fix(tests): merge base tests and our cache-read regression tests

The add/add conflict on test_lowest_cost.py arose because:
- base added framework tests for cost-key isolation and zero-division fix
- our branch added two regression tests for the cache-read price tie-break

Resolution: base tests first, our two regression tests appended. All 9 pass.

tests/test_litellm/router_strategy/test_lowest_cost.py

@@ -101,3 +101,84 @@ async def test_async_log_success_event_counts_a_response_with_no_completion_toke
     )
 
     assert _recorded_minute_counters(cache) == {"tpm": 12, "rpm": 1}
+
+
+@pytest.mark.parametrize("cheaper_cache_first", [True, False])
+@pytest.mark.asyncio
+async def test_cost_routing_breaks_input_output_tie_on_cache_read_cost(cheaper_cache_first):
+    """
+    Regression test for https://github.com/BerriAI/litellm/issues/38064
+
+    Two deployments with identical input+output price must be separated by their
+    cache-read price, not by whichever one happens to be listed first. The pricier
+    deployment omits a cache-read price to exercise the input-cost fallback.
+    """
+    cheaper = {
+        "model_name": "cache-tie-test",
+        "litellm_params": {
+            "model": "openai/tie-model-not-in-cost-map",
+            "input_cost_per_token": 1e-06,
+            "output_cost_per_token": 2e-06,
+            "cache_read_input_token_cost": 1e-08,
+        },
+        "model_info": {"id": "cheaper-cache"},
+    }
+    pricier = {
+        "model_name": "cache-tie-test",
+        "litellm_params": {
+            "model": "openai/tie-model-not-in-cost-map",
+            "input_cost_per_token": 1e-06,
+            "output_cost_per_token": 2e-06,
+        },
+        "model_info": {"id": "pricier-cache"},
+    }
+    model_list = [cheaper, pricier] if cheaper_cache_first else [pricier, cheaper]
+
+    logger = LowestCostLoggingHandler(router_cache=DualCache())
+
+    selected = await logger.async_get_available_deployments(
+        model_group="cache-tie-test", healthy_deployments=model_list
+    )
+
+    assert selected["model_info"]["id"] == "cheaper-cache"
+
+
+@pytest.mark.parametrize("free_cache_first", [True, False])
+@pytest.mark.asyncio
+async def test_cost_routing_honors_zero_deployment_cache_read_cost(free_cache_first):
+    """
+    A deployment-level cache_read_input_token_cost of 0 is a price, not a missing value.
+
+    Truthiness checks treat it as unset and fall through to the cost map / input-cost
+    fallback, which ranks a deployment whose cache reads are free as the priciest one
+    in a tie. Providers that do not charge for cache reads make this a real config.
+    """
+    free = {
+        "model_name": "cache-zero-test",
+        "litellm_params": {
+            "model": "openai/zero-model-not-in-cost-map",
+            "input_cost_per_token": 1e-06,
+            "output_cost_per_token": 2e-06,
+            "cache_read_input_token_cost": 0.0,
+        },
+        "model_info": {"id": "free-cache"},
+    }
+    paid = {
+        "model_name": "cache-zero-test",
+        "litellm_params": {
+            "model": "openai/zero-model-not-in-cost-map",
+            "input_cost_per_token": 1e-06,
+            "output_cost_per_token": 2e-06,
+            "cache_read_input_token_cost": 1e-08,
+        },
+        "model_info": {"id": "paid-cache"},
+    }
+    model_list = [free, paid] if free_cache_first else [paid, free]
+
+    logger = LowestCostLoggingHandler(router_cache=DualCache())
+
+    selected = await logger.async_get_available_deployments(
+        model_group="cache-zero-test", healthy_deployments=model_list
+    )
+
+    assert selected["model_info"]["id"] == "free-cache"