diff --git a/esphome/components/api/api.proto b/esphome/components/api/api.proto
index 4b3df62ec4..88af5957e7 100644
--- a/esphome/components/api/api.proto
+++ b/esphome/components/api/api.proto
@@ -19,6 +19,7 @@ service APIConnection {
   rpc device_info (DeviceInfoRequest) returns (DeviceInfoResponse) {
     option (needs_authentication) = false;
   }
+  rpc device_capabilities (DeviceCapabilitiesRequest) returns (DeviceCapabilitiesResponse) {}
   rpc list_entities (ListEntitiesRequest) returns (void) {}
   rpc subscribe_states (SubscribeStatesRequest) returns (void) {}
   rpc subscribe_logs (SubscribeLogsRequest) returns (void) {}
@@ -243,6 +244,12 @@ message SerialProxyInfo {
 //   model = 127 (core/config.BOARD_MAX_LENGTH, validated in platform schemas)
 //   project_name/project_version = 127 (core/config.PROJECT_MAX_LENGTH)
 //   suggested_area = 120 (core/config.FRIENDLY_NAME_MAX_LEN via AREA_SCHEMA)
+//
+// Some fields below are marked "Superseded by DeviceCapabilitiesResponse". They
+// have moved to that message as of API 1.15, but are still sent here so that
+// older clients keep working. Do NOT mark them (deprecated) until the removal
+// release: in this repo (deprecated) makes the generator drop the field
+// entirely, so the device would stop sending it.
 message DeviceInfoResponse {
   option (id) = 10;
   option (source) = SOURCE_SERVER;
@@ -280,6 +287,8 @@ message DeviceInfoResponse {
 
   // Deprecated in API version 1.9
   uint32 legacy_bluetooth_proxy_version = 11 [deprecated=true, (field_ifdef) = "USE_BLUETOOTH_PROXY"];
+
+  // Superseded by DeviceCapabilitiesResponse.bluetooth_proxy as of API 1.15.
   uint32 bluetooth_proxy_feature_flags = 15 [(field_ifdef) = "USE_BLUETOOTH_PROXY"];
 
   string manufacturer = 12 [(max_data_length) = 20, (force) = true];
@@ -288,11 +297,14 @@ message DeviceInfoResponse {
 
   // Deprecated in API version 1.10
   uint32 legacy_voice_assistant_version = 14 [deprecated=true, (field_ifdef) = "USE_VOICE_ASSISTANT"];
+
+  // Superseded by DeviceCapabilitiesResponse.voice_assistant as of API 1.15.
   uint32 voice_assistant_feature_flags = 17 [(field_ifdef) = "USE_VOICE_ASSISTANT"];
 
   string suggested_area = 16 [(max_data_length) = 120, (force) = true, (field_ifdef) = "USE_AREAS"];
 
   // The Bluetooth mac address of the device. For example "AC:BC:32:89:0E:AA"
+  // Superseded by DeviceCapabilitiesResponse.bluetooth_proxy.mac_address as of API 1.15.
   string bluetooth_mac_address = 18 [(max_data_length) = 17, (force) = true, (field_ifdef) = "USE_BLUETOOTH_PROXY"];
 
   // Supports receiving and saving api encryption key
@@ -305,10 +317,13 @@ message DeviceInfoResponse {
   AreaInfo area = 22 [(field_ifdef) = "USE_AREAS"];
 
   // Indicates if Z-Wave proxy support is available and features supported
+  // Superseded by DeviceCapabilitiesResponse.zwave_proxy as of API 1.15.
   uint32 zwave_proxy_feature_flags = 23 [(field_ifdef) = "USE_ZWAVE_PROXY"];
+  // Superseded by DeviceCapabilitiesResponse.zwave_proxy as of API 1.15.
   uint32 zwave_home_id = 24 [(field_ifdef) = "USE_ZWAVE_PROXY"];
 
   // Serial proxy instance metadata
+  // Superseded by DeviceCapabilitiesResponse.serial_proxies as of API 1.15.
   repeated SerialProxyInfo serial_proxies = 25 [(field_ifdef) = "USE_SERIAL_PROXY", (fixed_array_size_define) = "SERIAL_PROXY_COUNT"];
 
   // Device is unprovisioned and accepts Noise handshakes with the well-known
@@ -317,6 +332,63 @@ message DeviceInfoResponse {
   bool api_encryption_provisionable = 26 [(field_ifdef) = "USE_API_NOISE"];
 }
 
+// ==================== DEVICE CAPABILITIES ====================
+
+// Asks the device which optional features it supports.
+//
+// This message exists so that DeviceInfoResponse does not have to keep growing
+// a flat list of feature flags. DeviceInfoResponse is served before
+// authentication, so it is limited to identity information. Capabilities are
+// only served on an authenticated connection (encrypted as well, when
+// encryption is configured).
+//
+// Clients that see api_version >= 1.15 should read these values from
+// DeviceCapabilitiesResponse and ignore the matching DeviceInfoResponse fields.
+// Older clients keep reading DeviceInfoResponse, which still carries the same
+// values, so this is not a breaking change.
+message DeviceCapabilitiesRequest {
+  option (id) = 149;
+  option (source) = SOURCE_CLIENT;
+  // Empty
+}
+
+// Each feature gets its own sub-message so that it can gain fields over time
+// without crowding the top-level field numbering.
+//
+// Note: a sub-message whose fields are all at their default value is not sent
+// at all, so the presence of a sub-message is not a reliable test for "this
+// feature is compiled in". Clients should test a value inside it, for example
+// a non-zero feature_flags, exactly as they do today with DeviceInfoResponse.
+
+message BluetoothProxyCapabilities {
+  // Bitmask of the features this proxy supports
+  uint32 feature_flags = 1;
+  // The Bluetooth mac address of the device. For example "AC:BC:32:89:0E:AA"
+  string mac_address = 2 [(max_data_length) = 17, (force) = true];
+}
+
+message VoiceAssistantCapabilities {
+  // Bitmask of the features this voice assistant supports
+  uint32 feature_flags = 1;
+}
+
+message ZWaveProxyCapabilities {
+  // Bitmask of the features this proxy supports
+  uint32 feature_flags = 1;
+  uint32 home_id = 2;
+}
+
+message DeviceCapabilitiesResponse {
+  option (id) = 150;
+  option (source) = SOURCE_SERVER;
+
+  BluetoothProxyCapabilities bluetooth_proxy = 1 [(field_ifdef) = "USE_BLUETOOTH_PROXY"];
+  VoiceAssistantCapabilities voice_assistant = 2 [(field_ifdef) = "USE_VOICE_ASSISTANT"];
+  ZWaveProxyCapabilities zwave_proxy = 3 [(field_ifdef) = "USE_ZWAVE_PROXY"];
+  repeated SerialProxyInfo serial_proxies = 4
+      [(field_ifdef) = "USE_SERIAL_PROXY", (fixed_array_size_define) = "SERIAL_PROXY_COUNT"];
+}
+
 message ListEntitiesRequest {
   option (id) = 11;
   option (source) = SOURCE_CLIENT;
diff --git a/esphome/components/api/api_connection.cpp b/esphome/components/api/api_connection.cpp
index 2d03052d63..18eb2592ff 100644
--- a/esphome/components/api/api_connection.cpp
+++ b/esphome/components/api/api_connection.cpp
@@ -1736,7 +1736,7 @@ bool APIConnection::send_hello_response_(const HelloRequest &msg) {
 
   HelloResponse resp;
   resp.api_version_major = 1;
-  resp.api_version_minor = 14;
+  resp.api_version_minor = 15;
   // 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());
@@ -1904,6 +1904,35 @@ bool APIConnection::send_device_info_response_() {
 
   return this->send_message(resp);
 }
+bool APIConnection::send_device_capabilities_response_() {
+  // These are the same values DeviceInfoResponse still reports for older clients. Keep the blocks
+  // below in sync with send_device_info_response_() until those copies are removed.
+  DeviceCapabilitiesResponse resp;
+#ifdef USE_BLUETOOTH_PROXY
+  resp.bluetooth_proxy.feature_flags = bluetooth_proxy::global_bluetooth_proxy->get_feature_flags();
+  char bluetooth_mac[MAC_ADDRESS_PRETTY_BUFFER_SIZE];
+  bluetooth_proxy::global_bluetooth_proxy->get_bluetooth_mac_address_pretty(bluetooth_mac);
+  resp.bluetooth_proxy.mac_address = StringRef(bluetooth_mac);
+#endif
+#ifdef USE_VOICE_ASSISTANT
+  resp.voice_assistant.feature_flags = voice_assistant::global_voice_assistant->get_feature_flags();
+#endif
+#ifdef USE_ZWAVE_PROXY
+  resp.zwave_proxy.feature_flags = zwave_proxy::global_zwave_proxy->get_feature_flags();
+  resp.zwave_proxy.home_id = zwave_proxy::global_zwave_proxy->get_home_id();
+#endif
+#ifdef USE_SERIAL_PROXY
+  size_t serial_proxy_index = 0;
+  for (auto const &proxy : App.get_serial_proxies()) {
+    if (serial_proxy_index >= SERIAL_PROXY_COUNT)
+      break;
+    auto &info = resp.serial_proxies[serial_proxy_index++];
+    info.name = StringRef(proxy->get_name());
+    info.port_type = proxy->get_port_type();
+  }
+#endif
+  return this->send_message(resp);
+}
 void APIConnection::on_hello_request(const HelloRequest &msg) {
   if (!this->send_hello_response_(msg)) {
     this->on_fatal_error();
@@ -1925,6 +1954,11 @@ void APIConnection::on_device_info_request() {
     this->on_fatal_error();
   }
 }
+void APIConnection::on_device_capabilities_request() {
+  if (!this->send_device_capabilities_response_()) {
+    this->on_fatal_error();
+  }
+}
 
 #ifdef USE_API_HOMEASSISTANT_STATES
 void APIConnection::on_home_assistant_state_response(const HomeAssistantStateResponse &msg) {
diff --git a/esphome/components/api/api_connection.h b/esphome/components/api/api_connection.h
index 7df7ea1429..9ca1b8b6a4 100644
--- a/esphome/components/api/api_connection.h
+++ b/esphome/components/api/api_connection.h
@@ -266,6 +266,7 @@ class APIConnection final : public APIServerConnectionBase {
   void on_disconnect_request(const DisconnectRequest &msg);
   void on_ping_request();
   void on_device_info_request();
+  void on_device_capabilities_request();
   void on_list_entities_request() { this->begin_iterator_(ActiveIterator::LIST_ENTITIES); }
   void on_subscribe_states_request() {
     this->flags_.state_subscription = true;
@@ -385,6 +386,7 @@ class APIConnection final : public APIServerConnectionBase {
   bool send_disconnect_response_();
   bool send_ping_response_();
   bool send_device_info_response_();
+  bool send_device_capabilities_response_();
 #ifdef USE_API_NOISE
   bool send_noise_encryption_set_key_response_(const NoiseEncryptionSetKeyRequest &msg);
 #endif
diff --git a/esphome/components/api/api_frame_helper.h b/esphome/components/api/api_frame_helper.h
index 9cae6ba92e..9c49956bbd 100644
--- a/esphome/components/api/api_frame_helper.h
+++ b/esphome/components/api/api_frame_helper.h
@@ -149,7 +149,7 @@ class APIFrameHelper {
   // holding data too long waiting for Nagle's timer causes buffer exhaustion
   // and dropped messages.
   //
-  // ESP32 (TCP_SND_BUF=4×MSS+) / RP2040 (8×MSS) / LibreTiny (4×MSS): 4 logs per cycle
+  // ESP32 (TCP_SND_BUF=4×MSS+) / RP2040 (4×MSS) / LibreTiny (4×MSS): 4 logs per cycle
   // ESP8266 (2×MSS): 3 logs per cycle (tightest buffers)
   //
   // Flow (ESP32/RP2040/LT): Log 1 (Nagle on) -> Log 2 -> Log 3 -> Log 4 (NODELAY, flush)
@@ -312,7 +312,7 @@ class APIFrameHelper {
   // Values 1..LOG_NAGLE_COUNT count log messages in the current Nagle batch.
   // After LOG_NAGLE_COUNT logs, we flush by re-enabling NODELAY and resetting to 0.
   // ESP8266 has the tightest TCP send buffer (2×MSS) and needs conservative batching.
-  // ESP32 (4×MSS+), RP2040 (8×MSS), and LibreTiny (4×MSS) can coalesce more.
+  // ESP32 (4×MSS+), RP2040 (4×MSS), and LibreTiny (4×MSS) can coalesce more.
 #ifdef USE_ESP8266
   static constexpr uint8_t LOG_NAGLE_COUNT = 2;
 #else
diff --git a/esphome/components/api/api_pb2.cpp b/esphome/components/api/api_pb2.cpp
index 190bd32425..5776ec5c62 100644
--- a/esphome/components/api/api_pb2.cpp
+++ b/esphome/components/api/api_pb2.cpp
@@ -241,6 +241,82 @@ uint32_t DeviceInfoResponse::calculate_size() const {
 #endif
   return size;
 }
+#ifdef USE_BLUETOOTH_PROXY
+uint8_t *BluetoothProxyCapabilities::encode(ProtoWriteBuffer &buffer PROTO_ENCODE_DEBUG_PARAM) const {
+  uint8_t *__restrict__ pos = buffer.get_pos();
+  ProtoEncode::encode_uint32(pos PROTO_ENCODE_DEBUG_ARG, 1, this->feature_flags);
+  ProtoEncode::encode_short_string_force(pos PROTO_ENCODE_DEBUG_ARG, 18, this->mac_address);
+  return pos;
+}
+uint32_t BluetoothProxyCapabilities::calculate_size() const {
+  uint32_t size = 0;
+  size += ProtoSize::calc_uint32(1, this->feature_flags);
+  size += 2 + this->mac_address.size();
+  return size;
+}
+#endif
+#ifdef USE_VOICE_ASSISTANT
+uint8_t *VoiceAssistantCapabilities::encode(ProtoWriteBuffer &buffer PROTO_ENCODE_DEBUG_PARAM) const {
+  uint8_t *__restrict__ pos = buffer.get_pos();
+  ProtoEncode::encode_uint32(pos PROTO_ENCODE_DEBUG_ARG, 1, this->feature_flags);
+  return pos;
+}
+uint32_t VoiceAssistantCapabilities::calculate_size() const {
+  uint32_t size = 0;
+  size += ProtoSize::calc_uint32(1, this->feature_flags);
+  return size;
+}
+#endif
+#ifdef USE_ZWAVE_PROXY
+uint8_t *ZWaveProxyCapabilities::encode(ProtoWriteBuffer &buffer PROTO_ENCODE_DEBUG_PARAM) const {
+  uint8_t *__restrict__ pos = buffer.get_pos();
+  ProtoEncode::encode_uint32(pos PROTO_ENCODE_DEBUG_ARG, 1, this->feature_flags);
+  ProtoEncode::encode_uint32(pos PROTO_ENCODE_DEBUG_ARG, 2, this->home_id);
+  return pos;
+}
+uint32_t ZWaveProxyCapabilities::calculate_size() const {
+  uint32_t size = 0;
+  size += ProtoSize::calc_uint32(1, this->feature_flags);
+  size += ProtoSize::calc_uint32(1, this->home_id);
+  return size;
+}
+#endif
+uint8_t *DeviceCapabilitiesResponse::encode(ProtoWriteBuffer &buffer PROTO_ENCODE_DEBUG_PARAM) const {
+  uint8_t *__restrict__ pos = buffer.get_pos();
+#ifdef USE_BLUETOOTH_PROXY
+  ProtoEncode::encode_optional_sub_message(pos PROTO_ENCODE_DEBUG_ARG, buffer, 1, this->bluetooth_proxy);
+#endif
+#ifdef USE_VOICE_ASSISTANT
+  ProtoEncode::encode_optional_sub_message(pos PROTO_ENCODE_DEBUG_ARG, buffer, 2, this->voice_assistant);
+#endif
+#ifdef USE_ZWAVE_PROXY
+  ProtoEncode::encode_optional_sub_message(pos PROTO_ENCODE_DEBUG_ARG, buffer, 3, this->zwave_proxy);
+#endif
+#ifdef USE_SERIAL_PROXY
+  for (const auto &it : this->serial_proxies) {
+    ProtoEncode::encode_sub_message(pos PROTO_ENCODE_DEBUG_ARG, buffer, 4, it);
+  }
+#endif
+  return pos;
+}
+uint32_t DeviceCapabilitiesResponse::calculate_size() const {
+  uint32_t size = 0;
+#ifdef USE_BLUETOOTH_PROXY
+  size += ProtoSize::calc_message(1, this->bluetooth_proxy.calculate_size());
+#endif
+#ifdef USE_VOICE_ASSISTANT
+  size += ProtoSize::calc_message(1, this->voice_assistant.calculate_size());
+#endif
+#ifdef USE_ZWAVE_PROXY
+  size += ProtoSize::calc_message(1, this->zwave_proxy.calculate_size());
+#endif
+#ifdef USE_SERIAL_PROXY
+  for (const auto &it : this->serial_proxies) {
+    size += ProtoSize::calc_message_force(1, it.calculate_size());
+  }
+#endif
+  return size;
+}
 #ifdef USE_BINARY_SENSOR
 uint8_t *ListEntitiesBinarySensorResponse::encode(ProtoWriteBuffer &buffer PROTO_ENCODE_DEBUG_PARAM) const {
   uint8_t *__restrict__ pos = buffer.get_pos();
diff --git a/esphome/components/api/api_pb2.h b/esphome/components/api/api_pb2.h
index 4d5866da0b..f35f551060 100644
--- a/esphome/components/api/api_pb2.h
+++ b/esphome/components/api/api_pb2.h
@@ -600,6 +600,74 @@ class DeviceInfoResponse final : public ProtoMessage {
 
  protected:
 };
+#ifdef USE_BLUETOOTH_PROXY
+class BluetoothProxyCapabilities final : public ProtoMessage {
+ public:
+  uint32_t feature_flags{0};
+  StringRef mac_address{};
+  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_VOICE_ASSISTANT
+class VoiceAssistantCapabilities final : public ProtoMessage {
+ public:
+  uint32_t feature_flags{0};
+  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_ZWAVE_PROXY
+class ZWaveProxyCapabilities final : public ProtoMessage {
+ public:
+  uint32_t feature_flags{0};
+  uint32_t home_id{0};
+  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
+class DeviceCapabilitiesResponse final : public ProtoMessage {
+ public:
+  static constexpr uint8_t MESSAGE_TYPE = 150;
+  static constexpr uint8_t ESTIMATED_SIZE = 102;
+#ifdef HAS_PROTO_MESSAGE_DUMP
+  const LogString *message_name() const override { return LOG_STR("device_capabilities_response"); }
+#endif
+#ifdef USE_BLUETOOTH_PROXY
+  BluetoothProxyCapabilities bluetooth_proxy{};
+#endif
+#ifdef USE_VOICE_ASSISTANT
+  VoiceAssistantCapabilities voice_assistant{};
+#endif
+#ifdef USE_ZWAVE_PROXY
+  ZWaveProxyCapabilities zwave_proxy{};
+#endif
+#ifdef USE_SERIAL_PROXY
+  std::array<SerialProxyInfo, SERIAL_PROXY_COUNT> serial_proxies{};
+#endif
+  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:
+};
 class ListEntitiesDoneResponse final : public ProtoMessage {
  public:
   static constexpr uint8_t MESSAGE_TYPE = 19;
diff --git a/esphome/components/api/api_pb2_dump.cpp b/esphome/components/api/api_pb2_dump.cpp
index 09570b09e4..17ce7fba45 100644
--- a/esphome/components/api/api_pb2_dump.cpp
+++ b/esphome/components/api/api_pb2_dump.cpp
@@ -988,6 +988,55 @@ const char *DeviceInfoResponse::dump_to(DumpBuffer &out) const {
 #endif
   return out.c_str();
 }
+#ifdef USE_BLUETOOTH_PROXY
+const char *BluetoothProxyCapabilities::dump_to(DumpBuffer &out) const {
+  MessageDumpHelper helper(out, ESPHOME_PSTR("BluetoothProxyCapabilities"));
+  dump_field(out, ESPHOME_PSTR("feature_flags"), this->feature_flags);
+  dump_field(out, ESPHOME_PSTR("mac_address"), this->mac_address);
+  return out.c_str();
+}
+#endif
+#ifdef USE_VOICE_ASSISTANT
+const char *VoiceAssistantCapabilities::dump_to(DumpBuffer &out) const {
+  MessageDumpHelper helper(out, ESPHOME_PSTR("VoiceAssistantCapabilities"));
+  dump_field(out, ESPHOME_PSTR("feature_flags"), this->feature_flags);
+  return out.c_str();
+}
+#endif
+#ifdef USE_ZWAVE_PROXY
+const char *ZWaveProxyCapabilities::dump_to(DumpBuffer &out) const {
+  MessageDumpHelper helper(out, ESPHOME_PSTR("ZWaveProxyCapabilities"));
+  dump_field(out, ESPHOME_PSTR("feature_flags"), this->feature_flags);
+  dump_field(out, ESPHOME_PSTR("home_id"), this->home_id);
+  return out.c_str();
+}
+#endif
+const char *DeviceCapabilitiesResponse::dump_to(DumpBuffer &out) const {
+  MessageDumpHelper helper(out, ESPHOME_PSTR("DeviceCapabilitiesResponse"));
+#ifdef USE_BLUETOOTH_PROXY
+  out.append(2, ' ').append_p(ESPHOME_PSTR("bluetooth_proxy")).append(": ");
+  this->bluetooth_proxy.dump_to(out);
+  out.append("\n");
+#endif
+#ifdef USE_VOICE_ASSISTANT
+  out.append(2, ' ').append_p(ESPHOME_PSTR("voice_assistant")).append(": ");
+  this->voice_assistant.dump_to(out);
+  out.append("\n");
+#endif
+#ifdef USE_ZWAVE_PROXY
+  out.append(2, ' ').append_p(ESPHOME_PSTR("zwave_proxy")).append(": ");
+  this->zwave_proxy.dump_to(out);
+  out.append("\n");
+#endif
+#ifdef USE_SERIAL_PROXY
+  for (const auto &it : this->serial_proxies) {
+    out.append(4, ' ').append_p(ESPHOME_PSTR("serial_proxies")).append(": ");
+    it.dump_to(out);
+    out.append("\n");
+  }
+#endif
+  return out.c_str();
+}
 const char *ListEntitiesDoneResponse::dump_to(DumpBuffer &out) const {
   out.append_p(ESPHOME_PSTR("ListEntitiesDoneResponse {}"));
   return out.c_str();
diff --git a/esphome/components/api/api_pb2_service.cpp b/esphome/components/api/api_pb2_service.cpp
index 5c9df433dd..19dcbfb77c 100644
--- a/esphome/components/api/api_pb2_service.cpp
+++ b/esphome/components/api/api_pb2_service.cpp
@@ -705,6 +705,13 @@ void APIConnection::read_message_(uint32_t msg_size, uint32_t msg_type, const ui
       break;
     }
 #endif
+    case 149 /* DeviceCapabilitiesRequest is empty */: {
+#ifdef HAS_PROTO_MESSAGE_DUMP
+      this->log_receive_message_(LOG_STR("on_device_capabilities_request"));
+#endif
+      this->on_device_capabilities_request();
+      break;
+    }
     default:
       break;
   }
diff --git a/esphome/components/api/api_pb2_service.h b/esphome/components/api/api_pb2_service.h
index d1b51f4846..5ed78b3385 100644
--- a/esphome/components/api/api_pb2_service.h
+++ b/esphome/components/api/api_pb2_service.h
@@ -27,6 +27,8 @@ class APIServerConnectionBase {
   void on_ping_response(){};
   void on_device_info_request(){};
 
+  void on_device_capabilities_request(){};
+
   void on_list_entities_request(){};
 
   void on_subscribe_states_request(){};
diff --git a/esphome/components/rp2/__init__.py b/esphome/components/rp2/__init__.py
index 1bf01e6828..ea37134e27 100644
--- a/esphome/components/rp2/__init__.py
+++ b/esphome/components/rp2/__init__.py
@@ -351,6 +351,11 @@ async def to_code(config):
         ],
     )
 
+    # newlib-nano is the default libc for the arduino-pico toolchain and its
+    # printf silently drops %f unless _printf_float is force-linked. Components
+    # use %f widely in logging, so pull it in.
+    cg.add_build_flag("-Wl,-u,_printf_float")
+
     # Wrap FILE*-based printf functions to eliminate newlib's _vfprintf_r
     # (~9.2 KB). See printf_stubs.cpp for implementation.
     if config.get(CONF_ENABLE_FULL_PRINTF):
@@ -402,25 +407,39 @@ def _configure_lwip() -> None:
     ────────────────────────────────────────────────────────────────
     TCP_SND_BUF               2×MSS   4×MSS  8×MSS         4×MSS
     TCP_WND                   4×MSS   4×MSS  8×MSS         4×MSS
+    TCP_SND_QUEUELEN          ~8      17     32            17
     MEM_LIBC_MALLOC           1       1      0             0*
     MEMP_MEM_MALLOC           1       1      0             0**
-    MEM_SIZE                  N/A***  N/A*** 16KB          16KB
+    MEM_SIZE                  N/A***  N/A*** 16KB          32KB
     PBUF_POOL_SIZE            10      16     24            16
-    MEMP_NUM_TCP_SEG          10      16     32            17
+    MEMP_NUM_TCP_SEG          10      16     32            34****
     MEMP_NUM_TCP_PCB          5       16     5             dynamic
-    MEMP_NUM_TCP_PCB_LISTEN   4       16     8****         dynamic
+    MEMP_NUM_TCP_PCB_LISTEN   4       16     8*****        dynamic
     MEMP_NUM_UDP_PCB          4       16     7             dynamic
-    TCP_SND_QUEUELEN          ~8      17     32            17
 
     * MEM_LIBC_MALLOC must stay 0: arduino-pico uses
       PICO_CYW43_ARCH_THREADSAFE_BACKGROUND which runs lwIP callbacks from
       a low-priority pendsv IRQ. The pico-sdk explicitly blocks
       MEM_LIBC_MALLOC=1 because libc malloc uses mutexes (unsafe in IRQ).
-    ** MEMP_MEM_MALLOC must stay 0: the dedicated lwIP heap (MEM_SIZE=16KB)
-      is too small to hold all pools dynamically. The PBUF_POOL alone needs
-      ~24KB (16 × 1524 bytes). Increasing MEM_SIZE would negate BSS savings.
-    *** ESP8266/ESP32 use MEM_LIBC_MALLOC=1 (system heap, no dedicated pool).
-    **** opt.h default; arduino-pico doesn't override MEMP_NUM_TCP_PCB_LISTEN.
+    ** MEMP_MEM_MALLOC must stay 0, and the reason is IRQ safety, not size.
+      memp_malloc() pops the pool free list inside SYS_ARCH_PROTECT, but
+      lwIP's heap picks its protection from LWIP_ALLOW_MEM_FREE_FROM_OTHER_
+      CONTEXT (default 0), so under NO_SYS=1 mem_malloc()/mem_free() run
+      with no protection at all. Setting MEMP_MEM_MALLOC=1 also moves the
+      allocation outside the guard entirely (memp.c calls mem_malloc before
+      SYS_ARCH_PROTECT). RX pbufs would then be allocated from the pendsv
+      IRQ on the same unguarded free list the main loop uses for
+      tcp_write(), which corrupts the heap. Tried on hardware; it faults
+      within seconds on CYW43. Ethernet survives only because that path
+      polls from the main loop and never runs a second context.
+    *** ESP8266/ESP32 use MEM_LIBC_MALLOC=1 (system heap, no dedicated
+      pool), so their MEMP_NUM_* and PBUF_POOL_SIZE values are not caps.
+      Ours are hard limits; do not copy their numbers without that in mind.
+    **** MEMP_NUM_TCP_SEG is a *global* pool while TCP_SND_QUEUELEN is
+      *per-PCB*, so it must not be sized to the per-PCB value: one busy
+      connection would drain it for every other PCB. 2× covers two PCBs at
+      full depth, and MEM_SIZE is the real backing limit past that.
+    ***** opt.h default; arduino-pico doesn't override MEMP_NUM_TCP_PCB_LISTEN.
     "dynamic" = auto-calculated from component socket registrations via
     socket.get_socket_counts() with minimums of 8 TCP / 6 UDP / 2 TCP_LISTEN.
     """
@@ -446,35 +465,42 @@ def _configure_lwip() -> None:
     # TCP_WND: receive window. 4×MSS matches ESP32. Down from arduino-pico's 8×MSS.
     tcp_wnd = "(4*TCP_MSS)"
 
-    # TCP_SND_QUEUELEN: max pbufs queued for send buffer
+    # TCP_SND_QUEUELEN: max pbufs queued per PCB for the send buffer
     # ESP-IDF formula: (4 * TCP_SND_BUF + (TCP_MSS - 1)) / TCP_MSS
     # With 4×MSS: (4*5840 + 1459) / 1460 = 17 — match ESP32
     tcp_snd_queuelen = 17
-    # MEMP_NUM_TCP_SEG: segment pool, must be >= TCP_SND_QUEUELEN (lwIP sanity check)
-    memp_num_tcp_seg = tcp_snd_queuelen
+    # MEMP_NUM_TCP_SEG: segment pool shared by every PCB, so it cannot be the
+    # per-PCB queue length — lwIP's sanity check only demands >=, which is the
+    # floor for a single connection. 2× lets two PCBs queue fully before the
+    # rest start seeing ERR_MEM. Each entry is ~16 bytes.
+    memp_num_tcp_seg = 2 * tcp_snd_queuelen
 
     # PBUF_POOL_SIZE: RP2040 has 264KB RAM, more generous than LibreTiny.
-    # 16 matches ESP32 (vs arduino-pico's 24). With MEMP_MEM_MALLOC=1,
-    # this is a max count (allocated on demand from heap).
+    # 16 matches ESP32 (vs arduino-pico's 24). Receive side only; the send
+    # path copies into PBUF_RAM out of MEM_SIZE.
     pbuf_pool_size = 16
 
+    # MEM_SIZE: the lwIP heap that backs PBUF_RAM, which is where tcp_write()
+    # copies outgoing data. TCP_OVERSIZE defaults to TCP_MSS, so one PCB
+    # holding a full TCP_SND_BUF pins ~4 × 1.5KB. At arduino-pico's 16KB, two
+    # busy connections exhausted it while api's max_connections on rp2 is 4;
+    # that surfaces as ERR_MEM from tcp_write(), then a refused send_message().
+    # 32KB is arduino-pico's own next tier (its __LWIP_MEMMULT=2 boards).
+    # Must stay under 64000 or lwIP widens mem_size_t to u32_t.
+    mem_size = 32768
+
     # Build the lwIP override defines for the Jinja2 template.
     # The template uses #include_next to chain to the framework's original
     # lwipopts.h, then #undef/#define only the values we need to change.
     #
-    # Note: MEMP_MEM_MALLOC stays 0 (framework default). While the memp
-    # allocations use the dedicated lwIP heap (IRQ-safe), the 16KB MEM_SIZE
-    # is too small to hold all pools dynamically under stress. The PBUF_POOL
-    # alone needs ~24KB (16 × 1524 bytes). Increasing MEM_SIZE would negate
-    # the BSS savings.
-    #
-    # MEM_LIBC_MALLOC stays 0 (framework default): arduino-pico uses
-    # PICO_CYW43_ARCH_THREADSAFE_BACKGROUND which runs lwIP callbacks from
-    # a low-priority pendsv IRQ where libc malloc (mutex-based) is unsafe.
+    # Both malloc flags stay 0 (framework defaults); see the docstring. The
+    # static pools are the only IRQ-safe allocator on this platform, so the
+    # fix is to size them correctly rather than to make them dynamic.
     lwip_defines: dict[str, str] = {
         "TCP_SND_BUF": tcp_snd_buf,
         "TCP_WND": tcp_wnd,
         "TCP_SND_QUEUELEN": str(tcp_snd_queuelen),
+        "MEM_SIZE": str(mem_size),
         "MEMP_NUM_TCP_SEG": str(memp_num_tcp_seg),
         "PBUF_POOL_SIZE": str(pbuf_pool_size),
         "MEMP_NUM_TCP_PCB": str(tcp_sockets),
@@ -495,7 +521,8 @@ def _configure_lwip() -> None:
     udp_min = " (min)" if udp_sockets > sc.udp else ""
     listen_min = " (min)" if listening_tcp > sc.tcp_listen else ""
     _LOGGER.info(
-        "Configuring lwIP: TCP=%d%s [%s], UDP=%d%s [%s], TCP_LISTEN=%d%s [%s]",
+        "Configuring lwIP: %d byte heap; TCP=%d%s [%s], UDP=%d%s [%s], TCP_LISTEN=%d%s [%s]",
+        mem_size,
         tcp_sockets,
         tcp_min,
         sc.tcp_details,
diff --git a/esphome/components/rp2/lwipopts.h.jinja b/esphome/components/rp2/lwipopts.h.jinja
index 36d7d4da14..6563489d2e 100644
--- a/esphome/components/rp2/lwipopts.h.jinja
+++ b/esphome/components/rp2/lwipopts.h.jinja
@@ -20,13 +20,22 @@
 #undef TCP_WND
 #define TCP_WND                       {{ TCP_WND }}
 
-// Queued segment limits: derived from 4xMSS buffer size, matching ESP32
+// Per-PCB send queue: derived from 4xMSS buffer size, matching ESP32
 #undef TCP_SND_QUEUELEN
 #define TCP_SND_QUEUELEN              {{ TCP_SND_QUEUELEN }}
 
+// Segment pool: global across every PCB, so it is sized above the per-PCB
+// queue length rather than equal to it. lwIP's sanity check only requires
+// >= TCP_SND_QUEUELEN, which is the floor for a single connection.
 #undef MEMP_NUM_TCP_SEG
 #define MEMP_NUM_TCP_SEG              {{ MEMP_NUM_TCP_SEG }}
 
+// lwIP heap backing PBUF_RAM, which is what tcp_write() copies into.
+// Raised from arduino-pico's 16KB: TCP_OVERSIZE is TCP_MSS, so a single PCB
+// at a full TCP_SND_BUF pins about 6KB and two connections exhausted it.
+#undef MEM_SIZE
+#define MEM_SIZE                      {{ MEM_SIZE }}
+
 // Packet buffer pool: 16 matches ESP32 (down from 24)
 #undef PBUF_POOL_SIZE
 #define PBUF_POOL_SIZE                {{ PBUF_POOL_SIZE }}
diff --git a/tests/unit_tests/components/api/test_api_proto.py b/tests/unit_tests/components/api/test_api_proto.py
new file mode 100644
index 0000000000..35aa5ff529
--- /dev/null
+++ b/tests/unit_tests/components/api/test_api_proto.py
@@ -0,0 +1,371 @@
+"""Invariant tests for esphome/components/api/api.proto and its generated code.
+
+These guard the DeviceCapabilitiesRequest/DeviceCapabilitiesResponse addition
+(API 1.15) against regressions that protoc-based codegen would not catch on
+its own, without requiring protoc to be installed at test time:
+
+* script/api_protobuf/api_protobuf.py skips any field marked
+  `[deprecated = true]` completely -- it generates no C++ for it at all, so
+  the device silently stops sending that value. Six DeviceInfoResponse fields
+  were superseded by DeviceCapabilitiesResponse but must keep being sent for
+  backward compatibility with clients older than API 1.15. If a future edit
+  "tidies up" by marking one of them deprecated, this file breaks that field
+  for every existing client with nothing else in CI noticing.
+* Field numbers are the wire protocol, not the field names. Renaming a field
+  is harmless; renumbering it is a silent breaking change, because an old
+  client still decodes by number. This file pins the field number of each of
+  the six superseded DeviceInfoResponse fields and of every field on the new
+  DeviceCapabilitiesResponse/BluetoothProxyCapabilities/
+  VoiceAssistantCapabilities/ZWaveProxyCapabilities sub-messages, so a
+  well-intentioned reshuffle of api.proto gets caught here instead of on a
+  device in the field.
+* Message wire ids must be unique, and the new capabilities RPC must stay
+  authenticated-only.
+
+Group A below asserts on the checked-in generated files (api_pb2.h /
+api_pb2.cpp), since "the field is present in the generated C++" is exactly
+equivalent to "the device still sends it". Group B parses api.proto as plain
+text (no protoc). Group C checks the advertised API minor version.
+"""
+
+from __future__ import annotations
+
+from pathlib import Path
+import re
+
+import esphome
+
+API_DIR = Path(esphome.__file__).parent / "components" / "api"
+
+PROTO_TEXT = (API_DIR / "api.proto").read_text(encoding="utf-8")
+HEADER_TEXT = (API_DIR / "api_pb2.h").read_text(encoding="utf-8")
+CPP_TEXT = (API_DIR / "api_pb2.cpp").read_text(encoding="utf-8")
+API_CONNECTION_TEXT = (API_DIR / "api_connection.cpp").read_text(encoding="utf-8")
+
+# Fields on DeviceInfoResponse that were superseded by DeviceCapabilitiesResponse
+# as of API 1.15 but must still be generated (and therefore still sent) for
+# backward compatibility with older clients.
+SUPERSEDED_FIELDS: dict[str, int] = {
+    "bluetooth_proxy_feature_flags": 15,
+    "voice_assistant_feature_flags": 17,
+    "bluetooth_mac_address": 18,
+    "zwave_proxy_feature_flags": 23,
+    "zwave_home_id": 24,
+    "serial_proxies": 25,
+}
+
+# Field numbers on the new capability messages. These are a frozen wire
+# contract from the moment they ship: an old client decodes a sub-message
+# field purely by number, so renumbering any of these -- even without
+# touching a name -- silently corrupts what every already-deployed client
+# reads. Keyed by message name so the next capability sub-message is a
+# data-only addition here.
+NEW_CAPABILITY_FIELDS: dict[str, dict[str, int]] = {
+    "DeviceCapabilitiesResponse": {
+        "bluetooth_proxy": 1,
+        "voice_assistant": 2,
+        "zwave_proxy": 3,
+        "serial_proxies": 4,
+    },
+    "BluetoothProxyCapabilities": {
+        "feature_flags": 1,
+        "mac_address": 2,
+    },
+    "VoiceAssistantCapabilities": {
+        "feature_flags": 1,
+    },
+    "ZWaveProxyCapabilities": {
+        "feature_flags": 1,
+        "home_id": 2,
+    },
+}
+
+# Fields that are genuinely dead and are expected to carry `deprecated=true`.
+# Used to prove the deprecated-detection logic below actually detects
+# deprecation rather than trivially passing.
+GENUINELY_DEPRECATED_FIELDS: tuple[str, ...] = (
+    "legacy_bluetooth_proxy_version",
+    "legacy_voice_assistant_version",
+)
+
+DEPRECATED_FIELD_TRAP = (
+    "script/api_protobuf/api_protobuf.py skips fields marked `[deprecated = "
+    "true]` completely, generating no C++ for them at all. Marking this field "
+    "deprecated would silently stop the device from ever sending it, breaking "
+    "every existing client that still reads it from DeviceInfoResponse."
+)
+
+
+def _extract_braced_region(text: str, anchor_pattern: str) -> str:
+    """Return the region of `text` starting at the first match of
+    `anchor_pattern` up to the matching closing brace (inclusive), using
+    brace-depth counting so nested braces (e.g. a `for (...) { ... }` loop
+    inside a function body) don't cause a premature stop.
+    """
+    anchor_match = re.search(anchor_pattern, text)
+    if anchor_match is None:
+        raise AssertionError(f"could not find a match for {anchor_pattern!r}")
+    start = anchor_match.start()
+    open_brace = text.index("{", start)
+    depth = 0
+    for i in range(open_brace, len(text)):
+        if text[i] == "{":
+            depth += 1
+        elif text[i] == "}":
+            depth -= 1
+            if depth == 0:
+                return text[start : i + 1]
+    raise AssertionError(f"unbalanced braces while scanning after {anchor_pattern!r}")
+
+
+def _extract_class_body(header_text: str, class_name: str) -> str:
+    """Return the body of a generated C++ class, scoped so a field name that
+    also happens to exist on some other class cannot satisfy the assertion.
+    """
+    return _extract_braced_region(header_text, rf"class {re.escape(class_name)}\b")
+
+
+def _extract_function_body(cpp_text: str, qualified_name: str) -> str:
+    """Return the body of a generated `Class::method(...)` definition."""
+    return _extract_braced_region(cpp_text, rf"{re.escape(qualified_name)}\(")
+
+
+def _extract_proto_message(proto_text: str, message_name: str) -> str:
+    """Return the body of a top-level `message Name { ... }` block from the
+    .proto source. Proto message bodies here contain no nested `{`/`}` of
+    their own (options use parens, not braces), so a non-greedy match up to
+    the first line that is just `}` is sufficient and keeps the parsing
+    simple.
+    """
+    match = re.search(
+        rf"^message {re.escape(message_name)}\s*\{{(.*?)^\}}",
+        proto_text,
+        re.MULTILINE | re.DOTALL,
+    )
+    if match is None:
+        raise AssertionError(f"could not find `message {message_name}` in api.proto")
+    return match.group(1)
+
+
+def _extract_rpc_body(proto_text: str, rpc_name: str) -> str:
+    """Return the option body of an `rpc name (...) returns (...) { ... }`
+    declaration from the APIConnection service, robust to it being written
+    on one line (`{}`) or spread across several with options inside.
+    """
+    match = re.search(
+        rf"rpc\s+{re.escape(rpc_name)}\s*\([^)]*\)\s*returns\s*\([^)]*\)\s*\{{(.*?)\}}",
+        proto_text,
+        re.DOTALL,
+    )
+    if match is None:
+        raise AssertionError(f"could not find `rpc {rpc_name}` in api.proto")
+    return match.group(1)
+
+
+def _field_declaration_line(message_body: str, field_name: str) -> str:
+    """Return the single source line declaring `field_name` inside a proto
+    message body (all fields here are declared on one line).
+    """
+    for line in message_body.splitlines():
+        if re.search(rf"\b{re.escape(field_name)}\s*=\s*\d+", line):
+            return line
+    raise AssertionError(
+        f"could not find a field declaration for {field_name!r} in the given message body"
+    )
+
+
+# ==================== Group A: generated files ====================
+
+
+def test_superseded_device_info_fields_still_declared_in_header() -> None:
+    """Each superseded field must still be a real member of DeviceInfoResponse
+    in api_pb2.h -- not merely present somewhere in the file. Several of these
+    names (e.g. serial_proxies) also exist on DeviceCapabilitiesResponse, so an
+    unscoped substring search over the whole header would pass even if the
+    field were removed from DeviceInfoResponse.
+    """
+    class_body = _extract_class_body(HEADER_TEXT, "DeviceInfoResponse")
+    for field_name in SUPERSEDED_FIELDS:
+        assert re.search(rf"\b{field_name}\b", class_body), (
+            f"{field_name} is missing from the DeviceInfoResponse class body in "
+            f"api_pb2.h. {DEPRECATED_FIELD_TRAP}"
+        )
+
+
+def test_superseded_device_info_fields_still_encoded_and_sized() -> None:
+    """Each superseded field must still be touched by DeviceInfoResponse's
+    generated encode() and calculate_size(), i.e. it is still put on the wire.
+    """
+    encode_body = _extract_function_body(CPP_TEXT, "DeviceInfoResponse::encode")
+    size_body = _extract_function_body(CPP_TEXT, "DeviceInfoResponse::calculate_size")
+    for field_name in SUPERSEDED_FIELDS:
+        assert f"this->{field_name}" in encode_body, (
+            f"DeviceInfoResponse::encode() no longer references {field_name}. "
+            f"{DEPRECATED_FIELD_TRAP}"
+        )
+        assert f"this->{field_name}" in size_body, (
+            f"DeviceInfoResponse::calculate_size() no longer references "
+            f"{field_name}. {DEPRECATED_FIELD_TRAP}"
+        )
+
+
+def test_new_capability_classes_present_in_header() -> None:
+    """The new response message and its capability sub-messages must exist as
+    generated classes.
+    """
+    for class_name in (
+        "DeviceCapabilitiesResponse",
+        "BluetoothProxyCapabilities",
+        "VoiceAssistantCapabilities",
+        "ZWaveProxyCapabilities",
+    ):
+        assert re.search(rf"class {re.escape(class_name)}\b", HEADER_TEXT), (
+            f"expected a generated class named {class_name} in api_pb2.h"
+        )
+
+
+# ==================== Group B: api.proto source text ====================
+
+
+def test_all_message_ids_are_unique() -> None:
+    """Every `option (id) = N;` in api.proto must be unique. Two messages
+    sharing a wire id would make the client and server misinterpret each
+    other's messages -- nothing else currently checks this.
+    """
+    ids = [int(value) for value in re.findall(r"option \(id\) = (\d+);", PROTO_TEXT)]
+    assert ids, "did not find any `option (id) = N;` declarations in api.proto"
+    duplicates = sorted({value for value in ids if ids.count(value) > 1})
+    assert not duplicates, (
+        f"Duplicate `option (id)` values found in api.proto: {duplicates}. Each "
+        "message must have a unique wire id."
+    )
+
+
+def test_device_capabilities_request_has_id_149() -> None:
+    body = _extract_proto_message(PROTO_TEXT, "DeviceCapabilitiesRequest")
+    match = re.search(r"option \(id\) = (\d+);", body)
+    assert match is not None, "DeviceCapabilitiesRequest is missing `option (id)`"
+    assert int(match.group(1)) == 149, (
+        f"DeviceCapabilitiesRequest has id {match.group(1)}, expected 149. "
+        "Message ids are part of the wire protocol and must not change once "
+        "assigned."
+    )
+
+
+def test_device_capabilities_response_has_id_150() -> None:
+    body = _extract_proto_message(PROTO_TEXT, "DeviceCapabilitiesResponse")
+    match = re.search(r"option \(id\) = (\d+);", body)
+    assert match is not None, "DeviceCapabilitiesResponse is missing `option (id)`"
+    assert int(match.group(1)) == 150, (
+        f"DeviceCapabilitiesResponse has id {match.group(1)}, expected 150. "
+        "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
+    them (see module docstring). The second half of this test proves the
+    deprecated-detection itself works: two genuinely dead fields
+    (legacy_bluetooth_proxy_version, legacy_voice_assistant_version) must
+    still be detected as deprecated, so the first half isn't vacuously true.
+    """
+    body = _extract_proto_message(PROTO_TEXT, "DeviceInfoResponse")
+
+    for field_name in SUPERSEDED_FIELDS:
+        line = _field_declaration_line(body, field_name)
+        assert "deprecated" not in line, (
+            f"{field_name} in DeviceInfoResponse is marked deprecated in "
+            f"api.proto ({line.strip()!r}). {DEPRECATED_FIELD_TRAP}"
+        )
+
+    for field_name in GENUINELY_DEPRECATED_FIELDS:
+        line = _field_declaration_line(body, field_name)
+        assert "deprecated" in line, (
+            f"expected {field_name} to still carry `deprecated=true` in "
+            f"api.proto ({line.strip()!r}). If this fails, the deprecated "
+            "detection used above is broken, and the sibling assertion that "
+            "the superseded fields are NOT deprecated is not testing anything."
+        )
+
+
+def test_superseded_fields_keep_their_wire_numbers() -> None:
+    """Each superseded field must stay on the field number recorded in
+    SUPERSEDED_FIELDS. Old clients decode DeviceInfoResponse purely by field
+    number, so renumbering one of these -- even without touching its name --
+    would make an old client read a completely different value out of the
+    wire, with nothing else in CI noticing.
+    """
+    body = _extract_proto_message(PROTO_TEXT, "DeviceInfoResponse")
+
+    for field_name, field_number in SUPERSEDED_FIELDS.items():
+        line = _field_declaration_line(body, field_name)
+        assert re.search(rf"\b{field_name}\s*=\s*{field_number}\b", line), (
+            f"{field_name} in DeviceInfoResponse is no longer declared at "
+            f"field number {field_number} ({line.strip()!r}). Field numbers "
+            "are the wire protocol -- renumbering this field silently breaks "
+            "every existing client that still decodes DeviceInfoResponse by "
+            "the old numbering."
+        )
+
+
+def test_capability_message_fields_keep_their_wire_numbers() -> None:
+    """Every field on DeviceCapabilitiesResponse and its three capability
+    sub-messages must stay on the field number recorded in
+    NEW_CAPABILITY_FIELDS. These messages are brand new as of API 1.15, but
+    the moment a device ships with them, their field numbers are a frozen
+    wire contract -- a client decodes a sub-message field purely by number,
+    so a later "cleanup" that renumbers one of these would silently corrupt
+    what every already-deployed client reads, with nothing else in CI
+    noticing.
+    """
+    for message_name, fields in NEW_CAPABILITY_FIELDS.items():
+        body = _extract_proto_message(PROTO_TEXT, message_name)
+        for field_name, field_number in fields.items():
+            line = _field_declaration_line(body, field_name)
+            assert re.search(rf"\b{field_name}\s*=\s*{field_number}\b", line), (
+                f"{field_name} on {message_name} is no longer declared at "
+                f"field number {field_number} ({line.strip()!r}). Field "
+                "numbers are the wire protocol -- renumbering this field "
+                "silently breaks every existing client that decodes this "
+                "message by the old numbering."
+            )
+
+
+def test_device_capabilities_rpc_requires_authentication() -> None:
+    """The `device_capabilities` RPC must not set
+    `option (needs_authentication) = false;` (or set it to anything at all).
+    Leaving it unset makes it inherit needs_authentication = true, keeping
+    capability data behind authentication (and encryption, when configured).
+    """
+    body = _extract_rpc_body(PROTO_TEXT, "device_capabilities")
+    assert "needs_authentication" not in body, (
+        "rpc device_capabilities sets a `needs_authentication` option in "
+        "api.proto. It must stay unset so it inherits needs_authentication = "
+        "true; otherwise device capability data could be requested over an "
+        "unauthenticated connection."
+    )
+
+
+# ==================== Group C: advertised API version ====================
+
+
+def test_api_version_minor_is_at_least_15() -> None:
+    """Clients gate sending DeviceCapabilitiesRequest on seeing
+    api_version >= 1.15 in HelloResponse. Regressing api_version_minor below
+    15 would make every client believe capabilities are unsupported even
+    though the RPC exists, so this must never go backwards. Use >= rather
+    than == so the next unrelated minor-version bump doesn't need to touch
+    this test.
+    """
+    match = re.search(r"resp\.api_version_minor\s*=\s*(\d+);", API_CONNECTION_TEXT)
+    assert match is not None, (
+        "could not find `resp.api_version_minor = N;` in api_connection.cpp"
+    )
+    minor = int(match.group(1))
+    assert minor >= 15, (
+        f"api_version_minor is {minor}, but device_capabilities requires "
+        "clients to see api_version >= 1.15 in HelloResponse before they will "
+        "ever request it."
+    )