diff --git a/tests/oauth_client_real_operator.rs b/tests/oauth_client_real_operator.rs
index 5bc811d..978ac34 100644
--- a/tests/oauth_client_real_operator.rs
+++ b/tests/oauth_client_real_operator.rs
@@ -21,4 +21,15 @@ async fn run_starting_errors_when_stdout_not_tty() {
         report.code().map(|c| c.to_string()),
         Some("oauth_client::interactive::operator::not_a_tty".to_string()),
     );
+    // AC9.1 also pins the human-visible message string: the report
+    // must render the documented "interactive mode requires a TTY"
+    // phrase. Asserting this here keeps the wording stable across
+    // refactors — silently shortening the message (e.g. to "TTY
+    // required") would not change the diagnostic code but would
+    // regress the operator-facing prose.
+    let rendered = format!("{report:?}");
+    assert!(
+        rendered.contains("interactive mode requires a TTY"),
+        "expected the rendered miette report to contain the AC9.1 message string, got:\n{rendered}",
+    );
 }