diff --git a/next.config.ts b/next.config.ts
index 31d3ca9b..e51c52f0 100644
--- a/next.config.ts
+++ b/next.config.ts
@@ -53,6 +53,17 @@ const nextConfig: NextConfig = {
   // pin it so a future Next minor that flips defaults can't silently
   // split bench rankings between the two surface URLs.
   trailingSlash: false,
+  // Inject a build-time timestamp so the sitemap can emit a stable
+  // <lastmod> per deploy instead of `new Date()` at request time. The
+  // sitemap runs on force-dynamic (to bypass Next's 2 MB Data Cache
+  // limit), which means `new Date()` at module init evaluates anew on
+  // every Google crawl. Result: every URL in the sitemap got a freshly
+  // updated lastmod each visit, Google flagged the signal as unreliable
+  // and stopped using it to prioritise recrawls. This baking pins the
+  // value at build time so it changes only when a new deploy ships.
+  env: {
+    NEXT_PUBLIC_BUILD_TIME: new Date().toISOString(),
+  },
   turbopack: {
     root: __dirname,
   },
diff --git a/src/app/sitemap.ts b/src/app/sitemap.ts
index 403a05ba..aed5a6df 100644
--- a/src/app/sitemap.ts
+++ b/src/app/sitemap.ts
@@ -32,7 +32,15 @@ import type { Answer } from "@/lib/answers";
 // succeeds; the next ISR refresh recovers automatically.
 export const dynamic = "force-dynamic";
 
-const BUILD_TIME = new Date();
+// Stable per deploy, not per request. process.env.NEXT_PUBLIC_BUILD_TIME
+// is injected in next.config.ts at build time. Falling back to new Date()
+// keeps local dev working. Critical: must NOT be `new Date()` at request
+// time because the sitemap runs on force-dynamic (see header comment) so
+// every Google crawl would otherwise see a fresh lastmod on every URL
+// and Google would discard the signal as unreliable sitewide.
+const BUILD_TIME = process.env.NEXT_PUBLIC_BUILD_TIME
+  ? new Date(process.env.NEXT_PUBLIC_BUILD_TIME)
+  : new Date();
 // Vercel's build container doesn't preserve git-checkout mtimes — every
 // file gets reset to the build-system default (Oct 20 2018), which leaks
 // into the sitemap as `<lastmod>2018-10-20T...</lastmod>` for editorial