From f6bf6bd8ee0aff23bbc1961bac67e7d8beb52121 Mon Sep 17 00:00:00 2001
From: "J. Nick Koston" <nick@home-assistant.io>
Date: Sat, 8 Nov 2025 21:12:00 -0600
Subject: [PATCH 1/2] [uart] Store static data in flash and use function
 pointers for lambdas

---
 esphome/components/uart/__init__.py  |  7 ++++--
 esphome/components/uart/automation.h | 32 +++++++++++++++++-----------
 2 files changed, 24 insertions(+), 15 deletions(-)

diff --git a/esphome/components/uart/__init__.py b/esphome/components/uart/__init__.py
index eb911ed007..cbc11d0db0 100644
--- a/esphome/components/uart/__init__.py
+++ b/esphome/components/uart/__init__.py
@@ -31,7 +31,7 @@ from esphome.const import (
     PLATFORM_HOST,
     PlatformFramework,
 )
-from esphome.core import CORE
+from esphome.core import CORE, ID
 import esphome.final_validate as fv
 from esphome.yaml_util import make_data_base
 
@@ -446,7 +446,10 @@ async def uart_write_to_code(config, action_id, template_arg, args):
         templ = await cg.templatable(data, args, cg.std_vector.template(cg.uint8))
         cg.add(var.set_data_template(templ))
     else:
-        cg.add(var.set_data_static(cg.ArrayInitializer(*data)))
+        # Generate static array in flash to avoid RAM copy
+        arr_id = ID(f"{action_id}_data", is_declaration=True, type=cg.uint8)
+        arr = cg.static_const_array(arr_id, cg.ArrayInitializer(*data))
+        cg.add(var.set_data_static(arr, len(data)))
     return var
 
 
diff --git a/esphome/components/uart/automation.h b/esphome/components/uart/automation.h
index ad2c4d2bf1..7a3344c2f1 100644
--- a/esphome/components/uart/automation.h
+++ b/esphome/components/uart/automation.h
@@ -10,32 +10,38 @@ namespace uart {
 
 template<typename... Ts> class UARTWriteAction : public Action<Ts...>, public Parented<UARTComponent> {
  public:
-  void set_data_template(std::function<std::vector<uint8_t>(Ts...)> func) {
-    this->data_func_ = func;
+  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;
   }
-  void set_data_static(std::vector<uint8_t> &&data) {
-    this->data_static_ = std::move(data);
-    this->static_ = true;
-  }
-  void set_data_static(std::initializer_list<uint8_t> data) {
-    this->data_static_ = std::vector<uint8_t>(data);
+
+  // 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;
   }
 
   void play(const Ts &...x) override {
     if (this->static_) {
-      this->parent_->write_array(this->data_static_);
+      this->parent_->write_array(this->data_.static_data.ptr, this->data_.static_data.len);
     } else {
-      auto val = this->data_func_(x...);
+      auto val = this->data_.func(x...);
       this->parent_->write_array(val);
     }
   }
 
  protected:
-  bool static_{false};
-  std::function<std::vector<uint8_t>(Ts...)> data_func_{};
-  std::vector<uint8_t> data_static_{};
+  bool static_{true};  // Default to static mode (most common case)
+  union Data {
+    std::vector<uint8_t> (*func)(Ts...);  // Function pointer (stateless lambdas)
+    struct {
+      const uint8_t *ptr;
+      size_t len;
+    } static_data;
+  } data_;
 };
 
 }  // namespace uart

From c5014321a6e0d10eeda7d1b7ae861b1774e33a19 Mon Sep 17 00:00:00 2001
From: "J. Nick Koston" <nick@home-assistant.io>
Date: Sat, 8 Nov 2025 21:15:28 -0600
Subject: [PATCH 2/2] Expand uart.write tests

---
 tests/components/uart/test.esp32-idf.yaml | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/tests/components/uart/test.esp32-idf.yaml b/tests/components/uart/test.esp32-idf.yaml
index 9744a48409..6ffd0d7282 100644
--- a/tests/components/uart/test.esp32-idf.yaml
+++ b/tests/components/uart/test.esp32-idf.yaml
@@ -3,6 +3,8 @@ esphome:
     then:
       - uart.write: 'Hello World'
       - uart.write: [0x00, 0x20, 0x42]
+      - uart.write: !lambda |-
+          return {0xAA, 0xBB, 0xCC};
 
 uart:
   - id: uart_uart
@@ -46,6 +48,15 @@ switch:
       turn_on: "TURN_ON"
       turn_off: "TURN_OFF"
 
+number:
+  - platform: template
+    name: "Test Number"
+    id: test_number
+    optimistic: true
+    min_value: 0
+    max_value: 100
+    step: 1
+
 button:
   # Test uart button with array data
   - platform: uart
@@ -57,3 +68,10 @@ button:
     name: "UART Button String"
     uart_id: uart_uart
     data: "BUTTON_PRESS"
+  # Test uart button with lambda (function pointer)
+  - platform: template
+    name: "UART Lambda Test"
+    on_press:
+      - uart.write: !lambda |-
+          std::string cmd = "VALUE=" + str_sprintf("%.0f", id(test_number).state) + "\r\n";
+          return std::vector<uint8_t>(cmd.begin(), cmd.end());