diff --git a/crates/bugwarden/src/config.rs b/crates/bugwarden/src/config.rs
index 58912b7..26eed7c 100644
--- a/crates/bugwarden/src/config.rs
+++ b/crates/bugwarden/src/config.rs
@@ -571,13 +571,7 @@ mod tests {
         drop(custody.expect("key file resolves"));
         crate::testlog::assert_logged(&logs, "server-held");
         crate::testlog::assert_logged(&logs, &file.path().display().to_string());
-        // The I12 check below is a negative: on its own an empty capture would
-        // satisfy it. It is evidence only because the positive assertions above
-        // run first and fail loudly on one — keep them ahead of it.
-        assert!(
-            !logs.contains("hush-key"),
-            "the log must never carry key material (I12): {logs}"
-        );
+        logs.assert_not_contains("hush-key");
 
         let cli = base_cli("http");
         let (custody, logs) = crate::testlog::capture_logs(|| cli.resolve_key_custody());
diff --git a/crates/bugwarden/src/http_auth.rs b/crates/bugwarden/src/http_auth.rs
index 2df5f07..5070141 100644
--- a/crates/bugwarden/src/http_auth.rs
+++ b/crates/bugwarden/src/http_auth.rs
@@ -772,17 +772,15 @@ mod tests {
         });
         // Five refusals, three lines: 1, 2, 4. A mutation that logged every
         // refusal — an unauthenticated log-volume lever — fails here.
+        let text = logs.as_str();
         assert_eq!(
-            logs.matches("http bearer authentication refused a request")
+            text.matches("http bearer authentication refused a request")
                 .count(),
             3,
-            "{logs}"
-        );
-        assert!(logs.contains("refused=4"), "{logs}");
-        assert!(
-            !logs.contains(WRITE),
-            "the log must carry no token (I12): {logs}"
+            "{text}"
         );
+        crate::testlog::assert_logged(&logs, "refused=4");
+        logs.assert_not_contains(WRITE);
     }
 
     #[test]
@@ -795,14 +793,13 @@ mod tests {
         ] {
             let auth = HttpAuth::resolve(&e, insecure).expect("resolve");
             let ((), logs) = crate::testlog::capture_logs(|| auth.log_startup_mode());
-            // Positive first, so the I12 negative below is evidence and not
-            // just an empty capture passing.
+            let text = logs.as_str();
             assert!(
-                logs.contains("http authentication:") || logs.contains("--insecure-no-auth"),
-                "{logs}"
+                text.contains("http authentication:") || text.contains("--insecure-no-auth"),
+                "{text}"
             );
-            assert!(!logs.contains(WRITE), "{logs}");
-            assert!(!logs.contains(READ), "{logs}");
+            logs.assert_not_contains(WRITE);
+            logs.assert_not_contains(READ);
         }
     }
 
diff --git a/crates/bugwarden/src/server.rs b/crates/bugwarden/src/server.rs
index d512579..b69dbaa 100644
--- a/crates/bugwarden/src/server.rs
+++ b/crates/bugwarden/src/server.rs
@@ -4033,14 +4033,15 @@ mod tests {
             let (server, logs) =
                 crate::testlog::capture_logs(|| BugWarden::new(Arc::new(cli), guard, bz));
             drop(server.expect("server must build"));
-            // The `false` arm asserts an absence, which an empty capture
-            // satisfies while proving nothing. This is what separates "the
-            // warning was not emitted" from "nothing was captured at all".
+            // The `false` arm asserts an absence; as_str has already
+            // rejected a silent capture, so this is "not emitted" not
+            // "nothing captured".
             crate::testlog::assert_captured(&logs);
+            let text = logs.as_str();
             assert_eq!(
-                logs.contains("created_by_me describes that one account"),
+                text.contains("created_by_me describes that one account"),
                 expect_warn,
-                "startup logs: {logs}"
+                "startup logs: {text}"
             );
         }
     }
diff --git a/crates/bugwarden/src/testlog.rs b/crates/bugwarden/src/testlog.rs
index 24d74b2..7806a17 100644
--- a/crates/bugwarden/src/testlog.rs
+++ b/crates/bugwarden/src/testlog.rs
@@ -71,13 +71,58 @@ fn install() {
     });
 }
 
+/// Logs captured by [`capture_logs`].
+///
+/// There is no `Deref<Target = str>` and no `contains`. The text is reachable
+/// only through a path that has already established the capture is non-empty
+/// ([`assert_logged`], [`Self::assert_not_contains`], or [`Self::as_str`],
+/// which panics on empty). A negative I12 check over a silent capture
+/// therefore cannot compile, let alone pass.
+#[derive(Debug)]
+pub(crate) struct Captured(String);
+
+impl Captured {
+    /// The captured text. Panics if the subscriber saw no events.
+    #[track_caller]
+    pub(crate) fn as_str(&self) -> &str {
+        assert_captured(self);
+        &self.0
+    }
+
+    /// Assert that a non-empty capture does not contain `needle`.
+    ///
+    /// An empty capture fails as "captured nothing", not as a successful
+    /// absence. That is what makes a negative I12 check evidence.
+    #[track_caller]
+    pub(crate) fn assert_not_contains(&self, needle: &str) {
+        let logs = self.as_str();
+        assert!(
+            !logs.contains(needle),
+            "captured logs must not contain {needle:?}: {logs}"
+        );
+    }
+}
+
+/// Clears the thread-local capture slot, including when `f` panics, so the
+/// next capture on this thread cannot inherit residue. The `debug_assert!`
+/// on entry then only has to reject nesting, not also clean up a crash.
+struct ClearCaptureOnDrop;
+
+impl Drop for ClearCaptureOnDrop {
+    fn drop(&mut self) {
+        let _ = CAPTURE.try_with(|slot| {
+            slot.borrow_mut().take();
+        });
+    }
+}
+
 /// Run `f` with everything it logs *on this thread* captured, and return its
 /// result together with the captured output. Events emitted meanwhile by other
 /// threads go to their own (absent) capture and are discarded.
 ///
 /// Captures do not nest: an inner call would hand the outer one an empty
-/// string, so a nested capture is a debug assertion rather than a silent loss.
-pub(crate) fn capture_logs<T>(f: impl FnOnce() -> T) -> (T, String) {
+/// capture, so a nested capture is a debug assertion rather than a silent loss.
+pub(crate) fn capture_logs<T>(f: impl FnOnce() -> T) -> (T, Captured) {
     install();
     CAPTURE.with(|slot| {
         let mut slot = slot.borrow_mut();
@@ -87,12 +132,14 @@ pub(crate) fn capture_logs<T>(f: impl FnOnce() -> T) -> (T, String) {
         );
         *slot = Some(Vec::new());
     });
+    let guard = ClearCaptureOnDrop;
     let value = f();
     let captured = CAPTURE
         .with(|slot| slot.borrow_mut().take())
         .unwrap_or_default();
+    drop(guard);
     let logs = String::from_utf8(captured).expect("captured logs are UTF-8");
-    (value, logs)
+    (value, Captured(logs))
 }
 
 /// Assert that the capture is not empty.
@@ -103,9 +150,9 @@ pub(crate) fn capture_logs<T>(f: impl FnOnce() -> T) -> (T, String) {
 /// empty haystack. It is also what makes the negative I12 assertions evidence:
 /// "the log must never carry the key" holds vacuously over an empty capture.
 #[track_caller]
-pub(crate) fn assert_captured(logs: &str) {
+pub(crate) fn assert_captured(logs: &Captured) {
     assert!(
-        !logs.is_empty(),
+        !logs.0.is_empty(),
         "captured nothing: the subscriber saw no events"
     );
 }
@@ -113,8 +160,8 @@ pub(crate) fn assert_captured(logs: &str) {
 /// Assert that the capture carries `needle`, distinguishing an empty capture
 /// from one that merely lacks the text.
 #[track_caller]
-pub(crate) fn assert_logged(logs: &str, needle: &str) {
-    assert_captured(logs);
+pub(crate) fn assert_logged(logs: &Captured, needle: &str) {
+    let logs = logs.as_str();
     assert!(
         logs.contains(needle),
         "captured logs lack {needle:?}: {logs}"
@@ -147,9 +194,10 @@ mod tests {
         // Exactly one hit: the other thread's emit belongs to its own capture,
         // not to this one.
         assert_eq!(
-            logs.matches(PROBE).count(),
+            logs.as_str().matches(PROBE).count(),
             1,
-            "only this thread's events belong in the capture: {logs}"
+            "only this thread's events belong in the capture: {}",
+            logs.as_str()
         );
     }
 
@@ -157,12 +205,55 @@ mod tests {
     #[test]
     #[should_panic(expected = "captured nothing")]
     fn an_empty_capture_fails_as_an_empty_capture() {
-        assert_logged("", "anything");
+        assert_logged(&Captured(String::new()), "anything");
     }
 
     #[test]
     #[should_panic(expected = "captured logs lack")]
     fn a_non_empty_capture_without_the_needle_fails_as_a_missing_needle() {
-        assert_logged("something else entirely", "anything");
+        assert_logged(&Captured("something else entirely".into()), "anything");
+    }
+
+    #[test]
+    #[should_panic(expected = "captured nothing")]
+    fn as_str_on_an_empty_capture_fails_as_an_empty_capture() {
+        let _ = Captured(String::new()).as_str();
+    }
+
+    #[test]
+    #[should_panic(expected = "captured nothing")]
+    fn assert_not_contains_on_an_empty_capture_fails_as_an_empty_capture() {
+        Captured(String::new()).assert_not_contains("secret");
+    }
+
+    #[test]
+    #[should_panic(expected = "captured logs must not contain")]
+    fn assert_not_contains_fails_when_the_needle_is_present() {
+        Captured("leaked-token-material".into()).assert_not_contains("leaked-token-material");
+    }
+
+    #[test]
+    fn assert_not_contains_accepts_a_non_empty_capture_without_the_needle() {
+        Captured("harmless startup line".into()).assert_not_contains("secret");
+    }
+
+    #[test]
+    fn a_panicked_capture_clears_the_slot() {
+        let caught = std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| {
+            let _ = capture_logs(|| {
+                tracing::info!("residue-that-must-not-leak");
+                panic!("intentional capture panic");
+            });
+        }));
+        assert!(caught.is_err(), "the inner f must panic");
+        CAPTURE.with(|slot| {
+            assert!(
+                slot.borrow().is_none(),
+                "a panicked f must leave the capture slot empty"
+            );
+        });
+        let (_, logs) = capture_logs(probe);
+        assert_logged(&logs, PROBE);
+        logs.assert_not_contains("residue-that-must-not-leak");
     }
 }