fix: attribute realtime transcription cost in cost breakdown

Pass transcription_cost through additional_costs so cost_breakdown's
input_cost + output_cost + additional_costs sums to total_cost instead
of silently folding it into total_cost only.

Co-authored-by: Cursor <cursoragent@cursor.com>

litellm/cost_calculator.py

@@ -2356,6 +2356,7 @@ def handle_realtime_stream_cost_calculation(
         completion_tokens_cost_usd_dollar=output_cost_per_token,
         cost_for_built_in_tools_cost_usd_dollar=0.0,
         total_cost_usd_dollar=total_cost,
+        additional_costs={"transcription_cost": transcription_cost} if transcription_cost > 0 else None,
     )
 
     return total_cost

tests/test_litellm/test_cost_calculator.py

@@ -695,6 +695,18 @@ def test_realtime_transcription_duration_cost(monkeypatch):
     assert logging_obj.cost_breakdown is not None
     assert abs(logging_obj.cost_breakdown["total_cost"] - cost) < 1e-9
 
+    # The transcription cost must be attributed in the breakdown, not just folded
+    # into total_cost, or input_cost + output_cost + additional_costs won't sum to total_cost.
+    additional_costs = logging_obj.cost_breakdown.get("additional_costs")
+    assert additional_costs is not None
+    assert abs(additional_costs["transcription_cost"] - expected) < 1e-9
+    attributed_total = (
+        logging_obj.cost_breakdown["input_cost"]
+        + logging_obj.cost_breakdown["output_cost"]
+        + additional_costs["transcription_cost"]
+    )
+    assert abs(attributed_total - logging_obj.cost_breakdown["total_cost"]) < 1e-9
+
 
 def test_realtime_transcription_duration_cost_resolves_model_from_litellm_name(
     monkeypatch,