diff --git a/.github/workflows/dependabot-alerts.yml b/.github/workflows/dependabot-alerts.yml
index 4e1f3f38..7fe631f9 100644
--- a/.github/workflows/dependabot-alerts.yml
+++ b/.github/workflows/dependabot-alerts.yml
@@ -55,6 +55,16 @@ on:
     - cron: "17 6 * * *" # 06:17 UTC daily; alerts are not minute-sensitive
   workflow_dispatch:
 
+# The marker check is a read-before-write, not an atomic one, and nothing stops
+# a `workflow_dispatch` from landing on top of the scheduled run. Two overlapping
+# runs would both see no open issue and both file one, which is the exact
+# duplicate this sweep's whole idempotency design exists to prevent (Copilot).
+# `cancel-in-progress: false` because the queued run must WAIT and then re-read
+# the state the first run wrote — cancelling it would drop a sweep instead.
+concurrency:
+  group: dependabot-alert-sweep
+  cancel-in-progress: false
+
 permissions:
   contents: read
   issues: write
diff --git a/scripts/dependabot-alerts.mjs b/scripts/dependabot-alerts.mjs
index 1cb67411..dc6b596a 100644
--- a/scripts/dependabot-alerts.mjs
+++ b/scripts/dependabot-alerts.mjs
@@ -308,6 +308,27 @@ export function groupAlerts(alerts) {
     );
 }
 
+/**
+ * The milestone a new issue takes: the open one with the NEAREST due date.
+ *
+ * ⚠️ Selected here rather than in a `jq` expression because jq sorts `null`
+ * BEFORE every string, so a `sort_by(.due_on) | .[0]` over the raw list hands
+ * back an undated milestone in preference to every dated one (Copilot). An
+ * undated bucket has no due date and so cannot be the nearest; it is dropped
+ * rather than sorted last, and if nothing dated is open the issue is filed
+ * unmilestoned and triage places it.
+ *
+ * @param {Array<{title: string, state?: string, due_on?: string | null}>} milestones
+ * @returns {string | null}
+ */
+export function pickMilestone(milestones) {
+  const dated = (milestones ?? []).filter(
+    (m) => (m.state ?? "open") === "open" && m.due_on,
+  );
+  if (dated.length === 0) return null;
+  return dated.sort((a, b) => a.due_on.localeCompare(b.due_on))[0].title;
+}
+
 /**
  * Narrow a group to the advisories that actually apply to what is installed.
  *
@@ -683,16 +704,11 @@ function announcedAdvisories(repo, number, spawn) {
 }
 
 function currentMilestone(repo, spawn) {
-  const result = gh(spawn, [
-    "api",
-    `repos/${repo}/milestones`,
-    "--jq",
-    'map(select(.state=="open")) | sort_by(.due_on) | .[0].title // empty',
-  ]);
+  const result = gh(spawn, ["api", `repos/${repo}/milestones?state=open`]);
   if (result.status !== 0) {
     throw new Error(`milestone lookup failed: ${(result.stderr ?? "").trim()}`);
   }
-  return result.stdout.trim() || null;
+  return pickMilestone(JSON.parse(result.stdout || "[]"));
 }
 
 /**
diff --git a/scripts/dependabot-alerts.test.mjs b/scripts/dependabot-alerts.test.mjs
index 076d4a9d..08d2edc7 100644
--- a/scripts/dependabot-alerts.test.mjs
+++ b/scripts/dependabot-alerts.test.mjs
@@ -16,6 +16,7 @@ import { dirname, join } from "node:path";
 import { tmpdir } from "node:os";
 import {
   buildCommentMarker,
+  pickMilestone,
   narrowToApplicable,
   lockfileEntries,
   remediation,
@@ -334,6 +335,31 @@ test("buildIssueBody escapes a disjoint range so the table survives it", () => {
   assert.ok(row.includes(String.raw`\|\|`), "the range's own pipes survive");
 });
 
+test("pickMilestone takes the nearest due date, never an undated bucket", () => {
+  // jq sorts null before every string, so `sort_by(.due_on) | .[0]` over this
+  // list would return "Backlog" — an open bucket with no release date at all.
+  const milestones = [
+    { title: "Backlog", state: "open", due_on: null },
+    { title: "v2.7.0", state: "open", due_on: "2026-09-16T00:00:00Z" },
+    { title: "v2.6.0", state: "open", due_on: "2026-09-09T00:00:00Z" },
+  ];
+  assert.equal(pickMilestone(milestones), "v2.6.0");
+});
+
+test("pickMilestone ignores closed milestones and empty input", () => {
+  assert.equal(
+    pickMilestone([
+      { title: "v2.5.0", state: "closed", due_on: "2026-01-01T00:00:00Z" },
+    ]),
+    null,
+  );
+  // Nothing dated and open means no bucket to take: filed unmilestoned, and
+  // the board write is skipped so triage places it.
+  assert.equal(pickMilestone([{ title: "Backlog", due_on: null }]), null);
+  assert.equal(pickMilestone([]), null);
+  assert.equal(pickMilestone(undefined), null);
+});
+
 test("buildIssueTitle names the bump and pluralizes the advisory count", () => {
   const [many] = groupAlerts([
     alert({ ghsa: "GHSA-a" }),
@@ -530,7 +556,20 @@ function fakeSpawn({
       // `--slurp` yields one array PER PAGE; main() must flatten them.
       return ok(JSON.stringify(alertPages));
     }
-    if (joined.includes("milestones")) return ok(milestone);
+    if (joined.includes("milestones"))
+      return ok(
+        JSON.stringify(
+          milestone
+            ? [
+                {
+                  title: milestone,
+                  state: "open",
+                  due_on: "2026-09-09T00:00:00Z",
+                },
+              ]
+            : [],
+        ),
+      );
     // `--slurp`, so one array per page — `issues` may be a flat list or pages.
     if (joined.includes("/issues?"))
       return ok(JSON.stringify(Array.isArray(issues[0]) ? issues : [issues]));