This is M1's exit criterion and the first place the whole M0 → M1 pipeline runs end-to-end on a production wiki other than abstract.wikipedia.org. The mechanism is straightforward — set $wgWikiLambdaEnableAbstractClientMode = true on Test Wikipedia via InitialiseSettings.php, following the usual WMF deployment process — but the bullet's real substance is the set of preconditions that must be green before the config change lands, and the documented rollback if it isn't.

Acceptance criteria:
This bullet is M1's exit criterion; all of the below must be in place before M1 is considered complete and before M2 begins implementation work.

• The M0 prerequisites are verified in place: the durable cache is provisioned, abstract.wikipedia.org has completed its read-path migration, the pre-generation script has populated the cache with the pilot demo set in English (Test Wikipedia is English-only), and the observability dashboard is live.
• $wgWikiLambdaEnableAbstractClientMode = true is deployed to Test Wikipedia via InitialiseSettings.php, with the rollback being a one-line revert (no code redeploy required).
• The kill-switch drill has been performed by the person holding the deploy: flag on → reads work → flag off → special page cleanly unavailable → flag on again, with each step observed on the dashboard or in logs.
• The cross-wiki read path works end-to-end under realistic Test Wikipedia traffic, observed via traces sampled in the observability stack.
• Cold-path fallback has been exercised at least once on Test Wikipedia and the secondary banner variant rendered as expected.
• Banner renders correctly on desktop and mobile on Test Wikipedia, verified by eye.
• Cross-wiki tab links resolve via the abstract: interwiki prefix, not via hard-coded URLs, verified by clicking each tab at least once.
• A handover note to Product has been published: a one-pager explaining what is on Test Wikipedia, the URL to visit to see it, the topics and languages currently populated, and the contact point for problems.