diff --git a/CHANGELOG.md b/CHANGELOG.md
index 2f4fdfc..3328254 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,29 @@ together).
 
 ## Unreleased
 
+### Fixed
+
+- **A test that refused to pass vacuously failed instead, and it was
+  red-lighting every non-Linux machine.**
+  `a_run_that_was_not_contained_keeps_no_evidence_from_an_optimistic_probe`
+  asserts that a control record carries what the RUN achieved rather than
+  what the host probe optimistically reported. To stop that passing for the
+  wrong reason, it required the probe and the run to disagree - by asserting
+  the probe says `Enforced`.
+
+  That is only true on Linux. Everywhere else the assertion fired and the
+  test failed outright, so `cargo test --workspace` was red before anyone
+  had touched anything. The instinct was right and the mechanism was wrong:
+  a test that cannot be meaningful on this host should say so, not fail.
+
+  The disagreement is what the test needs, not which side of it the machine
+  is on. Off Linux the probe still says "not contained" - but for a
+  DIFFERENT reason - so comparing the two rendered lines keeps the test
+  honest everywhere and green everywhere it is honest.
+
+  A gate that is red by default is one people learn to read past, which is
+  the whole reason it matters that this one was.
+
 ### Added
 
 - **A contained run could delete your files and the report would not mention
diff --git a/crates/flowproof-cli/src/lib.rs b/crates/flowproof-cli/src/lib.rs
index fa7364e..0cbb28f 100644
--- a/crates/flowproof-cli/src/lib.rs
+++ b/crates/flowproof-cli/src/lib.rs
@@ -2026,15 +2026,22 @@ mod tests {
             "the fixture must actually carry a blocked lane, or this test proves nothing"
         );
 
-        // The probe on this host says `Enforced`; the RUN says it was not
-        // contained. They differ, so passing can only mean the run won.
-        assert!(
-            agent_flow::containment(&spec).is_enforced(),
-            "this test is only meaningful where the probe and the run DISAGREE; \
-             on a host whose probe already says 'not contained' it proves nothing"
-        );
         let achieved =
             flowproof_adapters::Containment::NotContained("the filters never installed".into());
+        // The probe and the RUN must DISAGREE, or passing proves nothing.
+        //
+        // This used to require the probe to say `Enforced`, which is true
+        // only on Linux - so the test failed outright on every other host
+        // and made `cargo test --workspace` red before anyone had touched
+        // anything. The disagreement is what matters, not which side of it
+        // this machine happens to be on: off Linux the probe still says
+        // "not contained", but for a DIFFERENT reason, and the record must
+        // carry the run's reason rather than the probe's.
+        assert_ne!(
+            agent_flow::containment(&spec).report_line(),
+            achieved.report_line(),
+            "the probe and the run must differ, or this test proves nothing"
+        );
         let record = build_control_record(
             &spec_path,
             &dir,