fix(proxy): don't let an all-null key router_settings shadow team settings

The admin UI persists the whole key/team router-settings form on save,
blank fields included as explicit nulls. Resolution is first-wins (Key
> Team, no per-field merge), so a key whose stored settings are only
nulls carried no opinion yet still shadowed every team setting - the
team's model_group_alias, routing strategy and friends were silently
dropped for all requests on that key.

Strip null entries when parsing stored key/team router_settings and
resolve an all-null form dump to None so the hierarchical fallback
applies. Falsy-but-set values ([] , 0, false) are preserved.

The merge-site null guard for request kwargs shipped separately in
#45227; this covers the source-side case it cannot see.

litellm/proxy/proxy_server.py

@@ -7885,12 +7885,18 @@ class ProxyConfig:
         """
         Parse a router_settings value that may be a dict or a JSON/YAML string.
 
+        Null entries are stripped: the admin UI persists the whole router-settings
+        form on save, including fields left blank as explicit nulls. A null means
+        "no opinion - keep the router default", never "override the default with
+        nothing". An all-null form dump must also resolve to None (not an
+        empty-looking dict) so the key->team hierarchical fallback still applies.
+
         Returns a non-empty dict if valid, otherwise None.
         """
         if value is None:
             return None
 
-        parsed: dict | None = None
+        parsed: dict[str, object] | None = None
         if isinstance(value, dict):
             parsed = value
         elif isinstance(value, str):
@@ -7904,9 +7910,11 @@ class ProxyConfig:
                 except json.JSONDecodeError:
                     pass
 
-        if isinstance(parsed, dict) and parsed:
-            return parsed
-        return None
+        if not isinstance(parsed, dict) or not parsed:
+            return None
+
+        stripped: Final = {k: v for k, v in parsed.items() if v is not None}
+        return stripped or None
 
     async def get_hierarchical_router_settings(
         self,

tests/unit/proxy/proxy_server/test_proxy_config.py

@@ -3747,6 +3747,39 @@ def test_ProxyConfig__parse_router_settings_value_invalid_returns_none():
     assert ProxyConfig._parse_router_settings_value({}) is None
 
 
+def test_ProxyConfig__parse_router_settings_value_strips_null_entries():
+    # The admin UI persists the whole router-settings form, blank fields included.
+    # Nulls must not survive parsing: as per-request kwargs they would shadow the
+    # Router's own defaults (e.g. fallbacks=None disables the global fallback chain).
+    ui_form_dump = {
+        "routing_strategy": "simple-shuffle",
+        "enable_tag_filtering": False,
+        "fallbacks": None,
+        "num_retries": None,
+        "timeout": None,
+        "retry_after": None,
+        "retry_policy": None,
+        "allowed_fails": None,
+        "cooldown_time": None,
+        "model_group_alias": None,
+        "routing_strategy_args": None,
+        "context_window_fallbacks": None,
+    }
+    assert ProxyConfig._parse_router_settings_value(ui_form_dump) == {
+        "routing_strategy": "simple-shuffle",
+        "enable_tag_filtering": False,
+    }
+    # Falsy-but-set values are NOT nulls and must be kept.
+    assert ProxyConfig._parse_router_settings_value({"fallbacks": [], "timeout": 0}) == {
+        "fallbacks": [],
+        "timeout": 0,
+    }
+    # A settings dict that is *only* nulls carries no opinion -> None.
+    assert ProxyConfig._parse_router_settings_value({"fallbacks": None, "timeout": None}) is None
+    # Also for the JSON-string storage shape.
+    assert ProxyConfig._parse_router_settings_value('{"num_retries": null, "timeout": 30}') == {"timeout": 30}
+
+
 # ---------------------------------------------------------------------------
 # ProxyConfig._get_hierarchical_router_settings
 # ---------------------------------------------------------------------------
@@ -3821,6 +3854,32 @@ async def test_ProxyConfig__get_hierarchical_router_settings_key_shadows_team_en
     team_lookup.assert_not_called()
 
 
+@pytest.mark.asyncio
+async def test_ProxyConfig__get_hierarchical_router_settings_all_null_key_falls_back_to_team(monkeypatch):
+    """A key whose stored settings are only nulls (an admin UI form save with
+    every field blank) carries no opinion and must not shadow the team's
+    configured settings, otherwise the team's model_group_alias is silently
+    dropped for every request on that key."""
+    pc = ProxyConfig()
+    fake_key = SimpleNamespace(
+        router_settings={"fallbacks": None, "num_retries": None, "timeout": None},
+        team_id="team-1",
+    )
+    team_settings = {"model_group_alias": {"group-a": "group-b"}, "routing_strategy": "simple-shuffle"}
+    monkeypatch.setattr(
+        "litellm.proxy.proxy_server.get_team_object",
+        AsyncMock(return_value=SimpleNamespace(router_settings=team_settings)),
+    )
+
+    out = await pc._get_hierarchical_router_settings(
+        user_api_key_dict=fake_key,
+        prisma_client=None,
+        proxy_logging_obj=None,
+    )
+
+    assert out == team_settings
+
+
 # ---------------------------------------------------------------------------
 # ProxyConfig._add_router_settings_from_db_config
 # ---------------------------------------------------------------------------