diff --git a/tests/model_executor/test_upstream_prompt_update_api.py b/tests/model_executor/test_upstream_prompt_update_api.py
new file mode 100644
index 000000000..da31980c2
--- /dev/null
+++ b/tests/model_executor/test_upstream_prompt_update_api.py
@@ -0,0 +1,64 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM-Omni project
+"""Guard that Omni only calls prompt-update helpers upstream still provides.
+
+vLLM 0.29 removed ``PromptUpdateDetails.select_text`` in favour of the
+token-oriented ``select_token_id`` / ``select_token_ids``. Three MiniCPM-o call
+sites survived the rebase and only failed at engine-core startup, where the
+traceback surfaced as an opaque "Engine core initialization failed" across eight
+CI jobs.
+
+These helpers are referenced from replacement closures that no unit test drives,
+so a static check is what actually catches the next removal. It resolves the
+attribute against the installed class rather than a hard-coded list, so it
+follows upstream automatically.
+"""
+
+from __future__ import annotations
+
+import ast
+from pathlib import Path
+
+import pytest
+from vllm.multimodal.processing.processor import PromptUpdateDetails
+
+pytestmark = [pytest.mark.core_model, pytest.mark.cpu]
+
+_OMNI_ROOT = Path(__file__).resolve().parents[2] / "vllm_omni"
+_GUARDED = "PromptUpdateDetails"
+
+
+def _referenced_attributes() -> dict[str, set[str]]:
+    """Map ``PromptUpdateDetails.<attr>`` -> source files referencing it."""
+    found: dict[str, set[str]] = {}
+    for path in _OMNI_ROOT.rglob("*.py"):
+        try:
+            tree = ast.parse(path.read_text(encoding="utf-8"))
+        except (SyntaxError, UnicodeDecodeError):  # pragma: no cover - defensive
+            continue
+        for node in ast.walk(tree):
+            if isinstance(node, ast.Attribute) and isinstance(node.value, ast.Name) and node.value.id == _GUARDED:
+                found.setdefault(node.attr, set()).add(str(path.relative_to(_OMNI_ROOT.parent)))
+    return found
+
+
+def test_omni_only_uses_prompt_update_helpers_that_exist_upstream() -> None:
+    referenced = _referenced_attributes()
+    assert referenced, "expected Omni to reference PromptUpdateDetails somewhere"
+
+    missing = {attr: sorted(files) for attr, files in referenced.items() if not hasattr(PromptUpdateDetails, attr)}
+
+    assert not missing, (
+        "Omni calls PromptUpdateDetails helpers that the installed vLLM does not "
+        f"provide: {missing}. Upstream moved prompt updates to token ids; use "
+        "select_token_id/select_token_ids with the encoded text."
+    )
+
+
+def test_token_oriented_helpers_are_the_ones_upstream_exposes() -> None:
+    """Pin the replacements, so their removal fails here and not at startup."""
+    assert hasattr(PromptUpdateDetails, "select_token_id")
+    assert hasattr(PromptUpdateDetails, "select_token_ids")
+    assert not hasattr(PromptUpdateDetails, "select_text"), (
+        "select_text is back upstream; re-check the MiniCPM-o replacements."
+    )