diff --git a/esphome/components/uart/automation.h b/esphome/components/uart/automation.h
index 7a3344c2f1..c2eb308eb8 100644
--- a/esphome/components/uart/automation.h
+++ b/esphome/components/uart/automation.h
@@ -12,36 +12,33 @@ template<typename... Ts> class UARTWriteAction : public Action<Ts...>, public Pa
  public:
   void set_data_template(std::vector<uint8_t> (*func)(Ts...)) {
     // Stateless lambdas (generated by ESPHome) implicitly convert to function pointers
-    this->data_.func = func;
-    this->static_ = false;
+    this->code_.func = func;
+    this->len_ = -1;  // Sentinel value indicates template mode
   }
 
   // Store pointer to static data in flash (no RAM copy)
   void set_data_static(const uint8_t *data, size_t len) {
-    // Simply set pointer and length - no construction needed for POD types
-    this->data_.static_data.ptr = data;
-    this->data_.static_data.len = len;
-    this->static_ = true;
+    this->code_.data = data;
+    this->len_ = len;  // Length >= 0 indicates static mode
   }
 
   void play(const Ts &...x) override {
-    if (this->static_) {
-      this->parent_->write_array(this->data_.static_data.ptr, this->data_.static_data.len);
+    if (this->len_ >= 0) {
+      // Static mode: use pointer and length
+      this->parent_->write_array(this->code_.data, static_cast<size_t>(this->len_));
     } else {
-      auto val = this->data_.func(x...);
+      // Template mode: call function
+      auto val = this->code_.func(x...);
       this->parent_->write_array(val);
     }
   }
 
  protected:
-  bool static_{true};  // Default to static mode (most common case)
-  union Data {
+  ssize_t len_{-1};  // -1 = template mode, >=0 = static mode with length
+  union Code {
     std::vector<uint8_t> (*func)(Ts...);  // Function pointer (stateless lambdas)
-    struct {
-      const uint8_t *ptr;
-      size_t len;
-    } static_data;
-  } data_;
+    const uint8_t *data;                  // Pointer to static data in flash
+  } code_;
 };
 
 }  // namespace uart