diff --git a/sdk/ai/azure-ai-projects/azure/ai/projects/models/_models.py b/sdk/ai/azure-ai-projects/azure/ai/projects/models/_models.py
index 27b70e37ce48..538af0d1dffe 100644
--- a/sdk/ai/azure-ai-projects/azure/ai/projects/models/_models.py
+++ b/sdk/ai/azure-ai-projects/azure/ai/projects/models/_models.py
@@ -3943,7 +3943,7 @@ class EasyInputMessage(InputItem, discriminator="message"):
     """The role of the message input. One of ``user``, ``assistant``, ``system``, or ``developer``.
      Required. Is one of the following types: Literal[\"user\"], Literal[\"assistant\"],
      Literal[\"system\"], Literal[\"developer\"]"""
-    content: Union[str, list["_models.InputContent"]] = rest_field(
+    content: Union["str", list["_models.InputContent"]] = rest_field(
         visibility=["read", "create", "update", "delete", "query"]
     )
     """Text, image, or audio input to the model, used to generate a response. Can also contain
@@ -3962,7 +3962,7 @@ def __init__(
         self,
         *,
         role: Literal["user", "assistant", "system", "developer"],
-        content: Union[str, list["_models.InputContent"]],
+        content: Union["str", list["_models.InputContent"]],
         status: Optional[Literal["in_progress", "completed", "incomplete"]] = None,
     ) -> None: ...
 
diff --git a/sdk/ai/azure-ai-projects/post-emitter-fixes.cmd b/sdk/ai/azure-ai-projects/post-emitter-fixes.cmd
index 34f29db3477a..a704810ce700 100644
--- a/sdk/ai/azure-ai-projects/post-emitter-fixes.cmd
+++ b/sdk/ai/azure-ai-projects/post-emitter-fixes.cmd
@@ -17,6 +17,9 @@ REM Rename `"items_property": items`, to `"items": items` in search_memories and
 powershell -Command "(Get-Content azure\ai\projects\aio\operations\_operations.py) -replace '\"items_property\": items', '\"items\": items' | Set-Content azure\ai\projects\aio\operations\_operations.py"
 powershell -Command "(Get-Content azure\ai\projects\operations\_operations.py) -replace '\"items_property\": items', '\"items\": items' | Set-Content azure\ai\projects\operations\_operations.py"
 
+REM Fix content type annotation: replace `content: Union[str, list[...]]` with `content: Union["str", list[...]]`. Otherwise serialization fails.
+powershell -Command "(Get-Content azure\ai\projects\models\_models.py) -replace 'content: Union\[str, list\[\"_models\.InputContent\"\]\]', 'content: Union[\"str\", list[\"_models.InputContent\"]]' | Set-Content azure\ai\projects\models\_models.py"
+
 REM Fix Sphinx issue in class ToolChoiceAllowed, in "tools" property doc string. Everything should be aligned including JSON example, like this:
 REM """A list of tool definitions that the model should be allowed to call. For the Responses API, the
 REM  list of tool definitions might look like: