diff --git a/desktop/src/features/agents/lib/agentAutocompleteEligibility.test.mjs b/desktop/src/features/agents/lib/agentAutocompleteEligibility.test.mjs
index 4e02b7bd68..7ccab6091a 100644
--- a/desktop/src/features/agents/lib/agentAutocompleteEligibility.test.mjs
+++ b/desktop/src/features/agents/lib/agentAutocompleteEligibility.test.mjs
@@ -7,6 +7,7 @@ import {
   getSharedChannelIds,
   isAgentIdentityInManagedList,
   relayAgentIsSharedWithUser,
+  shouldDropAgentMentionCandidate,
   shouldHideAgentFromMentions,
 } from "./agentAutocompleteEligibility.ts";
 
@@ -162,6 +163,44 @@ test("isAgentIdentityInManagedList: keeps people and only current managed agent
   );
 });
 
+test("shouldDropAgentMentionCandidate: keeps channel-member agents from other owners (#2508)", () => {
+  const managedAgentPubkeys = new Set([PUB_A]);
+
+  // A channel-member agent managed on someone else's machine must stay
+  // mentionable for every member of the channel.
+  assert.equal(
+    shouldDropAgentMentionCandidate(
+      { isAgent: true, isMember: true, pubkey: PUB_B },
+      managedAgentPubkeys,
+    ),
+    false,
+  );
+  // Locally managed agents stay mentionable even when not channel members.
+  assert.equal(
+    shouldDropAgentMentionCandidate(
+      { isAgent: true, isMember: false, pubkey: PUB_A },
+      managedAgentPubkeys,
+    ),
+    false,
+  );
+  // People are never dropped.
+  assert.equal(
+    shouldDropAgentMentionCandidate(
+      { isAgent: false, isMember: false, pubkey: PUB_B },
+      managedAgentPubkeys,
+    ),
+    false,
+  );
+  // Non-member agent identities from other owners remain restricted (#2149).
+  assert.equal(
+    shouldDropAgentMentionCandidate(
+      { isAgent: true, isMember: false, pubkey: PUB_B },
+      managedAgentPubkeys,
+    ),
+    true,
+  );
+});
+
 test("shouldHideAgentFromMentions: never hides non-agents", () => {
   assert.equal(
     shouldHideAgentFromMentions({
diff --git a/desktop/src/features/agents/lib/agentAutocompleteEligibility.ts b/desktop/src/features/agents/lib/agentAutocompleteEligibility.ts
index e4afe7fea4..ff2e0963d6 100644
--- a/desktop/src/features/agents/lib/agentAutocompleteEligibility.ts
+++ b/desktop/src/features/agents/lib/agentAutocompleteEligibility.ts
@@ -64,6 +64,25 @@ export function isAgentIdentityInManagedList(
   );
 }
 
+/**
+ * Composer drop gate for agent identities (#2508).
+ *
+ * A channel-member agent is never dropped: an agent that has been added to
+ * the channel must stay mentionable for every member, not only the person
+ * whose desktop manages it. The managed-list restriction (#2149) still
+ * applies to non-member agent identities (stale kind:0 discoveries, global
+ * search strays), which is the duplicate-noise case it was introduced for.
+ */
+export function shouldDropAgentMentionCandidate(
+  candidate: { isAgent?: boolean; isMember?: boolean; pubkey: string },
+  managedAgentPubkeys: ReadonlySet<string>,
+) {
+  return (
+    candidate.isMember !== true &&
+    !isAgentIdentityInManagedList(candidate, managedAgentPubkeys)
+  );
+}
+
 export function shouldHideAgentFromMentions({
   isAgent,
   isMember,
diff --git a/desktop/src/features/messages/lib/useMentions.ts b/desktop/src/features/messages/lib/useMentions.ts
index 0c73b75339..6b4f3c4efe 100644
--- a/desktop/src/features/messages/lib/useMentions.ts
+++ b/desktop/src/features/messages/lib/useMentions.ts
@@ -16,7 +16,7 @@ import {
   coalesceAutocompleteCandidatesByKey,
   getMentionableAgentPubkeys,
   getSharedChannelIds,
-  isAgentIdentityInManagedList,
+  shouldDropAgentMentionCandidate,
   shouldHideAgentFromMentions,
 } from "@/features/agents/lib/agentAutocompleteEligibility";
 import {
@@ -246,7 +246,7 @@ export function useMentions(
       if (isArchivedDiscovery(pubkey)) {
         return;
       }
-      if (!isAgentIdentityInManagedList(candidate, managedAgentPubkeys)) {
+      if (shouldDropAgentMentionCandidate(candidate, managedAgentPubkeys)) {
         return;
       }
       if (