diff --git a/litellm/cost_calculator.py b/litellm/cost_calculator.py
index 7990832dc48..1da51cd4246 100644
--- a/litellm/cost_calculator.py
+++ b/litellm/cost_calculator.py
@@ -964,6 +964,27 @@ def _extract_service_tier(source: object) -> str | None:
     return None
 
 
+def _completion_window_value(metadata: object) -> str | None:
+    """Return ``metadata["completion_window"]`` only when it names a billable tier
+    ("flex" or "balanced"). "asap" is the provider default and bills at standard
+    pricing, so it returns None."""
+    if not isinstance(metadata, dict):
+        return None
+    window: Final = cast(dict[str, object], metadata).get("completion_window")
+    if isinstance(window, str) and window in (ServiceTier.FLEX.value, ServiceTier.BALANCED.value):
+        return window
+    return None
+
+
+def _service_tier_from_completion_window(optional_params: dict[str, object]) -> str | None:
+    """Read ``metadata.completion_window`` from ``extra_body`` or a top-level ``metadata``
+    param (the two shapes callers use to pick a provider completion window directly)."""
+    extra_body: Final = optional_params.get("extra_body")
+    return _completion_window_value(
+        cast(dict[str, object], extra_body).get("metadata") if isinstance(extra_body, dict) else None
+    ) or _completion_window_value(optional_params.get("metadata"))
+
+
 def get_usage_object(
     completion_response: object,
 ) -> Usage | None:
@@ -1376,6 +1397,8 @@ def completion_cost(
         # Extract service_tier from optional_params if not provided directly
         if service_tier is None and optional_params is not None:
             service_tier = optional_params.get("service_tier")
+            if service_tier is None:
+                service_tier = _service_tier_from_completion_window(cast(dict[str, object], optional_params))
 
         service_tier = _normalize_service_tier(service_tier)