From 3162a16b0557c0b643c67517da21f3d8db6c60ee Mon Sep 17 00:00:00 2001
From: kbx81 <kbx81x@gmail.com>
Date: Wed, 2 Sep 2026 16:07:50 -0500
Subject: [PATCH] [serial_proxy] Add tap interface and port mode

Add SerialProxyTap, a protocol-agnostic observer interface that lets a
companion component watch the bytes flowing through a proxied port and
inject bytes of its own (protocol acknowledgements, for example) without
owning the port. The tap machinery is compiled in only when a tap
component defines USE_SERIAL_PROXY_TAP, so ports without one pay nothing.

Add a per-port mode (RAW or PROTOCOL) with a matching API message so
clients control whether the tap is active. The mode belongs to the client
session: it resets to RAW whenever the subscriber disconnects, and RAW is
guaranteed inert so a client can flash firmware through the port without
protocol bytes being injected. Bumps the API minor version to 17.

Co-Authored-By: puddly <32534428+puddly@users.noreply.github.com>
---
 esphome/components/api/api.proto              |  21 +++
 esphome/components/api/api_connection.cpp     |  11 +-
 esphome/components/api/api_connection.h       |   1 +
 esphome/components/api/api_pb2.cpp            |  13 ++
 esphome/components/api/api_pb2.h              |  20 +++
 esphome/components/api/api_pb2_dump.cpp       |  16 ++
 esphome/components/api/api_pb2_service.cpp    |  11 ++
 esphome/components/api/api_pb2_service.h      |   3 +
 esphome/components/serial_proxy/__init__.py   |  17 ++-
 .../components/serial_proxy/serial_proxy.cpp  | 141 +++++++++++++++---
 .../components/serial_proxy/serial_proxy.h    |  81 +++++++++-
 esphome/core/defines.h                        |   1 +
 tests/components/serial_proxy/common.yaml     |   1 +
 13 files changed, 315 insertions(+), 22 deletions(-)

diff --git a/esphome/components/api/api.proto b/esphome/components/api/api.proto
index 3a0e0abea9..2921d94d47 100644
--- a/esphome/components/api/api.proto
+++ b/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) {}
 }
 
 
@@ -2838,6 +2839,26 @@ 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;
+}
+
+message SerialProxySetModeRequest {
+  option (id) = 151;
+  option (source) = SOURCE_CLIENT;
+  option (ifdef) = "USE_SERIAL_PROXY";
+
+  uint32 instance = 1;
+  SerialProxyMode mode = 2;
+}
+
 // ==================== BLUETOOTH CONNECTION PARAMS ====================
 message BluetoothSetConnectionParamsRequest {
   option (id) = 145;
diff --git a/esphome/components/api/api_connection.cpp b/esphome/components/api/api_connection.cpp
index 9c609aa047..abe4d75841 100644
--- a/esphome/components/api/api_connection.cpp
+++ b/esphome/components/api/api_connection.cpp
@@ -1673,6 +1673,15 @@ 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);
+    return;
+  }
+  proxies[msg.instance]->set_mode(this, msg.mode);
+}
+
 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 +1808,7 @@ 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_connection.h b/esphome/components/api/api_connection.h
index a4c49dccf4..c19a33ca9b 100644
--- a/esphome/components/api/api_connection.h
+++ b/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
 
diff --git a/esphome/components/api/api_pb2.cpp b/esphome/components/api/api_pb2.cpp
index 2de1f0a15c..7f162d9c15 100644
--- a/esphome/components/api/api_pb2.cpp
+++ b/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) {
diff --git a/esphome/components/api/api_pb2.h b/esphome/components/api/api_pb2.h
index 5c3429a63a..fbfe5998df 100644
--- a/esphome/components/api/api_pb2.h
+++ b/esphome/components/api/api_pb2.h
@@ -366,6 +366,10 @@ 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 +3407,22 @@ class SerialProxyRequestResponse final : public ProtoMessage {
 
  protected:
 };
+class SerialProxySetModeRequest final : public ProtoDecodableMessage {
+ public:
+  static constexpr uint16_t MESSAGE_TYPE = 151;
+  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 {
diff --git a/esphome/components/api/api_pb2_dump.cpp b/esphome/components/api/api_pb2_dump.cpp
index dced81ee30..3d85b1276a 100644
--- a/esphome/components/api/api_pb2_dump.cpp
+++ b/esphome/components/api/api_pb2_dump.cpp
@@ -878,6 +878,16 @@ 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 +2815,12 @@ 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 {
diff --git a/esphome/components/api/api_pb2_service.cpp b/esphome/components/api/api_pb2_service.cpp
index 65c7b8858c..172062be63 100644
--- a/esphome/components/api/api_pb2_service.cpp
+++ b/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;
   }
diff --git a/esphome/components/api/api_pb2_service.h b/esphome/components/api/api_pb2_service.h
index 6abdf7093e..a4dfd6a366 100644
--- a/esphome/components/api/api_pb2_service.h
+++ b/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
diff --git a/esphome/components/serial_proxy/__init__.py b/esphome/components/serial_proxy/__init__.py
index 4186fcf8b1..158c9609e4 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_NAME
+from esphome.const import CONF_ID, CONF_MODE, CONF_NAME
 from esphome.core import CORE, coroutine_with_priority
 from esphome.coroutine import CoroPriority
 from esphome.types import ConfigType
@@ -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")
@@ -39,6 +40,16 @@ 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"
@@ -63,6 +74,9 @@ 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,
         }
@@ -87,6 +101,7 @@ 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 c1c1510643..5d0e9cbbd7 100644
--- a/esphome/components/serial_proxy/serial_proxy.cpp
+++ b/esphome/components/serial_proxy/serial_proxy.cpp
@@ -29,26 +29,55 @@ 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();
+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;
+}
 
+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
@@ -69,24 +98,67 @@ 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;
+  }
+  // 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()) {
+    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
+  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"
-                "  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"));
+  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"));
 }
 
 SerialProxyResult SerialProxy::configure(api::APIConnection *api_connection, uint32_t baudrate, bool flow_control,
@@ -159,6 +231,29 @@ 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) {
+#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;
+  }
+#endif
+  ESP_LOGD(TAG, "Serial proxy [%" PRIu32 "] mode set to %s", this->instance_index_,
+           mode == api::enums::SERIAL_PROXY_MODE_PROTOCOL ? "PROTOCOL" : "RAW");
+  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.
+  if (leaving_protocol_mode && this->tap_ != nullptr) {
+    this->tap_->on_protocol_disabled();
+  }
+#endif
+}
+
 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
@@ -171,6 +266,13 @@ void SerialProxy::write_from_client(api::APIConnection *api_connection, const ui
   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) {
@@ -264,6 +366,7 @@ SerialProxyResult SerialProxy::serial_proxy_request(api::APIConnection *api_conn
         return SerialProxyResult::SERIAL_PROXY_RESULT_OK;
       }
       this->api_connection_ = nullptr;
+      this->reset_mode_();
       this->disable_loop();
       ESP_LOGV(TAG, "API connection unsubscribed from serial proxy [%" PRIu32 "]", this->instance_index_);
       return SerialProxyResult::SERIAL_PROXY_RESULT_OK;
diff --git a/esphome/components/serial_proxy/serial_proxy.h b/esphome/components/serial_proxy/serial_proxy.h
index a0e47ee686..19a3a4f063 100644
--- a/esphome/components/serial_proxy/serial_proxy.h
+++ b/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 +53,35 @@ 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.
+  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 +107,15 @@ 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);
+
   /// Configure UART parameters and apply them
   /// @param api_connection The API connection requesting the change
   /// @param baudrate Baud rate in bits per second
@@ -121,15 +160,48 @@ 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, 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); }
+
+  /// 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.
+  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().
+  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;
 #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.
+  void reset_mode_();
+
+#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
   uint32_t instance_index_{0};
 
@@ -147,6 +219,9 @@ class SerialProxy final : public uart::UARTDevice, public Component {
   /// Port type
   api::enums::SerialProxyPortType port_type_{};
 
+  /// How the bytes passing through are treated; zero is SERIAL_PROXY_MODE_RAW
+  api::enums::SerialProxyMode mode_{};
+
   /// Optional GPIO pins for modem control
   GPIOPin *rts_pin_{nullptr};
   GPIOPin *dtr_pin_{nullptr};
@@ -154,6 +229,10 @@ 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
diff --git a/esphome/core/defines.h b/esphome/core/defines.h
index 7af41409fd..c5ac18a619 100644
--- a/esphome/core/defines.h
+++ b/esphome/core/defines.h
@@ -179,6 +179,7 @@
 #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
diff --git a/tests/components/serial_proxy/common.yaml b/tests/components/serial_proxy/common.yaml
index 6f03cf95df..a93d22e410 100644
--- a/tests/components/serial_proxy/common.yaml
+++ b/tests/components/serial_proxy/common.yaml
@@ -8,3 +8,4 @@ serial_proxy:
   - id: serial_proxy_1
     name: Test Serial Port
     port_type: RS232
+    mode: protocol