fix(cost): bill the fast service tier at the priority rate

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

litellm/litellm_core_utils/llm_cost_calc/utils.py

@@ -2,7 +2,8 @@
 ## Helper utilities for cost_per_token()
 
 from dataclasses import dataclass
-from typing import Any, Literal, Optional, Tuple, TypedDict, cast
+from types import MappingProxyType
+from typing import Any, Literal, Mapping, Optional, Tuple, TypedDict, cast
 
 import litellm
 from litellm._logging import verbose_logger
@@ -39,6 +40,14 @@ _VALID_DATA_RESIDENCIES = frozenset(r.value for r in DataResidency)
 # of being rebuilt for every model_info key on every call.
 _SERVICE_TIER_SUFFIXES: tuple[str, ...] = tuple(f"_{st.value}" for st in ServiceTier)
 
+_SERVICE_TIER_TO_COST_KEY_SUFFIX: Mapping[str, str] = MappingProxyType(
+    {
+        ServiceTier.FLEX.value: ServiceTier.FLEX.value,
+        ServiceTier.PRIORITY.value: ServiceTier.PRIORITY.value,
+        ServiceTier.FAST.value: ServiceTier.PRIORITY.value,
+    }
+)
+
 
 def _get_token_detail_value(details: object, key: str) -> Optional[int]:
     if isinstance(details, dict):
@@ -177,7 +186,7 @@ def _get_service_tier_cost_key(base_key: str, service_tier: Optional[str]) -> st
 
     Args:
         base_key: The base cost key (e.g., "input_cost_per_token")
-        service_tier: The service tier ("flex", "priority", or None for standard)
+        service_tier: The service tier ("flex", "priority", "fast", or None for standard)
 
     Returns:
         str: The cost key to use (e.g., "input_cost_per_token_flex" or "input_cost_per_token")
@@ -185,12 +194,11 @@ def _get_service_tier_cost_key(base_key: str, service_tier: Optional[str]) -> st
     if service_tier is None:
         return base_key
 
-    # Only use service tier specific keys for "flex" and "priority"
-    if service_tier.lower() in [ServiceTier.FLEX.value, ServiceTier.PRIORITY.value]:
-        return f"{base_key}_{service_tier.lower()}"
+    suffix = _SERVICE_TIER_TO_COST_KEY_SUFFIX.get(service_tier.lower())
+    if suffix is None:
+        return base_key
 
-    # For any other service tier, use standard pricing
-    return base_key
+    return f"{base_key}_{suffix}"
 
 
 def _parse_above_token_threshold(key: str) -> float:

litellm/types/utils.py

@@ -3847,6 +3847,7 @@ class ServiceTier(Enum):
     AUTO = "auto"
     FLEX = "flex"
     PRIORITY = "priority"
+    FAST = "fast"
 
 
 class DataResidency(Enum):

tests/test_litellm/litellm_core_utils/llm_cost_calc/test_llm_cost_calc_utils.py

@@ -2447,3 +2447,66 @@ def test_generic_cost_per_token_gemini_35_flash_lite():
     )
     assert prompt_cost == pytest.approx(0.0003)
     assert completion_cost == pytest.approx(0.00125)
+
+
+def test_fast_service_tier_bills_at_the_priority_rate(_local_model_cost_map):
+    """Regression: OpenAI's Fast mode replaced Priority Processing and costs 2x standard.
+
+    Before the fix "fast" fell through to standard pricing, so a Fast mode request
+    was billed at half of what it actually costs."""
+    from litellm.types.utils import Usage
+
+    usage = Usage(
+        prompt_tokens=1_000,
+        completion_tokens=500,
+        prompt_tokens_details=PromptTokensDetailsWrapper(cached_tokens=200),
+    )
+
+    standard = generic_cost_per_token(
+        model="gpt-5.6-sol", usage=usage, custom_llm_provider="openai", service_tier=None
+    )
+    priority = generic_cost_per_token(
+        model="gpt-5.6-sol", usage=usage, custom_llm_provider="openai", service_tier="priority"
+    )
+    fast = generic_cost_per_token(
+        model="gpt-5.6-sol", usage=usage, custom_llm_provider="openai", service_tier="fast"
+    )
+
+    expected_prompt = 800 * 1e-05 + 200 * 1e-06
+    expected_completion = 500 * 6e-05
+
+    assert fast == priority
+    assert fast[0] == pytest.approx(expected_prompt, rel=1e-9)
+    assert fast[1] == pytest.approx(expected_completion, rel=1e-9)
+    assert fast[0] == pytest.approx(standard[0] * 2, rel=1e-9)
+    assert fast[1] == pytest.approx(standard[1] * 2, rel=1e-9)
+
+
+def test_fast_service_tier_is_case_insensitive(_local_model_cost_map):
+    from litellm.types.utils import Usage
+
+    usage = Usage(prompt_tokens=1_000, completion_tokens=500)
+
+    assert generic_cost_per_token(
+        model="gpt-5.6-sol", usage=usage, custom_llm_provider="openai", service_tier="FAST"
+    ) == generic_cost_per_token(
+        model="gpt-5.6-sol", usage=usage, custom_llm_provider="openai", service_tier="fast"
+    )
+
+
+def test_fast_service_tier_matches_priority_above_the_context_threshold(_local_model_cost_map):
+    """The above-threshold branch resolves its own cost keys, so the alias has to hold there too."""
+    from litellm.types.utils import Usage
+
+    usage = Usage(prompt_tokens=300_000, completion_tokens=1_000)
+
+    fast = generic_cost_per_token(
+        model="gpt-5.6-sol", usage=usage, custom_llm_provider="openai", service_tier="fast"
+    )
+    priority = generic_cost_per_token(
+        model="gpt-5.6-sol", usage=usage, custom_llm_provider="openai", service_tier="priority"
+    )
+
+    assert fast == priority
+    assert fast[0] == pytest.approx(300_000 * 1e-05, rel=1e-9)
+    assert fast[1] == pytest.approx(1_000 * 4.5e-05, rel=1e-9)