diff --git a/esphome/components/api/api_connection.cpp b/esphome/components/api/api_connection.cpp
index d09b1107d2..ca6e2a2d56 100644
--- a/esphome/components/api/api_connection.cpp
+++ b/esphome/components/api/api_connection.cpp
@@ -248,25 +248,41 @@ void APIConnection::on_disconnect_response(const DisconnectResponse &value) {
 uint16_t APIConnection::encode_message_to_buffer(ProtoMessage &msg, uint16_t message_type, APIConnection *conn,
                                                  uint32_t remaining_size, bool is_single) {
   // Calculate size
-  uint32_t size = 0;
-  msg.calculate_size(size);
+  uint32_t calculated_size = 0;
+  msg.calculate_size(calculated_size);
+
+  // Cache frame sizes to avoid repeated virtual calls
+  const uint8_t header_padding = conn->helper_->frame_header_padding();
+  const uint8_t footer_size = conn->helper_->frame_footer_size();
 
   // Calculate total size with padding for buffer allocation
-  uint16_t total_size =
-      static_cast<uint16_t>(size) + conn->helper_->frame_header_padding() + conn->helper_->frame_footer_size();
+  size_t total_calculated_size = calculated_size + header_padding + footer_size;
 
   // Check if it fits
-  if (total_size > remaining_size) {
+  if (total_calculated_size > remaining_size) {
     return 0;  // Doesn't fit
   }
 
   // Allocate buffer space - pass payload size, allocation functions add header/footer space
-  ProtoWriteBuffer buffer =
-      is_single ? conn->allocate_single_message_buffer(size) : conn->allocate_batch_message_buffer(size);
+  ProtoWriteBuffer buffer = is_single ? conn->allocate_single_message_buffer(calculated_size)
+                                      : conn->allocate_batch_message_buffer(calculated_size);
+
+  // Get buffer size after allocation (which includes header padding)
+  std::vector<uint8_t> &shared_buf = conn->parent_->get_shared_buffer_ref();
+  size_t size_before_encode = shared_buf.size();
 
   // Encode directly into buffer
   msg.encode(buffer);
-  return total_size;
+
+  // Calculate actual encoded size (not including header that was already added)
+  size_t actual_payload_size = shared_buf.size() - size_before_encode;
+
+  // Return actual total size (header + actual payload + footer)
+  size_t actual_total_size = header_padding + actual_payload_size + footer_size;
+
+  // Verify that calculate_size() returned the correct value
+  assert(calculated_size == actual_payload_size);
+  return static_cast<uint16_t>(actual_total_size);
 }
 
 #ifdef USE_BINARY_SENSOR
diff --git a/tests/integration/conftest.py b/tests/integration/conftest.py
index 4c798c6b72..4eb1584c27 100644
--- a/tests/integration/conftest.py
+++ b/tests/integration/conftest.py
@@ -119,6 +119,21 @@ async def yaml_config(request: pytest.FixtureRequest, unused_tcp_port: int) -> s
         # Add port configuration after api:
         content = content.replace("api:", f"api:\n  port: {unused_tcp_port}")
 
+    # Add debug build flags for integration tests to enable assertions
+    if "esphome:" in content:
+        # Check if platformio_options already exists
+        if "platformio_options:" not in content:
+            # Add platformio_options with debug flags after esphome:
+            content = content.replace(
+                "esphome:",
+                "esphome:\n"
+                "  # Enable assertions for integration tests\n"
+                "  platformio_options:\n"
+                "    build_flags:\n"
+                '      - "-DDEBUG"  # Enable assert() statements\n'
+                '      - "-g"       # Add debug symbols',
+            )
+
     return content