[api] Break api_connection/api_server include cycle to drop custom unique_ptr deleter

api_connection.h now forward-declares APIServer instead of including
api_server.h, and api_server.h includes api_connection.h before clients_
is declared. With APIConnection complete at that point, libc++ no longer
trips the incomplete-type assert in default_delete<APIConnection> when
parsing std::array<unique_ptr<APIConnection>, MAX_API_CONNECTIONS>, so
the custom APIConnectionDeleter introduced in #16050 is no longer needed.

The two APIConnection inline methods that touched APIServer
(encode_to_buffer, get_batch_delay_ms_) are now defined at the bottom of
api_server.h, where APIServer is complete; api_connection.cpp gains a
direct include of api_server.h so those definitions are visible there.

Verified locally on macOS host platform with python3.14 -m esphome
compile (the path that reproduces the libc++ error). Noise and plaintext
integration tests pass.

esphome/components/api/api_connection.cpp

@@ -1,5 +1,6 @@
 #include "api_connection.h"
 #ifdef USE_API
+#include "api_server.h"  // brings APIServer complete for encode_to_buffer/get_batch_delay_ms_
 #ifdef USE_API_NOISE
 #include "api_frame_helper_noise.h"
 #endif

esphome/components/api/api_connection.h

@@ -11,7 +11,8 @@
 #endif
 #include "api_pb2.h"
 #include "api_pb2_service.h"
-#include "api_server.h"
+#include "list_entities.h"
+#include "subscribe_state.h"
 #include "esphome/core/application.h"
 #include "esphome/core/component.h"
 #ifdef USE_ESP32_CRASH_HANDLER
@@ -36,6 +37,11 @@ class ComponentIterator;
 
 namespace esphome::api {
 
+// Forward declaration to break the api_connection.h <-> api_server.h include cycle.
+// APIServer is only used through pointers/references in this header; methods that
+// need the complete type are defined inline at the bottom of api_server.h.
+class APIServer;
+
 // Keepalive timeout in milliseconds
 static constexpr uint32_t KEEPALIVE_TIMEOUT_MS = 60000;
 // Maximum number of entities to process in a single batch during initial state/info sending
@@ -413,42 +419,10 @@ class APIConnection final : public APIServerConnectionBase {
 
   // Core batch encoding logic. Computes header size, checks fit, resizes buffer, encodes.
   // ALWAYS_INLINE so the compiler can devirtualize encode_fn at hot call sites.
-  static inline uint16_t ESPHOME_ALWAYS_INLINE encode_to_buffer(uint32_t calculated_size, MessageEncodeFn encode_fn,
-                                                                const void *msg, APIConnection *conn,
-                                                                uint32_t remaining_size) {
-#ifdef HAS_PROTO_MESSAGE_DUMP
-    if (conn->flags_.log_only_mode) {
-      auto *proto_msg = static_cast<const ProtoMessage *>(msg);
-      DumpBuffer dump_buf;
-      conn->log_send_message_(proto_msg->message_name(), proto_msg->dump_to(dump_buf));
-      return 1;
-    }
-#endif
-    const uint8_t footer_size = conn->helper_->frame_footer_size();
-
-    // First message uses max padding (already in buffer), subsequent use exact header size
-    size_t to_add;
-    if (conn->flags_.batch_first_message) {
-      conn->flags_.batch_first_message = false;
-      conn->batch_header_size_ = conn->helper_->frame_header_padding();
-      to_add = calculated_size;
-    } else {
-      conn->batch_header_size_ = conn->helper_->frame_header_size(calculated_size, conn->batch_message_type_);
-      to_add = calculated_size + conn->batch_header_size_ + footer_size;
-    }
-
-    // Check if it fits (using actual header size, not max padding)
-    uint16_t total_calculated_size = calculated_size + conn->batch_header_size_ + footer_size;
-    if (total_calculated_size > remaining_size)
-      return 0;
-
-    auto &shared_buf = conn->parent_->get_shared_buffer_ref();
-    shared_buf.resize(shared_buf.size() + to_add);
-    ProtoWriteBuffer buffer{&shared_buf, shared_buf.size() - calculated_size};
-    encode_fn(msg, buffer PROTO_ENCODE_DEBUG_INIT(&shared_buf));
-
-    return total_calculated_size;
-  }
+  // Definition is at the bottom of api_server.h, where APIServer is a complete type
+  // (needed for conn->parent_->get_shared_buffer_ref()). Callers must include api_server.h.
+  static uint16_t ESPHOME_ALWAYS_INLINE encode_to_buffer(uint32_t calculated_size, MessageEncodeFn encode_fn,
+                                                         const void *msg, APIConnection *conn, uint32_t remaining_size);
 
   // Noinline version of encode_to_buffer for cold paths (entity info, zero-payload messages).
   // All cold callers share this single copy instead of each getting an ALWAYS_INLINE expansion.
@@ -792,7 +766,8 @@ class APIConnection final : public APIServerConnectionBase {
   // Read by process_batch_multi_ to pass into MessageInfo.
   uint8_t batch_header_size_{0};
 
-  uint32_t get_batch_delay_ms_() const { return this->parent_->get_batch_delay(); }
+  // Definition at the bottom of api_server.h (needs APIServer complete).
+  uint32_t get_batch_delay_ms_() const;
   // Message will use 8 more bytes than the minimum size, and typical
   // MTU is 1500. Sometimes users will see as low as 1460 MTU.
   // If its IPv6 the header is 40 bytes, and if its IPv4

esphome/components/api/api_server.cpp

@@ -30,11 +30,6 @@ APIServer *global_api_server = nullptr;  // NOLINT(cppcoreguidelines-avoid-non-c
 
 APIServer::APIServer() { global_api_server = this; }
 
-// Custom deleter defined here so `delete` sees the complete APIConnection type.
-// This prevents libc++ from emitting an "incomplete type" error when other
-// translation units only have the forward declaration of APIConnection.
-void APIServer::APIConnectionDeleter::operator()(APIConnection *p) const { delete p; }
-
 void APIServer::socket_failed_(const LogString *msg) {
   ESP_LOGW(TAG, "Socket %s: errno %d", LOG_STR_ARG(msg), errno);
   this->destroy_socket_();

esphome/components/api/api_server.h

@@ -3,6 +3,12 @@
 #include "esphome/core/defines.h"
 #ifdef USE_API
 #include "api_buffer.h"
+// api_connection.h must be included before clients_ is declared so APIConnection
+// is a complete type when std::unique_ptr<APIConnection> is instantiated. libc++
+// otherwise trips static_assert(sizeof(_Tp) >= 0, "cannot delete an incomplete
+// type") while parsing the array member. api_connection.h transitively brings in
+// list_entities.h and subscribe_state.h.
+#include "api_connection.h"
 #include "api_noise_context.h"
 #include "api_pb2.h"
 #include "api_pb2_service.h"
@@ -12,8 +18,6 @@
 #include "esphome/core/controller.h"
 #include "esphome/core/log.h"
 #include "esphome/core/string_ref.h"
-#include "list_entities.h"
-#include "subscribe_state.h"
 #ifdef USE_LOGGER
 #include "esphome/components/logger/logger.h"
 #endif
@@ -191,15 +195,9 @@ class APIServer final : public Component,
   bool is_connected_with_state_subscription() const;
 
   // Range-for view over the populated slice [0, api_connection_count_). Read-only with respect
-  // to ownership — callers get `const unique_ptr&` so they can invoke non-const methods on the
+  // to ownership; callers get `const unique_ptr&` so they can invoke non-const methods on the
   // APIConnection but cannot reset/move the slot and break the count invariant.
-  // Custom deleter is defined out-of-line in api_server.cpp so libc++ does not
-  // eagerly instantiate `delete static_cast<APIConnection *>(p)` here, where
-  // only the forward declaration of APIConnection is visible (incomplete type).
-  struct APIConnectionDeleter {
-    void operator()(APIConnection *p) const;
-  };
-  using APIConnectionPtr = std::unique_ptr<APIConnection, APIConnectionDeleter>;
+  using APIConnectionPtr = std::unique_ptr<APIConnection>;
   class ActiveClientsView {
     const APIConnectionPtr *begin_;
     const APIConnectionPtr *end_;
@@ -359,5 +357,49 @@ template<typename... Ts> class APIConnectedCondition : public Condition<Ts...> {
   }
 };
 
+// Inline definitions for APIConnection members declared in api_connection.h
+// that need the complete APIServer type. Defined here, after APIServer is
+// complete, so api_connection.h can break the include cycle by only
+// forward-declaring APIServer.
+
+inline uint16_t ESPHOME_ALWAYS_INLINE APIConnection::encode_to_buffer(uint32_t calculated_size,
+                                                                      MessageEncodeFn encode_fn, const void *msg,
+                                                                      APIConnection *conn, uint32_t remaining_size) {
+#ifdef HAS_PROTO_MESSAGE_DUMP
+  if (conn->flags_.log_only_mode) {
+    auto *proto_msg = static_cast<const ProtoMessage *>(msg);
+    DumpBuffer dump_buf;
+    conn->log_send_message_(proto_msg->message_name(), proto_msg->dump_to(dump_buf));
+    return 1;
+  }
+#endif
+  const uint8_t footer_size = conn->helper_->frame_footer_size();
+
+  // First message uses max padding (already in buffer), subsequent use exact header size
+  size_t to_add;
+  if (conn->flags_.batch_first_message) {
+    conn->flags_.batch_first_message = false;
+    conn->batch_header_size_ = conn->helper_->frame_header_padding();
+    to_add = calculated_size;
+  } else {
+    conn->batch_header_size_ = conn->helper_->frame_header_size(calculated_size, conn->batch_message_type_);
+    to_add = calculated_size + conn->batch_header_size_ + footer_size;
+  }
+
+  // Check if it fits (using actual header size, not max padding)
+  uint16_t total_calculated_size = calculated_size + conn->batch_header_size_ + footer_size;
+  if (total_calculated_size > remaining_size)
+    return 0;
+
+  auto &shared_buf = conn->parent_->get_shared_buffer_ref();
+  shared_buf.resize(shared_buf.size() + to_add);
+  ProtoWriteBuffer buffer{&shared_buf, shared_buf.size() - calculated_size};
+  encode_fn(msg, buffer PROTO_ENCODE_DEBUG_INIT(&shared_buf));
+
+  return total_calculated_size;
+}
+
+inline uint32_t APIConnection::get_batch_delay_ms_() const { return this->parent_->get_batch_delay(); }
+
 }  // namespace esphome::api
 #endif