diff --git a/esphome/components/api/api_connection.cpp b/esphome/components/api/api_connection.cpp
index faa39a01a9d..5ee432dc0c6 100644
--- a/esphome/components/api/api_connection.cpp
+++ b/esphome/components/api/api_connection.cpp
@@ -2021,19 +2021,21 @@ uint16_t APIConnection::encode_to_buffer(uint32_t calculated_size, MessageEncode
 
   auto &shared_buf = conn->parent_->get_shared_buffer_ref();
 
+  size_t write_start;
   if (conn->flags_.batch_first_message) {
     // First message - buffer already prepared by caller, just clear flag
     conn->flags_.batch_first_message = false;
+    write_start = shared_buf.size();
+    shared_buf.resize(write_start + calculated_size);
   } else {
     // Batch message second or later
-    // Add padding for previous message footer + this message header
+    // Reserve for full message, resize to include footer gap + header padding + payload
     size_t current_size = shared_buf.size();
-    shared_buf.reserve_and_resize(current_size + total_calculated_size, current_size + footer_size + header_padding);
+    shared_buf.reserve_and_resize(current_size + total_calculated_size,
+                                  current_size + footer_size + header_padding + calculated_size);
+    write_start = shared_buf.size() - calculated_size;
   }
 
-  // Pre-resize buffer to include payload, then encode through raw pointer
-  size_t write_start = shared_buf.size();
-  shared_buf.resize(write_start + calculated_size);
   ProtoWriteBuffer buffer{&shared_buf, write_start};
   encode_fn(msg, buffer);