From f6685b7858455ceddd16c6cc2c5e31521f6ef3cf Mon Sep 17 00:00:00 2001
From: mateo-berri <277851410+mateo-berri@users.noreply.github.com>
Date: Thu, 10 Sep 2026 08:12:16 -0700
Subject: [PATCH] fix(cost_calculator): keep base_model pricing off the
 regional row

A deployment with base_model set was priced from the region's own row once
completion_cost forwarded the response region into cost_per_token, which now
strips the provider prefix and finds bedrock/<region>/<base_model>. Explicit
pricing (base_model or custom pricing) suppresses the region for cost_per_token
the same way _select_model_name_for_cost_calc already does
---
 litellm/cost_calculator.py                 |  3 ++-
 tests/test_litellm/test_cost_calculator.py | 23 ++++++++++++++++++++++
 2 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/litellm/cost_calculator.py b/litellm/cost_calculator.py
index 1678c05c470..48880ef5611 100644
--- a/litellm/cost_calculator.py
+++ b/litellm/cost_calculator.py
@@ -1292,6 +1292,7 @@ def completion_cost(
 
         service_tier = _normalize_service_tier(service_tier)
 
+        explicit_pricing: Final = custom_pricing is True or base_model is not None
         selected_model: Final = _select_model_name_for_cost_calc(
             model=model,
             completion_response=completion_response,
@@ -1655,7 +1656,7 @@ def completion_cost(
                     completion_tokens=completion_tokens or 0,
                     custom_llm_provider=custom_llm_provider,
                     response_time_ms=total_time,
-                    region_name=region_name,
+                    region_name=None if explicit_pricing else region_name,
                     custom_cost_per_second=custom_cost_per_second,
                     custom_cost_per_token=custom_cost_per_token,
                     prompt_characters=prompt_characters,
diff --git a/tests/test_litellm/test_cost_calculator.py b/tests/test_litellm/test_cost_calculator.py
index d8c9b7cbc6e..4809b58be70 100644
--- a/tests/test_litellm/test_cost_calculator.py
+++ b/tests/test_litellm/test_cost_calculator.py
@@ -4363,6 +4363,29 @@ def test_select_model_name_keeps_base_model_free_of_region(_local_model_cost_map
     assert selected == "bedrock/moonshotai.kimi-k2.5"
 
 
+def test_completion_cost_base_model_ignores_regional_row(_local_model_cost_map):
+    """A deployment with base_model set is priced from that model's own row even when the response
+    carries a region whose regional row charges different rates."""
+
+    response = litellm.ModelResponse(
+        id="x",
+        choices=[{"index": 0, "message": {"role": "assistant", "content": "hi"}, "finish_reason": "stop"}],
+        model="my-bedrock-deployment",
+        usage={"prompt_tokens": 1000, "completion_tokens": 0, "total_tokens": 1000},
+    )
+    response._hidden_params = {"custom_llm_provider": "bedrock", "region_name": "eu-central-1"}
+    flat = litellm.model_cost["anthropic.claude-instant-v1"]
+    regional = litellm.model_cost["bedrock/eu-central-1/anthropic.claude-instant-v1"]
+    assert flat["input_cost_per_token"] != regional["input_cost_per_token"]
+
+    assert litellm.completion_cost(
+        completion_response=response,
+        model="my-bedrock-deployment",
+        custom_llm_provider="bedrock",
+        base_model="anthropic.claude-instant-v1",
+    ) == pytest.approx(1000 * flat["input_cost_per_token"])
+
+
 def test_completion_cost_nonzero_for_slash_alias_model_name(_local_model_cost_map):
     """End-to-end cost through a "/"-containing alias must price above zero (#38069)."""