db-platform-fit (M21) — version · pricing/lock-in · per-platform feature support

M21 audits the project against constraints imposed by the managed platform, not the raw engine. The same Postgres schema behaves differently on Neon (serverless, scale-to-zero, branching) vs RDS vs Supabase (RLS-centric, PostgREST). Findings feed both axes: feature gaps (FK support, RLS) are design; connection/cold-start/version issues are performance.

What it checks

1. Version currency — the declared/introspected engine version vs the platform's current supported range. Flag clearly-old majors. Never fabricate an EOL date or a "supported until" date. If the exact EOL is not known from a citable source, say "older major, confirm support window with the platform" and mark confidence: directional. With WebFetch you MAY cite the vendor's published version page; quote it, don't paraphrase a number you can't source.
2. Pricing / lock-in honesty — name the lock-in trade-off (proprietary features: Supabase Auth/RLS coupling, PlanetScale Vitess sharding, D1's SQLite dialect, DynamoDB single-table design, Aurora storage format). Never invent a price, a per-GB cost, or a row/request quota. State the category of cost (egress, branching compute, read/write units) and that exact figures must be confirmed on the vendor's current pricing page. Magnitude banded only.
3. Per-platform feature support — FK support is the headline check:
   • PlanetScale / Vitess — historically does not enforce foreign keys (and sharded FKs are restricted); a schema relying on DB-enforced FKs needs app-level integrity or FOREIGN_KEY_CHECKS awareness. → design + performance, sev 4 when FKs are declared but unenforceable.
   • Cloudflare D1 / SQLite — FKs exist but PRAGMA foreign_keys must be ON; limited concurrency, DB size ceilings.
   • DynamoDB / Firestore — no FKs/joins at all; integrity is an application concern (M3 re-homes).
   • Serverless drivers (Neon/PlanetScale HTTP, D1) — direct long-lived PG connections from serverless/edge are an anti-pattern → cross-checks db-connection-pooling (M15).
   • Supabase — RLS must be ON for any table exposed via the auto REST/Realtime API → cross-checks db-security-access (M10); RLS-off on an exposed table is sev 5 (design).
   • CockroachDB / Yugabyte — PG wire-compatible but distributed: serial/sequence hotspots, no certain PG extensions, different isolation defaults.

Tier-0 static checks

Detect the platform from references/detection-signals.md signals (supabase/ dir, @planetscale/database, @neondatabase/serverless, wrangler.toml [[d1_databases]], @libsql/client, cockroach/yugabyte in connection string/deps). Parse declared engine version from config/migrations. Cross-reference declared FK constraints against the platform's FK-support reality. Detect direct-PG drivers (pg, Pool) used in a serverless/edge entrypoint.

Tier-1 verification query

• Postgres family: SELECT version(); and SHOW server_version;.
• MySQL/PlanetScale: SELECT VERSION(); and SELECT @@foreign_key_checks;; inspect whether declared FKs actually exist: SELECT * FROM information_schema.table_constraints WHERE constraint_type='FOREIGN KEY';
• Supabase RLS: SELECT relname, relrowsecurity FROM pg_class WHERE relkind='r'; and pg_policies.
• D1/SQLite: PRAGMA foreign_keys;. When the engine/platform isn't reachable → needs_api, never a false pass.

Findings

Emit per schema/finding.schema.json. Example ids:

• M21.platform.fk_unenforced_on_vitess — FK constraints declared but PlanetScale/Vitess won't enforce them (fail, severity 4, axis both, confidence directional static / established with Tier-1).
• M21.platform.rls_off_supabase — table exposed via Supabase API with RLS disabled (fail, severity 5, axis design, confidence established with Tier-1).
• M21.platform.engine_version_old — clearly-old engine major (warn, severity 3, axis both, confidence directional; no fabricated EOL date — cite vendor page or defer).
• M21.platform.serverless_direct_pg — direct long-lived PG connection from edge/serverless (warn, severity 4, axis performance; cross-ref M15). Each finding: evidence.observed quotes the real config/DDL/version string verbatim (secrets redacted); verification.reproduce is a runnable command above using $DATABASE_URL; verification.method is connection_introspect, schema_introspect, or manual_review; expected_impact carries {axis, confidence, magnitude, rationale} — banded, never a naked %.

Honesty

• No fabricated EOL dates, prices, or quotas — ever, in findings or design recs. Cite the vendor's current page via WebFetch (quote it) or defer with directional and a "confirm on vendor page" note.
• Lock-in is a trade-off, not automatically a defect: name what you'd give up, don't moralize.
• Version "old" without a sourced EOL is directional and never caps. FK-unenforced caps only at sev 4 (not 5) unless it directly enables orphan financial/auth rows. Unreachable platform → needs_api.