From d4cce142c5352e94772777dfeaf668fbc1533cb4 Mon Sep 17 00:00:00 2001
From: "J. Nick Koston" <nick@koston.org>
Date: Wed, 8 Apr 2026 11:11:31 -1000
Subject: [PATCH] [api] Fix batch messages stuck in Nagle buffer (#15581)

---
 esphome/components/api/api_connection.cpp | 33 ++++++++++++++++-------
 esphome/components/api/api_connection.h   |  1 +
 2 files changed, 25 insertions(+), 9 deletions(-)

diff --git a/esphome/components/api/api_connection.cpp b/esphome/components/api/api_connection.cpp
index 17605345fd..7db423141c 100644
--- a/esphome/components/api/api_connection.cpp
+++ b/esphome/components/api/api_connection.cpp
@@ -315,6 +315,8 @@ void APIConnection::process_active_iterator_() {
       this->destroy_active_iterator_();
       if (this->flags_.state_subscription) {
         this->begin_iterator_(ActiveIterator::INITIAL_STATE);
+      } else {
+        this->finalize_iterator_sync_();
       }
     } else {
       this->process_iterator_batch_(this->iterator_storage_.list_entities);
@@ -322,21 +324,27 @@ void APIConnection::process_active_iterator_() {
   } else {  // INITIAL_STATE
     if (this->iterator_storage_.initial_state.completed()) {
       this->destroy_active_iterator_();
-      // Process any remaining batched messages immediately
-      if (!this->deferred_batch_.empty()) {
-        this->process_batch_();
-      }
-      // Now that everything is sent, enable immediate sending for future state changes
-      this->flags_.should_try_send_immediately = true;
-      // Release excess memory from buffers that grew during initial sync
-      this->deferred_batch_.release_buffer();
-      this->helper_->release_buffers();
+      this->finalize_iterator_sync_();
     } else {
       this->process_iterator_batch_(this->iterator_storage_.initial_state);
     }
   }
 }
 
+void APIConnection::finalize_iterator_sync_() {
+  // Flush any remaining batched messages immediately so clients
+  // receive completion responses (e.g. ListEntitiesDoneResponse)
+  // without waiting for the batch timer.
+  if (!this->deferred_batch_.empty()) {
+    this->process_batch_();
+  }
+  // Enable immediate sending for future state changes
+  this->flags_.should_try_send_immediately = true;
+  // Release excess memory from buffers that grew during initial sync
+  this->deferred_batch_.release_buffer();
+  this->helper_->release_buffers();
+}
+
 void APIConnection::process_iterator_batch_(ComponentIterator &iterator) {
   size_t initial_size = this->deferred_batch_.size();
   size_t max_batch = this->get_max_batch_size_();
@@ -2185,6 +2193,13 @@ void APIConnection::process_batch_multi_(APIBuffer &shared_buf, size_t num_items
       shared_buf.resize(shared_buf.size() + footer_size);
     }
 
+    // Ensure TCP_NODELAY is on before writing batch data.
+    // Log messages enable Nagle (NODELAY off) to coalesce small packets.
+    // Without this, batch data written to the socket sits in LWIP's Nagle
+    // buffer — the remote won't ACK until it sends its own data (e.g. a
+    // ping), which can take 20+ seconds.
+    this->helper_->set_nodelay_for_message(false);
+
     // Send all collected messages
     APIError err = this->helper_->write_protobuf_messages(ProtoWriteBuffer{&shared_buf},
                                                           std::span<const MessageInfo>(message_info, items_processed));
diff --git a/esphome/components/api/api_connection.h b/esphome/components/api/api_connection.h
index f227dbe2de..284c4475de 100644
--- a/esphome/components/api/api_connection.h
+++ b/esphome/components/api/api_connection.h
@@ -662,6 +662,7 @@ class APIConnection final : public APIServerConnectionBase {
   // Helper methods for iterator lifecycle management
   void destroy_active_iterator_();
   void begin_iterator_(ActiveIterator type);
+  void finalize_iterator_sync_();
 #ifdef USE_CAMERA
   std::unique_ptr<camera::CameraImageReader> image_reader_;
 #endif