From f4d32c7def97506ca93595280a35228f6d977c93 Mon Sep 17 00:00:00 2001
From: "J. Nick Koston" <nick@home-assistant.io>
Date: Wed, 29 Oct 2025 16:08:27 -0500
Subject: [PATCH 1/5] relo

---
 .../binary_sensor/template_binary_sensor.h    |   2 +-
 .../template/cover/template_cover.h           |   2 +-
 .../template/datetime/template_date.h         |   2 +-
 .../template/datetime/template_datetime.h     |   2 +-
 .../template/datetime/template_time.h         |   2 +-
 .../components/template/lock/template_lock.h  |   2 +-
 .../template/number/template_number.h         |   2 +-
 .../template/select/template_select.h         |   2 +-
 .../template/sensor/template_sensor.h         |   2 +-
 .../template/switch/template_switch.h         |   2 +-
 esphome/components/template/template_lambda.h |  51 ++++++++
 .../components/template/text/template_text.h  |   2 +-
 .../text_sensor/template_text_sensor.h        |   2 +-
 .../template/valve/template_valve.h           |   2 +-
 esphome/core/template_lambda.h                | 120 ------------------
 tests/components/template/common-base.yaml    |  16 +--
 16 files changed, 65 insertions(+), 148 deletions(-)
 create mode 100644 esphome/components/template/template_lambda.h
 delete mode 100644 esphome/core/template_lambda.h

diff --git a/esphome/components/template/binary_sensor/template_binary_sensor.h b/esphome/components/template/binary_sensor/template_binary_sensor.h
index 0373f898a8..f63738d93f 100644
--- a/esphome/components/template/binary_sensor/template_binary_sensor.h
+++ b/esphome/components/template/binary_sensor/template_binary_sensor.h
@@ -1,7 +1,7 @@
 #pragma once
 
 #include "esphome/core/component.h"
-#include "esphome/core/template_lambda.h"
+#include "../template_lambda.h"
 #include "esphome/components/binary_sensor/binary_sensor.h"
 
 namespace esphome {
diff --git a/esphome/components/template/cover/template_cover.h b/esphome/components/template/cover/template_cover.h
index 56ab61c3fb..57a5e11c5c 100644
--- a/esphome/components/template/cover/template_cover.h
+++ b/esphome/components/template/cover/template_cover.h
@@ -2,7 +2,7 @@
 
 #include "esphome/core/component.h"
 #include "esphome/core/automation.h"
-#include "esphome/core/template_lambda.h"
+#include "../template_lambda.h"
 #include "esphome/components/cover/cover.h"
 
 namespace esphome {
diff --git a/esphome/components/template/datetime/template_date.h b/esphome/components/template/datetime/template_date.h
index 7fed704d0e..b0fdbcbfbb 100644
--- a/esphome/components/template/datetime/template_date.h
+++ b/esphome/components/template/datetime/template_date.h
@@ -9,7 +9,7 @@
 #include "esphome/core/component.h"
 #include "esphome/core/preferences.h"
 #include "esphome/core/time.h"
-#include "esphome/core/template_lambda.h"
+#include "../template_lambda.h"
 
 namespace esphome {
 namespace template_ {
diff --git a/esphome/components/template/datetime/template_datetime.h b/esphome/components/template/datetime/template_datetime.h
index ec45bf0326..b1c94d7d34 100644
--- a/esphome/components/template/datetime/template_datetime.h
+++ b/esphome/components/template/datetime/template_datetime.h
@@ -9,7 +9,7 @@
 #include "esphome/core/component.h"
 #include "esphome/core/preferences.h"
 #include "esphome/core/time.h"
-#include "esphome/core/template_lambda.h"
+#include "../template_lambda.h"
 
 namespace esphome {
 namespace template_ {
diff --git a/esphome/components/template/datetime/template_time.h b/esphome/components/template/datetime/template_time.h
index ea7474c0ba..c6938fe7a0 100644
--- a/esphome/components/template/datetime/template_time.h
+++ b/esphome/components/template/datetime/template_time.h
@@ -9,7 +9,7 @@
 #include "esphome/core/component.h"
 #include "esphome/core/preferences.h"
 #include "esphome/core/time.h"
-#include "esphome/core/template_lambda.h"
+#include "../template_lambda.h"
 
 namespace esphome {
 namespace template_ {
diff --git a/esphome/components/template/lock/template_lock.h b/esphome/components/template/lock/template_lock.h
index 347c4effb3..2bb11bfef6 100644
--- a/esphome/components/template/lock/template_lock.h
+++ b/esphome/components/template/lock/template_lock.h
@@ -2,7 +2,7 @@
 
 #include "esphome/core/component.h"
 #include "esphome/core/automation.h"
-#include "esphome/core/template_lambda.h"
+#include "../template_lambda.h"
 #include "esphome/components/lock/lock.h"
 
 namespace esphome {
diff --git a/esphome/components/template/number/template_number.h b/esphome/components/template/number/template_number.h
index a9307e9246..1a6e9d964f 100644
--- a/esphome/components/template/number/template_number.h
+++ b/esphome/components/template/number/template_number.h
@@ -4,7 +4,7 @@
 #include "esphome/core/automation.h"
 #include "esphome/core/component.h"
 #include "esphome/core/preferences.h"
-#include "esphome/core/template_lambda.h"
+#include "../template_lambda.h"
 
 namespace esphome {
 namespace template_ {
diff --git a/esphome/components/template/select/template_select.h b/esphome/components/template/select/template_select.h
index 1c33153872..53cadfa303 100644
--- a/esphome/components/template/select/template_select.h
+++ b/esphome/components/template/select/template_select.h
@@ -4,7 +4,7 @@
 #include "esphome/core/automation.h"
 #include "esphome/core/component.h"
 #include "esphome/core/preferences.h"
-#include "esphome/core/template_lambda.h"
+#include "../template_lambda.h"
 
 namespace esphome {
 namespace template_ {
diff --git a/esphome/components/template/sensor/template_sensor.h b/esphome/components/template/sensor/template_sensor.h
index 793d754a0f..27980414f9 100644
--- a/esphome/components/template/sensor/template_sensor.h
+++ b/esphome/components/template/sensor/template_sensor.h
@@ -1,7 +1,7 @@
 #pragma once
 
 #include "esphome/core/component.h"
-#include "esphome/core/template_lambda.h"
+#include "../template_lambda.h"
 #include "esphome/components/sensor/sensor.h"
 
 namespace esphome {
diff --git a/esphome/components/template/switch/template_switch.h b/esphome/components/template/switch/template_switch.h
index 47154fd047..f436f657ae 100644
--- a/esphome/components/template/switch/template_switch.h
+++ b/esphome/components/template/switch/template_switch.h
@@ -2,7 +2,7 @@
 
 #include "esphome/core/component.h"
 #include "esphome/core/automation.h"
-#include "esphome/core/template_lambda.h"
+#include "../template_lambda.h"
 #include "esphome/components/switch/switch.h"
 
 namespace esphome {
diff --git a/esphome/components/template/template_lambda.h b/esphome/components/template/template_lambda.h
new file mode 100644
index 0000000000..894b5edffa
--- /dev/null
+++ b/esphome/components/template/template_lambda.h
@@ -0,0 +1,51 @@
+#pragma once
+
+#include "esphome/core/optional.h"
+
+namespace esphome {
+
+/** Lightweight wrapper for template platform lambdas (stateless function pointers only).
+ *
+ * This optimizes template platforms by storing only a function pointer (4 bytes on ESP32)
+ * instead of std::function (16-32 bytes).
+ *
+ * IMPORTANT: This only supports stateless lambdas (no captures). The set_template() method
+ * is an internal API used by YAML codegen, not intended for external use.
+ *
+ * Lambdas must return optional<T> to support the pattern:
+ *   return {};    // Don't publish a value
+ *   return 42.0;  // Publish this value
+ *
+ * operator() returns optional<T>, returning nullopt when no lambda is set (nullptr check).
+ *
+ * @tparam T The return type (e.g., float for sensor values)
+ * @tparam Args Optional arguments for the lambda
+ */
+template<typename T, typename... Args> class TemplateLambda {
+ public:
+  TemplateLambda() : f_(nullptr) {}
+
+  /** Set the lambda function pointer.
+   * INTERNAL API: Only for use by YAML codegen.
+   * Only stateless lambdas (no captures) are supported.
+   */
+  void set(optional<T> (*f)(Args...)) { this->f_ = f; }
+
+  /** Check if a lambda is set */
+  bool has_value() const { return this->f_ != nullptr; }
+
+  /** Call the lambda, returning nullopt if no lambda is set */
+  optional<T> operator()(Args... args) {
+    if (this->f_ == nullptr)
+      return nullopt;
+    return this->f_(args...);
+  }
+
+  /** Alias for operator() for compatibility */
+  optional<T> call(Args... args) { return (*this)(args...); }
+
+ protected:
+  optional<T> (*f_)(Args...);  // Function pointer (4 bytes on ESP32)
+};
+
+}  // namespace esphome
diff --git a/esphome/components/template/text/template_text.h b/esphome/components/template/text/template_text.h
index c12021f80e..fd28800bab 100644
--- a/esphome/components/template/text/template_text.h
+++ b/esphome/components/template/text/template_text.h
@@ -4,7 +4,7 @@
 #include "esphome/core/automation.h"
 #include "esphome/core/component.h"
 #include "esphome/core/preferences.h"
-#include "esphome/core/template_lambda.h"
+#include "../template_lambda.h"
 
 namespace esphome {
 namespace template_ {
diff --git a/esphome/components/template/text_sensor/template_text_sensor.h b/esphome/components/template/text_sensor/template_text_sensor.h
index 0d01c72023..fda28f53c7 100644
--- a/esphome/components/template/text_sensor/template_text_sensor.h
+++ b/esphome/components/template/text_sensor/template_text_sensor.h
@@ -2,7 +2,7 @@
 
 #include "esphome/core/component.h"
 #include "esphome/core/automation.h"
-#include "esphome/core/template_lambda.h"
+#include "../template_lambda.h"
 #include "esphome/components/text_sensor/text_sensor.h"
 
 namespace esphome {
diff --git a/esphome/components/template/valve/template_valve.h b/esphome/components/template/valve/template_valve.h
index 23a77ff918..7713091673 100644
--- a/esphome/components/template/valve/template_valve.h
+++ b/esphome/components/template/valve/template_valve.h
@@ -2,7 +2,7 @@
 
 #include "esphome/core/component.h"
 #include "esphome/core/automation.h"
-#include "esphome/core/template_lambda.h"
+#include "../template_lambda.h"
 #include "esphome/components/valve/valve.h"
 
 namespace esphome {
diff --git a/esphome/core/template_lambda.h b/esphome/core/template_lambda.h
deleted file mode 100644
index 1977265c33..0000000000
--- a/esphome/core/template_lambda.h
+++ /dev/null
@@ -1,120 +0,0 @@
-#pragma once
-
-#include <functional>
-#include <concepts>
-#include "esphome/core/optional.h"
-
-namespace esphome {
-
-/** Helper class for template platforms that stores either a stateless lambda (function pointer)
- * or a stateful lambda (std::function pointer).
- *
- * This provides backward compatibility with PR #11555 while maintaining the optimization:
- * - Stateless lambdas (no capture) → function pointer (4 bytes on ESP32)
- * - Stateful lambdas (with capture) → pointer to std::function (4 bytes on ESP32)
- * Total size: enum (1 byte) + union (4 bytes) + padding = 8 bytes (same as PR #11555)
- *
- * Both lambda types must return optional<T> (as YAML codegen does) to support the pattern:
- *   return {};  // Don't publish a value
- *   return 42.0;  // Publish this value
- *
- * operator() returns optional<T>, returning nullopt when no lambda is set (type == NONE).
- * This eliminates redundant "is lambda set" checks by reusing optional's discriminator.
- *
- * @tparam T The return type (e.g., float for TemplateLambda<optional<float>>)
- * @tparam Args Optional arguments for the lambda
- */
-template<typename T, typename... Args> class TemplateLambda {
- public:
-  TemplateLambda() : type_(NONE) {}
-
-  // For stateless lambdas: use function pointer
-  template<typename F>
-  requires std::invocable<F, Args...> && std::convertible_to < F, optional<T>(*)
-  (Args...) > void set(F f) {
-    this->reset_();
-    this->type_ = STATELESS_LAMBDA;
-    this->stateless_f_ = f;  // Implicit conversion to function pointer
-  }
-
-  // For stateful lambdas: use std::function pointer
-  template<typename F>
-  requires std::invocable<F, Args...> &&
-      (!std::convertible_to<F, optional<T> (*)(Args...)>) &&std::convertible_to<std::invoke_result_t<F, Args...>,
-                                                                                optional<T>> void set(F &&f) {
-    this->reset_();
-    this->type_ = LAMBDA;
-    this->f_ = new std::function<optional<T>(Args...)>(std::forward<F>(f));
-  }
-
-  ~TemplateLambda() { this->reset_(); }
-
-  // Copy constructor
-  TemplateLambda(const TemplateLambda &) = delete;
-  TemplateLambda &operator=(const TemplateLambda &) = delete;
-
-  // Move constructor
-  TemplateLambda(TemplateLambda &&other) noexcept : type_(other.type_) {
-    if (type_ == LAMBDA) {
-      this->f_ = other.f_;
-      other.f_ = nullptr;
-    } else if (type_ == STATELESS_LAMBDA) {
-      this->stateless_f_ = other.stateless_f_;
-    }
-    other.type_ = NONE;
-  }
-
-  TemplateLambda &operator=(TemplateLambda &&other) noexcept {
-    if (this != &other) {
-      this->reset_();
-      this->type_ = other.type_;
-      if (type_ == LAMBDA) {
-        this->f_ = other.f_;
-        other.f_ = nullptr;
-      } else if (type_ == STATELESS_LAMBDA) {
-        this->stateless_f_ = other.stateless_f_;
-      }
-      other.type_ = NONE;
-    }
-    return *this;
-  }
-
-  bool has_value() const { return this->type_ != NONE; }
-
-  // Returns optional<T>, returning nullopt if no lambda is set
-  optional<T> operator()(Args... args) {
-    switch (this->type_) {
-      case STATELESS_LAMBDA:
-        return this->stateless_f_(args...);  // Direct function pointer call
-      case LAMBDA:
-        return (*this->f_)(args...);  // std::function call via pointer
-      case NONE:
-      default:
-        return nullopt;  // No lambda set
-    }
-  }
-
-  optional<T> call(Args... args) { return (*this)(args...); }
-
- protected:
-  void reset_() {
-    if (this->type_ == LAMBDA) {
-      delete this->f_;
-      this->f_ = nullptr;
-    }
-    this->type_ = NONE;
-  }
-
-  enum : uint8_t {
-    NONE,
-    STATELESS_LAMBDA,
-    LAMBDA,
-  } type_;
-
-  union {
-    optional<T> (*stateless_f_)(Args...);     // Function pointer (4 bytes on ESP32)
-    std::function<optional<T>(Args...)> *f_;  // Pointer to std::function (4 bytes on ESP32)
-  };
-};
-
-}  // namespace esphome
diff --git a/tests/components/template/common-base.yaml b/tests/components/template/common-base.yaml
index 7d2cb19077..da449111a2 100644
--- a/tests/components/template/common-base.yaml
+++ b/tests/components/template/common-base.yaml
@@ -10,26 +10,12 @@ esphome:
         state: !lambda "return 42.0;"
 
     # Test C++ API: set_template() with stateless lambda (no captures)
+    # IMPORTANT: set_template() is an internal API. Only stateless lambdas are supported.
     - lambda: |-
         id(template_sens).set_template([]() -> esphome::optional<float> {
           return 123.0f;
         });
 
-    # Test C++ API: set_template() with stateful lambda (with captures)
-    # This is the regression test for issue #11555
-    - lambda: |-
-        float captured_value = 456.0f;
-        id(template_sens).set_template([captured_value]() -> esphome::optional<float> {
-          return captured_value;
-        });
-
-    # Test C++ API: set_template() with more complex capture
-    - lambda: |-
-        auto sensor_id = id(template_sens);
-        id(template_number).set_template([sensor_id]() -> esphome::optional<float> {
-          return sensor_id->state * 2.0f;
-        });
-
     - datetime.date.set:
         id: test_date
         date:

From a849ddd57d288d984dcd91c316104608a86a4b00 Mon Sep 17 00:00:00 2001
From: "J. Nick Koston" <nick@home-assistant.io>
Date: Wed, 29 Oct 2025 16:10:32 -0500
Subject: [PATCH 2/5] wip

---
 .../binary_sensor/template_binary_sensor.h    |  2 +-
 .../template/cover/template_cover.h           |  2 +-
 .../template/datetime/template_date.h         |  2 +-
 .../template/datetime/template_datetime.h     |  2 +-
 .../template/datetime/template_time.h         |  2 +-
 .../components/template/lock/template_lock.h  |  2 +-
 .../template/number/template_number.h         |  2 +-
 .../template/select/template_select.h         |  2 +-
 .../template/sensor/template_sensor.h         |  2 +-
 .../template/switch/template_switch.h         |  2 +-
 esphome/components/template/template_lambda.h | 51 -------------------
 .../components/template/text/template_text.h  |  2 +-
 .../text_sensor/template_text_sensor.h        |  2 +-
 .../template/valve/template_valve.h           |  2 +-
 tests/components/template/common-base.yaml    |  2 +-
 15 files changed, 14 insertions(+), 65 deletions(-)
 delete mode 100644 esphome/components/template/template_lambda.h

diff --git a/esphome/components/template/binary_sensor/template_binary_sensor.h b/esphome/components/template/binary_sensor/template_binary_sensor.h
index f63738d93f..0373f898a8 100644
--- a/esphome/components/template/binary_sensor/template_binary_sensor.h
+++ b/esphome/components/template/binary_sensor/template_binary_sensor.h
@@ -1,7 +1,7 @@
 #pragma once
 
 #include "esphome/core/component.h"
-#include "../template_lambda.h"
+#include "esphome/core/template_lambda.h"
 #include "esphome/components/binary_sensor/binary_sensor.h"
 
 namespace esphome {
diff --git a/esphome/components/template/cover/template_cover.h b/esphome/components/template/cover/template_cover.h
index 57a5e11c5c..56ab61c3fb 100644
--- a/esphome/components/template/cover/template_cover.h
+++ b/esphome/components/template/cover/template_cover.h
@@ -2,7 +2,7 @@
 
 #include "esphome/core/component.h"
 #include "esphome/core/automation.h"
-#include "../template_lambda.h"
+#include "esphome/core/template_lambda.h"
 #include "esphome/components/cover/cover.h"
 
 namespace esphome {
diff --git a/esphome/components/template/datetime/template_date.h b/esphome/components/template/datetime/template_date.h
index b0fdbcbfbb..7fed704d0e 100644
--- a/esphome/components/template/datetime/template_date.h
+++ b/esphome/components/template/datetime/template_date.h
@@ -9,7 +9,7 @@
 #include "esphome/core/component.h"
 #include "esphome/core/preferences.h"
 #include "esphome/core/time.h"
-#include "../template_lambda.h"
+#include "esphome/core/template_lambda.h"
 
 namespace esphome {
 namespace template_ {
diff --git a/esphome/components/template/datetime/template_datetime.h b/esphome/components/template/datetime/template_datetime.h
index b1c94d7d34..ec45bf0326 100644
--- a/esphome/components/template/datetime/template_datetime.h
+++ b/esphome/components/template/datetime/template_datetime.h
@@ -9,7 +9,7 @@
 #include "esphome/core/component.h"
 #include "esphome/core/preferences.h"
 #include "esphome/core/time.h"
-#include "../template_lambda.h"
+#include "esphome/core/template_lambda.h"
 
 namespace esphome {
 namespace template_ {
diff --git a/esphome/components/template/datetime/template_time.h b/esphome/components/template/datetime/template_time.h
index c6938fe7a0..ea7474c0ba 100644
--- a/esphome/components/template/datetime/template_time.h
+++ b/esphome/components/template/datetime/template_time.h
@@ -9,7 +9,7 @@
 #include "esphome/core/component.h"
 #include "esphome/core/preferences.h"
 #include "esphome/core/time.h"
-#include "../template_lambda.h"
+#include "esphome/core/template_lambda.h"
 
 namespace esphome {
 namespace template_ {
diff --git a/esphome/components/template/lock/template_lock.h b/esphome/components/template/lock/template_lock.h
index 2bb11bfef6..347c4effb3 100644
--- a/esphome/components/template/lock/template_lock.h
+++ b/esphome/components/template/lock/template_lock.h
@@ -2,7 +2,7 @@
 
 #include "esphome/core/component.h"
 #include "esphome/core/automation.h"
-#include "../template_lambda.h"
+#include "esphome/core/template_lambda.h"
 #include "esphome/components/lock/lock.h"
 
 namespace esphome {
diff --git a/esphome/components/template/number/template_number.h b/esphome/components/template/number/template_number.h
index 1a6e9d964f..a9307e9246 100644
--- a/esphome/components/template/number/template_number.h
+++ b/esphome/components/template/number/template_number.h
@@ -4,7 +4,7 @@
 #include "esphome/core/automation.h"
 #include "esphome/core/component.h"
 #include "esphome/core/preferences.h"
-#include "../template_lambda.h"
+#include "esphome/core/template_lambda.h"
 
 namespace esphome {
 namespace template_ {
diff --git a/esphome/components/template/select/template_select.h b/esphome/components/template/select/template_select.h
index 53cadfa303..1c33153872 100644
--- a/esphome/components/template/select/template_select.h
+++ b/esphome/components/template/select/template_select.h
@@ -4,7 +4,7 @@
 #include "esphome/core/automation.h"
 #include "esphome/core/component.h"
 #include "esphome/core/preferences.h"
-#include "../template_lambda.h"
+#include "esphome/core/template_lambda.h"
 
 namespace esphome {
 namespace template_ {
diff --git a/esphome/components/template/sensor/template_sensor.h b/esphome/components/template/sensor/template_sensor.h
index 27980414f9..793d754a0f 100644
--- a/esphome/components/template/sensor/template_sensor.h
+++ b/esphome/components/template/sensor/template_sensor.h
@@ -1,7 +1,7 @@
 #pragma once
 
 #include "esphome/core/component.h"
-#include "../template_lambda.h"
+#include "esphome/core/template_lambda.h"
 #include "esphome/components/sensor/sensor.h"
 
 namespace esphome {
diff --git a/esphome/components/template/switch/template_switch.h b/esphome/components/template/switch/template_switch.h
index f436f657ae..47154fd047 100644
--- a/esphome/components/template/switch/template_switch.h
+++ b/esphome/components/template/switch/template_switch.h
@@ -2,7 +2,7 @@
 
 #include "esphome/core/component.h"
 #include "esphome/core/automation.h"
-#include "../template_lambda.h"
+#include "esphome/core/template_lambda.h"
 #include "esphome/components/switch/switch.h"
 
 namespace esphome {
diff --git a/esphome/components/template/template_lambda.h b/esphome/components/template/template_lambda.h
deleted file mode 100644
index 894b5edffa..0000000000
--- a/esphome/components/template/template_lambda.h
+++ /dev/null
@@ -1,51 +0,0 @@
-#pragma once
-
-#include "esphome/core/optional.h"
-
-namespace esphome {
-
-/** Lightweight wrapper for template platform lambdas (stateless function pointers only).
- *
- * This optimizes template platforms by storing only a function pointer (4 bytes on ESP32)
- * instead of std::function (16-32 bytes).
- *
- * IMPORTANT: This only supports stateless lambdas (no captures). The set_template() method
- * is an internal API used by YAML codegen, not intended for external use.
- *
- * Lambdas must return optional<T> to support the pattern:
- *   return {};    // Don't publish a value
- *   return 42.0;  // Publish this value
- *
- * operator() returns optional<T>, returning nullopt when no lambda is set (nullptr check).
- *
- * @tparam T The return type (e.g., float for sensor values)
- * @tparam Args Optional arguments for the lambda
- */
-template<typename T, typename... Args> class TemplateLambda {
- public:
-  TemplateLambda() : f_(nullptr) {}
-
-  /** Set the lambda function pointer.
-   * INTERNAL API: Only for use by YAML codegen.
-   * Only stateless lambdas (no captures) are supported.
-   */
-  void set(optional<T> (*f)(Args...)) { this->f_ = f; }
-
-  /** Check if a lambda is set */
-  bool has_value() const { return this->f_ != nullptr; }
-
-  /** Call the lambda, returning nullopt if no lambda is set */
-  optional<T> operator()(Args... args) {
-    if (this->f_ == nullptr)
-      return nullopt;
-    return this->f_(args...);
-  }
-
-  /** Alias for operator() for compatibility */
-  optional<T> call(Args... args) { return (*this)(args...); }
-
- protected:
-  optional<T> (*f_)(Args...);  // Function pointer (4 bytes on ESP32)
-};
-
-}  // namespace esphome
diff --git a/esphome/components/template/text/template_text.h b/esphome/components/template/text/template_text.h
index fd28800bab..c12021f80e 100644
--- a/esphome/components/template/text/template_text.h
+++ b/esphome/components/template/text/template_text.h
@@ -4,7 +4,7 @@
 #include "esphome/core/automation.h"
 #include "esphome/core/component.h"
 #include "esphome/core/preferences.h"
-#include "../template_lambda.h"
+#include "esphome/core/template_lambda.h"
 
 namespace esphome {
 namespace template_ {
diff --git a/esphome/components/template/text_sensor/template_text_sensor.h b/esphome/components/template/text_sensor/template_text_sensor.h
index fda28f53c7..0d01c72023 100644
--- a/esphome/components/template/text_sensor/template_text_sensor.h
+++ b/esphome/components/template/text_sensor/template_text_sensor.h
@@ -2,7 +2,7 @@
 
 #include "esphome/core/component.h"
 #include "esphome/core/automation.h"
-#include "../template_lambda.h"
+#include "esphome/core/template_lambda.h"
 #include "esphome/components/text_sensor/text_sensor.h"
 
 namespace esphome {
diff --git a/esphome/components/template/valve/template_valve.h b/esphome/components/template/valve/template_valve.h
index 7713091673..23a77ff918 100644
--- a/esphome/components/template/valve/template_valve.h
+++ b/esphome/components/template/valve/template_valve.h
@@ -2,7 +2,7 @@
 
 #include "esphome/core/component.h"
 #include "esphome/core/automation.h"
-#include "../template_lambda.h"
+#include "esphome/core/template_lambda.h"
 #include "esphome/components/valve/valve.h"
 
 namespace esphome {
diff --git a/tests/components/template/common-base.yaml b/tests/components/template/common-base.yaml
index da449111a2..f101eea942 100644
--- a/tests/components/template/common-base.yaml
+++ b/tests/components/template/common-base.yaml
@@ -10,7 +10,7 @@ esphome:
         state: !lambda "return 42.0;"
 
     # Test C++ API: set_template() with stateless lambda (no captures)
-    # IMPORTANT: set_template() is an internal API. Only stateless lambdas are supported.
+    # NOTE: set_template() is not intended to be a public API, but we test it to ensure it doesn't break.
     - lambda: |-
         id(template_sens).set_template([]() -> esphome::optional<float> {
           return 123.0f;

From 922acda1a808235c8c252c156d4374a53c3e2e7c Mon Sep 17 00:00:00 2001
From: "J. Nick Koston" <nick@home-assistant.io>
Date: Wed, 29 Oct 2025 16:12:05 -0500
Subject: [PATCH 3/5] wip

---
 esphome/core/template_lambda.h | 51 ++++++++++++++++++++++++++++++++++
 1 file changed, 51 insertions(+)
 create mode 100644 esphome/core/template_lambda.h

diff --git a/esphome/core/template_lambda.h b/esphome/core/template_lambda.h
new file mode 100644
index 0000000000..894b5edffa
--- /dev/null
+++ b/esphome/core/template_lambda.h
@@ -0,0 +1,51 @@
+#pragma once
+
+#include "esphome/core/optional.h"
+
+namespace esphome {
+
+/** Lightweight wrapper for template platform lambdas (stateless function pointers only).
+ *
+ * This optimizes template platforms by storing only a function pointer (4 bytes on ESP32)
+ * instead of std::function (16-32 bytes).
+ *
+ * IMPORTANT: This only supports stateless lambdas (no captures). The set_template() method
+ * is an internal API used by YAML codegen, not intended for external use.
+ *
+ * Lambdas must return optional<T> to support the pattern:
+ *   return {};    // Don't publish a value
+ *   return 42.0;  // Publish this value
+ *
+ * operator() returns optional<T>, returning nullopt when no lambda is set (nullptr check).
+ *
+ * @tparam T The return type (e.g., float for sensor values)
+ * @tparam Args Optional arguments for the lambda
+ */
+template<typename T, typename... Args> class TemplateLambda {
+ public:
+  TemplateLambda() : f_(nullptr) {}
+
+  /** Set the lambda function pointer.
+   * INTERNAL API: Only for use by YAML codegen.
+   * Only stateless lambdas (no captures) are supported.
+   */
+  void set(optional<T> (*f)(Args...)) { this->f_ = f; }
+
+  /** Check if a lambda is set */
+  bool has_value() const { return this->f_ != nullptr; }
+
+  /** Call the lambda, returning nullopt if no lambda is set */
+  optional<T> operator()(Args... args) {
+    if (this->f_ == nullptr)
+      return nullopt;
+    return this->f_(args...);
+  }
+
+  /** Alias for operator() for compatibility */
+  optional<T> call(Args... args) { return (*this)(args...); }
+
+ protected:
+  optional<T> (*f_)(Args...);  // Function pointer (4 bytes on ESP32)
+};
+
+}  // namespace esphome

From 68d1a7e3effc93411549924234cedb6e6ef64ef8 Mon Sep 17 00:00:00 2001
From: "J. Nick Koston" <nick@home-assistant.io>
Date: Wed, 29 Oct 2025 16:15:15 -0500
Subject: [PATCH 4/5] wip

---
 .../components/template/binary_sensor/template_binary_sensor.h  | 2 +-
 esphome/components/template/cover/template_cover.h              | 2 +-
 esphome/components/template/datetime/template_date.h            | 2 +-
 esphome/components/template/datetime/template_datetime.h        | 2 +-
 esphome/components/template/datetime/template_time.h            | 2 +-
 esphome/components/template/lock/template_lock.h                | 2 +-
 esphome/components/template/number/template_number.h            | 2 +-
 esphome/components/template/select/template_select.h            | 2 +-
 esphome/components/template/sensor/template_sensor.h            | 2 +-
 esphome/components/template/switch/template_switch.h            | 2 +-
 esphome/{core => components/template}/template_lambda.h         | 0
 esphome/components/template/text/template_text.h                | 2 +-
 esphome/components/template/text_sensor/template_text_sensor.h  | 2 +-
 esphome/components/template/valve/template_valve.h              | 2 +-
 14 files changed, 13 insertions(+), 13 deletions(-)
 rename esphome/{core => components/template}/template_lambda.h (100%)

diff --git a/esphome/components/template/binary_sensor/template_binary_sensor.h b/esphome/components/template/binary_sensor/template_binary_sensor.h
index 0373f898a8..f63738d93f 100644
--- a/esphome/components/template/binary_sensor/template_binary_sensor.h
+++ b/esphome/components/template/binary_sensor/template_binary_sensor.h
@@ -1,7 +1,7 @@
 #pragma once
 
 #include "esphome/core/component.h"
-#include "esphome/core/template_lambda.h"
+#include "../template_lambda.h"
 #include "esphome/components/binary_sensor/binary_sensor.h"
 
 namespace esphome {
diff --git a/esphome/components/template/cover/template_cover.h b/esphome/components/template/cover/template_cover.h
index 56ab61c3fb..57a5e11c5c 100644
--- a/esphome/components/template/cover/template_cover.h
+++ b/esphome/components/template/cover/template_cover.h
@@ -2,7 +2,7 @@
 
 #include "esphome/core/component.h"
 #include "esphome/core/automation.h"
-#include "esphome/core/template_lambda.h"
+#include "../template_lambda.h"
 #include "esphome/components/cover/cover.h"
 
 namespace esphome {
diff --git a/esphome/components/template/datetime/template_date.h b/esphome/components/template/datetime/template_date.h
index 7fed704d0e..b0fdbcbfbb 100644
--- a/esphome/components/template/datetime/template_date.h
+++ b/esphome/components/template/datetime/template_date.h
@@ -9,7 +9,7 @@
 #include "esphome/core/component.h"
 #include "esphome/core/preferences.h"
 #include "esphome/core/time.h"
-#include "esphome/core/template_lambda.h"
+#include "../template_lambda.h"
 
 namespace esphome {
 namespace template_ {
diff --git a/esphome/components/template/datetime/template_datetime.h b/esphome/components/template/datetime/template_datetime.h
index ec45bf0326..b1c94d7d34 100644
--- a/esphome/components/template/datetime/template_datetime.h
+++ b/esphome/components/template/datetime/template_datetime.h
@@ -9,7 +9,7 @@
 #include "esphome/core/component.h"
 #include "esphome/core/preferences.h"
 #include "esphome/core/time.h"
-#include "esphome/core/template_lambda.h"
+#include "../template_lambda.h"
 
 namespace esphome {
 namespace template_ {
diff --git a/esphome/components/template/datetime/template_time.h b/esphome/components/template/datetime/template_time.h
index ea7474c0ba..c6938fe7a0 100644
--- a/esphome/components/template/datetime/template_time.h
+++ b/esphome/components/template/datetime/template_time.h
@@ -9,7 +9,7 @@
 #include "esphome/core/component.h"
 #include "esphome/core/preferences.h"
 #include "esphome/core/time.h"
-#include "esphome/core/template_lambda.h"
+#include "../template_lambda.h"
 
 namespace esphome {
 namespace template_ {
diff --git a/esphome/components/template/lock/template_lock.h b/esphome/components/template/lock/template_lock.h
index 347c4effb3..2bb11bfef6 100644
--- a/esphome/components/template/lock/template_lock.h
+++ b/esphome/components/template/lock/template_lock.h
@@ -2,7 +2,7 @@
 
 #include "esphome/core/component.h"
 #include "esphome/core/automation.h"
-#include "esphome/core/template_lambda.h"
+#include "../template_lambda.h"
 #include "esphome/components/lock/lock.h"
 
 namespace esphome {
diff --git a/esphome/components/template/number/template_number.h b/esphome/components/template/number/template_number.h
index a9307e9246..1a6e9d964f 100644
--- a/esphome/components/template/number/template_number.h
+++ b/esphome/components/template/number/template_number.h
@@ -4,7 +4,7 @@
 #include "esphome/core/automation.h"
 #include "esphome/core/component.h"
 #include "esphome/core/preferences.h"
-#include "esphome/core/template_lambda.h"
+#include "../template_lambda.h"
 
 namespace esphome {
 namespace template_ {
diff --git a/esphome/components/template/select/template_select.h b/esphome/components/template/select/template_select.h
index 1c33153872..53cadfa303 100644
--- a/esphome/components/template/select/template_select.h
+++ b/esphome/components/template/select/template_select.h
@@ -4,7 +4,7 @@
 #include "esphome/core/automation.h"
 #include "esphome/core/component.h"
 #include "esphome/core/preferences.h"
-#include "esphome/core/template_lambda.h"
+#include "../template_lambda.h"
 
 namespace esphome {
 namespace template_ {
diff --git a/esphome/components/template/sensor/template_sensor.h b/esphome/components/template/sensor/template_sensor.h
index 793d754a0f..27980414f9 100644
--- a/esphome/components/template/sensor/template_sensor.h
+++ b/esphome/components/template/sensor/template_sensor.h
@@ -1,7 +1,7 @@
 #pragma once
 
 #include "esphome/core/component.h"
-#include "esphome/core/template_lambda.h"
+#include "../template_lambda.h"
 #include "esphome/components/sensor/sensor.h"
 
 namespace esphome {
diff --git a/esphome/components/template/switch/template_switch.h b/esphome/components/template/switch/template_switch.h
index 47154fd047..f436f657ae 100644
--- a/esphome/components/template/switch/template_switch.h
+++ b/esphome/components/template/switch/template_switch.h
@@ -2,7 +2,7 @@
 
 #include "esphome/core/component.h"
 #include "esphome/core/automation.h"
-#include "esphome/core/template_lambda.h"
+#include "../template_lambda.h"
 #include "esphome/components/switch/switch.h"
 
 namespace esphome {
diff --git a/esphome/core/template_lambda.h b/esphome/components/template/template_lambda.h
similarity index 100%
rename from esphome/core/template_lambda.h
rename to esphome/components/template/template_lambda.h
diff --git a/esphome/components/template/text/template_text.h b/esphome/components/template/text/template_text.h
index c12021f80e..fd28800bab 100644
--- a/esphome/components/template/text/template_text.h
+++ b/esphome/components/template/text/template_text.h
@@ -4,7 +4,7 @@
 #include "esphome/core/automation.h"
 #include "esphome/core/component.h"
 #include "esphome/core/preferences.h"
-#include "esphome/core/template_lambda.h"
+#include "../template_lambda.h"
 
 namespace esphome {
 namespace template_ {
diff --git a/esphome/components/template/text_sensor/template_text_sensor.h b/esphome/components/template/text_sensor/template_text_sensor.h
index 0d01c72023..fda28f53c7 100644
--- a/esphome/components/template/text_sensor/template_text_sensor.h
+++ b/esphome/components/template/text_sensor/template_text_sensor.h
@@ -2,7 +2,7 @@
 
 #include "esphome/core/component.h"
 #include "esphome/core/automation.h"
-#include "esphome/core/template_lambda.h"
+#include "../template_lambda.h"
 #include "esphome/components/text_sensor/text_sensor.h"
 
 namespace esphome {
diff --git a/esphome/components/template/valve/template_valve.h b/esphome/components/template/valve/template_valve.h
index 23a77ff918..7713091673 100644
--- a/esphome/components/template/valve/template_valve.h
+++ b/esphome/components/template/valve/template_valve.h
@@ -2,7 +2,7 @@
 
 #include "esphome/core/component.h"
 #include "esphome/core/automation.h"
-#include "esphome/core/template_lambda.h"
+#include "../template_lambda.h"
 #include "esphome/components/valve/valve.h"
 
 namespace esphome {

From 5478fa69e9c550c18a54bdfa3f83e7a9b0ef2c10 Mon Sep 17 00:00:00 2001
From: "J. Nick Koston" <nick@home-assistant.io>
Date: Wed, 29 Oct 2025 16:20:11 -0500
Subject: [PATCH 5/5] twip

---
 .../components/template/binary_sensor/template_binary_sensor.h  | 2 +-
 esphome/components/template/cover/template_cover.h              | 2 +-
 esphome/components/template/datetime/template_date.h            | 2 +-
 esphome/components/template/datetime/template_datetime.h        | 2 +-
 esphome/components/template/datetime/template_time.h            | 2 +-
 esphome/components/template/lock/template_lock.h                | 2 +-
 esphome/components/template/number/template_number.h            | 2 +-
 esphome/components/template/select/template_select.h            | 2 +-
 esphome/components/template/sensor/template_sensor.h            | 2 +-
 esphome/components/template/switch/template_switch.h            | 2 +-
 esphome/components/template/text/template_text.h                | 2 +-
 esphome/components/template/text_sensor/template_text_sensor.h  | 2 +-
 esphome/components/template/valve/template_valve.h              | 2 +-
 esphome/{components/template => core}/template_lambda.h         | 0
 14 files changed, 13 insertions(+), 13 deletions(-)
 rename esphome/{components/template => core}/template_lambda.h (100%)

diff --git a/esphome/components/template/binary_sensor/template_binary_sensor.h b/esphome/components/template/binary_sensor/template_binary_sensor.h
index f63738d93f..0373f898a8 100644
--- a/esphome/components/template/binary_sensor/template_binary_sensor.h
+++ b/esphome/components/template/binary_sensor/template_binary_sensor.h
@@ -1,7 +1,7 @@
 #pragma once
 
 #include "esphome/core/component.h"
-#include "../template_lambda.h"
+#include "esphome/core/template_lambda.h"
 #include "esphome/components/binary_sensor/binary_sensor.h"
 
 namespace esphome {
diff --git a/esphome/components/template/cover/template_cover.h b/esphome/components/template/cover/template_cover.h
index 57a5e11c5c..56ab61c3fb 100644
--- a/esphome/components/template/cover/template_cover.h
+++ b/esphome/components/template/cover/template_cover.h
@@ -2,7 +2,7 @@
 
 #include "esphome/core/component.h"
 #include "esphome/core/automation.h"
-#include "../template_lambda.h"
+#include "esphome/core/template_lambda.h"
 #include "esphome/components/cover/cover.h"
 
 namespace esphome {
diff --git a/esphome/components/template/datetime/template_date.h b/esphome/components/template/datetime/template_date.h
index b0fdbcbfbb..7fed704d0e 100644
--- a/esphome/components/template/datetime/template_date.h
+++ b/esphome/components/template/datetime/template_date.h
@@ -9,7 +9,7 @@
 #include "esphome/core/component.h"
 #include "esphome/core/preferences.h"
 #include "esphome/core/time.h"
-#include "../template_lambda.h"
+#include "esphome/core/template_lambda.h"
 
 namespace esphome {
 namespace template_ {
diff --git a/esphome/components/template/datetime/template_datetime.h b/esphome/components/template/datetime/template_datetime.h
index b1c94d7d34..ec45bf0326 100644
--- a/esphome/components/template/datetime/template_datetime.h
+++ b/esphome/components/template/datetime/template_datetime.h
@@ -9,7 +9,7 @@
 #include "esphome/core/component.h"
 #include "esphome/core/preferences.h"
 #include "esphome/core/time.h"
-#include "../template_lambda.h"
+#include "esphome/core/template_lambda.h"
 
 namespace esphome {
 namespace template_ {
diff --git a/esphome/components/template/datetime/template_time.h b/esphome/components/template/datetime/template_time.h
index c6938fe7a0..ea7474c0ba 100644
--- a/esphome/components/template/datetime/template_time.h
+++ b/esphome/components/template/datetime/template_time.h
@@ -9,7 +9,7 @@
 #include "esphome/core/component.h"
 #include "esphome/core/preferences.h"
 #include "esphome/core/time.h"
-#include "../template_lambda.h"
+#include "esphome/core/template_lambda.h"
 
 namespace esphome {
 namespace template_ {
diff --git a/esphome/components/template/lock/template_lock.h b/esphome/components/template/lock/template_lock.h
index 2bb11bfef6..347c4effb3 100644
--- a/esphome/components/template/lock/template_lock.h
+++ b/esphome/components/template/lock/template_lock.h
@@ -2,7 +2,7 @@
 
 #include "esphome/core/component.h"
 #include "esphome/core/automation.h"
-#include "../template_lambda.h"
+#include "esphome/core/template_lambda.h"
 #include "esphome/components/lock/lock.h"
 
 namespace esphome {
diff --git a/esphome/components/template/number/template_number.h b/esphome/components/template/number/template_number.h
index 1a6e9d964f..a9307e9246 100644
--- a/esphome/components/template/number/template_number.h
+++ b/esphome/components/template/number/template_number.h
@@ -4,7 +4,7 @@
 #include "esphome/core/automation.h"
 #include "esphome/core/component.h"
 #include "esphome/core/preferences.h"
-#include "../template_lambda.h"
+#include "esphome/core/template_lambda.h"
 
 namespace esphome {
 namespace template_ {
diff --git a/esphome/components/template/select/template_select.h b/esphome/components/template/select/template_select.h
index 53cadfa303..1c33153872 100644
--- a/esphome/components/template/select/template_select.h
+++ b/esphome/components/template/select/template_select.h
@@ -4,7 +4,7 @@
 #include "esphome/core/automation.h"
 #include "esphome/core/component.h"
 #include "esphome/core/preferences.h"
-#include "../template_lambda.h"
+#include "esphome/core/template_lambda.h"
 
 namespace esphome {
 namespace template_ {
diff --git a/esphome/components/template/sensor/template_sensor.h b/esphome/components/template/sensor/template_sensor.h
index 27980414f9..793d754a0f 100644
--- a/esphome/components/template/sensor/template_sensor.h
+++ b/esphome/components/template/sensor/template_sensor.h
@@ -1,7 +1,7 @@
 #pragma once
 
 #include "esphome/core/component.h"
-#include "../template_lambda.h"
+#include "esphome/core/template_lambda.h"
 #include "esphome/components/sensor/sensor.h"
 
 namespace esphome {
diff --git a/esphome/components/template/switch/template_switch.h b/esphome/components/template/switch/template_switch.h
index f436f657ae..47154fd047 100644
--- a/esphome/components/template/switch/template_switch.h
+++ b/esphome/components/template/switch/template_switch.h
@@ -2,7 +2,7 @@
 
 #include "esphome/core/component.h"
 #include "esphome/core/automation.h"
-#include "../template_lambda.h"
+#include "esphome/core/template_lambda.h"
 #include "esphome/components/switch/switch.h"
 
 namespace esphome {
diff --git a/esphome/components/template/text/template_text.h b/esphome/components/template/text/template_text.h
index fd28800bab..c12021f80e 100644
--- a/esphome/components/template/text/template_text.h
+++ b/esphome/components/template/text/template_text.h
@@ -4,7 +4,7 @@
 #include "esphome/core/automation.h"
 #include "esphome/core/component.h"
 #include "esphome/core/preferences.h"
-#include "../template_lambda.h"
+#include "esphome/core/template_lambda.h"
 
 namespace esphome {
 namespace template_ {
diff --git a/esphome/components/template/text_sensor/template_text_sensor.h b/esphome/components/template/text_sensor/template_text_sensor.h
index fda28f53c7..0d01c72023 100644
--- a/esphome/components/template/text_sensor/template_text_sensor.h
+++ b/esphome/components/template/text_sensor/template_text_sensor.h
@@ -2,7 +2,7 @@
 
 #include "esphome/core/component.h"
 #include "esphome/core/automation.h"
-#include "../template_lambda.h"
+#include "esphome/core/template_lambda.h"
 #include "esphome/components/text_sensor/text_sensor.h"
 
 namespace esphome {
diff --git a/esphome/components/template/valve/template_valve.h b/esphome/components/template/valve/template_valve.h
index 7713091673..23a77ff918 100644
--- a/esphome/components/template/valve/template_valve.h
+++ b/esphome/components/template/valve/template_valve.h
@@ -2,7 +2,7 @@
 
 #include "esphome/core/component.h"
 #include "esphome/core/automation.h"
-#include "../template_lambda.h"
+#include "esphome/core/template_lambda.h"
 #include "esphome/components/valve/valve.h"
 
 namespace esphome {
diff --git a/esphome/components/template/template_lambda.h b/esphome/core/template_lambda.h
similarity index 100%
rename from esphome/components/template/template_lambda.h
rename to esphome/core/template_lambda.h