fix(cost): bedrock_mantle inherits bedrock regional pricing

bedrock_mantle deployments fell back to the bare (US) price-map entry
because no bedrock_mantle/<region>/<model> key exists. Resolve pricing
through the bedrock keys when bedrock_mantle has no entry of its own, so
a mantle deployment bills the same regional rate as the equivalent
bedrock deployment.

Fixes #35953

litellm/cost_calculator.py

@@ -450,14 +450,20 @@ def cost_per_token(
     model_with_provider = model
     if caller_supplied_provider:
         _prov_prefix: Final = f"{custom_llm_provider}/"
-        if model_is_str and model.startswith(_prov_prefix):
-            model_with_provider = model
-        else:
-            model_with_provider = f"{custom_llm_provider}/{model}"
-        if region_name is not None:
-            model_with_provider_and_region: Final = f"{custom_llm_provider}/{region_name}/{model}"
-            if model_with_provider_and_region in model_cost_ref:  # use region based pricing, if it's available
-                model_with_provider = model_with_provider_and_region
+        _has_prov_prefix: Final = model_is_str and model.startswith(_prov_prefix)
+        _bare_model: Final = model[len(_prov_prefix) :] if _has_prov_prefix else model
+        _pricing_providers: Final[tuple[str, ...]] = (
+            (custom_llm_provider, "bedrock") if custom_llm_provider == "bedrock_mantle" else (custom_llm_provider,)
+        )
+        _regional_keys: Final[tuple[str, ...]] = (
+            tuple(f"{_p}/{region_name}/{_bare_model}" for _p in _pricing_providers) if region_name is not None else ()
+        )
+        _flat_keys: Final[tuple[str, ...]] = tuple(f"{_p}/{_bare_model}" for _p in _pricing_providers)
+        _default_key: Final = model if _has_prov_prefix else f"{custom_llm_provider}/{model}"
+        model_with_provider = next(
+            (k for k in (*_regional_keys, *_flat_keys) if k in model_cost_ref),
+            _default_key,
+        )
     else:
         _, custom_llm_provider, _, _ = litellm.get_llm_provider(model=model)
 

tests/test_litellm/test_cost_calculator.py

@@ -42,6 +42,68 @@ def test_cost_per_token_duplicate_openai_prefix_matches_model_cost(monkeypatch):
     assert prompt_usd + completion_usd > 0
 
 
+def test_cost_per_token_bedrock_mantle_inherits_bedrock_regional_pricing(monkeypatch):
+    """
+    A bedrock_mantle deployment has no price-map key of its own for models that
+    are also served over bedrock-runtime, so it must inherit the bedrock regional
+    rate instead of falling back to the bare (US) entry. Regression for #35953.
+    """
+    monkeypatch.setenv("LITELLM_LOCAL_MODEL_COST_MAP", "True")
+    monkeypatch.setattr(litellm, "model_cost", litellm.get_model_cost_map(url=""))
+
+    eu_key = "bedrock/eu-north-1/moonshotai.kimi-k2.5"
+    us_key = "moonshotai.kimi-k2.5"
+    eu_in = litellm.model_cost[eu_key]["input_cost_per_token"]
+    eu_out = litellm.model_cost[eu_key]["output_cost_per_token"]
+    us_in = litellm.model_cost[us_key]["input_cost_per_token"]
+    assert eu_in != us_in  # otherwise the test cannot distinguish the bug
+
+    prompt_usd, completion_usd = cost_per_token(
+        model="moonshotai.kimi-k2.5",
+        prompt_tokens=1_000_000,
+        completion_tokens=1_000_000,
+        custom_llm_provider="bedrock_mantle",
+        region_name="eu-north-1",
+    )
+
+    assert prompt_usd == pytest.approx(eu_in * 1_000_000)
+    assert completion_usd == pytest.approx(eu_out * 1_000_000)
+
+    bedrock_prompt_usd, bedrock_completion_usd = cost_per_token(
+        model="moonshotai.kimi-k2.5",
+        prompt_tokens=1_000_000,
+        completion_tokens=1_000_000,
+        custom_llm_provider="bedrock",
+        region_name="eu-north-1",
+    )
+    assert (prompt_usd, completion_usd) == (bedrock_prompt_usd, bedrock_completion_usd)
+
+
+def test_cost_per_token_bedrock_mantle_keeps_its_own_pricing(monkeypatch):
+    """
+    The bedrock fallback must not override a model that has its own
+    bedrock_mantle price-map entry (e.g. mantle-exclusive gpt-oss). Guards the
+    #35953 fallback from becoming over-broad.
+    """
+    monkeypatch.setenv("LITELLM_LOCAL_MODEL_COST_MAP", "True")
+    monkeypatch.setattr(litellm, "model_cost", litellm.get_model_cost_map(url=""))
+
+    mantle_key = "bedrock_mantle/openai.gpt-oss-120b"
+    mantle_in = litellm.model_cost[mantle_key]["input_cost_per_token"]
+    mantle_out = litellm.model_cost[mantle_key]["output_cost_per_token"]
+
+    prompt_usd, completion_usd = cost_per_token(
+        model="openai.gpt-oss-120b",
+        prompt_tokens=1_000_000,
+        completion_tokens=1_000_000,
+        custom_llm_provider="bedrock_mantle",
+        region_name="eu-north-1",
+    )
+
+    assert prompt_usd == pytest.approx(mantle_in * 1_000_000)
+    assert completion_usd == pytest.approx(mantle_out * 1_000_000)
+
+
 def test_cost_per_token_non_string_model_does_not_hang():
     """
     The provider-prefix dedup loop must not spin forever when `model` is a