diff --git a/desktop/src/features/agents/lib/agentAutocompleteEligibility.test.mjs b/desktop/src/features/agents/lib/agentAutocompleteEligibility.test.mjs
index 4e02b7bd68..aa8241af6a 100644
--- a/desktop/src/features/agents/lib/agentAutocompleteEligibility.test.mjs
+++ b/desktop/src/features/agents/lib/agentAutocompleteEligibility.test.mjs
@@ -227,6 +227,26 @@ test("shouldHideAgentFromMentions: shows member agents with unknown invocability
   );
 });
 
+test("shouldHideAgentFromMentions: #4187 shows a channel-member agent managed by another Desktop instance", () => {
+  // Regression for #4187: a managed agent running on another Desktop instance
+  // appears here as a channel member (isAgent via role "bot") that is neither
+  // in this client's managed-agent list nor in the relay directory
+  // (mentionableAgentPubkeys/directoryAgentPubkeys both empty). It MUST remain
+  // visible in the mention picker. The mention path (useMentions) must rely on
+  // this policy alone and not additionally pre-filter with
+  // isAgentIdentityInManagedList, which would drop it.
+  assert.equal(
+    shouldHideAgentFromMentions({
+      isAgent: true,
+      isMember: true,
+      pubkey: PUB_A,
+      mentionableAgentPubkeys: new Set(),
+      directoryAgentPubkeys: new Set(),
+    }),
+    false,
+  );
+});
+
 test("shouldHideAgentFromMentions: normalizes the pubkey before lookup", () => {
   const mixedCase = "Ab".repeat(32);
   const normalized = mixedCase.toLowerCase();
diff --git a/desktop/src/features/messages/lib/useMentions.ts b/desktop/src/features/messages/lib/useMentions.ts
index 0c73b75339..abaaf0e211 100644
--- a/desktop/src/features/messages/lib/useMentions.ts
+++ b/desktop/src/features/messages/lib/useMentions.ts
@@ -16,7 +16,6 @@ import {
   coalesceAutocompleteCandidatesByKey,
   getMentionableAgentPubkeys,
   getSharedChannelIds,
-  isAgentIdentityInManagedList,
   shouldHideAgentFromMentions,
 } from "@/features/agents/lib/agentAutocompleteEligibility";
 import {
@@ -246,9 +245,16 @@ export function useMentions(
       if (isArchivedDiscovery(pubkey)) {
         return;
       }
-      if (!isAgentIdentityInManagedList(candidate, managedAgentPubkeys)) {
-        return;
-      }
+      // Agent visibility in the mention picker is governed solely by
+      // `shouldHideAgentFromMentions` (the "Option B" member/directory policy
+      // below). We deliberately do NOT pre-filter with
+      // `isAgentIdentityInManagedList` here: that gate keeps only *locally*
+      // managed agents, which drops channel-member agents managed by another
+      // Desktop instance before the member/directory policy can show them —
+      // the root cause of #4187 (cross-desktop managed agents missing from the
+      // `@mention` autocomplete). `isAgentIdentityInManagedList` still belongs
+      // in the "add member" search (MembersSidebar), where you may only add
+      // agents you manage; it does not belong on the mention path.
       if (
         shouldHideAgentFromMentions({
           isAgent: candidate.isAgent === true,
@@ -420,7 +426,6 @@ export function useMentions(
     managedAgentNamesByPubkey,
     managedAgentPersonaIds,
     managedAgentPersonaIdsByPubkey,
-    managedAgentPubkeys,
     managedAgentsQuery.data,
     memberPubkeys,
     members,