diff --git a/homeassistant/components/conversation/agent_manager.py b/homeassistant/components/conversation/agent_manager.py
index ce3a0cf028d..5ff47977d88 100644
--- a/homeassistant/components/conversation/agent_manager.py
+++ b/homeassistant/components/conversation/agent_manager.py
@@ -79,6 +79,9 @@ async def async_converse(
     extra_system_prompt: str | None = None,
 ) -> ConversationResult:
     """Process text and get intent."""
+    if agent_id is None:
+        agent_id = HOME_ASSISTANT_AGENT
+
     agent = async_get_agent(hass, agent_id)
 
     if agent is None:
diff --git a/homeassistant/components/conversation/default_agent.py b/homeassistant/components/conversation/default_agent.py
index bd7450e5a0f..23c201d7579 100644
--- a/homeassistant/components/conversation/default_agent.py
+++ b/homeassistant/components/conversation/default_agent.py
@@ -381,7 +381,7 @@ class DefaultAgent(ConversationEntity):
             speech: str = response.speech.get("plain", {}).get("speech", "")
             chat_log.async_add_assistant_content_without_tools(
                 AssistantContent(
-                    agent_id=user_input.agent_id,  # type: ignore[arg-type]
+                    agent_id=user_input.agent_id,
                     content=speech,
                 )
             )
diff --git a/homeassistant/components/conversation/http.py b/homeassistant/components/conversation/http.py
index 8134ecb0eee..4d8526a4fd4 100644
--- a/homeassistant/components/conversation/http.py
+++ b/homeassistant/components/conversation/http.py
@@ -195,7 +195,7 @@ async def websocket_hass_agent_debug(
             conversation_id=None,
             device_id=msg.get("device_id"),
             language=msg.get("language", hass.config.language),
-            agent_id=None,
+            agent_id=agent.entity_id,
         )
         result_dict: dict[str, Any] | None = None
 
diff --git a/homeassistant/components/conversation/models.py b/homeassistant/components/conversation/models.py
index 9462c597f23..08a68fa0164 100644
--- a/homeassistant/components/conversation/models.py
+++ b/homeassistant/components/conversation/models.py
@@ -37,7 +37,7 @@ class ConversationInput:
     language: str
     """Language of the request."""
 
-    agent_id: str | None = None
+    agent_id: str
     """Agent to use for processing."""
 
     extra_system_prompt: str | None = None
diff --git a/homeassistant/components/google_generative_ai_conversation/conversation.py b/homeassistant/components/google_generative_ai_conversation/conversation.py
index 8a6c5563601..0f26c93da25 100644
--- a/homeassistant/components/google_generative_ai_conversation/conversation.py
+++ b/homeassistant/components/google_generative_ai_conversation/conversation.py
@@ -261,8 +261,6 @@ class GoogleGenerativeAIConversationEntity(
         chat_log: conversation.ChatLog,
     ) -> conversation.ConversationResult:
         """Call the API."""
-
-        assert user_input.agent_id
         options = self.entry.options
 
         try:
diff --git a/homeassistant/components/openai_conversation/conversation.py b/homeassistant/components/openai_conversation/conversation.py
index 73dafa1c48d..eaa62bd1adc 100644
--- a/homeassistant/components/openai_conversation/conversation.py
+++ b/homeassistant/components/openai_conversation/conversation.py
@@ -198,7 +198,6 @@ class OpenAIConversationEntity(
         chat_log: conversation.ChatLog,
     ) -> conversation.ConversationResult:
         """Call the API."""
-        assert user_input.agent_id
         options = self.entry.options
 
         try:
diff --git a/tests/components/conversation/test_init.py b/tests/components/conversation/test_init.py
index 6900ba2d419..9ac5c7d16a4 100644
--- a/tests/components/conversation/test_init.py
+++ b/tests/components/conversation/test_init.py
@@ -271,6 +271,7 @@ async def test_async_handle_sentence_triggers(
             text="my trigger",
             context=Context(),
             conversation_id=None,
+            agent_id=conversation.HOME_ASSISTANT_AGENT,
             device_id=device_id,
             language=hass.config.language,
         ),
@@ -306,6 +307,7 @@ async def test_async_handle_intents(hass: HomeAssistant) -> None:
         ConversationInput(
             text="I'd like to order a stout",
             context=Context(),
+            agent_id=conversation.HOME_ASSISTANT_AGENT,
             conversation_id=None,
             device_id=None,
             language=hass.config.language,
@@ -321,6 +323,7 @@ async def test_async_handle_intents(hass: HomeAssistant) -> None:
         hass,
         ConversationInput(
             text="this sentence does not exist",
+            agent_id=conversation.HOME_ASSISTANT_AGENT,
             context=Context(),
             conversation_id=None,
             device_id=None,
diff --git a/tests/components/conversation/test_trigger.py b/tests/components/conversation/test_trigger.py
index 9b57bb43b58..3aa8ae2939f 100644
--- a/tests/components/conversation/test_trigger.py
+++ b/tests/components/conversation/test_trigger.py
@@ -5,7 +5,7 @@ import logging
 import pytest
 import voluptuous as vol
 
-from homeassistant.components.conversation import default_agent
+from homeassistant.components.conversation import HOME_ASSISTANT_AGENT, default_agent
 from homeassistant.components.conversation.const import DATA_DEFAULT_ENTITY
 from homeassistant.components.conversation.models import ConversationInput
 from homeassistant.core import Context, HomeAssistant, ServiceCall
@@ -82,7 +82,7 @@ async def test_if_fires_on_event(
         "details": {},
         "device_id": None,
         "user_input": {
-            "agent_id": None,
+            "agent_id": HOME_ASSISTANT_AGENT,
             "context": context.as_dict(),
             "conversation_id": None,
             "device_id": None,
@@ -230,7 +230,7 @@ async def test_response_same_sentence(
         "details": {},
         "device_id": None,
         "user_input": {
-            "agent_id": None,
+            "agent_id": HOME_ASSISTANT_AGENT,
             "context": context.as_dict(),
             "conversation_id": None,
             "device_id": None,
@@ -408,7 +408,7 @@ async def test_same_trigger_multiple_sentences(
         "details": {},
         "device_id": None,
         "user_input": {
-            "agent_id": None,
+            "agent_id": HOME_ASSISTANT_AGENT,
             "context": context.as_dict(),
             "conversation_id": None,
             "device_id": None,
@@ -636,7 +636,7 @@ async def test_wildcards(hass: HomeAssistant, service_calls: list[ServiceCall])
         },
         "device_id": None,
         "user_input": {
-            "agent_id": None,
+            "agent_id": HOME_ASSISTANT_AGENT,
             "context": context.as_dict(),
             "conversation_id": None,
             "device_id": None,