diff --git a/esphome/components/api/api_connection.cpp b/esphome/components/api/api_connection.cpp
index e4e8333cc71..dc665da27c2 100644
--- a/esphome/components/api/api_connection.cpp
+++ b/esphome/components/api/api_connection.cpp
@@ -331,12 +331,8 @@ uint16_t APIConnection::encode_message_to_buffer(ProtoMessage &msg, uint8_t mess
   std::vector<uint8_t> &shared_buf = conn->parent_->get_shared_buffer_ref();
 
   if (conn->flags_.batch_first_message) {
-    // First message - clear flag
+    // First message - buffer already prepared by caller, just clear flag
     conn->flags_.batch_first_message = false;
-    // If buffer not prepped by caller (batch pre-reserves with size == header_padding), prep now
-    if (shared_buf.size() != header_padding) {
-      conn->prepare_first_message_buffer(shared_buf, header_padding, total_calculated_size);
-    }
   } else {
     // Batch message second or later
     // Add padding for previous message footer + this message header
@@ -1863,6 +1859,7 @@ void APIConnection::process_batch_() {
   // Fast path for single message - allocate exact size needed
   if (num_items == 1) {
     const auto &item = this->deferred_batch_[0];
+    this->prepare_first_message_buffer(shared_buf, item.estimated_size);
 
     // Let dispatch_message_ calculate size and encode if it fits
     uint16_t payload_size = this->dispatch_message_(item, std::numeric_limits<uint16_t>::max(), true);
@@ -1902,9 +1899,10 @@ void APIConnection::process_batch_() {
     total_estimated_size += item.estimated_size;
   }
 
-  // Calculate total overhead for all messages
-  // Reserve based on estimated size (much more accurate than 24-byte worst-case)
+  // Prepare buffer with total estimated size for all messages (already cleared above)
   shared_buf.reserve(total_estimated_size);
+  shared_buf.resize(header_padding);
+  this->flags_.batch_first_message = true;
 
   size_t items_processed = 0;
   uint16_t remaining_size = std::numeric_limits<uint16_t>::max();
diff --git a/esphome/components/api/api_connection.h b/esphome/components/api/api_connection.h
index 2c9202b561b..5f7975d5e74 100644
--- a/esphome/components/api/api_connection.h
+++ b/esphome/components/api/api_connection.h
@@ -653,9 +653,11 @@ class APIConnection final : public APIServerConnection {
   bool send_message_smart_(EntityBase *entity, uint8_t message_type, uint8_t estimated_size,
                            uint8_t aux_data_index = DeferredBatch::AUX_DATA_UNUSED) {
     if (this->should_send_immediately_(message_type) && this->helper_->can_write_without_blocking()) {
+      auto &shared_buf = this->parent_->get_shared_buffer_ref();
+      this->prepare_first_message_buffer(shared_buf, estimated_size);
       DeferredBatch::BatchItem item{entity, message_type, estimated_size, aux_data_index};
       if (this->dispatch_message_(item, MAX_BATCH_PACKET_SIZE, true) &&
-          this->send_buffer(ProtoWriteBuffer{&this->parent_->get_shared_buffer_ref()}, message_type)) {
+          this->send_buffer(ProtoWriteBuffer{&shared_buf}, message_type)) {
 #ifdef HAS_PROTO_MESSAGE_DUMP
         this->log_batch_item_(item);
 #endif