fix(router): register a router in the live set when it gains a deployment

_live_routers was only joined when a router was constructed with a model_list,
but a router built empty is populated through add_deployment, and the empty
branch exists for exactly that. Such a router was invisible to the live-router
scan, so deleting the deployment from another router released the shared
cost-map key while it was still serving that id.

Joining the set where a deployment enters the list covers every path, and it
also lets a price reload rebuild what a dynamically built router serves.

litellm/router.py

@@ -9586,6 +9586,9 @@ class Router:
         """
         idx: Final = len(self.model_list)
         self.model_list.append(model)
+        # A router built without a model_list joins the registry here instead, so a price
+        # reload rebuilds what it serves and delete_deployment can see it still holds an id.
+        _live_routers.add(self)
         self._invalidate_model_group_info_cache()
         self._invalidate_access_groups_cache()
 

tests/test_litellm/test_router_model_cost_isolation.py

@@ -382,6 +382,36 @@ def test_should_keep_the_cost_map_key_while_another_router_still_serves_it():
         _restore_model_cost_entries(original)
 
 
+def test_should_keep_the_cost_map_key_while_a_dynamically_built_router_serves_it():
+    """A router built with no model_list still serves whatever add_deployment gives it, so it
+    counts when deciding whether the shared cost-map claim can be released."""
+    from litellm.router import _DEPLOYMENT_COST_MAP_KEYS
+
+    model_id = "deployment-added-dynamically"
+    original = {model_id: litellm.model_cost.get(model_id)}
+    entry = {
+        "model_name": "added-dynamically",
+        "litellm_params": {"model": "gpt-4o-mini", "mock_response": "ok"},
+        "model_info": {"id": model_id, "input_cost_per_token": 0.005},
+    }
+    configured = Router(model_list=[entry])
+    dynamic = Router()
+    dynamic.add_deployment(deployment=Deployment(**entry))
+
+    try:
+        assert configured.delete_deployment(id=model_id) is not None
+
+        assert model_id in _DEPLOYMENT_COST_MAP_KEYS, (
+            "the claim was released while a dynamically built router still served the deployment"
+        )
+
+        assert dynamic.delete_deployment(id=model_id) is not None
+        assert model_id not in _DEPLOYMENT_COST_MAP_KEYS
+    finally:
+        _DEPLOYMENT_COST_MAP_KEYS.discard(model_id)
+        _restore_model_cost_entries(original)
+
+
 def test_should_preserve_builtin_pricing_regardless_of_deployment_order():
     """
     The built-in pricing should be preserved no matter which deployment