diff --git a/.changeset/ci-node-eol-guard.md b/.changeset/ci-node-eol-guard.md
new file mode 100644
index 0000000000..91d1111e34
--- /dev/null
+++ b/.changeset/ci-node-eol-guard.md
@@ -0,0 +1,34 @@
+---
+---
+
+ci: fail the Node pin guard when the pinned runtime is out of support (#3825)
+
+`check-node-version` proved all 18 workflows agree on a Node version. It would
+have said `OK` just as cheerfully when all 18 agreed on **Node 20, three months
+after that line went EOL** — which is exactly the state #3825 found the repo in.
+Consistency was only half the invariant; the other half is that the thing they
+agree on is still receiving patches.
+
+The guard now also checks the pin's lifecycle:
+
+- **past EOL → error.** An unpatched runtime is guarding every merge.
+- **within 180 days of EOL → `::warning::`** on the PR, so the bump is scheduled
+  work rather than an emergency. It stays a warning until support actually ends —
+  failing months early would block unrelated PRs.
+- **a major the guard has no dates for → error.** Adopting Node 26 forces you to
+  record its dates rather than silently validating on an unknown runtime.
+
+Dates come from `nodejs/Release` `schedule.json`, hardcoded deliberately: a
+required gate must not depend on the network, and they move once a year.
+
+The success line now reports the runway, which surfaces something worth knowing:
+
+```
+check-node-version: OK (18 setup-node step(s) across 16 workflow(s), all on Node 22).
+  Node 22 is in maintenance; supported until 2027-04-30 (276 days).
+```
+
+**Node 22 entered maintenance on 2025-10-21** — Node 24 has been Active LTS
+since 2025-10-28. Moving to 24 is now a one-line `.nvmrc` edit, and this guard
+will list the workflows to follow. That is a separate decision; nothing here
+forces it.
diff --git a/scripts/check-node-version.mjs b/scripts/check-node-version.mjs
index 3a4717359d..eaf3f021e2 100644
--- a/scripts/check-node-version.mjs
+++ b/scripts/check-node-version.mjs
@@ -1,7 +1,8 @@
 #!/usr/bin/env node
 // Copyright (c) 2025 ObjectStack. Licensed under the Apache-2.0 license.
 //
-// check-node-version -- every workflow must run the Node version in .nvmrc.
+// check-node-version -- every workflow must run the Node version in .nvmrc,
+// and that version must still be supported by Node.
 //
 // Before #3825 the repo ran two Node versions at once, and nobody had decided
 // that: all 12 PR gates were on Node 20 while release.yml, publish-smoke.yml,
@@ -51,6 +52,77 @@ if (!pin) {
   process.exit(1);
 }
 
+// --- Lifecycle: the pin must not be a runtime Node has stopped patching. ------
+//
+// Consistency alone does not make the pin correct. The guard below proves all
+// 18 workflows agree; it would have said OK just as cheerfully when all 18
+// agreed on Node 20, three months after that line went EOL. That is the exact
+// state #3825 found the repo in, so "they match" is only half the invariant --
+// the other half is "and the thing they match is still supported".
+//
+// Dates are from nodejs/Release schedule.json, hardcoded on purpose: a required
+// gate must not depend on the network, and these move once a year. An
+// unrecognised major is an ERROR rather than a pass, so adopting Node 26 forces
+// you to record its dates here instead of silently validating on a runtime this
+// guard knows nothing about.
+const NODE_LIFECYCLE = {
+  18: { maintenance: '2023-10-18', end: '2025-04-30' },
+  20: { maintenance: '2024-10-22', end: '2026-04-30' },
+  22: { maintenance: '2025-10-21', end: '2027-04-30' },
+  24: { maintenance: '2026-10-20', end: '2028-04-30' },
+  26: { maintenance: '2027-10-20', end: '2029-04-30' },
+};
+
+// Warn this far ahead of EOL. Long enough that the bump is scheduled work
+// rather than an emergency, and it stays a warning until the day support
+// actually ends -- a hard failure months early would block unrelated PRs.
+const WARN_WITHIN_DAYS = 180;
+const DAY = 24 * 60 * 60 * 1000;
+
+const major = Number.parseInt(String(pin).replace(/^v/, ''), 10);
+const lifecycle = NODE_LIFECYCLE[major];
+
+if (!Number.isInteger(major) || !lifecycle) {
+  console.error(
+    `check-node-version: ${PIN_FILE} pins Node "${pin}", which this guard has no support dates for.\n\n` +
+      `Add it to NODE_LIFECYCLE in ${'scripts/check-node-version.mjs'} using the dates from\n` +
+      `https://github.com/nodejs/Release/blob/main/schedule.json, then re-run.\n` +
+      `Known: ${Object.keys(NODE_LIFECYCLE).join(', ')}.`,
+  );
+  process.exit(1);
+}
+
+const today = new Date();
+const eol = new Date(`${lifecycle.end}T00:00:00Z`);
+const daysLeft = Math.round((eol - today) / DAY);
+const inMaintenance = today >= new Date(`${lifecycle.maintenance}T00:00:00Z`);
+
+if (daysLeft <= 0) {
+  console.error(
+    `check-node-version: ${PIN_FILE} pins Node ${major}, which reached end-of-life on ${lifecycle.end} ` +
+      `(${Math.abs(daysLeft)} days ago).\n\n` +
+      `An EOL runtime receives no security patches, and every PR gate in this repo runs on it --\n` +
+      `so the runtime guarding each merge is the one nobody is fixing. That is #3825 verbatim.\n\n` +
+      `Bump ${PIN_FILE} to a supported major and update the workflows this guard then lists.\n` +
+      `Supported today: ${Object.entries(NODE_LIFECYCLE)
+        .filter(([, l]) => new Date(`${l.end}T00:00:00Z`) > today)
+        .map(([m, l]) => `${m} (until ${l.end})`)
+        .join(', ')}.`,
+  );
+  process.exit(1);
+}
+
+// GitHub renders ::warning:: in the job summary and on the PR, so this is
+// visible without opening logs -- unlike a plain console.warn.
+if (daysLeft <= WARN_WITHIN_DAYS) {
+  console.log(
+    `::warning file=${PIN_FILE}::Node ${major} reaches end-of-life on ${lifecycle.end} ` +
+      `(${daysLeft} days). Plan the bump before then -- when it lands, every PR gate will be ` +
+      `validating on an unpatched runtime. Bumping is a one-line ${PIN_FILE} edit plus the ` +
+      `workflows check-node-version lists.`,
+  );
+}
+
 const files = readdirSync(join(root, WORKFLOW_DIR))
   .filter((f) => f.endsWith('.yml') || f.endsWith('.yaml'))
   .sort();
@@ -140,8 +212,10 @@ for (const file of files) {
 }
 
 if (offenders.length === 0) {
+  const phase = inMaintenance ? 'maintenance' : 'active LTS';
   console.log(
-    `check-node-version: OK (${steps} setup-node step(s) across ${files.length} workflow(s), all on Node ${pin}).`,
+    `check-node-version: OK (${steps} setup-node step(s) across ${files.length} workflow(s), all on Node ${pin}).\n` +
+      `  Node ${major} is in ${phase}; supported until ${lifecycle.end} (${daysLeft} days).`,
   );
   process.exit(0);
 }