diff --git a/openhands-agent-server/openhands/agent_server/api.py b/openhands-agent-server/openhands/agent_server/api.py
index 0e8eb9acb3..0b10848dfa 100644
--- a/openhands-agent-server/openhands/agent_server/api.py
+++ b/openhands-agent-server/openhands/agent_server/api.py
@@ -29,11 +29,15 @@
 )
 from openhands.agent_server.sockets import sockets_router
 from openhands.agent_server.tool_router import tool_router
+from openhands.agent_server.utils import patch_fastapi_discriminated_union_support
 from openhands.agent_server.vscode_router import vscode_router
 from openhands.agent_server.vscode_service import get_vscode_service
 from openhands.sdk.logger import DEBUG, get_logger
 
 
+# Apply FastAPI patch for discriminated union support
+patch_fastapi_discriminated_union_support()
+
 logger = get_logger(__name__)
 
 
diff --git a/openhands-agent-server/openhands/agent_server/utils.py b/openhands-agent-server/openhands/agent_server/utils.py
index b95138403b..b874ef5f07 100644
--- a/openhands-agent-server/openhands/agent_server/utils.py
+++ b/openhands-agent-server/openhands/agent_server/utils.py
@@ -156,16 +156,7 @@ def patch_fastapi_discriminated_union_support():
 
     Also extracts inline discriminated unions as separate schema components for
     better OpenAPI documentation and Swagger UI display.
-
-    Skips patching if SKIP_FASTAPI_DISCRIMINATED_UNION_FIX environment variable is set.
     """
-    # Skip patching if environment variable flag is defined
-    if os.environ.get("SKIP_FASTAPI_DISCRIMINATED_UNION_FIX"):
-        logger.debug(
-            "Skipping FastAPI discriminated union patch due to environment variable"
-        )
-        return
-
     try:
         import fastapi._compat.v2 as fastapi_v2
         from fastapi import FastAPI
diff --git a/openhands-sdk/openhands/sdk/utils/models.py b/openhands-sdk/openhands/sdk/utils/models.py
index 39e520a29d..4e053d2350 100644
--- a/openhands-sdk/openhands/sdk/utils/models.py
+++ b/openhands-sdk/openhands/sdk/utils/models.py
@@ -12,8 +12,6 @@
     TypeAdapter,
 )
 
-from openhands.agent_server.utils import patch_fastapi_discriminated_union_support
-
 
 logger = logging.getLogger(__name__)
 _rebuild_required = True
@@ -302,7 +300,3 @@ def __init_subclass__(cls, **kwargs):
 def _rebuild_if_required():
     if _rebuild_required:
         rebuild_all()
-
-
-# Always call the FastAPI patch after DiscriminatedUnionMixin definition
-patch_fastapi_discriminated_union_support()