diff --git a/tests/functional/models/test_union_compatibility.py b/tests/functional/models/test_union_compatibility.py
new file mode 100644
index 000000000..a0a038322
--- /dev/null
+++ b/tests/functional/models/test_union_compatibility.py
@@ -0,0 +1,96 @@
+"""Compatibility tests for model field union annotations.
+
+These tests exercise the boundary between ``mode.utils.objects`` union
+detection and Faust's model type-expression compiler.  They cover both
+``typing.Union`` and PEP 604 (``X | Y``) syntax so dependency upgrades cannot
+silently change which annotations reach ``UnionNode``.
+"""
+
+from datetime import datetime
+from decimal import Decimal
+from typing import Any, Dict, List, Optional, Union
+
+import pytest
+
+from faust import Record
+from faust.models.typing import TypeExpression
+
+
+class Child(Record, namespace="test.union.Child"):
+    value: int
+
+
+@pytest.mark.parametrize(
+    "annotation,value,expected",
+    [
+        (str | int | None, "hello", "hello"),
+        (str | int | None, 42, 42),
+        (str | int | None, None, None),
+        (Union[str, int, None], "hello", "hello"),
+        (Union[str, int, None], 42, 42),
+        (Union[str, int, None], None, None),
+        (Child | None, {"value": 3}, Child(value=3)),
+        (Optional[Child], {"value": 3}, Child(value=3)),
+        (List[Optional[int]], [1, None, 2], [1, None, 2]),
+        (
+            Dict[str, Optional[int]],
+            {"one": 1, "none": None},
+            {"one": 1, "none": None},
+        ),
+    ],
+)
+def test_supported_union_type_expressions_compile(annotation, value, expected):
+    """Existing supported union forms must continue compiling."""
+    converter = TypeExpression(annotation).as_function()
+    assert converter(value) == expected
+
+
+JSON_PASSTHROUGH_UNIONS = [
+    str | list | dict | None,
+    Union[str, list, dict, None],
+    str | list[str] | dict[str, Any] | None,
+    Union[str, List[str], Dict[str, Any], None],
+]
+
+
+@pytest.mark.parametrize("annotation", JSON_PASSTHROUGH_UNIONS)
+@pytest.mark.xfail(
+    reason="mode#80: Faust does not yet pass through heterogeneous JSON unions",
+    strict=True,
+)
+def test_json_native_heterogeneous_unions_compile(annotation):
+    """JSON-shaped heterogeneous unions should be safe pass-through fields."""
+    converter = TypeExpression(annotation).as_function()
+    for value in ("secret", ["secret"], {"secret": True}, None):
+        assert converter(value) == value
+
+
+@pytest.mark.xfail(
+    reason="mode#80: class creation currently rejects the reported PEP 604 union",
+    strict=True,
+)
+def test_issue_80_record_declaration_regression():
+    """Exercise the exact model declaration reported in mode issue #80."""
+
+    class SensitiveInfo(Record, namespace="test.union.SensitiveInfo"):
+        data: str | list | dict | None
+        meta: dict
+
+    value = SensitiveInfo(data={"token": "redacted"}, meta={"source": "test"})
+    assert value.data == {"token": "redacted"}
+
+
+@pytest.mark.parametrize(
+    "annotation",
+    [
+        str | Child,
+        Union[str, Child],
+        str | Decimal,
+        Union[str, datetime],
+        list[Child] | dict[str, Child],
+    ],
+)
+def test_ambiguous_or_coercion_sensitive_unions_remain_unsupported(annotation):
+    """Do not hide unions that require runtime type selection or coercion."""
+    with pytest.raises(NotImplementedError, match="Union of types"):
+        TypeExpression(annotation).as_function()