[CI] Re-enable Prefix Cache Test (#2869)

Signed-off-by: Alex Brooks <albrooks@redhat.com>

tests/e2e/online_serving/test_qwen3_omni.py

@@ -180,14 +180,19 @@ def test_text_to_text_001(omni_server, openai_client) -> None:
 @pytest.mark.omni
 @hardware_test(res={"cuda": "H100", "rocm": "MI325"}, num_cards=2)
 @pytest.mark.parametrize("omni_server", prefix_test_params, indirect=True)
-@pytest.mark.skip(reason="issue: #2833")
 def test_thinker_prefix_caching(omni_server, openai_client) -> None:
     """
     Test thinker prefix caching by sending identical requests with an image (i.e.,
     a large shared prefix) and verifying that the second request uses cached tokens
-    & produces the same output.
+    & produces the same output with greedy decoding.
+
+    NOTE: The seed for this test is used as a regression test for the issue linked below;
+    https://github.com/vllm-project/vllm-omni/issues/2833; without passing the sampling
+    params, this test will fail with the current default stage configs.
     """
-    image_data_url = f"data:image/jpeg;base64,{generate_synthetic_image(224, 224)['base64']}"
+    seed = 10
+    img_res = generate_synthetic_image(224, 224, seed=seed)
+    image_data_url = f"data:image/jpeg;base64,{img_res['base64']}"
     messages = dummy_messages_from_mix_data(
         system_prompt=get_system_prompt(),
         image_data_url=image_data_url,
@@ -199,16 +204,14 @@ def test_thinker_prefix_caching(omni_server, openai_client) -> None:
         "messages": messages,
         "stream": False,
         "modalities": ["text"],
+        "sampling_params_list": [{"seed": seed, "temperature": 0, "max_tokens": 16}] * 3,
     }
 
     response_1 = openai_client.send_omni_request(request_config, request_num=1)[0]
     response_2 = openai_client.send_omni_request(request_config, request_num=1)[0]
 
-    assert response_1.success
-    assert response_2.success
-    assert response_2.cached_tokens is not None
     # We should cache the vast majority of the prompt (image + up to last full block),
-    # and set seed in the CI config, so the second request should give an identical
+    # and set seed + temperature, so the second request should give an identical
     # response for the generated input image, even if we use dummy weights
-    assert response_2.cached_tokens > 0
+    assert response_2.cached_tokens is not None and response_2.cached_tokens > 0
     assert response_1.text_content == response_2.text_content

tests/helpers/media.py

@@ -447,12 +447,16 @@ def generate_synthetic_image(
     *,
     force_regenerate: bool = False,
     cache_dir: Path | str | None = None,
+    seed: int | None = None,
 ) -> dict[str, Any]:
     """
     Random colored squares on white background. Caches JPEG by ``width`` /
     ``height`` unless ``force_regenerate`` is true. Cache root: ``cache_dir``
     if given, else the default temp subdirectory.
     """
+    if seed is not None:
+        random.seed(seed)
+
     root = _resolve_synthetic_media_cache_dir(cache_dir)
     root.mkdir(parents=True, exist_ok=True)
     cache_path = root / f"synth_image_w{width}_h{height}.jpg"

tests/helpers/runtime.py

@@ -620,6 +620,9 @@ class OpenAIClientHandler:
             mm = dict(extra_body.get("mm_processor_kwargs") or {})
             mm["use_audio_in_video"] = True
             extra_body["mm_processor_kwargs"] = mm
+        if "sampling_params_list" in request_config:
+            extra_body["sampling_params_list"] = request_config["sampling_params_list"]
+
         create_kwargs: dict[str, Any] = {
             "model": request_config.get("model"),
             "messages": request_config.get("messages"),