Multi-Tenant Platform Architecture (Cloudflare or Vercel)

• IS: platform choice, domain strategy and PSL, tenant identification, compute and data isolation, hostname routing, tenant context propagation, custom domains and SSL, per-tenant static files, and mapping platform limits to plans.
• IS NOT: general folder structure or module contracts (use codebase-architecture), scaffolding a new repo (use scaffold-nextjs), or the content of per-tenant SEO files once routing serves them dynamically: sitemap entries, canonical URLs, structured data, indexing policy (use seo).

Contents

• Platform dispatch (decide first)
• Reference files
• Workflow (order matters)
• Gotchas
• Output schema
• Evidence commands
• Related skills

Platform dispatch (decide first)

Signals	Platform	Model
Tenants upload or generate their own code; code-level isolation; edge compute on KV, D1, Durable Objects, R2	Cloudflare	Dispatch Worker in front of a dispatch namespace of per-tenant Workers; Cloudflare for SaaS for custom hostnames
Every tenant runs the same Next.js codebase and differs by content, branding, and plan; ISR, Server Components, Vercel deploys	Vercel	One deployment; proxy.ts resolves the tenant from the hostname; wildcard plus custom domains on the project

• Pick one platform per product. Fronting a Vercel app with a Cloudflare proxy doubles the TLS and redirect layers and is the usual cause of redirect loops and failed certificate issuance.
• Tenants shipping their own code on Vercel is the multi-project model (one Vercel project per tenant, created with the SDK). It follows the Cloudflare row's isolation reasoning; this skill's Vercel references cover the single-deployment model only.

Reference files

File	Read when
cloudflare-platform.md	Cloudflare chosen: dispatch namespaces, routing, Cloudflare for SaaS custom hostnames, isolation modes, KV and D1 (steps 3 to 7)
vercel-platform.md	Vercel chosen: proxy.ts resolution, App Router layout, Global Config lookups, per-tenant static files, custom subpaths, local dev (steps 4 to 7)
vercel-domains.md	Vercel chosen: SDK domain lifecycle, DNS targets, verification, wildcard nameservers, SSL, troubleshooting (step 7)
data-isolation.md	Step 3 on either platform: shared schema with RLS, schema-per-tenant, database-per-tenant, and the Postgres/Supabase/Drizzle policy pattern
psl.md	Step 1 when tenants publish content or run code on sibling subdomains: eligibility, submission, interim cookie controls
limits-and-quotas.md	Step 8: Cloudflare, Vercel, and Neon limit names with the official source for each, to map onto plans

Workflow (order matters)

Copy this checklist to track progress:

Multi-tenant progress:
- [ ] Step 1: Domain strategy and PSL decision
- [ ] Step 2: Tenant identification strategy
- [ ] Step 3: Isolation model (compute and data)
- [ ] Step 4: Deterministic routing
- [ ] Step 5: Tenant context propagation
- [ ] Step 6: Tenant config and least-privilege bindings
- [ ] Step 7: Custom domains and per-tenant static files
- [ ] Step 8: Limits mapped to plans, evidence captured

1. Choose the domain strategy

• Put tenant workloads on a dedicated registrable domain (acme.app for tenants, acme.com for brand). One phishing tenant on x.acme.com puts the whole domain on blocklists, and a tenant cookie with Domain=acme.com reaches your dashboard.
• Keep the dashboard and auth on a different apex from tenant subdomains (app.acme.com for the console, *.acme.app for tenants).
• If tenants publish content or run code on sibling subdomains, submit the label directly above the tenant name (acme.app, or sites.acme.app for <tenant>.sites.acme.app) to the PSL and start now: there is no SLA. Tenant-owned custom domains need no PSL entry. Otherwise record No PSL with the reason.

2. Choose tenant identification (one primary; custom domain as the upgrade path)

• Subdomain tenant.acme.app: wildcard DNS plus wildcard certificate. The default.
• Custom domain tenant.com: the tenant CNAMEs to you. Paying tenants; reputation shifts to them; needs the onboarding lifecycle in step 7.
• Path acme.app/tenant: no per-tenant DNS or certificates, but no cookie isolation and no branding. Choose it only when tenants will never get a hostname.

3. Define the isolation model

• Compute, Cloudflare: one dispatch namespace in untrusted mode; per-invocation cpuMs and subRequests limits per plan; an outbound Worker if tenant code may call the internet.
• Compute, Vercel: one deployment, tenant code never executes. If tenants must ship code, move to Vercel multi-project or Cloudflare rather than sandboxing inside the app.
• Data: shared schema with tenant_id on every tenant-aware table plus RLS is the default; database-per-tenant for regulated or noisy tenants, selectable per plan. See data-isolation.md.

4. Route deterministically (tenants never influence routing or see each other)

• Cloudflare: a single */* route on the SaaS zone to the dispatch Worker; hostname -> tenant record (KV, D1 on miss) -> env.DISPATCHER.get(script); Worker not found -> 404.
• Vercel: proxy.ts (Next.js 16; middleware.ts with runtime: 'nodejs' on 15) reads host, looks the tenant up in Global Config or the database, rewrites into the tenant segment; unknown hostname -> 404, never the brand site.
• Let /.well-known through before any tenant rewrite. Route robots.txt, sitemap.xml, and llms.txt into the tenant segment so they vary per tenant.

5. Propagate tenant context from one authority

• Delete every inbound x-tenant-* header, set x-tenant-id, x-tenant-slug, x-tenant-plan from the resolved tenant, and forward them on the request (NextResponse.next({ request: { headers } })). Server Components read await headers(); route handlers read request.headers. Cloudflare: the dispatch Worker sets headers or passes parameters before fetch.
• The proxy is routing, not authorization. Server Functions, route handlers, and jobs re-derive the tenant from the session and the data layer enforces it (RLS or tenant_id predicates).

6. Bind only what the tenant needs

• Cloudflare: each user Worker gets its own bindings (KV namespace, D1 database, R2 prefix); adding a binding is an explicit redeploy. No shared globals.
• Vercel: Global Config holds only hostname -> { id, slug, plan }; the database is the source of truth and write-through happens when a domain verifies. Feature flags and branding come from the database keyed by tenant id.

7. Support custom domains and per-tenant static files

• Lifecycle to design and record: add domain -> show DNS target -> verify ownership -> certificate issued -> mapping activated -> removal or failure path.
• Cloudflare: Cloudflare for SaaS custom hostname on the SaaS zone, proxied fallback origin, customers.<you>.com CNAME target, http or txt validation, pre-validate before DNS cutover. See cloudflare-platform.md.
• Vercel: projectsAddProjectDomain -> DNS values from the project's domain card -> _vercel TXT only if the domain is already on Vercel -> projectsVerifyProjectDomain -> Let's Encrypt HTTP-01. See vercel-domains.md.
• robots.txt, sitemap.xml, llms.txt are route handlers inside the tenant segment with explicit Content-Type; nothing tenant-specific lives in /public. Their content is seo territory.

8. Surface limits as plans and capture evidence

• Fill the limits-to-plan table from limits-and-quotas.md, reading each current value from its source URL and dating it; enforce at the routing layer (Cloudflare limits, Vercel plan header plus server checks).
• Nothing long-running in the request path: Cloudflare Queues or Workflows, Vercel background functions or cron.
• Every tenant operation (create tenant, add domain, verify, remove) works over HTTP with the same authority as the UI; if it only works in the dashboard, the platform leaks into the UI.
• Run the evidence commands and paste results into the output.

Gotchas

Platform and data gotchas live in the reference for that step; these fire before any reference loads.

• Forwarding inbound tenant headers: curl -H "x-tenant-id: <other>" then serves another tenant's data. Delete or overwrite x-tenant-* on every path through the proxy or dispatch Worker, including paths that skip resolution (/.well-known, the brand site).
• A hostname map keyed in a restricted alphabet (Global Config allows only ^[\w-]+$) needs a collision-free encoding or a hash; replacing dots with underscores maps a-b.example.com and a.b-example.com to one key. Keep the original hostname in the record and compare it on read.
• Starting path-based with custom domains on the roadmap means URL rewrites, cookie changes, and DNS migration later.

Output schema

Length follows the decisions: drop any section the project does not face rather than filling it.

# Multi-tenant architecture

## Platform decision
- Platform: Cloudflare | Vercel
- Why this platform:
- Rejected platform and reason:

## Domain map
- Brand domain:
- Tenant domain:
- Tenant subdomains:
- Custom domains:
- PSL decision: Submit (suffix, owner, PR link, _psl TXT date) | No PSL (reason)

## Routing matrix
| Host pattern | Resolver | Destination | Unknown tenant behavior |
|---|---|---|---|

## Tenant context flow
- Authority: proxy.ts | dispatch Worker
- Headers set and stripped:
- Server read path:
- Data-layer enforcement:

## Isolation model
- Compute isolation:
- Data isolation (and per-plan variant):
- Config/binding isolation:

## Custom-domain lifecycle
1. DNS target:
2. Ownership verification:
3. Certificate provisioning:
4. Routing activation:
5. Removal/failure path:

## Limits-to-plan table
| Limit | Source URL / access date | Free | Pro | Enterprise | Enforcement point |
|---|---|---:|---:|---:|---|

## Validation evidence
| Check | Command | Expected | Result |
|---|---|---|---|

Evidence commands

Run against local or preview before calling the design done; mark N/A with a reason.

Check	Command	Expected
Tenant boundary exists in code	rg -n "x-tenant-id|CREATE POLICY|FORCE ROW LEVEL SECURITY|DISPATCHER.get" .	Hits in proxy or dispatch Worker and in the schema
Unknown host is 404	curl -sI -H "Host: nope.acme.app" <url>	404
Forged header ignored	curl -s -H "Host: a.acme.app" -H "x-tenant-id: tenant-b" <url>/api/whoami	Tenant A
Static files vary	curl -s -H "Host: a.acme.app" <url>/robots.txt vs -H "Host: b.acme.app"	Different bodies, Content-Type: text/plain
RLS holds for the app role	psql -c "BEGIN; SET LOCAL ROLE app_user; SELECT set_config('app.tenant_id','<t1>',true); SELECT count(*) FROM posts; ROLLBACK;"	Only tenant t1's rows
ACME path reachable	curl -sI -H "Host: tenant.com" <url>/.well-known/acme-challenge/test	Not a redirect into the tenant segment
Limits current	Access date next to each URL in the limits table	Dated within the planning window

Related skills

• codebase-architecture: folder structure, module contracts, and the request-context pipeline for the application itself.
• scaffold-nextjs: bootstrap the Next.js turborepo before applying these tenancy patterns.
• seo: content of per-tenant robots.txt, sitemap.xml, llms.txt, canonical URLs, and structured data once routing serves them.

Maintenance only: evals/evals.json contains regression scenarios for changes to this skill; it does not load during a user task.