diff --git a/esphome/components/api/api_connection.cpp b/esphome/components/api/api_connection.cpp
index d9bb2f5fd4..e21af787cb 100644
--- a/esphome/components/api/api_connection.cpp
+++ b/esphome/components/api/api_connection.cpp
@@ -1705,10 +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
+  // const_cast is safe: msg references mutable rx_buf_ data; the const& handler
   // signature is a generated protobuf pattern, not a true immutability contract.
   if (!msg.state.empty()) {
-    const_cast<StringRef &>(msg.state).null_terminate_in_place();
+    const_cast<char *>(msg.state.c_str())[msg.state.size()] = '\0';
   }
 
   for (auto &it : this->parent_->get_state_subs()) {
@@ -1731,11 +1731,11 @@ 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.
-  // const_cast is safe: msg references rx_buf_ data which is mutable; the const& handler
+  // const_cast is safe: msg references mutable rx_buf_ data; 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();
+      const_cast<char *>(arg.string_.c_str())[arg.string_.size()] = '\0';
     }
   }
   bool found = false;
diff --git a/esphome/components/api/api_frame_helper_noise.cpp b/esphome/components/api/api_frame_helper_noise.cpp
index 48694702b1..3ae35e9be8 100644
--- a/esphome/components/api/api_frame_helper_noise.cpp
+++ b/esphome/components/api/api_frame_helper_noise.cpp
@@ -194,18 +194,19 @@ APIError APINoiseFrameHelper::try_read_frame_() {
   uint16_t msg_size = (((uint16_t) rx_header_buf_[1]) << 8) | rx_header_buf_[2];
 
   // Check against size limits to prevent OOM: MAX_HANDSHAKE_SIZE for handshake, MAX_MESSAGE_SIZE for data
-  uint16_t limit = (state_ == State::DATA) ? MAX_MESSAGE_SIZE : MAX_HANDSHAKE_SIZE;
+  bool is_data = (state_ == State::DATA);
+  uint16_t limit = is_data ? MAX_MESSAGE_SIZE : MAX_HANDSHAKE_SIZE;
   if (msg_size > limit) {
     state_ = State::FAILED;
     HELPER_LOG("Bad packet: message size %u exceeds maximum %u", msg_size, limit);
-    return (state_ == State::DATA) ? APIError::BAD_DATA_PACKET : APIError::BAD_HANDSHAKE_PACKET_LEN;
+    return is_data ? APIError::BAD_DATA_PACKET : APIError::BAD_HANDSHAKE_PACKET_LEN;
   }
 
   // Reserve space for body (+ null terminator in DATA state so protobuf
   // StringRef fields can be safely null-terminated in-place after decode.
   // During handshake, rx_buf_.size() is used in prologue construction, so
   // the buffer must be exactly msg_size to avoid prologue mismatch.)
-  uint16_t alloc_size = msg_size + (state_ == State::DATA ? RX_BUF_NULL_TERMINATOR : 0);
+  uint16_t alloc_size = msg_size + (is_data ? RX_BUF_NULL_TERMINATOR : 0);
   if (this->rx_buf_.size() != alloc_size) {
     this->rx_buf_.resize(alloc_size);
   }
diff --git a/esphome/core/string_ref.h b/esphome/core/string_ref.h
index d25b59632c..d502c4d27f 100644
--- a/esphome/core/string_ref.h
+++ b/esphome/core/string_ref.h
@@ -81,11 +81,6 @@ class StringRef {
 
   operator std::string() const { return str(); }
 
-  /// 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).
-  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.
   size_type find(const char *s, size_type pos = 0) const {