diff --git a/homeassistant/components/anthropic/entity.py b/homeassistant/components/anthropic/entity.py
index 62f39bd4a02c..658267219e35 100644
--- a/homeassistant/components/anthropic/entity.py
+++ b/homeassistant/components/anthropic/entity.py
@@ -858,6 +858,11 @@ class AnthropicBaseLLMEntity(Entity):
                     ]
                 )
                 messages.extend(new_messages)
+            except anthropic.AuthenticationError as err:
+                self.entry.async_start_reauth(self.hass)
+                raise HomeAssistantError(
+                    "Authentication error with Anthropic API, reauthentication required"
+                ) from err
             except anthropic.AnthropicError as err:
                 raise HomeAssistantError(
                     f"Sorry, I had a problem talking to Anthropic: {err}"
diff --git a/tests/components/anthropic/test_conversation.py b/tests/components/anthropic/test_conversation.py
index b3aa18265817..8c3327b6cc96 100644
--- a/tests/components/anthropic/test_conversation.py
+++ b/tests/components/anthropic/test_conversation.py
@@ -4,7 +4,7 @@ import datetime
 from typing import Any
 from unittest.mock import AsyncMock, Mock, patch
 
-from anthropic import RateLimitError
+from anthropic import AuthenticationError, RateLimitError
 from anthropic.types import (
     CitationsWebSearchResultLocation,
     CitationWebSearchResultLocationParam,
@@ -36,8 +36,10 @@ from homeassistant.components.anthropic.const import (
     CONF_WEB_SEARCH_REGION,
     CONF_WEB_SEARCH_TIMEZONE,
     CONF_WEB_SEARCH_USER_LOCATION,
+    DOMAIN,
 )
 from homeassistant.components.anthropic.entity import CitationDetails, ContentDetails
+from homeassistant.config_entries import SOURCE_REAUTH
 from homeassistant.const import CONF_LLM_HASS_API
 from homeassistant.core import Context, HomeAssistant
 from homeassistant.exceptions import HomeAssistantError
@@ -107,7 +109,7 @@ async def test_error_handling(
     mock_init_component,
     mock_create_stream: AsyncMock,
 ) -> None:
-    """Test that the default prompt works."""
+    """Test error handling."""
     mock_create_stream.side_effect = RateLimitError(
         message=None,
         response=Response(status_code=429, request=Request(method="POST", url=URL())),
@@ -122,6 +124,38 @@ async def test_error_handling(
     assert result.response.error_code == "unknown", result
 
 
+async def test_auth_error_handling(
+    hass: HomeAssistant,
+    mock_config_entry: MockConfigEntry,
+    mock_init_component,
+    mock_create_stream: AsyncMock,
+) -> None:
+    """Test reauth after authentication error during conversation."""
+    mock_create_stream.side_effect = AuthenticationError(
+        message="Invalid API key",
+        response=Response(status_code=403, request=Request(method="POST", url=URL())),
+        body=None,
+    )
+
+    result = await conversation.async_converse(
+        hass, "hello", None, Context(), agent_id="conversation.claude_conversation"
+    )
+
+    assert result.response.response_type == intent.IntentResponseType.ERROR
+    assert result.response.error_code == "unknown", result
+
+    await hass.async_block_till_done()
+    flows = hass.config_entries.flow.async_progress()
+    assert len(flows) == 1
+
+    flow = flows[0]
+    assert flow["step_id"] == "reauth_confirm"
+    assert flow["handler"] == DOMAIN
+    assert "context" in flow
+    assert flow["context"]["source"] == SOURCE_REAUTH
+    assert flow["context"]["entry_id"] == mock_config_entry.entry_id
+
+
 async def test_template_error(
     hass: HomeAssistant,
     mock_config_entry: MockConfigEntry,