[entity_transport] Reject a timeout on a packet interface with a session layer

With a session and a reliability layer, the update interval defaults to
never, and unchanged states are not sent again, so a receiver's timeout
cleared them on a working link. The session layer already clears the
states when the other device cannot be reached.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

esphome/components/entity_transport/__init__.py

@@ -7,7 +7,8 @@ import esphome.codegen as cg
 from esphome.components import packet_interface
 from esphome.components.binary_sensor import BinarySensor
 from esphome.components.const import CONF_PROVIDER, CONF_REMOTE_ID
-from esphome.components.packet_interface import CONF_PACKET_INTERFACE
+from esphome.components.packet_interface import CONF_PACKET_INTERFACE, interface_layers
+from esphome.components.packet_interface.layers import CONF_SESSION
 from esphome.components.sensor import Sensor
 import esphome.config_validation as cv
 from esphome.const import (
@@ -95,6 +96,21 @@ CONFIG_SCHEMA = cv.Schema(
 ).extend(cv.COMPONENT_SCHEMA)
 
 
+def _final_validate(config: ConfigType) -> ConfigType:
+    """With a session layer, which clears the states when the peer is lost, nothing would refresh an unchanged state."""
+    layers = interface_layers(config[CONF_PACKET_INTERFACE])
+    if CONF_TIMEOUT in config and layers is not None and CONF_SESSION in layers:
+        raise cv.Invalid(
+            f"Not allowed with a '{CONF_SESSION}' layer, which already clears the states when the other device "
+            "cannot be reached; unchanged states may never be sent again, so they would time out on a working link",
+            path=[CONF_TIMEOUT],
+        )
+    return config
+
+
+FINAL_VALIDATE_SCHEMA = _final_validate
+
+
 def _validate_remote_id(config: ConfigType) -> ConfigType:
     if CONF_REMOTE_ID not in config and not config[CONF_ID].is_manual:
         raise cv.Invalid(f"'{CONF_REMOTE_ID}' is required unless '{CONF_ID}' is given")

esphome/components/packet_interface/__init__.py

@@ -6,7 +6,9 @@ import esphome.codegen as cg
 from esphome.components.const import CONF_MAX_PACKET_SIZE
 import esphome.config_validation as cv
 from esphome.const import CONF_ID
+from esphome.core import ID
 from esphome.cpp_generator import MockObj, MockObjClass
+import esphome.final_validate as fv
 from esphome.types import ConfigType
 
 from . import layers
@@ -34,6 +36,7 @@ LINK_LAYER_SCHEMAS = stack_schema()
 __all__ = [
     "CONF_MAX_PACKET_SIZE",
     "CONF_PACKET_INTERFACE",
+    "interface_layers",
     "PacketInterface",
     "new_packet_interface",
     "packet_interface_ns",
@@ -59,6 +62,14 @@ def _validate_layers(config: ConfigType) -> ConfigType:
     return config
 
 
+def interface_layers(interface_id: ID) -> ConfigType | None:
+    """During final validation, the configuration holding the layers of the packet interface with this id."""
+    for interface in fv.full_config.get().get(DOMAIN, []):
+        if interface[CONF_ID].id == interface_id.id:
+            return interface
+    return None
+
+
 def packet_interface_schema(
     class_: MockObjClass,
     default_max_packet_size: int,

tests/components/entity_transport/common.yaml

@@ -2,7 +2,6 @@ packet_interface:
   - platform: uart
     id: entity_transport_link
     reliability:
-    session:
 
 sensor:
   - platform: template

tests/unit_tests/test_entity_transport.py

@@ -4,7 +4,11 @@ import pytest
 
 from esphome import config_validation as cv
 from esphome.components.const import CONF_PROVIDER, CONF_REMOTE_ID
-from esphome.components.entity_transport import CONFIG_SCHEMA, remote_entity_schema
+from esphome.components.entity_transport import (
+    CONFIG_SCHEMA,
+    FINAL_VALIDATE_SCHEMA,
+    remote_entity_schema,
+)
 from esphome.components.sensor import sensor_schema
 from esphome.const import (
     CONF_ID,
@@ -14,6 +18,7 @@ from esphome.const import (
     SCHEDULER_DONT_RUN,
 )
 from esphome.core import ID
+import esphome.final_validate as fv
 
 
 def test_remote_id_required_without_manual_id(setup_core) -> None:
@@ -51,6 +56,29 @@ def test_timeout(setup_core) -> None:
         CONFIG_SCHEMA({**base, CONF_TIMEOUT: "500ms"})
 
 
+@pytest.mark.parametrize(
+    ("layers", "allowed"),
+    [({}, True), ({"reliability": None}, True), ({"session": {}}, False)],
+)
+def test_timeout_not_with_session(setup_core, layers: dict, allowed: bool) -> None:
+    """With a session layer, unchanged states are never sent again, so a timeout would clear them on a working link."""
+    config = CONFIG_SCHEMA({"packet_interface": "link", CONF_TIMEOUT: "1min"})
+    token = fv.full_config.set(
+        {"packet_interface": [{CONF_ID: ID("link", is_manual=True), **layers}]}
+    )
+    try:
+        if allowed:
+            FINAL_VALIDATE_SCHEMA(config)
+        else:
+            with pytest.raises(cv.Invalid, match="session") as error:
+                FINAL_VALIDATE_SCHEMA(config)
+            assert error.value.path == [CONF_TIMEOUT]
+        # Without a timeout, any layers are fine
+        FINAL_VALIDATE_SCHEMA(CONFIG_SCHEMA({"packet_interface": "link"}))
+    finally:
+        fv.full_config.reset(token)
+
+
 def test_advertised_ids_unique(setup_core) -> None:
     """Two entities of one type cannot be sent with the same id."""
     base = {"packet_interface": "link"}