fix(cost_calculator): sum mirrored cache token fields once in combine_usage_objects

combine_usage_objects iterates prompt_tokens_details model_fields and sums each;
with cache_write_tokens and cache_creation_tokens now mirroring each other via
__setattr__, the pair was summed twice, doubling cache creation counts for
Anthropic batch cost calc, mid-stream fallback usage merges, and realtime usage.
Collapse the mirrored pair to one representative before summing.

litellm/cost_calculator.py

@@ -2191,6 +2191,13 @@ def batch_cost_calculator(
     return total_prompt_cost, total_completion_cost
 
 
+def _summable_prompt_token_fields(prompt_tokens_details: BaseModel) -> List[str]:
+    field_names = list(type(prompt_tokens_details).model_fields)
+    if getattr(prompt_tokens_details, "cache_write_tokens", None) is None:
+        return field_names
+    return [attr for attr in field_names if attr != "cache_creation_tokens"]
+
+
 class BaseTokenUsageProcessor:
     @staticmethod
     def combine_usage_objects(usage_objects: List[Usage]) -> Usage:
@@ -2225,7 +2232,7 @@ class BaseTokenUsageProcessor:
 
                 # Check what keys exist in the model's prompt_tokens_details
                 # Access model_fields on the class, not the instance, to avoid Pydantic 2.11+ deprecation warnings
-                for attr in type(usage.prompt_tokens_details).model_fields:
+                for attr in _summable_prompt_token_fields(usage.prompt_tokens_details):
                     if (
                         hasattr(usage.prompt_tokens_details, attr)
                         and not attr.startswith("_")

tests/test_litellm/test_cost_calculator.py

@@ -12,6 +12,7 @@ from pydantic import BaseModel
 
 import litellm
 from litellm.cost_calculator import (
+    BaseTokenUsageProcessor,
     RealtimeAPITokenUsageProcessor,
     completion_cost,
     cost_per_token,
@@ -3479,3 +3480,32 @@ def test_batch_cost_calculator_cache_creation_falls_back_to_input_rate():
     )
 
     assert prompt_cost == pytest.approx((1000 * 3e-6 + 8000 * 3e-7 + 2000 * 3e-6) / 2)
+
+
+def test_combine_usage_objects_sums_mirrored_cache_write_fields_once():
+    """
+    cache_write_tokens and cache_creation_tokens mirror each other on
+    PromptTokensDetailsWrapper, so field-iterating aggregation must sum the pair
+    once: a single 50-token usage stays 50 and two combine to 100, not double.
+    """
+    single = Usage(
+        prompt_tokens=100,
+        completion_tokens=10,
+        total_tokens=110,
+        prompt_tokens_details=PromptTokensDetailsWrapper(cache_write_tokens=50),
+    )
+    combined = BaseTokenUsageProcessor.combine_usage_objects([single])
+    assert combined.prompt_tokens_details is not None
+    assert combined.prompt_tokens_details.cache_write_tokens == 50
+    assert combined.prompt_tokens_details.cache_creation_tokens == 50
+
+    anthropic_style = Usage(
+        prompt_tokens=100,
+        completion_tokens=10,
+        total_tokens=110,
+        cache_creation_input_tokens=50,
+    )
+    combined_pair = BaseTokenUsageProcessor.combine_usage_objects([anthropic_style, anthropic_style])
+    assert combined_pair.prompt_tokens_details is not None
+    assert combined_pair.prompt_tokens_details.cache_write_tokens == 100
+    assert combined_pair.prompt_tokens_details.cache_creation_tokens == 100