[ble_device_base] Bind the GATT backend at compile time (#18185)

esphome/components/ble_device_base/ble_gatt_client.h

@@ -2,12 +2,13 @@
 //
 // Platform-neutral GATT client connection contract.
 //
-// A platform's GATT client backend (bluetooth_connection/esp32,
-// bluetooth_connection/rp2) implements BLEGattConnection; consumers
-// (bluetooth_proxy) drive it through this interface and receive
-// completions through GattClientEventListener. All listener callbacks are
-// delivered on the ESPHome main loop; borrowed data pointers are valid only
-// for the duration of the call.
+// Exactly one GATT backend exists per build, so BLEGattConnection is a
+// compile-time alias (bluetooth_connection_gatt_backend.h), not an abstract
+// interface.
+// The hub BluetoothConnection wrapper drives it and receives completions
+// through its event-sink methods, which the backend calls directly. All sink
+// calls are delivered on the ESPHome main loop; borrowed data pointers are
+// valid only for the duration of the call.
 //
 // Error domain (plain int, forwarded to the API without translation):
 //   0            success
@@ -29,6 +30,7 @@
 #include "ble_client_state.h"
 #include "ble_device.h"
 
+#include <concepts>
 #include <cstdint>
 
 namespace esphome::ble_device_base {
@@ -76,67 +78,53 @@ struct GattServiceTable {
   uint16_t descriptor_count{0};
 };
 
-/// Completion/event sink for a GATT connection. Implemented by the consumer
-/// (bluetooth_proxy's connection wrapper). Every callback runs on the main loop.
-class GattClientEventListener {
- public:
-  virtual ~GattClientEventListener() = default;
-
-  /// Connected (with negotiated MTU) or disconnected/connect-failed
-  /// (error = HCI status or disconnect reason).
-  virtual void on_connection_state(bool connected, uint16_t mtu, int error) = 0;
-  /// Service discovery finished; on success the service table is populated.
-  virtual void on_service_discovery_done(int error) = 0;
-  /// Characteristic or descriptor read finished. data/len valid during the call.
-  virtual void on_read_result(uint16_t handle, const uint8_t *data, uint16_t len, int error) = 0;
-  /// Characteristic write-with-response or descriptor write finished.
-  virtual void on_write_result(uint16_t handle, int error) = 0;
-  /// Notification/indication registration state changed.
-  virtual void on_notify_state(uint16_t handle, bool enabled, int error) = 0;
-  /// Notification/indication data from the peer. data/len valid during the call.
-  virtual void on_notify_data(uint16_t handle, const uint8_t *data, uint16_t len) = 0;
-  virtual void on_pairing_result(int status) {}
+// The BLEGattConnection op surface, asserted where the alias binds
+// (bluetooth_connection_gatt_backend.h). Operations return 0 when accepted (completion arrives
+// through the sink) or a synchronous error (busy, not connected, stack
+// rejection); one operation may be outstanding at a time. Semantics beyond
+// the signatures:
+// - connect: addr_type is a BLE_ADDR_TYPE_* constant (ble_device.h).
+// - disconnect: also cancels a connect in progress.
+// - notify_characteristic: local registration only; the CCCD write is the
+//   API client's responsibility (a plain write_descriptor).
+// - get_service_table/release_services: backend-owned transient storage,
+//   released after streaming (release is idempotent).
+// - completions: connect and disconnect land in on_connection_state,
+//   discover_services in on_service_discovery_done, pair in
+//   on_pairing_result, reads in on_read_result, notify_characteristic in
+//   on_notify_state, characteristic writes with response and descriptor
+//   writes in on_write_result.
+template<typename T, typename Sink>
+concept BLEGattConnectionContract = requires(T conn, Sink *sink, const uint8_t *data) {
+  conn.set_listener(sink);
+  { conn.connect(uint64_t{}, uint8_t{}) } -> std::same_as<int>;
+  { conn.disconnect() } -> std::same_as<int>;
+  { conn.discover_services() } -> std::same_as<int>;
+  { conn.read_characteristic(uint16_t{}) } -> std::same_as<int>;
+  { conn.write_characteristic(uint16_t{}, data, uint16_t{}, true) } -> std::same_as<int>;
+  { conn.read_descriptor(uint16_t{}) } -> std::same_as<int>;
+  { conn.write_descriptor(uint16_t{}, data, uint16_t{}) } -> std::same_as<int>;
+  { conn.notify_characteristic(uint16_t{}, true) } -> std::same_as<int>;
+  { conn.pair() } -> std::same_as<int>;
+  { conn.update_connection_params(uint16_t{}, uint16_t{}, uint16_t{}, uint16_t{}) } -> std::same_as<int>;
+  { conn.get_service_table() } -> std::same_as<GattServiceTable>;
+  { conn.release_services() } -> std::same_as<void>;
 };
 
-/// One GATT client connection slot. Operations return 0 when accepted
-/// (completion arrives via the listener) or a synchronous error code
-/// (busy, not connected, stack rejection). One operation may be outstanding
-/// at a time; callers see a synchronous error otherwise.
-class BLEGattConnection {
- public:
-  virtual ~BLEGattConnection() = default;
-
-  void set_listener(GattClientEventListener *listener) { this->listener_ = listener; }
-
-  /// Start connecting to a peer. addr_type is a BLE_ADDR_TYPE_* constant
-  /// (ble_device.h). Completion: on_connection_state().
-  virtual int connect(uint64_t address, uint8_t addr_type) = 0;
-  /// Disconnect (or cancel a connect in progress). Completion: on_connection_state().
-  virtual int disconnect() = 0;
-  /// Discover the peer's services/characteristics/descriptors into the
-  /// service table. Completion: on_service_discovery_done().
-  virtual int discover_services() = 0;
-  virtual int read_characteristic(uint16_t handle) = 0;
-  virtual int write_characteristic(uint16_t handle, const uint8_t *data, uint16_t len, bool response) = 0;
-  virtual int read_descriptor(uint16_t handle) = 0;
-  virtual int write_descriptor(uint16_t handle, const uint8_t *data, uint16_t len) = 0;
-  /// Enable/disable delivery of on_notify_data() for a characteristic value
-  /// handle. Local registration only — the CCCD write is the API client's
-  /// responsibility (it arrives as a plain write_descriptor).
-  virtual int notify_characteristic(uint16_t handle, bool enable) = 0;
-  /// Initiate pairing on the live link. Completion: on_pairing_result().
-  virtual int pair() { return GATT_ERR_NOT_CONNECTED; }
-  virtual int update_connection_params(uint16_t min_interval, uint16_t max_interval, uint16_t latency,
-                                       uint16_t timeout) = 0;
-
-  /// Backend-owned service table (see GattServiceTable lifetime).
-  virtual GattServiceTable get_service_table() = 0;
-  /// Free the transient service table storage. Call after streaming;
-  /// idempotent (a call with no table held is a no-op).
-  virtual void release_services() = 0;
-
- protected:
-  GattClientEventListener *listener_{nullptr};
+// The event sink the backend calls directly (the hub BluetoothConnection
+// wrapper), asserted where the wrapper is defined: on_connection_state
+// carries the negotiated MTU and an HCI status/disconnect reason. The
+// requirements check call validity, not exact parameter types; keep sink
+// parameters at the documented widths (uint16_t handles and lengths).
+template<typename S>
+concept GattClientEventSinkContract = requires(S sink, const uint8_t *data) {
+  { sink.on_connection_state(true, uint16_t{}, int{}) } -> std::same_as<void>;
+  { sink.on_service_discovery_done(int{}) } -> std::same_as<void>;
+  { sink.on_read_result(uint16_t{}, data, uint16_t{}, int{}) } -> std::same_as<void>;
+  { sink.on_write_result(uint16_t{}, int{}) } -> std::same_as<void>;
+  { sink.on_notify_state(uint16_t{}, true, int{}) } -> std::same_as<void>;
+  { sink.on_notify_data(uint16_t{}, data, uint16_t{}) } -> std::same_as<void>;
+  { sink.on_pairing_result(int{}) } -> std::same_as<void>;
 };
 
 }  // namespace esphome::ble_device_base

esphome/components/ble_device_base/ble_hub.h

@@ -58,8 +58,9 @@ struct HubCapabilities {
   /// may only see them where the receiver merges per address (Home Assistant does).
   bool merges_scan_response;
   /// GATT client connections are available: the platform has a
-  /// bluetooth_connection backend implementing ble_device_base::BLEGattConnection
-  /// (ble_gatt_client.h). Today: esp32 and rp2.
+  /// bluetooth_connection backend (rp2 binds the BLEGattConnection alias in
+  /// bluetooth_connection_gatt_backend.h; esp32 uses its Bluedroid client).
+  /// Today: esp32 and rp2.
   bool gatt;
   /// request_scan_mode() is honored at runtime. Distinct from active_scan:
   /// a passive-only controller (bk72xx) can never switch, and a hub may

esphome/components/bluetooth_connection/bluetooth_connection_gatt_backend.h

@@ -0,0 +1,64 @@
+// bluetooth_connection_gatt_backend.h
+//
+// Binds ble_device_base::BLEGattConnection to the build's one GATT backend.
+// Backend and consumer both live in this component, so the ladder does too;
+// backends implement ble_gatt_client.h (the neutral contract).
+
+#pragma once
+
+#include "esphome/core/defines.h"
+
+#ifdef USE_BLE_GATT_CLIENT
+
+#include "esphome/components/ble_device_base/ble_gatt_client.h"
+
+#if defined(USE_RP2040_BLE)
+#include "bluetooth_connection_rp2.h"
+#define ESPHOME_BLE_GATT_CONNECTION_TYPE bluetooth_connection::RP2GattClient
+#elif defined(USE_BLE_GATT_CLIENT_STUB_BACKEND)
+// Emitted only by the host unit-test manifest: the tests compile the hub
+// wrapper standalone, so bind a do-nothing backend. Every other backend-less
+// build hits the #error below.
+namespace esphome::bluetooth_connection {
+
+class BluetoothConnection;
+
+class StubGattBackend {
+ public:
+  void set_listener(BluetoothConnection *listener) {}
+  int connect(uint64_t address, uint8_t addr_type) { return ble_device_base::GATT_ERR_NOT_CONNECTED; }
+  int disconnect() { return ble_device_base::GATT_ERR_NOT_CONNECTED; }
+  int discover_services() { return ble_device_base::GATT_ERR_NOT_CONNECTED; }
+  int read_characteristic(uint16_t handle) { return ble_device_base::GATT_ERR_NOT_CONNECTED; }
+  int write_characteristic(uint16_t handle, const uint8_t *data, uint16_t len, bool response) {
+    return ble_device_base::GATT_ERR_NOT_CONNECTED;
+  }
+  int read_descriptor(uint16_t handle) { return ble_device_base::GATT_ERR_NOT_CONNECTED; }
+  int write_descriptor(uint16_t handle, const uint8_t *data, uint16_t len) {
+    return ble_device_base::GATT_ERR_NOT_CONNECTED;
+  }
+  int notify_characteristic(uint16_t handle, bool enable) { return ble_device_base::GATT_ERR_NOT_CONNECTED; }
+  int pair() { return ble_device_base::GATT_ERR_NOT_CONNECTED; }
+  int update_connection_params(uint16_t min_interval, uint16_t max_interval, uint16_t latency, uint16_t timeout) {
+    return ble_device_base::GATT_ERR_NOT_CONNECTED;
+  }
+  ble_device_base::GattServiceTable get_service_table() { return {}; }
+  void release_services() {}
+};
+
+}  // namespace esphome::bluetooth_connection
+#define ESPHOME_BLE_GATT_CONNECTION_TYPE bluetooth_connection::StubGattBackend
+#else
+#error "USE_BLE_GATT_CLIENT is set but this build has no GATT backend; add an alias arm here"
+#endif
+
+namespace esphome::ble_device_base {
+
+using BLEGattConnection = ESPHOME_BLE_GATT_CONNECTION_TYPE;
+static_assert(BLEGattConnectionContract<BLEGattConnection, bluetooth_connection::BluetoothConnection>,
+              "The build's GATT backend is missing part of the BLEGattConnection surface (ble_gatt_client.h)");
+#undef ESPHOME_BLE_GATT_CONNECTION_TYPE
+
+}  // namespace esphome::ble_device_base
+
+#endif  // USE_BLE_GATT_CLIENT

esphome/components/bluetooth_connection/bluetooth_connection_hub.cpp

@@ -96,7 +96,7 @@ void BluetoothConnection::reset_connection_(conn_err_t reason) {
   this->proxy_->reset_connection_slot_(this, reason);
 }
 
-// ---- GattClientEventListener ----
+// ---- backend event sink ----
 
 void BluetoothConnection::on_connection_state(bool connected, uint16_t mtu, int error) {
   if (connected && this->address_ == 0) {

esphome/components/bluetooth_connection/bluetooth_connection_hub.h

@@ -1,6 +1,6 @@
-// Hub-platform BluetoothConnection: drives a platform GATT client backend
-// through the neutral ble_device_base::BLEGattConnection interface and
-// translates its events into the same API messages the esp32 class emits.
+// Hub-platform BluetoothConnection: drives the build's GATT backend (the
+// ble_device_base::BLEGattConnection alias) and translates its events into
+// the same API messages the esp32 class emits.
 // Presents the identical method surface, so the proxy's GATT dispatch
 // compiles against either class unchanged.
 
@@ -13,7 +13,7 @@
 #include "bluetooth_connection.h"
 
 #include "esphome/components/ble_device_base/ble_client_state.h"
-#include "esphome/components/ble_device_base/ble_gatt_client.h"
+#include "bluetooth_connection_gatt_backend.h"
 #include "esphome/core/helpers.h"
 
 namespace esphome::bluetooth_proxy {
@@ -25,7 +25,7 @@ namespace esphome::bluetooth_connection {
 using ClientState = ble_device_base::ClientState;
 using ConnectionType = ble_device_base::ConnectionType;
 
-class BluetoothConnection final : public ble_device_base::GattClientEventListener {
+class BluetoothConnection final {
  public:
   /// Wire the platform backend. Called from codegen before setup.
   void set_backend(ble_device_base::BLEGattConnection *backend) {
@@ -83,14 +83,14 @@ class BluetoothConnection final : public ble_device_base::GattClientEventListene
     this->check_disconnect_timeout_();
   }
 
-  // ---- ble_device_base::GattClientEventListener ----
-  void on_connection_state(bool connected, uint16_t mtu, int error) override;
-  void on_service_discovery_done(int error) override;
-  void on_read_result(uint16_t handle, const uint8_t *data, uint16_t len, int error) override;
-  void on_write_result(uint16_t handle, int error) override;
-  void on_notify_state(uint16_t handle, bool enabled, int error) override;
-  void on_notify_data(uint16_t handle, const uint8_t *data, uint16_t len) override;
-  void on_pairing_result(int status) override;
+  // ---- backend event sink (called directly by the backend, main loop) ----
+  void on_connection_state(bool connected, uint16_t mtu, int error);
+  void on_service_discovery_done(int error);
+  void on_read_result(uint16_t handle, const uint8_t *data, uint16_t len, int error);
+  void on_write_result(uint16_t handle, int error);
+  void on_notify_state(uint16_t handle, bool enabled, int error);
+  void on_notify_data(uint16_t handle, const uint8_t *data, uint16_t len);
+  void on_pairing_result(int status);
 
  protected:
   friend class bluetooth_proxy::BluetoothProxy;
@@ -102,8 +102,7 @@ class BluetoothConnection final : public ble_device_base::GattClientEventListene
   conn_err_t check_connected_op_(const char *action, const char *type) const;
   void log_gatt_operation_error_(const char *operation, uint16_t handle, int status);
 
-  // Memory optimized layout for 32-bit systems (a vptr precedes: pointers and
-  // 2-byte members first fill to an 8-byte boundary before address_)
+  // Memory optimized layout for 32-bit systems
   // Group 1: Pointers (4 bytes each, naturally aligned)
   bluetooth_proxy::BluetoothProxy *proxy_{nullptr};
   ble_device_base::BLEGattConnection *backend_{nullptr};
@@ -129,6 +128,9 @@ class BluetoothConnection final : public ble_device_base::GattClientEventListene
   bool services_discovered_{false};
 };
 
+static_assert(ble_device_base::GattClientEventSinkContract<BluetoothConnection>,
+              "The hub wrapper is missing part of the event-sink surface (ble_gatt_client.h)");
+
 }  // namespace esphome::bluetooth_connection
 
 #endif  // !USE_ESP32 && USE_BLE_GATT_CLIENT

esphome/components/bluetooth_connection/bluetooth_connection_rp2.cpp

@@ -1,4 +1,6 @@
 #include "bluetooth_connection_rp2.h"
+
+#include "bluetooth_connection_hub.h"
 #include "bluetooth_connection.h"
 
 #if defined(USE_RP2040_BLE) && defined(USE_BLE_GATT_CLIENT)

esphome/components/bluetooth_connection/bluetooth_connection_rp2.h

@@ -1,11 +1,10 @@
 // RP2 (Pico W / Pico 2 W) GATT client backend over BTstack.
 //
-// Implements ble_device_base::BLEGattConnection for the hub BluetoothConnection
-// wrapper. BTstack packet handlers run in the CYW43 async-context low-priority
-// IRQ (or on the main-loop stack during BluetoothLock release), so handlers
-// only copy into per-instance lock-free queues/storage; loop() drains them and
-// drives the state machine. Every BTstack call issued from the main loop is
-// wrapped in BluetoothLock.
+// The build's ble_device_base::BLEGattConnection backend (bound by alias in
+// bluetooth_connection_gatt_backend.h) for the hub BluetoothConnection wrapper. BTstack packet handlers run in the
+// CYW43 async-context low-priority IRQ (or on the main-loop stack during BluetoothLock release), so handlers only copy
+// into per-instance lock-free queues/storage; loop() drains them and drives the state machine. Every BTstack call
+// issued from the main loop is wrapped in BluetoothLock.
 
 #pragma once
 
@@ -27,6 +26,8 @@
 
 namespace esphome::bluetooth_connection {
 
+class BluetoothConnection;
+
 // Caps for the transient service table. Sized generously for real devices
 // (typical peripherals expose < 8 services / < 30 characteristics); a peer
 // exceeding a cap fails discovery with INSUFFICIENT_RESOURCES rather than
@@ -72,29 +73,28 @@ static constexpr uint8_t RP2_GATT_EVENT_QUEUE_SIZE = 8;
 // full 512 B ATT payload, so depth buys burst tolerance at ~516 B per slot.
 static constexpr uint8_t RP2_GATT_NOTIFY_QUEUE_SIZE = 4;
 
-class RP2GattClient final : public Component,
-                            public ble_device_base::BLEGattConnection,
-                            public Parented<rp2040_ble::RP2040BLE> {
+class RP2GattClient final : public Component, public Parented<rp2040_ble::RP2040BLE> {
  public:
   void setup() override;
   void loop() override;
   void dump_config() override;
   float get_setup_priority() const override;
 
-  // ---- ble_device_base::BLEGattConnection ----
-  int connect(uint64_t address, uint8_t addr_type) override;
-  int disconnect() override;
-  int discover_services() override;
-  int read_characteristic(uint16_t handle) override;
-  int write_characteristic(uint16_t handle, const uint8_t *data, uint16_t len, bool response) override;
-  int read_descriptor(uint16_t handle) override;
-  int write_descriptor(uint16_t handle, const uint8_t *data, uint16_t len) override;
-  int notify_characteristic(uint16_t handle, bool enable) override;
-  int pair() override;
-  int update_connection_params(uint16_t min_interval, uint16_t max_interval, uint16_t latency,
-                               uint16_t timeout) override;
-  ble_device_base::GattServiceTable get_service_table() override;
-  void release_services() override;
+  void set_listener(BluetoothConnection *listener) { this->listener_ = listener; }
+
+  // ---- ble_device_base::BLEGattConnection contract ----
+  int connect(uint64_t address, uint8_t addr_type);
+  int disconnect();
+  int discover_services();
+  int read_characteristic(uint16_t handle);
+  int write_characteristic(uint16_t handle, const uint8_t *data, uint16_t len, bool response);
+  int read_descriptor(uint16_t handle);
+  int write_descriptor(uint16_t handle, const uint8_t *data, uint16_t len);
+  int notify_characteristic(uint16_t handle, bool enable);
+  int pair();
+  int update_connection_params(uint16_t min_interval, uint16_t max_interval, uint16_t latency, uint16_t timeout);
+  ble_device_base::GattServiceTable get_service_table();
+  void release_services();
 
  protected:
   // Link/engine state. Discovery and GATT ops have their own cursors below —
@@ -150,6 +150,7 @@ class RP2GattClient final : public Component,
   }
 
   // Group 1: containers / large storage
+  BluetoothConnection *listener_{nullptr};
   ServiceArena *arena_{nullptr};
   esphome::LockFreeQueue<RP2GattEvent, RP2_GATT_EVENT_QUEUE_SIZE> event_queue_;
   esphome::EventPool<RP2GattEvent, RP2_GATT_EVENT_QUEUE_SIZE - 1> event_pool_;

tests/components/ble_device_base/test_gatt_client_contract.cpp

@@ -1,5 +1,8 @@
 // The GATT client contract compiles in no real build until a hub backend is
 // configured; this TU pins it on the host so the header cannot rot unseen.
+// The contract is a concept (BLEGattConnection is a per-platform alias), so
+// the minimal backend here proves the concept stays satisfiable and routes
+// events through the duck-typed sink the way a real backend does.
 #define USE_BLE_GATT_CLIENT
 
 #include "esphome/components/ble_device_base/ble_gatt_client.h"
@@ -8,57 +11,57 @@
 
 namespace esphome::ble_device_base::testing {
 
-class RecordingListener : public GattClientEventListener {
- public:
-  void on_connection_state(bool connected, uint16_t mtu, int error) override { this->connected_ = connected; }
-  void on_service_discovery_done(int error) override { this->discovery_error_ = error; }
-  void on_read_result(uint16_t handle, const uint8_t *data, uint16_t len, int error) override {}
-  void on_write_result(uint16_t handle, int error) override {}
-  void on_notify_state(uint16_t handle, bool enabled, int error) override {}
-  void on_notify_data(uint16_t handle, const uint8_t *data, uint16_t len) override {}
+struct RecordingSink {
+  void on_connection_state(bool connected, uint16_t mtu, int error) { this->connected_ = connected; }
+  void on_service_discovery_done(int error) { this->discovery_error_ = error; }
+  void on_read_result(uint16_t handle, const uint8_t *data, uint16_t len, int error) {}
+  void on_write_result(uint16_t handle, int error) {}
+  void on_notify_state(uint16_t handle, bool enabled, int error) {}
+  void on_notify_data(uint16_t handle, const uint8_t *data, uint16_t len) {}
+  void on_pairing_result(int status) {}
   bool connected_{false};
   int discovery_error_{0};
 };
 
-class MinimalConnection : public BLEGattConnection {
+static_assert(GattClientEventSinkContract<RecordingSink>, "the recording sink must cover the full event-sink surface");
+
+class MinimalConnection {
  public:
-  int connect(uint64_t address, uint8_t addr_type) override {
+  void set_listener(RecordingSink *listener) { this->listener_ = listener; }
+
+  int connect(uint64_t address, uint8_t addr_type) {
     if (this->listener_ != nullptr)
       this->listener_->on_connection_state(true, 517, 0);
     return 0;
   }
-  int disconnect() override { return 0; }
-  int discover_services() override {
+  int disconnect() { return 0; }
+  int discover_services() {
     if (this->listener_ != nullptr)
       this->listener_->on_service_discovery_done(0);
     return 0;
   }
-  int read_characteristic(uint16_t handle) override { return GATT_ERR_NOT_CONNECTED; }
-  int write_characteristic(uint16_t handle, const uint8_t *data, uint16_t len, bool response) override { return 0; }
-  int read_descriptor(uint16_t handle) override { return 0; }
-  int write_descriptor(uint16_t handle, const uint8_t *data, uint16_t len) override { return 0; }
-  int notify_characteristic(uint16_t handle, bool enable) override { return 0; }
-  int update_connection_params(uint16_t min_interval, uint16_t max_interval, uint16_t latency,
-                               uint16_t timeout) override {
+  int read_characteristic(uint16_t handle) { return GATT_ERR_NOT_CONNECTED; }
+  int write_characteristic(uint16_t handle, const uint8_t *data, uint16_t len, bool response) { return 0; }
+  int read_descriptor(uint16_t handle) { return 0; }
+  int write_descriptor(uint16_t handle, const uint8_t *data, uint16_t len) { return 0; }
+  int notify_characteristic(uint16_t handle, bool enable) { return 0; }
+  int pair() { return GATT_ERR_NOT_CONNECTED; }
+  int update_connection_params(uint16_t min_interval, uint16_t max_interval, uint16_t latency, uint16_t timeout) {
     return 0;
   }
-  GattServiceTable get_service_table() override { return {}; }
-  void release_services() override {}
+  GattServiceTable get_service_table() { return {}; }
+  void release_services() {}
+
+ protected:
+  RecordingSink *listener_{nullptr};
 };
 
-TEST(BleGattClientContract, PairingDefaultsAreSafeForNonPairingBackends) {
-  // pair() defaults to not-connected and on_pairing_result() to a no-op, so
-  // a backend without pairing still answers the client through the dispatch.
-  MinimalConnection conn;
-  RecordingListener listener;
-  conn.set_listener(&listener);
-  EXPECT_EQ(conn.pair(), GATT_ERR_NOT_CONNECTED);
-  listener.on_pairing_result(0);  // must not crash: default body
-}
+static_assert(BLEGattConnectionContract<MinimalConnection, RecordingSink>,
+              "a minimal backend must satisfy the contract the alias asserts");
 
 TEST(BleGattClientContract, MinimalImplementerCompilesAndRoutesEvents) {
   MinimalConnection connection;
-  RecordingListener listener;
+  RecordingSink listener;
   connection.set_listener(&listener);
   EXPECT_EQ(connection.connect(0xAABBCCDDEEFFULL, 0), 0);
   EXPECT_TRUE(listener.connected_);

tests/components/bluetooth_connection/__init__.py

@@ -9,6 +9,7 @@ def override_manifest(manifest: ComponentManifestOverride) -> None:
         # These defines are global to the merged host test binary; safe
         # because no co-compiled test observes them.
         cg.add_define("USE_BLE_GATT_CLIENT")
+        cg.add_define("USE_BLE_GATT_CLIENT_STUB_BACKEND")
         cg.add_define("USE_BLUETOOTH_PROXY")
         cg.add_define("BLUETOOTH_PROXY_ADVERTISEMENT_BATCH_SIZE", 16)
         cg.add_define("BLUETOOTH_PROXY_MAX_CONNECTIONS", 1)