diff --git a/esphome/core/entity_base.cpp b/esphome/core/entity_base.cpp
index 17254c78c8..dc27c1e56a 100644
--- a/esphome/core/entity_base.cpp
+++ b/esphome/core/entity_base.cpp
@@ -57,9 +57,10 @@ void EntityBase::configure_entity_(const char *name, uint32_t object_id_hash, ui
 }
 
 void EntityBase::set_internal(bool internal) {
+  // Remove the after-setup path in 2027.3.0 and ignore the call instead.
   if (App.is_setup_complete()) {
-    ESP_LOGE(TAG, "'%s': set_internal() called after setup, ignored", this->get_name().c_str());
-    return;
+    ESP_LOGE(TAG, "'%s': set_internal() after setup is undefined behavior, stops working in 2027.3.0",
+             this->get_name().c_str());
   }
   this->flags_.internal = internal;
 }
diff --git a/esphome/core/entity_base.h b/esphome/core/entity_base.h
index c95d7184d3..d383466fee 100644
--- a/esphome/core/entity_base.h
+++ b/esphome/core/entity_base.h
@@ -92,7 +92,8 @@ class EntityBase {
   // whenever possible: it is guaranteed and has none of the limitations below. Use this only when
   // the decision can only be made at boot. Must be called before MQTT and the API read the flag:
   // from on_boot at the default priority, or a setup() that runs above setup_priority::AFTER_WIFI.
-  // Calls after setup finishes are ignored and log an error.
+  // Calls after setup finishes are undefined behavior: the flag is still written and an error is
+  // logged, and from 2027.3.0 the call will be ignored.
   //
   // Known limitations. Not bugs, so no issue reports please; a PR that removes one with no RAM
   // or performance cost would be considered.
diff --git a/tests/integration/test_set_internal_at_boot.py b/tests/integration/test_set_internal_at_boot.py
index b2c1cc670e..68b0bd1080 100644
--- a/tests/integration/test_set_internal_at_boot.py
+++ b/tests/integration/test_set_internal_at_boot.py
@@ -14,7 +14,7 @@ async def test_set_internal_at_boot(
     run_compiled: RunCompiledFunction,
     api_client_connected: APIClientConnectedFactory,
 ) -> None:
-    """set_internal() in on_boot changes API exposure, later calls are rejected."""
+    """set_internal() in on_boot changes API exposure, later calls log an error."""
     waiter = LineWaiter()
 
     async with (
@@ -31,8 +31,11 @@ async def test_set_internal_at_boot(
         late = next(s for s in services if s.name == "set_internal_late")
         await client.execute_service(late, {})
         await waiter.wait_for(
-            "'Untouched'", "set_internal() called after setup", timeout=5.0
+            "'Untouched'",
+            "set_internal() after setup is undefined behavior",
+            timeout=5.0,
         )
 
+        # Still written during the deprecation window, ignored from 2027.3.0
         entities, _ = await client.list_entities_services()
-        assert "Untouched" in {entity.name for entity in entities}
+        assert "Untouched" not in {entity.name for entity in entities}