Stripe webhook fires before Supabase row exists checkout session

The browser creates a Checkout Session and may insert a "pending" row. Stripe's checkout.session.completed webhook often arrives before that insert commits — or the insert fails silently. If your webhook does select where user_id = ? and 404s, you drop a paid event. The seam is ordering across two writers (client + Stripe), not Stripe reliability.

import Stripe from "stripe";
import { createClient } from "@supabase/supabase-js";

export async function onCheckoutCompleted(event: Stripe.Event) {
  const session = event.data.object as Stripe.Checkout.Session;
  const supabase = createClient(
    process.env.NEXT_PUBLIC_SUPABASE_URL!,
    process.env.SUPABASE_SERVICE_ROLE_KEY!
  );

  const { error } = await supabase.from("subscriptions").upsert(
    {
      stripe_customer_id: session.customer as string,
      stripe_subscription_id: session.subscription as string,
      status: "active",
      user_id: session.metadata?.user_id,
    },
    { onConflict: "stripe_subscription_id" }
  );

  if (error) throw error;
}