ -29,26 +29,57 @@ void SerialProxy::setup() {
 #ifdef USE_API
   // instance_index_ is fixed at registration time; pre-set it so loop() only needs to update data
   this->outgoing_msg_.instance = this->instance_index_;
+#endif
+#ifdef USE_SERIAL_PROXY_TAP
+  // A tap sets itself up before this runs (its setup priority is higher), so it may
+  // already be waiting on the port -- a boot-time handshake with the device, say. Leaving
+  // the loop enabled is what lets that finish; without it the tap would stall until a
+  // client happened to subscribe.
+  if (this->tap_ != nullptr && this->tap_->tap_needs_port()) {
+    return;
+  }
 #endif
   // No subscriber at startup; disable loop until a client subscribes
   this->disable_loop();
 }
 
-void SerialProxy::loop() {
-#ifdef USE_API
-  // Safety check — loop should only run when subscribed, but guard against races
-  if (this->api_connection_ == nullptr) [[unlikely]] {
-    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
+  // them -- a firmware upload, or any client built before this request existed and so
+  // unable to turn it off. Guessing RAW is the safe direction: a client that wanted
+  // protocol handling and did not ask for it merely sends its own acknowledgements.
+  if (this->mode_ == api::enums::SERIAL_PROXY_MODE_RAW) {
     return;
   }
+  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
   // Detect subscriber disconnect
-  if (this->api_connection_->is_marked_for_removal() || !this->api_connection_->is_connection_setup() ||
-      !api_is_connected()) {
+  if (this->api_connection_ != nullptr && (this->api_connection_->is_marked_for_removal() ||
+                                           !this->api_connection_->is_connection_setup() || !api_is_connected())) {
     ESP_LOGW(TAG, "Subscriber disconnected");
     this->api_connection_ = nullptr;
+    this->reset_mode_();
+  }
+
+  // With no subscriber there is normally nothing to do, but a tap may still need the port
+  // read -- it does its protocol work precisely while nobody else is listening.
+  if (this->api_connection_ == nullptr) [[unlikely]] {
+#ifdef USE_SERIAL_PROXY_TAP
+    if (this->tap_ == nullptr || !this->tap_->tap_needs_port()) {
+      this->disable_loop();
+      return;
+    }
+#else
     this->disable_loop();
     return;
+#endif
   }
 
   // Read available data from UART and forward to subscribed client
@@ -69,11 +100,54 @@ void __attribute__((noinline)) SerialProxy::read_and_send_(size_t available) {
   if (!this->read_array(buffer, to_read))
     return;
 
+#ifdef USE_SERIAL_PROXY_TAP
+  // Before forwarding, so a tap that answers the device (an acknowledgement, say) is not
+  // waiting on the network round trip to a subscriber that may not even exist.
+  if (this->tap_observing_()) {
+    this->tap_->on_device_rx(buffer, to_read);
+  }
+#endif
+
+  if (this->api_connection_ == nullptr) {
+    return;
+  }
   this->outgoing_msg_.set_data(buffer, to_read);
   this->api_connection_->send_serial_proxy_data(this->outgoing_msg_);
 }
 #endif
 
+#ifdef USE_SERIAL_PROXY_TAP
+
+bool SerialProxy::tap_observing_() const {
+  if (this->tap_ == nullptr) {
+    return false;
+  }
+  // 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
+  // flashing firmware is entitled to a byte pipe with nothing injecting protocol bytes
+  // into it, and "the tap turned out not to recognise the stream" is not good enough.
+  return this->mode_ == api::enums::SERIAL_PROXY_MODE_PROTOCOL;
+}
+
+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);
+  }
+#endif
+}
+#endif
+
 void SerialProxy::dump_config() {
   ESP_LOGCONFIG(TAG,
                 "Serial Proxy [%" PRIu32 "]:\n"
@@ -92,8 +166,9 @@ void SerialProxy::dump_config() {
 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
@@ -159,24 +234,80 @@ SerialProxyResult SerialProxy::configure(api::APIConnection *api_connection, uin
   return SerialProxyResult::SERIAL_PROXY_RESULT_OK;
 }
 
+SerialProxyResult SerialProxy::set_mode_from_client(api::APIConnection *api_connection,
+                                                    api::enums::SerialProxyMode mode) {
+#ifdef USE_API
+  // 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 ? 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;
+
+  // 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.
+  if (leaving_protocol_mode && this->tap_ != nullptr) {
+    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
   if (data == nullptr || len == 0)
     return;
   this->write_array(data, len);
+
+#ifdef USE_SERIAL_PROXY_TAP
+  // After the write, so the tap observes the same ordering the device does
+  if (this->tap_observing_()) {
+    this->tap_->on_client_tx(data, len);
+  }
+#endif
 }
 
 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
@@ -210,8 +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
@@ -230,11 +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) {
@@ -252,6 +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();
@@ -264,7 +394,15 @@ SerialProxyResult SerialProxy::serial_proxy_request(api::APIConnection *api_conn
         return SerialProxyResult::SERIAL_PROXY_RESULT_OK;
       }
       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:

esphome/components/serial_proxy/serial_proxy.h

@@ -26,6 +26,7 @@ class APIConnection;
 namespace enums {
 enum SerialProxyPortType : uint32_t;
 enum SerialProxyRequestType : uint32_t;
+enum SerialProxyMode : uint32_t;
 }  // namespace enums
 }  // namespace esphome::api
 
@@ -52,6 +53,36 @@ enum class SerialProxyResult : uint8_t {
 /// Maximum bytes to read from UART in a single loop iteration
 inline constexpr size_t SERIAL_PROXY_MAX_READ_SIZE = 256;
 
+#ifdef USE_SERIAL_PROXY_TAP
+/// Observes a port's traffic without owning it, and may inject bytes of its own.
+///
+/// This exists so protocol-aware behaviour can be layered onto a plain byte pipe without
+/// the pipe knowing anything about the protocol: the tap is compiled in only when some
+/// component asks for one, so a proxy carrying an RS485 meter pays nothing for it.
+///
+/// A tap is an observer, never a gatekeeper -- it cannot suppress or alter the bytes
+/// flowing in either direction, so a misbehaving tap cannot corrupt the stream.
+class SerialProxyTap {
+ public:
+  /// Bytes read from the device, before they are forwarded to any subscriber.
+  virtual void on_device_rx(const uint8_t *data, size_t len) = 0;
+
+  /// Bytes a subscriber sent towards the device, after they have been written.
+  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. 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
+  /// automatic reset when a session ends: this one means a client intends to do something
+  /// else with the device -- reflash it, most likely -- so anything the tap believes about
+  /// it should be treated as suspect.
+  virtual void on_protocol_disabled() = 0;
+};
+#endif
+
 class SerialProxy final : public uart::UARTDevice, public Component {
  public:
   void setup() override;
@@ -77,6 +108,9 @@ class SerialProxy final : public uart::UARTDevice, public Component {
   /// Get the port type
   api::enums::SerialProxyPortType get_port_type() const { return this->port_type_; }
 
+  /// Handle a mode change requested by an API client
+  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
   /// @param baudrate Baud rate in bits per second
@@ -121,13 +155,67 @@ class SerialProxy final : public uart::UARTDevice, public Component {
   /// Set the DTR GPIO pin (from YAML configuration)
   void set_dtr_pin(GPIOPin *pin) { this->dtr_pin_ = pin; }
 
+#ifdef USE_SERIAL_PROXY_TAP
+  /// Attach a traffic observer. At most one, set once at setup time.
+  void set_tap(SerialProxyTap *tap) { this->tap_ = tap; }
+
+  /// Write bytes originating from the tap rather than from a client. Bypasses the
+  /// 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 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
+  /// a tap can notice the device being unplugged and plugged back in.
+  bool is_device_connected() const { return this->parent_->is_connected(); }
+
+  /// 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(). 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
+
  protected:
 #ifdef USE_API
-  /// Read from UART and send to API client (slow path with 256-byte stack buffer)
+  /// Read from UART, hand the bytes to any tap, and forward them to a subscriber
+  /// (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
+
+#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
+  bool tap_observing_() const;
 #endif
 
   /// Instance index for identifying this proxy in API messages
@@ -147,6 +235,11 @@ 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};
   GPIOPin *dtr_pin_{nullptr};
@@ -154,6 +247,10 @@ class SerialProxy final : public uart::UARTDevice, public Component {
   /// Current modem pin states
   bool rts_state_{false};
   bool dtr_state_{false};
+
+#ifdef USE_SERIAL_PROXY_TAP
+  SerialProxyTap *tap_{nullptr};
+#endif
 };
 
 }  // namespace esphome::serial_proxy

esphome/core/defines.h

@@ -181,6 +181,7 @@
 #define USE_SENSOR
 #define USE_SENSOR_FILTER
 #define USE_SERIAL_PROXY
+#define USE_SERIAL_PROXY_TAP
 #define USE_SETUP_PRIORITY_OVERRIDE
 #define USE_STATUS_LED
 #define USE_STATUS_SENSOR

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;
   }

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