fix(openrouter): let aspect_ratio or resolution win over a pixel size on /images

On the chat path, an image_config from litellm_params replaced the one built
from size, so a caller that also sent size "1024x1024" still got the
configured aspect ratio or image size. /images answers a pixel size next to a
mismatched aspect_ratio or resolution with a 400 ('size "1024x1024" conflicts
with resolution "2K".', seen on 2026-10-03), so the same config started failing

When the body carries aspect_ratio or resolution, an explicit pixel size is now
left out. A tier size such as "2K" still goes with aspect_ratio

litellm/llms/openrouter/image_generation/transformation.py

@@ -16,6 +16,7 @@ Response shape:
 }
 """
 
+import re
 from types import MappingProxyType
 from typing import TYPE_CHECKING, Any, Final
 
@@ -53,6 +54,8 @@ LEGACY_IMAGE_CONFIG_FIELDS: Final = MappingProxyType({"aspect_ratio": "aspect_ra
 NON_BODY_PARAMS: Final = frozenset(
     {"model", "prompt", "messages", "modalities", "stream", "image_config", "extra_headers"}
 )
+PIXEL_SIZE: Final = re.compile(r"\d+x\d+")
+SIZE_OVERRIDING_FIELDS: Final = frozenset({"aspect_ratio", "resolution"})
 
 
 class OpenRouterImageGenerationConfig(BaseImageGenerationConfig):
@@ -180,9 +183,12 @@ class OpenRouterImageGenerationConfig(BaseImageGenerationConfig):
         """
         image_config is the request shape of the older chat-based path. Its fields map onto the
         /images names so existing configs keep working, and explicit top-level values win
+
+        A configured aspect_ratio or resolution wins over an OpenAI pixel size, the way image_config
+        won over size on the chat path, because /images answers that pair with a 400
         """
         legacy_image_config: Final = optional_params.get("image_config") or {}
-        return {
+        body: Final[dict[str, object]] = {
             "model": model,
             "prompt": prompt,
             **{
@@ -192,6 +198,10 @@ class OpenRouterImageGenerationConfig(BaseImageGenerationConfig):
             },
             **{key: value for key, value in optional_params.items() if key not in NON_BODY_PARAMS},
         }
+        drop_pixel_size: Final = not SIZE_OVERRIDING_FIELDS.isdisjoint(body) and (
+            PIXEL_SIZE.fullmatch(str(body.get("size", ""))) is not None
+        )
+        return {key: value for key, value in body.items() if not (drop_pixel_size and key == "size")}
 
     def transform_image_generation_response(
         self,

tests/unit/llms/openrouter/image_generation/test_openrouter_image_gen_transformation.py

@@ -236,6 +236,33 @@ def test_transform_request_maps_legacy_image_config_and_explicit_fields_win(
     assert body == {"model": HYBRID_MODEL, "prompt": PROMPT, **expected_fields}
 
 
+# On 2026-10-03 POST https://openrouter.ai/api/v1/images returned 400 for size "1024x1024" with aspect_ratio
+# "3:2" (openai/gpt-image-1-mini) and with resolution "2K" (google/gemini-2.5-flash-image). The size field in
+# https://openrouter.ai/openapi.json says a tier size such as "2K" combines with aspect_ratio
+@pytest.mark.parametrize(
+    ("optional_params", "expected_fields"),
+    [
+        ({"size": "1024x1024", "image_config": {"aspect_ratio": "16:9"}}, {"aspect_ratio": "16:9"}),
+        ({"size": "1024x1024", "resolution": "4K"}, {"resolution": "4K"}),
+        ({"size": "1024x1024", "aspect_ratio": "1:1"}, {"aspect_ratio": "1:1"}),
+        ({"size": "2K", "aspect_ratio": "16:9"}, {"size": "2K", "aspect_ratio": "16:9"}),
+        ({"size": "1024x1024"}, {"size": "1024x1024"}),
+    ],
+)
+def test_transform_request_lets_aspect_ratio_or_resolution_win_over_a_pixel_size(
+    optional_params: dict[str, object], expected_fields: dict[str, object]
+):
+    body = CONFIG.transform_image_generation_request(
+        model=HYBRID_MODEL,
+        prompt=PROMPT,
+        optional_params=optional_params,
+        litellm_params={},
+        headers={},
+    )
+
+    assert body == {"model": HYBRID_MODEL, "prompt": PROMPT, **expected_fields}
+
+
 def test_transform_response_returns_every_image_in_order():
     response = _transform_response(httpx.Response(200, json=_images_response("aW1hZ2Ux", "aW1hZ2Uy")))
 
@@ -332,6 +359,22 @@ def test_hybrid_image_text_model_uses_the_same_images_endpoint():
     assert json.loads(request.content) == {"model": HYBRID_MODEL, "prompt": PROMPT}
 
 
+def test_legacy_image_config_with_an_openai_pixel_size_sends_only_the_aspect_ratio():
+    recorder = RequestRecorder(_images_response("aW1hZ2Ux"))
+
+    litellm.image_generation(
+        model=f"openrouter/{HYBRID_MODEL}",
+        prompt=PROMPT,
+        size="1024x1024",
+        image_config={"aspect_ratio": "16:9"},
+        api_key="sk-test",
+        client=_client(recorder),
+    )
+
+    (request,) = recorder.requests
+    assert json.loads(request.content) == {"model": HYBRID_MODEL, "prompt": PROMPT, "aspect_ratio": "16:9"}
+
+
 def test_legacy_chat_completions_api_base_still_reaches_the_images_endpoint():
     recorder = RequestRecorder(_images_response("aW1hZ2Ux"))