diff --git a/docs/subdomain-setup.md b/docs/subdomain-setup.md
new file mode 100644
index 0000000..550a49f
--- /dev/null
+++ b/docs/subdomain-setup.md
@@ -0,0 +1,89 @@
+# Subdomain Setup — freno.me
+
+This document records the DNS and Vercel domain configuration for the four product subdomains.
+
+## DNS Configuration
+
+**DNS Provider:** Google Domains (nameservers: `ns-cloud-a1` through `ns-cloud-a4.googledomains.com`)
+
+Add the following CNAME records in the [Google Domains DNS console](https://domains.google.com/registrar/freno.me/dns):
+
+| Subdomain | Type | Target | TTL |
+|---|---|---|---|
+| `nessa` | CNAME | `cname.vercel-dns.com` | Automatic |
+| `lineage` | CNAME | `cname.vercel-dns.com` | Automatic |
+| `gaze` | CNAME | `cname.vercel-dns.com` | Automatic |
+| `inputhalo` | CNAME | `cname.vercel-dns.com` | Automatic |
+
+**After adding records:** Wait for DNS propagation (typically minutes) and for Vercel to auto-issue SSL certificates for each subdomain.
+
+## Vercel Project Domains
+
+Add the following domains in the [Vercel project Settings → Domains](https://vercel.com/your-team/freno-dev/settings/domains):
+
+| Domain | Redirects to |
+|---|---|
+| `nessa.freno.me` | (no redirect — serves `src/routes/nessa/*` via vercel.json rewrite) |
+| `lineage.freno.me` | (no redirect — serves `src/routes/lineage/*` via vercel.json rewrite) |
+| `gaze.freno.me` | (no redirect — serves `src/routes/gaze/*` via vercel.json rewrite) |
+| `inputhalo.freno.me` | (no redirect — serves `src/routes/inputhalo/*` via vercel.json rewrite) |
+
+**Do NOT set any of these as the Production Branch domain** — `freno.me` remains the production domain.
+
+## Rewrite Architecture (`vercel.json`)
+
+The rewrites are defined in `vercel.json` with **two groups, ordered precisely**:
+
+### Group 1: `/api/*` pass-throughs (must come first)
+
+```json
+{ "source": "/api/(.*)", "has": [{ "type": "host", "value": "gaze.freno.me" }], "destination": "/api/$1" }
+{ "source": "/api/(.*)", "has": [{ "type": "host", "value": "inputhalo.freno.me" }], "destination": "/api/$1" }
+{ "source": "/api/(.*)", "has": [{ "type": "host", "value": "nessa.freno.me" }], "destination": "/api/$1" }
+{ "source": "/api/(.*)", "has": [{ "type": "host", "value": "lineage.freno.me" }], "destination": "/api/$1" }
+```
+
+These pass API requests on subdomains straight through to the existing `/api/*` routes. This enables **dual-host Sparkle appcast support**: `gaze.freno.me/api/Gaze/appcast.xml` hits the same `src/routes/api/Gaze/appcast.xml.ts` route as `freno.me/api/Gaze/appcast.xml`.
+
+### Group 2: `/(.*)` catch-all rewrites
+
+```json
+{ "source": "/(.*)", "has": [{ "type": "host", "value": "nessa.freno.me" }], "destination": "/nessa/$1" }
+{ "source": "/(.*)", "has": [{ "type": "host", "value": "lineage.freno.me" }], "destination": "/lineage/$1" }
+{ "source": "/(.*)", "has": [{ "type": "host", "value": "gaze.freno.me" }], "destination": "/gaze/$1" }
+{ "source": "/(.*)", "has": [{ "type": "host", "value": "inputhalo.freno.me" }], "destination": "/inputhalo/$1" }
+```
+
+These rewrite non-API requests on each subdomain to its internal route prefix. Vercel matches top-to-bottom, so the `/api/*` rules above catch API paths first.
+
+## Verification Checklist
+
+After DNS propagation and Vercel certificate issuance:
+
+- [ ] `dig nessa.freno.me` returns the Vercel CNAME
+- [ ] `dig lineage.freno.me` returns the Vercel CNAME
+- [ ] `dig gaze.freno.me` returns the Vercel CNAME
+- [ ] `dig inputhalo.freno.me` returns the Vercel CNAME
+- [ ] `curl -sI https://nessa.freno.me/ | head -1` → `HTTP/2 200`
+- [ ] `curl -sI https://lineage.freno.me/ | head -1` → `HTTP/2 200`
+- [ ] `curl -sI https://gaze.freno.me/ | head -1` → `HTTP/2 200`
+- [ ] `curl -sI https://inputhalo.freno.me/ | head -1` → `HTTP/2 200`
+- [ ] `curl -sI https://freno.me/api/Gaze/appcast.xml | head -1` → `HTTP/2 200` (regression)
+- [ ] `curl -sI https://freno.me/api/InputHalo/appcast.xml | head -1` → `HTTP/2 200` (regression)
+- [ ] `curl -sI https://freno.me/ | head -1` → `HTTP/2 200` (regression)
+- [ ] All four subdomains have valid SSL certificates (no browser warnings)
+
+## Auth Boundaries
+
+Auth remains **host-scoped** — no cookie domain broadening:
+
+- `freno.me` web JWT cookies: host-only on `freno.me`
+- Nessa: Clerk session tokens (independent)
+- Lineage: mobile JWT (independent)
+- Gaze/InputHalo: no web auth
+
+## Notes
+
+- DNS records must be added at **Google Domains** (not Vercel's DNS) since freno.me uses Google's nameservers.
+- Subdomains will 404 until route files exist in `src/routes/<prefix>/*` (content tasks 05–11).
+- The `bun run build` gate is worktree-friendly; run it before deploying.