Fix /key/update rejecting budget_limits clear requests with HTTP 400

Sending budget_limits: [] or null to /key/update returned HTTP 400, so
once a key had budget windows the last one could never be removed.

prepare_key_update_data only json.dumps'd budget_limits when the value
was truthy, so [] and None passed through raw to the Prisma Json?
column; jsonify_object only serializes dicts, and prisma-client-py has
no DbNull sentinel for Json? writes, so Prisma rejected both shapes.

Serialize the clear case explicitly as the JSON literal null, matching
how memory_endpoints encodes metadata for the same column type. Truthy
values keep the existing reset_at window initialization path.

Fixes #30067.

litellm/proxy/management_endpoints/key_management_endpoints.py

@@ -1861,18 +1861,23 @@ async def prepare_key_update_data(
             non_default_values["budget_reset_at"] = key_reset_at
             non_default_values["budget_duration"] = budget_duration
 
-    if "budget_limits" in non_default_values and non_default_values["budget_limits"]:
-        from litellm.proxy.common_utils.timezone_utils import get_budget_reset_time
-
+    if "budget_limits" in non_default_values:
         raw_windows = non_default_values["budget_limits"]
-        initialized_windows = []
-        for window in raw_windows:
-            w = window if isinstance(window, dict) else window.model_dump()
-            w["reset_at"] = get_budget_reset_time(
-                budget_duration=w["budget_duration"]
-            ).isoformat()
-            initialized_windows.append(w)
-        non_default_values["budget_limits"] = json.dumps(initialized_windows)
+        if raw_windows:
+            from litellm.proxy.common_utils.timezone_utils import get_budget_reset_time
+
+            initialized_windows = []
+            for window in raw_windows:
+                w = window if isinstance(window, dict) else window.model_dump()
+                w["reset_at"] = get_budget_reset_time(
+                    budget_duration=w["budget_duration"]
+                ).isoformat()
+                initialized_windows.append(w)
+            non_default_values["budget_limits"] = json.dumps(initialized_windows)
+        else:
+            # [] / None clears the field; prisma-client-py has no DbNull
+            # sentinel for Json? columns, so store the JSON literal null
+            non_default_values["budget_limits"] = json.dumps(None)
 
     if "object_permission" in non_default_values:
         non_default_values = await _handle_update_object_permission(

tests/test_litellm/proxy/management_endpoints/test_key_management_endpoints.py

@@ -1488,6 +1488,65 @@ async def test_prepare_key_update_data_duration_none_never_expires():
     assert result["expires"] is None
 
 
+@pytest.mark.asyncio
+@pytest.mark.parametrize("cleared_value", [[], None])
+async def test_prepare_key_update_data_budget_limits_clears_field(cleared_value):
+    """budget_limits=[] / None must serialize to JSON null, never reach Prisma raw."""
+    from litellm.proxy._types import UpdateKeyRequest
+    from litellm.proxy.management_endpoints.key_management_endpoints import (
+        prepare_key_update_data,
+    )
+
+    existing_key = LiteLLM_VerificationToken(
+        token="test-token",
+        key_alias="test-key",
+        models=["gpt-3.5-turbo"],
+        user_id="test-user",
+        team_id=None,
+        metadata={},
+    )
+
+    update_request = UpdateKeyRequest(key="test-token", budget_limits=cleared_value)
+
+    result = await prepare_key_update_data(
+        data=update_request, existing_key_row=existing_key
+    )
+
+    assert result["budget_limits"] == json.dumps(None)
+
+
+@pytest.mark.asyncio
+async def test_prepare_key_update_data_budget_limits_serializes_windows():
+    """Non-empty budget_limits stay JSON-encoded with reset_at initialized."""
+    from litellm.proxy._types import UpdateKeyRequest
+    from litellm.proxy.management_endpoints.key_management_endpoints import (
+        prepare_key_update_data,
+    )
+
+    existing_key = LiteLLM_VerificationToken(
+        token="test-token",
+        key_alias="test-key",
+        models=["gpt-3.5-turbo"],
+        user_id="test-user",
+        team_id=None,
+        metadata={},
+    )
+
+    update_request = UpdateKeyRequest(
+        key="test-token",
+        budget_limits=[{"budget_duration": "1d", "max_budget": 10.0}],
+    )
+
+    result = await prepare_key_update_data(
+        data=update_request, existing_key_row=existing_key
+    )
+
+    windows = json.loads(result["budget_limits"])
+    assert isinstance(result["budget_limits"], str)
+    assert windows[0]["max_budget"] == 10.0
+    assert windows[0]["reset_at"] is not None
+
+
 @pytest.mark.asyncio
 async def test_validate_team_id_used_in_service_account_request_requires_team_id():
     """