diff --git a/litellm/proxy/management_endpoints/key_management_endpoints.py b/litellm/proxy/management_endpoints/key_management_endpoints.py
index 4fa6a1d0571..455afdefe7a 100644
--- a/litellm/proxy/management_endpoints/key_management_endpoints.py
+++ b/litellm/proxy/management_endpoints/key_management_endpoints.py
@@ -2267,6 +2267,11 @@ async def prepare_key_update_data(
 
     if "duration" in non_default_values:
         duration: Final = non_default_values.pop("duration")
+        if duration is None or isinstance(duration, str):
+            # Reject garbage here so bulk/team-bulk/regenerate callers that
+            # skip _validate_update_key_data still 400 instead of 500ing
+            # inside duration math below.
+            validate_key_duration(duration)
         if duration is None or duration == "-1":
             # Set expires to None to indicate the key never expires
             non_default_values["expires"] = None
@@ -2277,6 +2282,9 @@ async def prepare_key_update_data(
 
     if "budget_duration" in non_default_values:
         budget_duration: Final = non_default_values.pop("budget_duration")
+        if budget_duration is None or isinstance(budget_duration, str):
+            # Same write-boundary guarantee as duration above.
+            validate_budget_duration(budget_duration)
         if budget_duration is None:
             non_default_values["budget_duration"] = None
             non_default_values["budget_reset_at"] = None
diff --git a/tests/test_litellm/proxy/management_endpoints/test_key_management_endpoints.py b/tests/test_litellm/proxy/management_endpoints/test_key_management_endpoints.py
index 65cc23ea67f..f06f40c98dd 100644
--- a/tests/test_litellm/proxy/management_endpoints/test_key_management_endpoints.py
+++ b/tests/test_litellm/proxy/management_endpoints/test_key_management_endpoints.py
@@ -1796,6 +1796,32 @@ async def test_update_service_account_works_with_team_id():
     await prepare_key_update_data(data=data, existing_key_row=existing_key)
 
 
+@pytest.mark.asyncio
+async def test_update_key_garbage_duration_400s_instead_of_500():
+    """Regression: bulk/team-bulk/regenerate skip _validate_update_key_data, so
+    prepare_key_update_data itself must reject garbage durations (#39711)."""
+    data = UpdateKeyRequest(key="sk-1", duration="not-a-duration")
+    existing_key = LiteLLM_VerificationToken(token="hashed")
+
+    with pytest.raises(HTTPException) as exc_info:
+        await prepare_key_update_data(data=data, existing_key_row=existing_key)
+    assert exc_info.value.status_code == 400
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize("duration", ["7d", None, "-1"])
+async def test_update_key_valid_durations_still_flow_through(duration):
+    data = UpdateKeyRequest(key="sk-1", duration=duration)
+    existing_key = LiteLLM_VerificationToken(token="hashed")
+
+    updated = await prepare_key_update_data(data=data, existing_key_row=existing_key)
+
+    if duration in (None, "-1"):
+        assert updated["expires"] is None
+    else:
+        assert updated["expires"] is not None
+
+
 @pytest.mark.asyncio
 @pytest.mark.parametrize("flag_value", [True, False])
 async def test_update_key_enable_prompt_caching_folds_into_metadata(flag_value):