Test your Supabase RLS before you ship: a free red/green fixture and the 9 SQL checks a linter cannot run

If you built a Supabase app quickly - with an AI coding tool or by hand - the row-level-security policies were often written last, or generated for you. That is fine. What is not fine is shipping without knowing whether those policies actually isolate one user's rows from another. Supabase ships a database linter, and you should run it first - it is free and it catches the obvious cases: RLS switched off, and RLS switched on with no policy behind it. But a linter checks whether a policy exists , not whether the policy is correct . Those are different questions, and the second one is where cross-user leaks live. The 2-second test I put a minimal, synthetic reproduction on GitHub: supabase-rls-leak-demo . Same test suite on two branches, differing only by db/policies.sql : broken -> 4 failed, 1 passed (an authenticated user reads another user's row) fixed -> 5 passed npm ci npm run test :ci No Docker, no Supabase project, no credentials. The tests run PostgreSQL in PGlite locally and exercise database-level row security. They do not model Supabase Auth, PostgREST, the Data API, or the network path - the result proves only the row-level gate in the fixture, which is exactly the gate people get wrong. On broken , the failing assertion is readable on purpose: x does not let user B read any row owned by user A -> user B received 1 row(s) belonging to another user: ["A: card ending 4471, expiry 09/29"] (That is synthetic seed data, not a real card.) Run the free checks against your own database The repo also ships audit/rls-audit.sql - nine read-only queries against the system catalogs, MIT-licensed, nothing to install and nothing to send anywhere. Every one is SELECT -only, so it is safe to paste into the Supabase SQL editor. They tell you: RLS coverage per table Every policy and the roles it actually applies to (an empty roles array means no TO clause, so the policy is evaluated for anon too) The effective write check, and which columns its predicate never mentions What