diff --git a/fleetd/src/main/java/dev/ltms/fleet/mcp/FleetMcp.java b/fleetd/src/main/java/dev/ltms/fleet/mcp/FleetMcp.java
index 16e19a8..e351f8a 100644
--- a/fleetd/src/main/java/dev/ltms/fleet/mcp/FleetMcp.java
+++ b/fleetd/src/main/java/dev/ltms/fleet/mcp/FleetMcp.java
@@ -994,7 +994,11 @@ public final class FleetMcp {
         if (isBlank(target) || isBlank(msgId)) {
             return error("target and msgId are required");
         }
-        messages.ackReply(target, msgId);
+        if (!messages.ackReply(target, msgId)) {
+            return error(msgId + " is not in " + target + "'s reply inbox (wrong id, wrong target, "
+                    + "or already acked). Held lead-to-lead (peer) mail cannot be acked this way — "
+                    + "read it with fleet_poll{coordId}.");
+        }
         return text("acknowledged " + msgId);
     }
 
@@ -1766,7 +1770,10 @@ public final class FleetMcp {
                         + "has processed a reply and wants to confirm it, leaving other pending replies "
                         + "in the inbox for later drain.",
                 objectSchema(Map.of(
-                        "target", stringProp("Worker session id whose inbox to ack from"),
+                        "target", stringProp("Worker session id whose inbox to ack from. Must name a "
+                                + "reply actually queued for it — an id in no inbox, or a coord-id "
+                                + "(peer held mail, read with fleet_poll{coordId} instead), errors "
+                                + "rather than reporting a false success"),
                         "msgId", stringProp("The message id to acknowledge")),
                         List.of("target", "msgId")));
     }
diff --git a/fleetd/src/main/java/dev/ltms/fleet/msg/AmqpReplyInbox.java b/fleetd/src/main/java/dev/ltms/fleet/msg/AmqpReplyInbox.java
index 95b984a..3c631f8 100644
--- a/fleetd/src/main/java/dev/ltms/fleet/msg/AmqpReplyInbox.java
+++ b/fleetd/src/main/java/dev/ltms/fleet/msg/AmqpReplyInbox.java
@@ -394,17 +394,17 @@ public final class AmqpReplyInbox implements ReplyInbox, AutoCloseable {
     }
 
     @Override
-    public void ack(String target, String msgId) {
+    public boolean ack(String target, String msgId) {
         var perTarget = held.get(target);
         if (perTarget == null || perTarget == RELEASED) {
-            return;
+            return false;
         }
         Held h;
         synchronized (perTarget) {
             h = perTarget.remove(msgId);
         }
         if (h == null) {
-            return;                                // never held (or already acked) — no-op
+            return false;                          // never held (or already acked) — no-op
         }
         try {
             synchronized (channelLock) {
@@ -418,6 +418,7 @@ public final class AmqpReplyInbox implements ReplyInbox, AutoCloseable {
             }
             throw new IllegalStateException("cannot ack reply " + msgId + " on " + queueName(target), e);
         }
+        return true;
     }
 
     private DeliverCallback deliverCallback(String target) {
diff --git a/fleetd/src/main/java/dev/ltms/fleet/msg/InMemoryReplyInbox.java b/fleetd/src/main/java/dev/ltms/fleet/msg/InMemoryReplyInbox.java
index 69d72c9..efab1ad 100644
--- a/fleetd/src/main/java/dev/ltms/fleet/msg/InMemoryReplyInbox.java
+++ b/fleetd/src/main/java/dev/ltms/fleet/msg/InMemoryReplyInbox.java
@@ -59,16 +59,17 @@ public final class InMemoryReplyInbox implements ReplyInbox {
     }
 
     @Override
-    public void ack(String target, String msgId) {
+    public boolean ack(String target, String msgId) {
         if (!owned.contains(target)) {
-            return;
+            return false;
         }
         var perTarget = store.get(target);
-        if (perTarget != null) {
-            //noinspection SynchronizationOnLocalVariableOrMethodParameter
-            synchronized (perTarget) {
-                perTarget.remove(msgId);
-            }
+        if (perTarget == null) {
+            return false;
+        }
+        //noinspection SynchronizationOnLocalVariableOrMethodParameter
+        synchronized (perTarget) {
+            return perTarget.remove(msgId) != null;
         }
     }
 }
diff --git a/fleetd/src/main/java/dev/ltms/fleet/msg/MessageService.java b/fleetd/src/main/java/dev/ltms/fleet/msg/MessageService.java
index 04ebc21..cf5a76f 100644
--- a/fleetd/src/main/java/dev/ltms/fleet/msg/MessageService.java
+++ b/fleetd/src/main/java/dev/ltms/fleet/msg/MessageService.java
@@ -826,9 +826,13 @@ public final class MessageService {
     /**
      * Acknowledge a specific reply by {@code msgId} for {@code target}. Removes it from the inbox
      * so that a subsequent drain or peek no longer returns it.
+     *
+     * @return {@code true} if an entry was actually removed, {@code false} if {@code msgId} was not
+     *     in {@code target}'s inbox (wrong id, wrong target, or already acked). The caller —
+     *     {@link dev.ltms.fleet.mcp.FleetMcp#ack} — must not report success on {@code false}.
      */
-    public void ackReply(String target, String msgId) {
-        inbox.ack(target, msgId);
+    public boolean ackReply(String target, String msgId) {
+        return inbox.ack(target, msgId);
     }
 
     /**
diff --git a/fleetd/src/main/java/dev/ltms/fleet/msg/ReplyInbox.java b/fleetd/src/main/java/dev/ltms/fleet/msg/ReplyInbox.java
index 38ce4b9..dca7e8b 100644
--- a/fleetd/src/main/java/dev/ltms/fleet/msg/ReplyInbox.java
+++ b/fleetd/src/main/java/dev/ltms/fleet/msg/ReplyInbox.java
@@ -46,6 +46,13 @@ public interface ReplyInbox {
     /** Non-destructive snapshot of pending replies for {@code target} (FIFO), empty list if none. */
     List<InboxMessage> peek(String target);
 
-    /** Remove the reply {@code msgId} for {@code target} once the primary has taken it. No-op if absent. */
-    void ack(String target, String msgId);
+    /**
+     * Remove the reply {@code msgId} for {@code target} once the primary has taken it.
+     *
+     * @return {@code true} if an entry was actually removed, {@code false} if there was nothing to
+     *     remove (unknown {@code target}, unowned {@code target}, or a {@code msgId} not held for
+     *     it). A {@code false} is not an error — acking a {@code target} this daemon does not own is
+     *     part of the normal contract, not a failure.
+     */
+    boolean ack(String target, String msgId);
 }
diff --git a/fleetd/src/test/java/dev/ltms/fleet/mcp/FleetMcpTest.java b/fleetd/src/test/java/dev/ltms/fleet/mcp/FleetMcpTest.java
index 2186371..d1aac0d 100644
--- a/fleetd/src/test/java/dev/ltms/fleet/mcp/FleetMcpTest.java
+++ b/fleetd/src/test/java/dev/ltms/fleet/mcp/FleetMcpTest.java
@@ -352,7 +352,7 @@ class FleetMcpTest {
                 fail("a lead fleet_reply must not publish to the worker inbox");
             }
             @Override public List<InboxMessage> peek(String target) { return List.of(); }
-            @Override public void ack(String target, String msgId) { }
+            @Override public boolean ack(String target, String msgId) { return false; }
         };
         MessageService leadMessages = new MessageService(agents, new Injector(agents), new Rendezvous(),
                 inboxThatRejectsPublishes);
@@ -1386,6 +1386,10 @@ class FleetMcpTest {
 
     @Test
     void bridgeAckReturnsConfirmationForValidArgs() {
+        // fleet_ack only reports success for a msgId actually queued in the target's inbox
+        // (fleetd #437) — publish one via the inbox directly rather than asserting on a
+        // fabricated id nothing ever queued.
+        inbox.publish("term_a", "msg-1", "queued reply");
         McpSchema.CallToolResult res = FleetMcp.ack(messages, "term_a", "msg-1");
         assertNotEquals(Boolean.TRUE, res.isError());
         assertTrue(textOf(res).contains("msg-1"), "response should mention the msgId");
@@ -1398,6 +1402,26 @@ class FleetMcpTest {
         assertTrue(FleetMcp.ack(messages, "  ", "msg-1").isError());
     }
 
+    @Test
+    void bridgeAckOfAnIdInNoInboxIsAnError() {
+        // fleetd #437: fleet_ack used to say "acknowledged <msgId>" for a message it never
+        // touched, because nothing in the chain reported hit vs. miss. "never-queued" is in no
+        // inbox at all, so this must error rather than claim success.
+        McpSchema.CallToolResult res = FleetMcp.ack(messages, "term_a", "never-queued");
+        assertTrue(res.isError());
+        assertTrue(textOf(res).contains("never-queued"), textOf(res));
+    }
+
+    @Test
+    void bridgeAckOfACoordIdTargetIsAnErrorNamingFleetPoll() {
+        // A coord-id names a peer lead's held mailbox (LeadChannel/LeadMailbox), never a
+        // worker's ReplyInbox — fleet_ack has no route to it and must say so, pointing at
+        // fleet_poll{coordId} instead of reporting a false "acknowledged".
+        McpSchema.CallToolResult res = FleetMcp.ack(messages, "coord-some-peer", "msg-1");
+        assertTrue(res.isError());
+        assertTrue(textOf(res).contains("fleet_poll{coordId}"), textOf(res));
+    }
+
     @Test
     void bridgeAckRemovesSpecificReply() {
         // Queue a reply and capture its msgId.
@@ -1411,8 +1435,18 @@ class FleetMcpTest {
         var peeked = messages.drainReplies("term_a");
         assertEquals(1, peeked.size(), "one fresh reply in the inbox");
 
-        // ackReply works (no-op since published with a different UUID, but callable).
-        assertDoesNotThrow(() -> messages.ackReply("term_a", msgId));
+        // fleetd #437: msgId was already drained above (a fresh UUID each publish), so it is no
+        // longer in the inbox — ackReply must now report that miss instead of pretending to ack.
+        assertFalse(messages.ackReply("term_a", msgId));
+    }
+
+    @Test
+    void bridgeAckRemovingARealQueuedReplyReportsSuccessAndRemovesIt() {
+        // The worker path must not change behaviour: acking a reply that IS still in the inbox
+        // still succeeds and still removes it (fleetd #437).
+        inbox.publish("term_a", "real-1", "still queued");
+        assertTrue(messages.ackReply("term_a", "real-1"), "ack of a real queued reply must report true");
+        assertTrue(inbox.peek("term_a").isEmpty(), "the acked reply must be gone from the inbox");
     }
 
     @Test
diff --git a/fleetd/src/test/java/dev/ltms/fleet/msg/AmqpReplyInboxContractTest.java b/fleetd/src/test/java/dev/ltms/fleet/msg/AmqpReplyInboxContractTest.java
index b310062..8f2e35a 100644
--- a/fleetd/src/test/java/dev/ltms/fleet/msg/AmqpReplyInboxContractTest.java
+++ b/fleetd/src/test/java/dev/ltms/fleet/msg/AmqpReplyInboxContractTest.java
@@ -16,6 +16,7 @@ import java.util.List;
 import java.util.concurrent.TimeUnit;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertThrows;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 
@@ -221,6 +222,31 @@ class AmqpReplyInboxContractTest {
         }
     }
 
+    @Test
+    void ackReportsHitVsMissAgainstARealBroker() throws Exception {
+        // fleetd #437: fleet_ack said "acknowledged <msgId>" for a message it never touched,
+        // because ReplyInbox.ack() (void) could not tell a hit from a miss. Pin the fixed
+        // boolean contract against a real broker — the adapter fleetd actually runs live.
+        String target = "worker-ack-contract-" + System.nanoTime();
+        try (AmqpReplyInbox inbox = AmqpReplyInbox.open(uri())) {
+            inbox.own(target);
+
+            // Never held for this target at all: must report false, not throw.
+            assertFalse(inbox.ack(target, "never-held"),
+                    "acking a msgId never held for an owned target must report false");
+
+            // A real message: first ack removes it and reports true...
+            inbox.publish(target, "m1", "ack me");
+            assertEquals(1, awaitPeek(inbox, target).size(), "the published reply should be held");
+            assertTrue(inbox.ack(target, "m1"), "acking a held reply must report true");
+            assertTrue(inbox.peek(target).isEmpty(), "an acked reply is dropped");
+
+            // ...and the second ack of the SAME msgId has nothing left to remove: false.
+            assertFalse(inbox.ack(target, "m1"),
+                    "acking the same msgId twice must report false the second time");
+        }
+    }
+
     @Test
     void confirmedPublishDeliversNormally() throws Exception {
         String target = "worker-confirm-" + System.nanoTime();
diff --git a/fleetd/src/test/java/dev/ltms/fleet/msg/InMemoryReplyInboxTest.java b/fleetd/src/test/java/dev/ltms/fleet/msg/InMemoryReplyInboxTest.java
index f48e4c8..ec4a021 100644
--- a/fleetd/src/test/java/dev/ltms/fleet/msg/InMemoryReplyInboxTest.java
+++ b/fleetd/src/test/java/dev/ltms/fleet/msg/InMemoryReplyInboxTest.java
@@ -41,20 +41,20 @@ class InMemoryReplyInboxTest {
     @Test
     void ackRemovesTheMessage() {
         inbox.publish("term_a", "m1", "hello");
-        inbox.ack("term_a", "m1");
+        assertTrue(inbox.ack("term_a", "m1"), "fleetd #437: ack of a real entry must report true");
         assertTrue(inbox.peek("term_a").isEmpty(), "after ack, the message is gone");
     }
 
     @Test
     void ackForUnknownMsgIdIsNoOp() {
         inbox.publish("term_a", "m1", "hello");
-        inbox.ack("term_a", "no-such-id"); // no-op
+        assertFalse(inbox.ack("term_a", "no-such-id"), "fleetd #437: a miss must report false"); // no-op
         assertEquals(1, inbox.peek("term_a").size(), "the published message is still there");
     }
 
     @Test
     void ackForUnknownTargetIsNoOp() {
-        inbox.ack("no-such-target", "m1"); // no-op, should not throw
+        assertFalse(inbox.ack("no-such-target", "m1"), "fleetd #437: a miss must report false"); // no-op, should not throw
     }
 
     @Test
@@ -167,7 +167,7 @@ class InMemoryReplyInboxTest {
     @Test
     void peekAndAckAreNoOpsForUnownedTarget() {
         assertTrue(inbox.peek("term_not_owned").isEmpty());
-        inbox.ack("term_not_owned", "m1"); // no-op, should not throw
+        assertFalse(inbox.ack("term_not_owned", "m1"), "fleetd #437: a miss must report false"); // no-op, should not throw
     }
 
     @Test