diff --git a/messagefoundry/_diffcov_probe.py b/messagefoundry/_diffcov_probe.py
new file mode 100644
index 00000000..bbe6a783
--- /dev/null
+++ b/messagefoundry/_diffcov_probe.py
@@ -0,0 +1,34 @@
+"""TEMPORARY diff-coverage probe — DELETE BEFORE MERGE. Not part of the engine.
+
+Exists for one reason: four merged PRs surfaced the advisory quality gates, and the headline claim of
+that work — that diff-cover emits INLINE `::notice` annotations on the Files changed tab — has never
+been observed on a real diff, because none of those PRs touched `messagefoundry/` and `--cov` only
+measures this package. The mechanism is proven locally and the CI path provably executes, but "it
+reports correctly that there was nothing to report" is not the same as "it renders".
+
+This module supplies real changed lines under coverage: `probe_covered` is exercised by
+tests/test_diffcov_probe.py, `probe_uncovered` deliberately is not. A correct run annotates only the
+latter — which proves both that the surface renders AND that it is scoped to genuinely uncovered
+changed lines rather than to the whole diff.
+
+Delete this file and its test once the annotation has been observed.
+"""
+
+
+def probe_covered(value: int) -> int:
+    """Exercised by the probe test, so these lines must NOT be annotated."""
+    doubled = value * 2
+    return doubled
+
+
+def probe_uncovered(value: int) -> str:
+    """Deliberately unexercised. Every line below should come back as a `Missing Coverage` notice,
+    coalesced into as few ranges as diff-cover can manage."""
+    if value > 100:
+        label = "large"
+    elif value > 10:
+        label = "medium"
+    else:
+        label = "small"
+    suffix = "!" if value < 0 else ""
+    return f"{label}{suffix}"
diff --git a/tests/test_diffcov_probe.py b/tests/test_diffcov_probe.py
new file mode 100644
index 00000000..983613f0
--- /dev/null
+++ b/tests/test_diffcov_probe.py
@@ -0,0 +1,13 @@
+"""TEMPORARY — companion to messagefoundry/_diffcov_probe.py. DELETE BEFORE MERGE.
+
+Covers `probe_covered` and pointedly not `probe_uncovered`, so the diff-coverage job has a real diff
+with a mix of covered and uncovered changed lines to annotate.
+"""
+
+from messagefoundry._diffcov_probe import probe_covered
+
+
+def test_probe_covered_doubles() -> None:
+    assert probe_covered(21) == 42
+    assert probe_covered(0) == 0
+    assert probe_covered(-3) == -6