diff --git a/esphome/components/api/api_connection.cpp b/esphome/components/api/api_connection.cpp
index 9714f8bd609..d9bb2f5fd44 100644
--- a/esphome/components/api/api_connection.cpp
+++ b/esphome/components/api/api_connection.cpp
@@ -1705,8 +1705,10 @@ void APIConnection::on_home_assistant_state_response(const HomeAssistantStateRes
   // Null-terminate state in-place for safe c_str() usage (e.g., parse_number in callbacks).
   // Safe: decode is complete, byte after string data was already consumed during parse,
   // and frame helpers reserve RX_BUF_NULL_TERMINATOR extra byte in rx_buf_.
+  // const_cast is safe: msg references rx_buf_ data which is mutable; the const& handler
+  // signature is a generated protobuf pattern, not a true immutability contract.
   if (!msg.state.empty()) {
-    msg.state.null_terminate_in_place();
+    const_cast<StringRef &>(msg.state).null_terminate_in_place();
   }
 
   for (auto &it : this->parent_->get_state_subs()) {
@@ -1729,7 +1731,9 @@ void APIConnection::on_execute_service_request(const ExecuteServiceRequest &msg)
   // Null-terminate string args in-place for safe c_str() usage in YAML service triggers.
   // Safe: full ExecuteServiceRequest decode is complete, all bytes in rx_buf_ consumed,
   // and frame helpers reserve RX_BUF_NULL_TERMINATOR extra byte for the last field.
-  for (auto &arg : msg.args) {
+  // const_cast is safe: msg references rx_buf_ data which is mutable; the const& handler
+  // signature is a generated protobuf pattern, not a true immutability contract.
+  for (auto &arg : const_cast<ExecuteServiceRequest &>(msg).args) {
     if (!arg.string_.empty()) {
       arg.string_.null_terminate_in_place();
     }
diff --git a/esphome/core/string_ref.h b/esphome/core/string_ref.h
index 3a66f3d9d59..d25b59632cd 100644
--- a/esphome/core/string_ref.h
+++ b/esphome/core/string_ref.h
@@ -84,9 +84,7 @@ class StringRef {
   /// Write a null terminator at base_[len_] in-place.
   /// Caller must guarantee that the byte at base_[len_] is writable memory
   /// (e.g., the RX_BUF_NULL_TERMINATOR byte reserved by frame helpers after decode).
-  /// Marked const because StringRef itself is not modified; the underlying buffer
-  /// (owned by frame helper rx_buf_) is mutated via const_cast.
-  void null_terminate_in_place() const { const_cast<char *>(base_)[len_] = '\0'; }
+  void null_terminate_in_place() { const_cast<char *>(base_)[len_] = '\0'; }
 
   /// Find first occurrence of substring, returns std::string::npos if not found.
   /// Note: Requires the underlying string to be null-terminated.