diff --git a/server/src/__tests__/issue-comment-reopen-routes.test.ts b/server/src/__tests__/issue-comment-reopen-routes.test.ts
index 594fdef5..30693674 100644
--- a/server/src/__tests__/issue-comment-reopen-routes.test.ts
+++ b/server/src/__tests__/issue-comment-reopen-routes.test.ts
@@ -552,6 +552,83 @@ describe.sequential("issue comment reopen routes", () => {
     expect(mockHeartbeatService.wakeup).not.toHaveBeenCalled();
   });
 
+  it("allows mention-granted non-assignee agent POST comments on closed issues without reopening", async () => {
+    const mentionedAgentId = "33333333-3333-4333-8333-333333333333";
+    mockIssueService.getById.mockResolvedValue(makeIssue("done"));
+    mockIssueService.addComment.mockResolvedValue({
+      id: "comment-1",
+      issueId: "11111111-1111-4111-8111-111111111111",
+      companyId: "company-1",
+      body: "I can answer the mention without reopening.",
+      createdAt: new Date(),
+      updatedAt: new Date(),
+      authorAgentId: mentionedAgentId,
+      authorUserId: null,
+    });
+    mockAccessService.decide.mockImplementation(async (input: { action?: string }) => {
+      const allowed = input.action === "issue:comment";
+      return {
+        allowed,
+        action: input.action,
+        reason: allowed ? "allow_issue_mention_grant" : "deny_missing_grant",
+        explanation: allowed ? "Allowed by a mention-scoped issue comment grant." : "Missing permission.",
+      };
+    });
+
+    const res = await request(await installActor(createApp(), agentActor(mentionedAgentId)))
+      .post("/api/issues/11111111-1111-4111-8111-111111111111/comments")
+      .send({ body: "I can answer the mention without reopening." });
+
+    expect(res.status, JSON.stringify(res.body)).toBe(201);
+    expect(mockIssueService.addComment).toHaveBeenCalled();
+    expect(mockIssueService.update).not.toHaveBeenCalled();
+    expect(mockHeartbeatService.wakeup).not.toHaveBeenCalled();
+    expect(mockAccessService.decide).not.toHaveBeenCalledWith(expect.objectContaining({ action: "issue:mutate" }));
+  });
+
+  it.each([
+    ["resume", { resume: true }],
+    ["reopen", { reopen: true }],
+  ])(
+    // Mention grants are append-only; explicit lifecycle intent still requires mutation authority.
+    "denies mention-granted non-assignee agent POST comments on closed issues with %s intent",
+    async (_name, intent) => {
+      const mentionedAgentId = "33333333-3333-4333-8333-333333333333";
+      mockIssueService.getById.mockResolvedValue(makeIssue("done"));
+      mockIssueService.addComment.mockResolvedValue({
+        id: "comment-1",
+        issueId: "11111111-1111-4111-8111-111111111111",
+        companyId: "company-1",
+        body: "Please continue this closed issue.",
+        createdAt: new Date(),
+        updatedAt: new Date(),
+        authorAgentId: mentionedAgentId,
+        authorUserId: null,
+      });
+      mockAccessService.decide.mockImplementation(async (input: { action?: string }) => {
+        const allowed = input.action === "issue:comment";
+        return {
+          allowed,
+          action: input.action,
+          reason: allowed ? "allow_issue_mention_grant" : "deny_missing_grant",
+          explanation: allowed ? "Allowed by a mention-scoped issue comment grant." : "Missing permission.",
+        };
+      });
+
+      const res = await request(await installActor(createApp(), agentActor(mentionedAgentId)))
+        .post("/api/issues/11111111-1111-4111-8111-111111111111/comments")
+        .send({ body: "Please continue this closed issue.", ...intent });
+
+      expect(res.status, JSON.stringify(res.body)).toBe(403);
+      expect(res.body).toEqual({ error: "Issue is outside this actor's authorization boundary" });
+      expect(mockAccessService.decide).toHaveBeenCalledWith(expect.objectContaining({ action: "issue:comment" }));
+      expect(mockAccessService.decide).toHaveBeenCalledWith(expect.objectContaining({ action: "issue:mutate" }));
+      expect(mockIssueService.update).not.toHaveBeenCalled();
+      expect(mockIssueService.addComment).not.toHaveBeenCalled();
+      expect(mockHeartbeatService.wakeup).not.toHaveBeenCalled();
+    },
+  );
+
   // POST self-comment from the assignee agent on a done issue with explicit
   // reopen=true is the same log-class signal — the guard must suppress reopen.
   it("does not reopen via POST comment+reopen when the assignee agent is the actor on a done issue", async () => {
diff --git a/server/src/routes/issues.ts b/server/src/routes/issues.ts
index 1a4897b0..fbce62cc 100644
--- a/server/src/routes/issues.ts
+++ b/server/src/routes/issues.ts
@@ -1915,7 +1915,11 @@ export function issueRoutes(
       res.status(403).json({ error: "Issue is outside this actor's authorization boundary" });
       return false;
     }
-    return true;
+    return boundaryDecision;
+  }
+
+  function isIssueMentionGrantDecision(decision: true | Awaited<ReturnType<typeof decideIssueAccess>>) {
+    return decision !== true && decision.reason === "allow_issue_mention_grant";
   }
 
   async function filterIssuesForActor<T extends Parameters<typeof decideIssueAccess>[1]>(req: Request, rows: T[]) {
@@ -7359,7 +7363,8 @@ export function issueRoutes(
       return;
     }
     assertCompanyAccess(req, issue.companyId);
-    if (!(await assertAgentIssueCommentAllowed(req, res, issue))) return;
+    const commentAccessDecision = await assertAgentIssueCommentAllowed(req, res, issue);
+    if (!commentAccessDecision) return;
     if (!assertStructuredCommentFieldsAllowed(req, res, {
       presentation: req.body.presentation,
       metadata: req.body.metadata,
@@ -7376,19 +7381,30 @@ export function issueRoutes(
     const interruptRequested = req.body.interrupt === true;
     const isClosed = isClosedIssueStatus(issue.status);
     const isBlocked = issue.status === "blocked";
+    const mentionGrantedPeerAgentCommentOnly =
+      isClosed &&
+      req.actor.type === "agent" &&
+      issue.assigneeAgentId !== null &&
+      issue.assigneeAgentId !== req.actor.agentId &&
+      !reopenRequested &&
+      !resumeRequested &&
+      isIssueMentionGrantDecision(commentAccessDecision);
+    const effectiveReopenRequested = mentionGrantedPeerAgentCommentOnly ? false : reopenRequested;
+    const effectiveResumeRequested = mentionGrantedPeerAgentCommentOnly ? false : resumeRequested;
     if (
       isClosed &&
       req.actor.type === "agent" &&
       issue.assigneeAgentId !== null &&
-      issue.assigneeAgentId !== req.actor.agentId
+      issue.assigneeAgentId !== req.actor.agentId &&
+      !mentionGrantedPeerAgentCommentOnly
     ) {
       if (!(await assertAgentIssueMutationAllowed(req, res, issue))) return;
     }
-    if (resumeRequested === true && !(await assertExplicitResumeIntentAllowed(req, res, issue))) return;
-    if (resumeRequested !== true && reopenRequested === true && req.actor.type === "agent") {
+    if (effectiveResumeRequested === true && !(await assertExplicitResumeIntentAllowed(req, res, issue))) return;
+    if (effectiveResumeRequested !== true && effectiveReopenRequested === true && req.actor.type === "agent") {
       if (!(await assertExplicitResumeIntentAllowed(req, res, issue))) return;
     }
-    const explicitMoveToTodoRequested = reopenRequested || resumeRequested === true;
+    const explicitMoveToTodoRequested = effectiveReopenRequested || effectiveResumeRequested === true;
     const scheduledRetryForHumanComment =
       shouldHumanCommentResumeInProgressScheduledRetry({
         hasComment: true,