diff --git a/fleetd/src/test/java/dev/ltms/fleet/testing/CapturedLogTest.java b/fleetd/src/test/java/dev/ltms/fleet/testing/CapturedLogTest.java
new file mode 100644
index 0000000..456b8a5
--- /dev/null
+++ b/fleetd/src/test/java/dev/ltms/fleet/testing/CapturedLogTest.java
@@ -0,0 +1,96 @@
+package dev.ltms.fleet.testing;
+
+import ch.qos.logback.classic.Level;
+import ch.qos.logback.classic.Logger;
+import org.junit.jupiter.api.Test;
+import org.slf4j.LoggerFactory;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+/**
+ * fleetd #537: pins {@link CapturedLog#close}'s own contract — the appender detach half, the level
+ * restore half, and that {@link CapturedLog#setLevel} does not change what {@code close} restores.
+ * Before this, only the level-restore half was pinned (by {@code
+ * WorktreeSessionManagerTest.sharedSessionManagerLoggerLevelIsRestoredAfterDirtyWorktreeReleasePinsWarn}).
+ * Measured: deleting {@code logger.detachAppender(appender);} from {@code close()} still left
+ * {@code mvn clean install} green — 1701 tests, 0 failures — before this file existed.
+ *
+ * <p>Every test below uses a logger name no production class uses, and unique per test, so this
+ * file cannot become the next entry in fleetd #525's leak family: {@link CapturedLog}'s own
+ * javadoc re-measure commands (top of that file) would otherwise need to start naming this class.
+ */
+class CapturedLogTest {
+
+    /**
+     * The appender must be detached on close: an event logged through the raw logger after close
+     * must not land in {@link CapturedLog#events()}. Asserting on the observable list (rather than
+     * {@code logger.iteratorForAppenders()}) is what the ticket asked for, and it is also what a
+     * real leak would actually break — a later test's own {@code ListAppender} silently gaining
+     * events emitted by code under test that has nothing to do with it.
+     */
+    @Test
+    void closeDetachesTheAppenderSoALaterLogIsNotCaptured() {
+        String loggerName = "capturedlog-test-only.appender-detach";
+        Logger rawLogger = (Logger) LoggerFactory.getLogger(loggerName);
+
+        CapturedLog log = CapturedLog.of(loggerName);
+        rawLogger.info("while open");
+        int eventsWhileOpen = log.events().size();
+        assertEquals(1, eventsWhileOpen, "the event logged while open must be captured");
+
+        log.close();
+        rawLogger.info("after close");
+
+        assertEquals(eventsWhileOpen, log.events().size(),
+                "close() must detach the appender: an event logged after close must not be "
+                        + "captured, but the captured list grew from " + eventsWhileOpen + " to "
+                        + log.events().size());
+    }
+
+    /**
+     * The helper's own headline contract, pinned in one place independent of any production
+     * class's behaviour: {@code close()} restores the level the logger had before {@link
+     * CapturedLog#at} pinned it.
+     */
+    @Test
+    void closeRestoresTheLevelCapturedAtOpen() {
+        String loggerName = "capturedlog-test-only.level-restore";
+        Logger rawLogger = (Logger) LoggerFactory.getLogger(loggerName);
+        rawLogger.setLevel(Level.DEBUG);
+
+        CapturedLog log = CapturedLog.at(loggerName, Level.ERROR);
+        assertEquals(Level.ERROR, rawLogger.getLevel(), "the pinned level took effect while open");
+
+        log.close();
+
+        assertEquals(Level.DEBUG, rawLogger.getLevel(),
+                "close() must restore the level captured when at() was called (DEBUG), not leave "
+                        + "the pinned level (ERROR) in place");
+    }
+
+    /**
+     * {@link CapturedLog#setLevel}'s javadoc claims that re-pinning the level mid-capture does not
+     * change what {@code close()} restores — that restore always uses the level captured when the
+     * instance was created, never a value set through {@code setLevel}. Nothing checked this
+     * before: open with a pinned WARN, call {@code setLevel(TRACE)}, close, and the result must be
+     * the level from BEFORE {@code at} — neither WARN nor TRACE.
+     */
+    @Test
+    void setLevelDuringCaptureDoesNotChangeWhatCloseRestores() {
+        String loggerName = "capturedlog-test-only.setlevel-no-effect";
+        Logger rawLogger = (Logger) LoggerFactory.getLogger(loggerName);
+        rawLogger.setLevel(Level.DEBUG);
+
+        CapturedLog log = CapturedLog.at(loggerName, Level.WARN);
+        log.setLevel(Level.TRACE);
+        assertEquals(Level.TRACE, rawLogger.getLevel(), "setLevel took effect immediately");
+
+        log.close();
+
+        assertEquals(Level.DEBUG, rawLogger.getLevel(),
+                "close() must restore the level captured at open (DEBUG) regardless of any "
+                        + "later setLevel() call: it must be neither WARN (the level pinned by "
+                        + "at()) nor TRACE (the level set via setLevel() mid-capture), but got "
+                        + rawLogger.getLevel());
+    }
+}