diff --git a/litellm/litellm_core_utils/llm_cost_calc/tool_call_cost_tracking.py b/litellm/litellm_core_utils/llm_cost_calc/tool_call_cost_tracking.py
index 164ba14b200..1e5bab7ea3c 100644
--- a/litellm/litellm_core_utils/llm_cost_calc/tool_call_cost_tracking.py
+++ b/litellm/litellm_core_utils/llm_cost_calc/tool_call_cost_tracking.py
@@ -277,7 +277,7 @@ class StandardBuiltInToolCostTracking:
                 custom_llm_provider=custom_llm_provider or "openai",
                 quality=quality if isinstance(quality, str) and quality != "auto" else None,
                 n=1,
-                size=size if isinstance(size, str) else None,
+                size=size if isinstance(size, str) and size != "auto" else None,
             )
         except Exception as e:  # noqa: BLE001  # pricing helpers raise bare Exception for unmapped models; bill 0.0
             verbose_logger.debug("Could not price Responses API image_generation_call item: %s", e)
@@ -294,19 +294,20 @@ class StandardBuiltInToolCostTracking:
             model_info: Final = litellm.get_model_info(
                 model=tool_model, custom_llm_provider=custom_llm_provider or "openai"
             )
-        except Exception as e:  # noqa: BLE001  # get_model_info raises bare Exception for unmapped models; bill 0.0
+        except Exception as e:  # noqa: BLE001  # get_model_info raises bare Exception for unmapped models; fall back
             verbose_logger.debug("Could not resolve pricing for image tool model %s: %s", tool_model, e)
-            return 0.0
+            return None
         image_gen: Final = tool_usage.image_gen
         input_details: Final = image_gen.input_tokens_details
         output_details: Final = image_gen.output_tokens_details
-        return (
+        token_cost: Final = (
             (input_details.text_tokens if input_details else 0) * (model_info.get("input_cost_per_token") or 0)
             + (input_details.image_tokens if input_details else 0) * (model_info.get("input_cost_per_image_token") or 0)
             + (output_details.image_tokens if output_details else 0)
             * (model_info.get("output_cost_per_image_token") or 0)
             + (output_details.text_tokens if output_details else 0) * (model_info.get("output_cost_per_token") or 0)
         )
+        return token_cost if token_cost > 0 else None
 
     @staticmethod
     def _handle_image_generation_cost(response_object: object, custom_llm_provider: str | None) -> float:
diff --git a/tests/test_litellm/litellm_core_utils/llm_cost_calc/test_tool_call_cost_tracking.py b/tests/test_litellm/litellm_core_utils/llm_cost_calc/test_tool_call_cost_tracking.py
index 570b701450d..6d525bca654 100644
--- a/tests/test_litellm/litellm_core_utils/llm_cost_calc/test_tool_call_cost_tracking.py
+++ b/tests/test_litellm/litellm_core_utils/llm_cost_calc/test_tool_call_cost_tracking.py
@@ -990,3 +990,89 @@ def test_responses_zero_image_gen_tokens_fall_back_to_per_image_pricing(local_mo
         )
     )
     assert cost > 0
+
+
+def test_responses_auto_size_image_generation_call_billed_at_default_size(local_model_cost_map):
+    """An image_generation_call with size "auto" bills at the default size instead of erroring to $0."""
+    from litellm.cost_calculator import default_image_cost_calculator
+
+    response = _openai_responses_response(
+        "gpt-5",
+        [
+            {
+                "type": "image_generation_call",
+                "id": "ig_1",
+                "status": "completed",
+                "quality": "high",
+                "size": "auto",
+                "result": "AAAA",
+            }
+        ],
+    )
+
+    cost = StandardBuiltInToolCostTracking.get_cost_for_built_in_tools(
+        model="gpt-5",
+        response_object=response,
+        usage=None,
+        custom_llm_provider="openai",
+        standard_built_in_tools_params=None,
+    )
+
+    assert cost == pytest.approx(
+        default_image_cost_calculator(
+            model="gpt-image-1",
+            custom_llm_provider="openai",
+            quality="high",
+            n=1,
+            size=None,
+        )
+    )
+    assert cost > 0
+
+
+def test_responses_image_gen_total_without_token_details_falls_back_to_per_image(local_model_cost_map):
+    """A positive image_gen total with no token details falls back to per-image pricing, not $0."""
+    tool_model = "gpt-image-1"
+    quality = "low"
+    size = "1024x1024"
+    response = _openai_responses_response(
+        "gpt-5",
+        [
+            {
+                "type": "image_generation_call",
+                "id": "ig_1",
+                "status": "completed",
+                "quality": quality,
+                "size": size,
+                "result": "AAAA",
+            }
+        ],
+        usage={
+            "input_tokens": 10,
+            "output_tokens": 5,
+            "total_tokens": 15,
+            "tool_usage": {"image_gen": {"input_tokens": 5, "output_tokens": 5, "total_tokens": 10}},
+        },
+        tools=[{"type": "image_generation", "model": tool_model, "quality": quality, "size": size}],
+    )
+
+    cost = StandardBuiltInToolCostTracking.get_cost_for_built_in_tools(
+        model="gpt-5",
+        response_object=response,
+        usage=None,
+        custom_llm_provider="openai",
+        standard_built_in_tools_params=None,
+    )
+
+    from litellm.cost_calculator import default_image_cost_calculator
+
+    assert cost == pytest.approx(
+        default_image_cost_calculator(
+            model=tool_model,
+            custom_llm_provider="openai",
+            quality=quality,
+            n=1,
+            size=size,
+        )
+    )
+    assert cost > 0