diff --git a/src/schemas/models.py b/src/schemas/models.py
index fdc004b..673a1df 100644
--- a/src/schemas/models.py
+++ b/src/schemas/models.py
@@ -1,13 +1,22 @@
-from pydantic import BaseModel, Field
-from typing import Any, Literal, Optional
+from pydantic import BaseModel, BeforeValidator, Field
+from typing import Annotated, Any, Literal, Optional
 
 ClientMode = Literal["direct", "browser_addon"]
 
+
+def normalize_client_mode(value: Any) -> Any:
+    if value is None or (isinstance(value, str) and value.strip() == ""):
+        return "direct"
+    return value
+
+
+NormalizedClientMode = Annotated[ClientMode, BeforeValidator(normalize_client_mode)]
+
 class ProcessRequest_chat(BaseModel):
     sessionId: str = Field(..., max_length=64)
     timeStamp: str = Field(..., max_length=32)
     text: str = Field(...)
-    clientMode: ClientMode = "direct"
+    clientMode: NormalizedClientMode = "direct"
     needFormUpdate: bool = False
     useTextChunk: bool = False
 
diff --git a/test/test_client_mode.py b/test/test_client_mode.py
index a0764b9..1afa949 100644
--- a/test/test_client_mode.py
+++ b/test/test_client_mode.py
@@ -36,6 +36,18 @@ def test_unknown_client_mode_is_rejected():
         )
 
 
+@pytest.mark.parametrize("client_mode", ["", "  ", None])
+def test_empty_client_mode_defaults_to_direct(client_mode):
+    request = ProcessRequest_chat(
+        sessionId="session-1",
+        timeStamp="1",
+        text="你好",
+        clientMode=client_mode,
+    )
+
+    assert request.clientMode == "direct"
+
+
 @pytest.mark.parametrize(
     ("client_mode", "raw_code", "expected"),
     [