diff --git a/litellm/router.py b/litellm/router.py
index dd1cb51d9af..c50fab5fe0f 100644
--- a/litellm/router.py
+++ b/litellm/router.py
@@ -8176,16 +8176,19 @@ class Router:
         entry only when the deployment entry carries a base pricing field, and
         ``off_peak_pricing`` is deliberately kept off the shared entry, so a
         deployment spelling out only its off-peak schedule would otherwise
-        never receive the discount. Every price-bearing backend field is
-        copied, not just the flat token rates: threshold, tiered, service-tier,
-        cache, character, and per-second rates all carry over, so peak-hour
-        billing through the deployment entry matches the shared backend entry
-        exactly. Values are deep-copied to keep the builtin entry isolated, and
-        a flat token rate ``get_model_info`` synthesized as zero for a backend
-        without one is rejected, like ``_inherit_builtin_tiered_output_rate``
-        does, so a tiered-only backend is never marked explicitly priced free.
-        User-specified rates always win; no-op when any base pricing field is
-        already set or the backend model has no canonical entry.
+        never receive the discount. The backend model's entire canonical cost
+        map entry is copied, field by field, so threshold, tiered,
+        service-tier, cache, character, and per-second rates as well as
+        companion billing fields like ``web_search_billing_unit`` and the
+        regional uplift multipliers all carry over, and peak-hour billing
+        through the deployment entry matches the shared backend entry exactly.
+        The raw ``litellm.model_cost`` entry is the copy source rather than
+        ``get_model_info``'s view of it, since that view synthesizes zero flat
+        token rates for backends without one and storing those would mark a
+        tiered-only backend explicitly priced free. Values are deep-copied to
+        keep the builtin entry isolated. User-specified fields always win;
+        no-op when any base pricing field is already set or the backend model
+        has no canonical entry.
         """
         if not model_info.get("off_peak_pricing"):
             return
@@ -8198,13 +8201,12 @@ class Router:
             backend_info: Final = litellm.get_model_info(model=backend_model, custom_llm_provider=custom_llm_provider)
         except Exception:  # noqa: BLE001  # get_model_info raises plain Exception for an unmapped backend model
             return
-        for field, backend_value in backend_info.items():
-            if "cost" not in field and field != "tiered_pricing":
-                continue
+        backend_entry: Final = litellm.model_cost.get(backend_info.get("key") or "")
+        if not isinstance(backend_entry, dict):
+            return
+        for field, backend_value in backend_entry.items():
             if model_info.get(field) is not None or backend_value is None:
                 continue
-            if field in ("input_cost_per_token", "output_cost_per_token") and not backend_value:
-                continue
             model_info[field] = copy.deepcopy(backend_value)
 
     @staticmethod
diff --git a/tests/test_litellm/test_router_model_cost_isolation.py b/tests/test_litellm/test_router_model_cost_isolation.py
index a1040e972f2..7b7a962bf00 100644
--- a/tests/test_litellm/test_router_model_cost_isolation.py
+++ b/tests/test_litellm/test_router_model_cost_isolation.py
@@ -595,6 +595,30 @@ def test_inherit_builtin_base_rates_for_off_peak_carries_threshold_rates():
     )
 
 
+def test_inherit_builtin_base_rates_for_off_peak_carries_companion_billing_fields():
+    """Billing rules that are not literal cost rates, like the web search
+    billing unit, must ride along, or grounding and regional uplifts would
+    bill differently through the deployment entry than through the shared
+    backend entry.
+    """
+    backend_model = "gemini-3-pro-image"
+    raw_entry = litellm.model_cost[backend_model]
+    assert raw_entry.get("web_search_billing_unit") is not None
+
+    model_info = {
+        "off_peak_pricing": {"hours_utc": "00:00-00:00", "input_cost_per_token": 5e-07},
+    }
+
+    Router._inherit_builtin_base_rates_for_off_peak(
+        model_info=model_info,
+        backend_model=backend_model,
+        custom_llm_provider=None,
+    )
+
+    assert model_info["web_search_billing_unit"] == raw_entry["web_search_billing_unit"]
+    assert model_info["input_cost_per_token"] == raw_entry["input_cost_per_token"]
+
+
 def test_inherit_builtin_base_rates_for_off_peak_tiered_only_backend_stores_no_zero():
     """A tiered-only backend has no flat token rates; get_model_info synthesizes
     zeros for them, and storing those would mark the deployment explicitly