diff --git a/bifromq-inbox/bifromq-inbox-server/src/main/java/org/apache/bifromq/inbox/server/InboxFetchPipeline.java b/bifromq-inbox/bifromq-inbox-server/src/main/java/org/apache/bifromq/inbox/server/InboxFetchPipeline.java
index f06e56040..7259742c9 100644
--- a/bifromq-inbox/bifromq-inbox-server/src/main/java/org/apache/bifromq/inbox/server/InboxFetchPipeline.java
+++ b/bifromq-inbox/bifromq-inbox-server/src/main/java/org/apache/bifromq/inbox/server/InboxFetchPipeline.java
@@ -25,8 +25,7 @@
 
 import io.grpc.stub.StreamObserver;
 import io.reactivex.rxjava3.disposables.Disposable;
-import java.util.Collections;
-import java.util.HashSet;
+import java.util.Iterator;
 import java.util.Map;
 import java.util.Set;
 import java.util.concurrent.CompletableFuture;
@@ -90,7 +89,7 @@ public InboxFetchPipeline(StreamObserver<InboxFetched> responseObserver,
                                 fetchHint.getSessionId());
                             inboxSessionMap.computeIfAbsent(
                                 new InboxId(fetchHint.getInboxId(), fetchHint.getIncarnation()),
-                                k1 -> new HashSet<>()).add(fetchHint.getSessionId());
+                                k1 -> ConcurrentHashMap.newKeySet()).add(fetchHint.getSessionId());
                         }
                         v.lastFetchQoS0Seq.set(
                             Math.max(fetchHint.getLastFetchQoS0Seq(), v.lastFetchQoS0Seq.get()));
@@ -131,16 +130,31 @@ public void send(InboxFetched message) {
     public boolean signalFetch(String inboxId, long incarnation, long now) {
         log.trace("Signal fetch: tenantId={}, inboxId={}", tenantId, inboxId);
         // signal fetch won't refresh expiry
-        Set<Long> sessionIds = inboxSessionMap.getOrDefault(new InboxId(inboxId, incarnation), Collections.emptySet());
-        for (Long sessionId : sessionIds) {
+        InboxId inboxKey = new InboxId(inboxId, incarnation);
+        Set<Long> sessionIds = inboxSessionMap.get(inboxKey);
+        if (sessionIds == null || sessionIds.isEmpty()) {
+            return false;
+        }
+        boolean triggered = false;
+        Iterator<Long> itr = sessionIds.iterator();
+        while (itr.hasNext()) {
+            Long sessionId = itr.next();
             FetchState fetchState = inboxFetchSessions.get(sessionId);
-            if (fetchState != null && fetchState.signalFetchTS.get() < now) {
+            if (fetchState == null) {
+                itr.remove();
+                continue;
+            }
+            triggered = true;
+            if (fetchState.signalFetchTS.get() < now) {
                 fetchState.hasMore.set(true);
                 fetchState.signalFetchTS.set(now);
                 fetch(fetchState);
             }
         }
-        return !sessionIds.isEmpty();
+        if (sessionIds.isEmpty()) {
+            inboxSessionMap.remove(inboxKey, sessionIds);
+        }
+        return triggered;
     }
 
     @Override
diff --git a/bifromq-inbox/bifromq-inbox-server/src/test/java/org/apache/bifromq/inbox/server/InboxFetchPipelineMappingTest.java b/bifromq-inbox/bifromq-inbox-server/src/test/java/org/apache/bifromq/inbox/server/InboxFetchPipelineMappingTest.java
index 071fab65c..cff671dfa 100644
--- a/bifromq-inbox/bifromq-inbox-server/src/test/java/org/apache/bifromq/inbox/server/InboxFetchPipelineMappingTest.java
+++ b/bifromq-inbox/bifromq-inbox-server/src/test/java/org/apache/bifromq/inbox/server/InboxFetchPipelineMappingTest.java
@@ -25,20 +25,27 @@
 import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.Mockito.doAnswer;
 import static org.testng.Assert.assertEquals;
+import static org.testng.Assert.assertFalse;
 import static org.testng.Assert.assertNotNull;
+import static org.testng.Assert.assertNull;
 import static org.testng.Assert.assertTrue;
 
 import io.grpc.Context;
 import io.grpc.stub.ServerCallStreamObserver;
 import io.micrometer.core.instrument.Timer;
 import io.micrometer.core.instrument.simple.SimpleMeterRegistry;
+import java.lang.reflect.Field;
 import java.util.ArrayList;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.Set;
 import java.util.concurrent.BlockingQueue;
 import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.LinkedBlockingQueue;
+import java.util.concurrent.atomic.AtomicInteger;
+import java.util.concurrent.atomic.AtomicReference;
 import lombok.SneakyThrows;
 import org.apache.bifromq.baserpc.RPCContext;
 import org.apache.bifromq.baserpc.metrics.IRPCMeter;
@@ -205,12 +212,96 @@ public void shouldNotRewindStartAfterWhenHintIsStale() {
         pipeline.close();
     }
 
+    @Test
+    public void shouldCleanStaleSessionIdWhenFetchStateMissing() throws Exception {
+        InboxFetcherRegistry registry = new InboxFetcherRegistry();
+        InboxFetchPipeline pipeline = new InboxFetchPipeline(responseObserver, noopFetcher(), registry);
+
+        long sessionId = 4004L;
+        pipeline.onNext(hint(sessionId, 1));
+
+        Map<Long, ?> fetchSessions = fetchSessions(pipeline);
+        fetchSessions.remove(sessionId);
+
+        Map<?, Set<Long>> sessionMap = inboxSessionMap(pipeline);
+        Set<Long> sessionIds = sessionMap.values().iterator().next();
+        assertTrue(sessionIds.contains(sessionId));
+
+        boolean signalled = pipeline.signalFetch(INBOX, INCARNATION, System.nanoTime());
+
+        assertFalse(signalled);
+        assertTrue(sessionMap.isEmpty());
+    }
+
+    @Test
+    public void shouldNotThrowWhenSignalFetchConcurrentWithSessionRemoval() throws Exception {
+        InboxFetcherRegistry registry = new InboxFetcherRegistry();
+        CountingFetcher fetcher = new CountingFetcher();
+        InboxFetchPipeline pipeline = new InboxFetchPipeline(responseObserver, fetcher, registry);
+
+        long sessionA = 5005L;
+        long sessionB = 6006L;
+
+        pipeline.onNext(hint(sessionA, 5));
+        pipeline.onNext(hint(sessionB, 5));
+
+        CountDownLatch latch = new CountDownLatch(1);
+        AtomicReference<Throwable> error = new AtomicReference<>();
+
+        Thread signalThread = new Thread(() -> {
+            try {
+                latch.await();
+                for (int i = 0; i < 500; i++) {
+                    pipeline.signalFetch(INBOX, INCARNATION, System.nanoTime());
+                }
+            } catch (Throwable t) {
+                error.compareAndSet(null, t);
+            }
+        });
+
+        Thread removeThread = new Thread(() -> {
+            try {
+                latch.await();
+                for (int i = 0; i < 500; i++) {
+                    pipeline.onNext(hint(sessionB, -1));
+                    pipeline.onNext(hint(sessionB, 5));
+                }
+            } catch (Throwable t) {
+                error.compareAndSet(null, t);
+            }
+        });
+
+        signalThread.start();
+        removeThread.start();
+        latch.countDown();
+        signalThread.join();
+        removeThread.join();
+
+        assertNull(error.get());
+        await().until(() -> fetcher.fetchCount.get() > 0);
+        pipeline.close();
+    }
+
     private InboxFetched lastReceived() {
         synchronized (received) {
             return received.get(received.size() - 1);
         }
     }
 
+    @SuppressWarnings("unchecked")
+    private Map<Long, ?> fetchSessions(InboxFetchPipeline pipeline) throws Exception {
+        Field field = InboxFetchPipeline.class.getDeclaredField("inboxFetchSessions");
+        field.setAccessible(true);
+        return (Map<Long, ?>) field.get(pipeline);
+    }
+
+    @SuppressWarnings("unchecked")
+    private Map<?, Set<Long>> inboxSessionMap(InboxFetchPipeline pipeline) throws Exception {
+        Field field = InboxFetchPipeline.class.getDeclaredField("inboxSessionMap");
+        field.setAccessible(true);
+        return (Map<?, Set<Long>>) field.get(pipeline);
+    }
+
     private static class TestFetcher implements InboxFetchPipeline.Fetcher {
         private final BlockingQueue<FetchRequest> requests = new LinkedBlockingQueue<>();
         private final BlockingQueue<CompletableFuture<Fetched>> responses = new LinkedBlockingQueue<>();
@@ -237,4 +328,16 @@ void completeNext(Fetched fetched) {
             future.complete(fetched);
         }
     }
+
+    private static class CountingFetcher implements InboxFetchPipeline.Fetcher {
+        private final AtomicInteger fetchCount = new AtomicInteger();
+
+        @Override
+        public CompletableFuture<Fetched> fetch(FetchRequest request) {
+            fetchCount.incrementAndGet();
+            return CompletableFuture.completedFuture(Fetched.newBuilder()
+                .setResult(Fetched.Result.OK)
+                .build());
+        }
+    }
 }