diff --git a/shiny/ui/_chat.py b/shiny/ui/_chat.py
index d1f0c6104..d728bcc7d 100644
--- a/shiny/ui/_chat.py
+++ b/shiny/ui/_chat.py
@@ -489,7 +489,7 @@ def messages(
                     transform_user == "last" and i == len(messages) - 1
                 )
             content_key = m["transform_key" if transform else "pre_transform_key"]
-            chat_msg = ChatMessage(content=m[content_key], role=m["role"])
+            chat_msg = ChatMessage(content=str(m[content_key]), role=m["role"])
             if not isinstance(format, MISSING_TYPE):
                 chat_msg = as_provider_message(chat_msg, format)
             res.append(chat_msg)
@@ -635,7 +635,7 @@ async def _send_append_message(
         content_type = "html" if isinstance(content, HTML) else "markdown"
 
         msg = ClientMessage(
-            content=content,
+            content=str(content),
             role=message["role"],
             content_type=content_type,
             chunk_type=chunk_type,
@@ -790,7 +790,7 @@ async def _transform_message(
         if content is None:
             return None
 
-        res[key] = content
+        res[key] = content  # type: ignore
 
         return res
 
@@ -950,7 +950,7 @@ def user_input(self, transform: bool = False) -> str | None:
         if msg is None:
             return None
         key = "content_server" if transform else "content_client"
-        return msg[key]
+        return str(msg[key])
 
     def _user_input(self) -> str:
         id = self.user_input_id
diff --git a/shiny/ui/_chat_normalize.py b/shiny/ui/_chat_normalize.py
index 2d5063324..7bec8102c 100644
--- a/shiny/ui/_chat_normalize.py
+++ b/shiny/ui/_chat_normalize.py
@@ -2,6 +2,8 @@
 from abc import ABC, abstractmethod
 from typing import TYPE_CHECKING, Any, Optional, cast
 
+from htmltools import HTML
+
 from ._chat_types import ChatMessage
 
 if TYPE_CHECKING:
@@ -49,10 +51,10 @@ def normalize_chunk(self, chunk: Any) -> ChatMessage:
         return ChatMessage(content=x or "", role="assistant")
 
     def can_normalize(self, message: Any) -> bool:
-        return isinstance(message, str) or message is None
+        return isinstance(message, (str, HTML)) or message is None
 
     def can_normalize_chunk(self, chunk: Any) -> bool:
-        return isinstance(chunk, str) or chunk is None
+        return isinstance(chunk, (str, HTML)) or chunk is None
 
 
 class DictNormalizer(BaseMessageNormalizer):
diff --git a/shiny/ui/_chat_types.py b/shiny/ui/_chat_types.py
index 458286720..34924904e 100644
--- a/shiny/ui/_chat_types.py
+++ b/shiny/ui/_chat_types.py
@@ -2,6 +2,8 @@
 
 from typing import Literal, TypedDict
 
+from htmltools import HTML
+
 Role = Literal["assistant", "user", "system"]
 
 
@@ -14,7 +16,7 @@ class ChatMessage(TypedDict):
 
 # A message once transformed have been applied
 class TransformedMessage(TypedDict):
-    content_client: str
+    content_client: str | HTML
     content_server: str
     role: Role
     transform_key: Literal["content_client", "content_server"]
diff --git a/tests/playwright/shiny/components/chat/transform_assistant/app.py b/tests/playwright/shiny/components/chat/transform_assistant/app.py
index fe89d84b5..17f46a2d3 100644
--- a/tests/playwright/shiny/components/chat/transform_assistant/app.py
+++ b/tests/playwright/shiny/components/chat/transform_assistant/app.py
@@ -1,3 +1,5 @@
+from typing import Union
+
 from shiny.express import render, ui
 
 # Set some Shiny page options
@@ -12,7 +14,7 @@
 
 # TODO: test with append_message_stream() as well
 @chat.transform_assistant_response
-def transform(content: str) -> str:
+def transform(content: str) -> Union[str, ui.HTML]:
     if content == "return HTML":
         return ui.HTML(f"<b>Transformed response</b>: {content}")
     else:
diff --git a/tests/playwright/shiny/components/chat/transform_assistant/test_chat_transform_assistant.py b/tests/playwright/shiny/components/chat/transform_assistant/test_chat_transform_assistant.py
index d38f35807..b0d177f15 100644
--- a/tests/playwright/shiny/components/chat/transform_assistant/test_chat_transform_assistant.py
+++ b/tests/playwright/shiny/components/chat/transform_assistant/test_chat_transform_assistant.py
@@ -1,7 +1,6 @@
 from playwright.sync_api import Page, expect
 from utils.deploy_utils import skip_on_webkit
 
-from shiny import ui
 from shiny.playwright import controller
 from shiny.run import ShinyAppProc
 
@@ -48,7 +47,7 @@ def test_validate_chat_transform_assistant(page: Page, local_app: ShinyAppProc)
             {"content": "Transformed response: `hello`", "role": "assistant"},
             {"content": "return HTML", "role": "user"},
             {
-                "content": ui.HTML("<b>Transformed response</b>: return HTML"),
+                "content": "<b>Transformed response</b>: return HTML",
                 "role": "assistant",
             },
         ]