Sitemap lastmod vs IndexNow for recrawl signaling

Two ways to tell engines "this changed, come back." Use both, but know what each does.

1. Sitemap lastmod: passive pull signal. Google reads it on its own schedule; honest lastmod gently nudges recrawl priority. Slow, but zero per-URL cost.
2. IndexNow (Bing, Yandex, others — not Google): active push. You ping the moment a URL changes; near-instant recrawl on supporting engines.
3. Decision rule: bulk steady-state freshness → trustworthy lastmod. Time-sensitive single-URL updates (price drop, new data) → push via IndexNow for the engines that honor it.
4. Guardrail: don't fire IndexNow on cosmetic changes — abuse gets your key throttled. Only ping on meaningful content change.
5. Guardrail: lastmod that updates on every build (even with no content change) trains Google to ignore it. Gate: lastmod derives from content hash, not deploy time.

Rollback: both signals are advisory; stop sending, no harm done.

Ship gate: don't publish until all boxes are checked.