Design an A/B test for a Celebrate reaction

WeChat Moments considers adding a 'Celebrate' reaction. 1) Define success metrics that credibly tie to business outcomes (e.g., 7/30/90‑day retention, posting rate) and guardrails (e.g., session length, hide/mute/report rates, feed quality). 2) Choose the randomization unit (user, post, user→post edge) and defend it under network interference; propose a cluster or switchback design to mitigate spillovers. 3) Power the test (state effect size assumptions) and specify a staged rollout with sequential monitoring that controls Type I error. 4) Plan heterogeneity analysis (e.g., by country, age, new vs. existing users). 5) Detect novelty and saturation effects; propose ramp, cooldown, and difference‑in‑differences diagnostics. 6) Define a follow‑up experiment to ensure the feature doesn’t incentivize low‑quality posting; include pre‑registered decision rules for ship/iterate/rollback.