diff --git a/tests/engine/test_omni_request_output.py b/tests/engine/test_omni_request_output.py
index fa25bba9e..eb1bdfd41 100644
--- a/tests/engine/test_omni_request_output.py
+++ b/tests/engine/test_omni_request_output.py
@@ -270,3 +270,56 @@ class TestMsgpackRoundTrip:
         # Content merged from legacy nested key
         assert result.prompt == "legacy prompt"
         assert result.prompt_token_ids == [1, 2]
+
+
+class TestRequestOutputFieldParity:
+    """``OmniRequestOutput`` must expose everything ``RequestOutput`` sets.
+
+    ``RequestOutput`` is a plain class, so the dataclass-generated ``__init__``
+    replaces its ``__init__`` and the inherited attributes only exist because
+    they are redeclared as fields. Any vLLM field that is not redeclared simply
+    does not exist on an omni output, and vLLM's own serving code reads those
+    attributes unconditionally — ``/v1/completions`` returned HTTP 500 with
+    ``AttributeError: 'OmniRequestOutput' object has no attribute
+    'ec_transfer_params'`` for exactly that reason. This test fails on the next
+    vLLM bump that adds a field, instead of leaving it for an endpoint to hit.
+    """
+
+    def test_carries_every_attribute_request_output_sets(self) -> None:
+        reference = _make_text_request_output()
+        omni = OmniRequestOutput(request_id="req-parity")
+
+        missing = sorted(name for name in vars(reference) if not name.startswith("_") and not hasattr(omni, name))
+        assert not missing, (
+            f"OmniRequestOutput is missing RequestOutput attributes {missing}; "
+            "redeclare them as dataclass fields in vllm_omni/outputs/__init__.py"
+        )
+
+    def test_transfer_param_fields_default_to_none(self) -> None:
+        omni = OmniRequestOutput(request_id="req-defaults")
+        assert omni.kv_transfer_params is None
+        assert omni.ec_transfer_params is None
+        assert omni.num_cached_tokens is None
+        assert omni.num_cache_creation_tokens is None
+
+    def test_transfer_param_fields_are_copied_from_the_stage_output(self) -> None:
+        """Declaring the fields is not enough — they must also be copied.
+
+        ``from_stage_output`` only carries the names listed in
+        ``_REQUEST_OUTPUT_CONTENT_ATTRS``. With the fields declared but absent
+        from that list the endpoint stops raising ``AttributeError`` while
+        silently dropping external-connector metadata and cache-creation
+        accounting, which is harder to notice than the crash it replaced.
+        """
+        source = _make_text_request_output()
+        source.num_cache_creation_tokens = 7
+        source.ec_transfer_params = {"connector": "shm", "handle": "abc"}
+        source.num_cached_tokens = 3
+        source.kv_transfer_params = {"role": "producer"}
+
+        omni = OmniRequestOutput.from_stage_output(source, stage_id=1)
+
+        assert omni.num_cache_creation_tokens == 7
+        assert omni.ec_transfer_params == {"connector": "shm", "handle": "abc"}
+        assert omni.num_cached_tokens == 3
+        assert omni.kv_transfer_params == {"role": "producer"}
diff --git a/vllm_omni/outputs/__init__.py b/vllm_omni/outputs/__init__.py
index 105d0222d..90197f6e6 100644
--- a/vllm_omni/outputs/__init__.py
+++ b/vllm_omni/outputs/__init__.py
@@ -85,7 +85,9 @@ _REQUEST_OUTPUT_CONTENT_ATTRS = (
     "encoder_prompt",
     "encoder_prompt_token_ids",
     "num_cached_tokens",
+    "num_cache_creation_tokens",
     "kv_transfer_params",
+    "ec_transfer_params",
 )
 
 # Omni-specific content copied when the wrapped stage output is itself an
@@ -147,7 +149,9 @@ class OmniRequestOutput(RequestOutput):
     encoder_prompt: str | None = None
     encoder_prompt_token_ids: list[int] | None = None
     num_cached_tokens: int | None = None
+    num_cache_creation_tokens: int | None = None
     kv_transfer_params: dict[str, Any] | None = None
+    ec_transfer_params: dict[str, Any] | None = None
 
     # --- Pipeline stage fields ---
     stage_id: int | None = None