[api] Replace std::vector<uint8_t> with APIBuffer to skip zero-fill

Replace std::vector<uint8_t> with a minimal APIBuffer class for the
shared protobuf write buffer, frame helper receive buffer (rx_buf_),
and noise handshake prologue buffer.

std::vector::resize() zero-fills new bytes via memset. Every byte is
immediately overwritten by the protobuf encoder or socket reads,
making the zero-fill pure waste. APIBuffer skips zero-initialization
on resize() and uses make_unique_for_overwrite where available.

Also removes a dead write_raw_ template overload from api_frame_helper.h.

esphome/components/api/api_buffer.cpp

@@ -0,0 +1,13 @@
+#include "api_buffer.h"
+
+namespace esphome::api {
+
+void APIBuffer::grow_(size_t n) {
+  auto new_data = make_buffer(n);
+  if (this->size_)
+    std::memcpy(new_data.get(), this->data_.get(), this->size_);
+  this->data_ = std::move(new_data);
+  this->capacity_ = n;
+}
+
+}  // namespace esphome::api

esphome/components/api/api_buffer.h

@@ -0,0 +1,62 @@
+#pragma once
+
+#include <cstdint>
+#include <cstring>
+#include <memory>
+
+#include "esphome/core/defines.h"
+#include "esphome/core/helpers.h"
+
+namespace esphome::api {
+
+/// Helper to use make_unique_for_overwrite where available (skips zero-fill),
+/// falling back to make_unique on older GCC (ESP8266, BK72xx, LN882x).
+inline std::unique_ptr<uint8_t[]> make_buffer(size_t n) {
+#if defined(USE_ESP8266) || defined(USE_BK72XX) || defined(USE_LN882X)
+  return std::make_unique<uint8_t[]>(n);
+#else
+  return std::make_unique_for_overwrite<uint8_t[]>(n);
+#endif
+}
+
+/// Byte buffer that skips zero-initialization on resize().
+///
+/// std::vector<uint8_t>::resize() zero-fills new bytes via memset. For the
+/// shared protobuf write buffer, every byte is overwritten by the encoder,
+/// making the zero-fill pure waste. For the receive buffer, bytes are
+/// overwritten by socket reads.
+///
+/// Safe because: callers always write exactly the number of bytes they
+/// resize for, and debug_check_bounds_ validates writes in debug builds.
+class APIBuffer {
+ public:
+  void clear() { this->size_ = 0; }
+  inline void reserve(size_t n) ESPHOME_ALWAYS_INLINE {
+    if (n > this->capacity_)
+      this->grow_(n);
+  }
+  inline void resize(size_t n) ESPHOME_ALWAYS_INLINE {
+    this->reserve(n);
+    this->size_ = n;  // no zero-fill
+  }
+  uint8_t *data() { return this->data_.get(); }
+  const uint8_t *data() const { return this->data_.get(); }
+  size_t size() const { return this->size_; }
+  bool empty() const { return this->size_ == 0; }
+  uint8_t &operator[](size_t i) { return this->data_[i]; }
+  const uint8_t &operator[](size_t i) const { return this->data_[i]; }
+  /// Release all memory (equivalent to std::vector swap trick).
+  void release() {
+    this->data_.reset();
+    this->size_ = 0;
+    this->capacity_ = 0;
+  }
+
+ protected:
+  void grow_(size_t n);
+  std::unique_ptr<uint8_t[]> data_;
+  size_t size_{0};
+  size_t capacity_{0};
+};
+
+}  // namespace esphome::api

esphome/components/api/api_connection.cpp

@@ -1915,7 +1915,7 @@ uint16_t APIConnection::encode_to_buffer(uint32_t calculated_size, MessageEncode
   if (total_calculated_size > remaining_size)
     return 0;  // Doesn't fit
 
-  std::vector<uint8_t> &shared_buf = conn->parent_->get_shared_buffer_ref();
+  auto &shared_buf = conn->parent_->get_shared_buffer_ref();
 
   if (conn->flags_.batch_first_message) {
     // First message - buffer already prepared by caller, just clear flag
@@ -2083,7 +2083,7 @@ void APIConnection::process_batch_() {
 
 // Separated from process_batch_() so the single-message fast path gets a minimal
 // stack frame without the MAX_MESSAGES_PER_BATCH * sizeof(MessageInfo) array.
-void APIConnection::process_batch_multi_(std::vector<uint8_t> &shared_buf, size_t num_items, uint8_t header_padding,
+void APIConnection::process_batch_multi_(APIBuffer &shared_buf, size_t num_items, uint8_t header_padding,
                                          uint8_t footer_size) {
   // Ensure MessageInfo remains trivially destructible for our placement new approach
   static_assert(std::is_trivially_destructible<MessageInfo>::value,

esphome/components/api/api_connection.h

@@ -278,7 +278,7 @@ class APIConnection final : public APIServerConnectionBase {
     }
   }
 
-  void prepare_first_message_buffer(std::vector<uint8_t> &shared_buf, size_t header_padding, size_t total_size) {
+  void prepare_first_message_buffer(APIBuffer &shared_buf, size_t header_padding, size_t total_size) {
     shared_buf.clear();
     // Reserve space for header padding + message + footer
     // - Header padding: space for protocol headers (7 bytes for Noise, 6 for Plaintext)
@@ -289,7 +289,7 @@ class APIConnection final : public APIServerConnectionBase {
   }
 
   // Convenience overload - computes frame overhead internally
-  void prepare_first_message_buffer(std::vector<uint8_t> &shared_buf, size_t payload_size) {
+  void prepare_first_message_buffer(APIBuffer &shared_buf, size_t payload_size) {
     const uint8_t header_padding = this->helper_->frame_header_padding();
     const uint8_t footer_size = this->helper_->frame_footer_size();
     this->prepare_first_message_buffer(shared_buf, header_padding, payload_size + header_padding + footer_size);
@@ -669,8 +669,8 @@ class APIConnection final : public APIServerConnectionBase {
 
   bool schedule_batch_();
   void process_batch_();
-  void process_batch_multi_(std::vector<uint8_t> &shared_buf, size_t num_items, uint8_t header_padding,
-                            uint8_t footer_size) __attribute__((noinline));
+  void process_batch_multi_(APIBuffer &shared_buf, size_t num_items, uint8_t header_padding, uint8_t footer_size)
+      __attribute__((noinline));
   void clear_batch_() {
     this->deferred_batch_.clear();
     this->flags_.batch_scheduled = false;

esphome/components/api/api_frame_helper.h

@@ -5,10 +5,10 @@
 #include <memory>
 #include <span>
 #include <utility>
-#include <vector>
 
 #include "esphome/core/defines.h"
 #ifdef USE_API
+#include "esphome/components/api/api_buffer.h"
 #include "esphome/components/socket/socket.h"
 #include "esphome/core/application.h"
 #include "esphome/core/log.h"
@@ -179,7 +179,7 @@ class APIFrameHelper {
     // and clearing would lose partially received data.
     if (this->rx_buf_len_ == 0) {
       // Use swap trick since shrink_to_fit() is non-binding and may be ignored
-      std::vector<uint8_t>().swap(this->rx_buf_);
+      this->rx_buf_.release();
     }
   }
 
@@ -206,9 +206,6 @@ class APIFrameHelper {
 
   // Common socket write error handling
   APIError handle_socket_write_error_();
-  template<typename StateEnum>
-  APIError write_raw_(const struct iovec *iov, int iovcnt, socket::Socket *socket, std::vector<uint8_t> &tx_buf,
-                      const std::string &info, StateEnum &state, StateEnum failed_state);
 
   // Socket ownership (4 bytes on 32-bit, 8 bytes on 64-bit)
   std::unique_ptr<socket::Socket> socket_;
@@ -234,7 +231,7 @@ class APIFrameHelper {
 
   // Containers (size varies, but typically 12+ bytes on 32-bit)
   std::array<std::unique_ptr<SendBuffer>, API_MAX_SEND_QUEUE> tx_buf_;
-  std::vector<uint8_t> rx_buf_;
+  APIBuffer rx_buf_;
 
   // Client name buffer - stores name from Hello message or initial peername
   char client_name_[CLIENT_INFO_NAME_MAX_LEN]{};

esphome/components/api/api_frame_helper_noise.cpp

@@ -579,8 +579,7 @@ APIError APINoiseFrameHelper::init_handshake_() {
   if (aerr != APIError::OK)
     return aerr;
   // set_prologue copies it into handshakestate, so we can get rid of it now
-  // Use swap idiom to actually release memory (= {} only clears size, not capacity)
-  std::vector<uint8_t>().swap(prologue_);
+  prologue_.release();
 
   err = noise_handshakestate_start(handshake_);
   aerr = handle_noise_error_(err, LOG_STR("noise_handshakestate_start"), APIError::HANDSHAKESTATE_SETUP_FAILED);

esphome/components/api/api_frame_helper_noise.h

@@ -43,8 +43,8 @@ class APINoiseFrameHelper final : public APIFrameHelper {
   // Reference to noise context (4 bytes on 32-bit)
   APINoiseContext &ctx_;
 
-  // Vector (12 bytes on 32-bit)
-  std::vector<uint8_t> prologue_;
+  // Buffer for noise handshake prologue (released after handshake)
+  APIBuffer prologue_;
 
   // NoiseProtocolId (size depends on implementation)
   NoiseProtocolId nid_;

esphome/components/api/api_server.h

@@ -65,7 +65,7 @@ class APIServer : public Component,
   void set_max_connections(uint8_t max_connections) { this->max_connections_ = max_connections; }
 
   // Get reference to shared buffer for API connections
-  std::vector<uint8_t> &get_shared_buffer_ref() { return shared_write_buffer_; }
+  APIBuffer &get_shared_buffer_ref() { return shared_write_buffer_; }
 
 #ifdef USE_API_NOISE
   bool save_noise_psk(psk_t psk, bool make_active = true);
@@ -276,7 +276,7 @@ class APIServer : public Component,
   // Not pre-allocated: all send paths call prepare_first_message_buffer() which
   // reserves the exact needed size. Pre-allocating here would cause heap fragmentation
   // since the buffer would almost always reallocate on first use.
-  std::vector<uint8_t> shared_write_buffer_;
+  APIBuffer shared_write_buffer_;
 #ifdef USE_API_HOMEASSISTANT_STATES
   std::vector<HomeAssistantStateSubscription> state_subs_;
 #endif

esphome/components/api/proto.h

@@ -1,6 +1,7 @@
 #pragma once
 
 #include "api_pb2_defines.h"
+#include "api_buffer.h"
 #include "esphome/core/component.h"
 #include "esphome/core/helpers.h"
 #include "esphome/core/log.h"
@@ -237,9 +238,8 @@ class Proto32Bit {
 
 class ProtoWriteBuffer {
  public:
-  ProtoWriteBuffer(std::vector<uint8_t> *buffer) : buffer_(buffer), pos_(buffer->data() + buffer->size()) {}
-  ProtoWriteBuffer(std::vector<uint8_t> *buffer, size_t write_pos)
-      : buffer_(buffer), pos_(buffer->data() + write_pos) {}
+  ProtoWriteBuffer(APIBuffer *buffer) : buffer_(buffer), pos_(buffer->data() + buffer->size()) {}
+  ProtoWriteBuffer(APIBuffer *buffer, size_t write_pos) : buffer_(buffer), pos_(buffer->data() + write_pos) {}
   void encode_varint_raw(uint32_t value) {
     while (value > 0x7F) {
       this->debug_check_bounds_(1);
@@ -375,7 +375,7 @@ class ProtoWriteBuffer {
   // Non-template core for encode_optional_sub_message.
   void encode_optional_sub_message(uint32_t field_id, uint32_t nested_size, const void *value,
                                    void (*encode_fn)(const void *, ProtoWriteBuffer &));
-  std::vector<uint8_t> *get_buffer() const { return buffer_; }
+  APIBuffer *get_buffer() const { return buffer_; }
 
  protected:
 #ifdef ESPHOME_DEBUG_API
@@ -385,7 +385,7 @@ class ProtoWriteBuffer {
   void debug_check_bounds_([[maybe_unused]] size_t bytes) {}
 #endif
 
-  std::vector<uint8_t> *buffer_;
+  APIBuffer *buffer_;
   uint8_t *pos_;
 };
 

tests/components/api/test.ln882x-ard.yaml

@@ -0,0 +1,5 @@
+<<: !include common.yaml
+
+wifi:
+  ssid: MySSID
+  password: password1