diff --git a/evals/cli/oaieval.py b/evals/cli/oaieval.py
index e48a09ac19..05b7b99fd9 100644
--- a/evals/cli/oaieval.py
+++ b/evals/cli/oaieval.py
@@ -5,6 +5,7 @@
 import logging
 import shlex
 import sys
+from numbers import Number
 from typing import Any, Mapping, Optional, Union, cast
 
 import evals
@@ -18,6 +19,26 @@
 logger = logging.getLogger(__name__)
 
 
+def _flatten_usage_metrics(usage: Any, prefix: str = "") -> dict[str, Number]:
+    if usage is None:
+        return {}
+
+    if isinstance(usage, Number):
+        return {prefix: usage} if prefix else {}
+
+    if hasattr(usage, "model_dump"):
+        return _flatten_usage_metrics(usage.model_dump(exclude_none=True), prefix)
+
+    if isinstance(usage, Mapping):
+        flattened: dict[str, Number] = {}
+        for key, value in usage.items():
+            nested_prefix = f"{prefix}_{key}" if prefix else str(key)
+            flattened.update(_flatten_usage_metrics(value, nested_prefix))
+        return flattened
+
+    return {}
+
+
 def _purple(str: str) -> str:
     return f"\033[1;35m{str}\033[0m"
 
@@ -274,13 +295,14 @@ def add_token_usage_to_result(result: dict[str, Any], recorder: RecorderBase) ->
     sampling_events = recorder.get_events("sampling")
     for event in sampling_events:
         if "usage" in event.data:
-            usage_events.append(dict(event.data["usage"]))
+            usage_events.append(_flatten_usage_metrics(event.data["usage"]))
     logger.info(f"Found {len(usage_events)}/{len(sampling_events)} sampling events with usage data")
     if usage_events:
-        # Sum up the usage of all samples (assumes the usage is the same for all samples)
+        # Sum up token usage across all sampling events, including nested usage breakdowns.
+        usage_keys = set().union(*(usage_event.keys() for usage_event in usage_events))
         total_usage = {
-            key: sum(u[key] if u[key] is not None else 0 for u in usage_events)
-            for key in usage_events[0]
+            key: sum(usage_event.get(key, 0) for usage_event in usage_events)
+            for key in sorted(usage_keys)
         }
         total_usage_str = "\n".join(f"{key}: {value:,}" for key, value in total_usage.items())
         logger.info(f"Token usage from {len(usage_events)} sampling events:\n{total_usage_str}")
diff --git a/evals/cli/oaieval_test.py b/evals/cli/oaieval_test.py
new file mode 100644
index 0000000000..6330ef4069
--- /dev/null
+++ b/evals/cli/oaieval_test.py
@@ -0,0 +1,54 @@
+from openai.types.completion_usage import CompletionTokensDetails, CompletionUsage, PromptTokensDetails
+
+from evals.base import RunSpec
+from evals.cli.oaieval import add_token_usage_to_result
+from evals.record import DummyRecorder
+
+
+def test_add_token_usage_to_result_flattens_nested_usage_details() -> None:
+    spec = RunSpec(
+        completion_fns=[""],
+        eval_name="",
+        base_eval="",
+        split="",
+        run_config={},
+        created_by="",
+        run_id="",
+        created_at="",
+    )
+    recorder = DummyRecorder(spec)
+
+    with recorder.as_default_recorder("sample-1"):
+        recorder.record_sampling(
+            prompt="prompt-1",
+            sampled="answer-1",
+            usage=CompletionUsage(
+                prompt_tokens=10,
+                completion_tokens=5,
+                total_tokens=15,
+                completion_tokens_details=CompletionTokensDetails(reasoning_tokens=2),
+                prompt_tokens_details=PromptTokensDetails(cached_tokens=4),
+            ),
+        )
+
+    with recorder.as_default_recorder("sample-2"):
+        recorder.record_sampling(
+            prompt="prompt-2",
+            sampled="answer-2",
+            usage=CompletionUsage(
+                prompt_tokens=7,
+                completion_tokens=6,
+                total_tokens=13,
+                completion_tokens_details=CompletionTokensDetails(reasoning_tokens=3),
+                prompt_tokens_details=PromptTokensDetails(cached_tokens=1),
+            ),
+        )
+
+    result: dict[str, int] = {}
+    add_token_usage_to_result(result, recorder)
+
+    assert result["usage_prompt_tokens"] == 17
+    assert result["usage_completion_tokens"] == 11
+    assert result["usage_total_tokens"] == 28
+    assert result["usage_completion_tokens_details_reasoning_tokens"] == 5
+    assert result["usage_prompt_tokens_details_cached_tokens"] == 5