diff --git a/server/src/__tests__/routines-service.test.ts b/server/src/__tests__/routines-service.test.ts
index 7bdb69023d4..4770f843406 100644
--- a/server/src/__tests__/routines-service.test.ts
+++ b/server/src/__tests__/routines-service.test.ts
@@ -301,6 +301,23 @@ describeEmbeddedPostgres("routine service live-execution coalescing", () => {
     return { issue, runId, heartbeatRunId };
   }
 
+  async function waitForBlockedQuery(...patterns: string[]) {
+    for (let attempt = 0; attempt < 80; attempt += 1) {
+      const [waiting] = await db.execute<{ waiting: boolean }>(sql`
+        select exists (
+          select 1
+          from pg_stat_activity
+          where state = 'active'
+            and wait_event_type = 'Lock'
+            and ${sql.join(patterns.map((pattern) => sql`query ilike ${pattern}`), sql` and `)}
+        ) as waiting
+      `);
+      if (waiting?.waiting) return true;
+      await new Promise((resolve) => setTimeout(resolve, 25));
+    }
+    return false;
+  }
+
   it("supersedes and links every older unfinished instance after an opted-in newer success", async () => {
     const { agentId, companyId, issueSvc, routine, svc } = await seedFixture();
     const optedInRoutine = await svc.update(routine.id, { lifecyclePolicy: "latest_success_wins" }, {});
@@ -768,6 +785,90 @@ describeEmbeddedPostgres("routine service live-execution coalescing", () => {
     }
   });
 
+  it("rejects a concurrent nested child when the supersession sweep locks the root first", async () => {
+    const { agentId, companyId, issueSvc, routine, svc } = await seedFixture();
+    const optedInRoutine = await svc.update(routine.id, { lifecyclePolicy: "latest_success_wins" }, {});
+    const revisionId = optedInRoutine!.latestRevisionId!;
+    const candidate = await createExecutionInstance({
+      companyId,
+      agentId,
+      routine: optedInRoutine!,
+      issueSvc,
+      triggeredAt: new Date("2026-08-23T09:00:00.000Z"),
+      routineRevisionId: revisionId,
+    });
+    const terminalChild = await issueSvc.create(companyId, {
+      projectId: routine.projectId,
+      parentId: candidate.issue.id,
+      title: "Completed direct child",
+      status: "done",
+      priority: "medium",
+      assigneeAgentId: agentId,
+    });
+    const winner = await createExecutionInstance({
+      companyId,
+      agentId,
+      routine: optedInRoutine!,
+      issueSvc,
+      triggeredAt: new Date("2026-08-23T10:00:00.000Z"),
+      routineRevisionId: revisionId,
+    });
+    await issueSvc.update(winner.issue.id, { status: "done" });
+
+    let releaseCandidateLock!: () => void;
+    const candidateLockReleased = new Promise<void>((resolve) => {
+      releaseCandidateLock = resolve;
+    });
+    let markCandidateLocked!: () => void;
+    const candidateLocked = new Promise<void>((resolve) => {
+      markCandidateLocked = resolve;
+    });
+    const lockTransaction = db.transaction(async (tx) => {
+      await tx.execute(sql`
+        select ${issues.id}
+        from ${issues}
+        where ${issues.companyId} = ${companyId}
+          and ${issues.id} = ${candidate.issue.id}
+        for update
+      `);
+      markCandidateLocked();
+      await candidateLockReleased;
+    });
+    await candidateLocked;
+
+    const sweepPromise = svc.syncRunStatusForIssue(winner.issue.id);
+    await expect(waitForBlockedQuery("%issues%", "%for update%")).resolves.toBe(true);
+    const grandchildPromise = issueSvc.create(companyId, {
+      projectId: routine.projectId,
+      parentId: terminalChild.id,
+      title: "Nested child created during supersession",
+      status: "in_progress",
+      priority: "medium",
+      assigneeAgentId: agentId,
+    });
+    await expect(waitForBlockedQuery("%ancestor_ids%", "%for key share%")).resolves.toBe(true);
+
+    releaseCandidateLock();
+    await lockTransaction;
+    const [sweepOutcome, grandchildOutcome] = await Promise.allSettled([
+      sweepPromise,
+      grandchildPromise,
+    ]);
+
+    expect(sweepOutcome.status).toBe("fulfilled");
+    expect(grandchildOutcome.status).toBe("rejected");
+    await expect(issueSvc.getById(candidate.issue.id)).resolves.toMatchObject({
+      status: "cancelled",
+      supersededByIssueId: winner.issue.id,
+    });
+    await expect(
+      db
+        .select()
+        .from(issues)
+        .where(and(eq(issues.companyId, companyId), eq(issues.parentId, terminalChild.id))),
+    ).resolves.toHaveLength(0);
+  });
+
   it("defers supersession when a concurrent direct child commits before the sweep lock", async () => {
     const { agentId, companyId, issueSvc, routine, svc } = await seedFixture();
     const optedInRoutine = await svc.update(routine.id, { lifecyclePolicy: "latest_success_wins" }, {});
@@ -790,23 +891,6 @@ describeEmbeddedPostgres("routine service live-execution coalescing", () => {
     });
     await issueSvc.update(winner.issue.id, { status: "done" });
 
-    const waitForBlockedQuery = async (...patterns: string[]) => {
-      for (let attempt = 0; attempt < 80; attempt += 1) {
-        const [waiting] = await db.execute<{ waiting: boolean }>(sql`
-          select exists (
-            select 1
-            from pg_stat_activity
-            where state = 'active'
-              and wait_event_type = 'Lock'
-              and ${sql.join(patterns.map((pattern) => sql`query ilike ${pattern}`), sql` and `)}
-          ) as waiting
-        `);
-        if (waiting?.waiting) return true;
-        await new Promise((resolve) => setTimeout(resolve, 25));
-      }
-      return false;
-    };
-
     let releaseCompanyLock!: () => void;
     const companyLockReleased = new Promise<void>((resolve) => {
       releaseCompanyLock = resolve;
diff --git a/server/src/services/issues.ts b/server/src/services/issues.ts
index 483657e0db6..79a15cb8470 100644
--- a/server/src/services/issues.ts
+++ b/server/src/services/issues.ts
@@ -7142,25 +7142,45 @@ export function issueService(db: Db) {
         }
 
         if (issueData.parentId) {
-          // This conflicts with the supersession sweep's FOR UPDATE lock. When
-          // creation wins the lock order, it commits before the sweep proceeds;
-          // the sweep's following descendant query then sees the new child in
-          // its fresh READ COMMITTED snapshot.
-          const parent = await tx
-            .select({
-              id: issues.id,
-              status: issues.status,
-              supersededByIssueId: issues.supersededByIssueId,
-            })
-            .from(issues)
-            .where(and(eq(issues.companyId, companyId), eq(issues.id, issueData.parentId)))
-            .for("key share")
-            .then((rows) => rows[0] ?? null);
+          // Lock the complete ancestor chain so nested child creation also
+          // conflicts with a supersession sweep's root FOR UPDATE lock. When
+          // creation wins, it commits before the sweep proceeds and the
+          // sweep's fresh READ COMMITTED descendant snapshot sees the child.
+          const ancestors = Array.from(await tx.execute(sql`
+            with recursive ancestor_ids(id, parent_id, depth) as (
+              select ${issues.id}, ${issues.parentId}, 0
+              from ${issues}
+              where ${issues.companyId} = ${companyId}
+                and ${issues.id} = ${issueData.parentId}
+              union all
+              select ancestor_issue.id, ancestor_issue.parent_id, ancestor_ids.depth + 1
+              from ${issues} ancestor_issue
+              join ancestor_ids on ancestor_issue.id = ancestor_ids.parent_id
+              where ancestor_issue.company_id = ${companyId}
+            )
+            select ancestor_issue.id,
+                   ancestor_issue.status as issue_status,
+                   ancestor_issue.superseded_by_issue_id
+            from ${issues} ancestor_issue
+            join ancestor_ids on ancestor_ids.id = ancestor_issue.id
+            where ancestor_issue.company_id = ${companyId}
+            order by ancestor_ids.depth desc
+            for key share of ancestor_issue
+          `)) as Array<{
+            id: string;
+            issue_status: string;
+            superseded_by_issue_id: string | null;
+          }>;
+          const parent = ancestors.find((ancestor) => ancestor.id === issueData.parentId) ?? null;
           if (!parent) throw notFound("Parent issue not found");
-          if (parent.status === "cancelled" && parent.supersededByIssueId) {
-            throw conflict("Cannot create a child issue under a superseded issue", {
+          const supersededAncestor = ancestors.find(
+            (ancestor) => ancestor.issue_status === "cancelled" && ancestor.superseded_by_issue_id,
+          );
+          if (supersededAncestor) {
+            throw conflict("Cannot create a descendant issue under a superseded issue", {
               parentIssueId: parent.id,
-              supersededByIssueId: parent.supersededByIssueId,
+              ancestorIssueId: supersededAncestor.id,
+              supersededByIssueId: supersededAncestor.superseded_by_issue_id,
             });
           }
         }