diff --git a/src/app/sitemap.xml/route.ts b/src/app/sitemap.xml/route.ts
index 69348443..70935422 100644
--- a/src/app/sitemap.xml/route.ts
+++ b/src/app/sitemap.xml/route.ts
@@ -15,11 +15,15 @@ import { buildSitemap } from "@/lib/sitemap-builder";
 // serialized to XML here and shipped with a real edge cache header.
 export const runtime = "nodejs";
 export const dynamic = "force-dynamic";
-// Hard platform ceiling so Vercel never holds the connection open longer
-// than this when the SRH/Redis fan-out runs over budget. The internal
-// buildSitemap() timeout is 20 s; this gives 40 s of headroom and still
-// replies well within the smoke-test's 90 s limit.
-export const maxDuration = 60;
+// 300 s: the aggregate blob is 4.25 MB — larger than Next.js Data Cache's
+// 2 MB limit, so unstable_cache can never store it and every request must
+// re-fetch from the network. The /api/aggregate ISR proxy responds in
+// 18-65 s on a cold cache. With maxDuration=60 the process received
+// SIGKILL before the fetch resolved, discarding the buffered response and
+// returning 500. 300 s gives the full build time to complete on cold
+// starts; the internal JS timeout (240 s) fires first and falls back to
+// the static sitemap if the full build hangs.
+export const maxDuration = 300;
 
 function escapeXml(s: string): string {
   return s
diff --git a/src/lib/sitemap-builder.ts b/src/lib/sitemap-builder.ts
index 84664741..957b39f8 100644
--- a/src/lib/sitemap-builder.ts
+++ b/src/lib/sitemap-builder.ts
@@ -577,14 +577,13 @@ async function buildFullSitemap(): Promise<MetadataRoute.Sitemap> {
 
 export async function buildSitemap(): Promise<MetadataRoute.Sitemap> {
   try {
-    // 55 s: the Vercel maxDuration on the route is 60 s. With /api/aggregate
-    // ISR-cached (revalidate=60 on that route) warm requests return in <1 ms
-    // and the full sitemap completes in ~10 s. Cold-start requests (first hit
-    // after deployment) may take 18-49 s for the aggregate; 55 s gives enough
-    // headroom while leaving 5 s for buildStaticFallback() before Vercel kills
-    // the function.
+    // 240 s: maxDuration on the route is 300 s. The aggregate blob (4.25 MB)
+    // exceeds Next.js Data Cache's 2 MB cap, so every sitemap request
+    // re-fetches from /api/aggregate whose FETCH_TIMEOUT_MS is 65 s. A 240 s
+    // budget covers cold aggregate + HL-builder Prometheus sweeps with 60 s of
+    // headroom for buildStaticFallback() before the 300 s SIGKILL ceiling.
     const timeout = new Promise<never>((_, reject) =>
-      setTimeout(() => reject(new Error("sitemap build timeout")), 55_000),
+      setTimeout(() => reject(new Error("sitemap build timeout")), 240_000),
     );
     // Attach a no-op .catch() to prevent unhandled-rejection crashes if
     // buildFullSitemap() rejects AFTER the race has already settled (via