Decision (ADR) — one origin per hostname: which host answers for each public surface, and what becomes of motir-marketing

Type: decision · Executor: coding_agent · Write docs/decisions/public-surface-hosts.md in motir-core, and AMEND docs/decisions/marketing-site-hosting.md to point at it. No application behaviour ships in this subtask.

The QUESTION

marketing-site-hosting.md (Accepted 2026-08-27) answered "where does the marketing SITE run?" and answered it correctly. It never asked "which host answers for each PUBLIC SURFACE?", because when it was written the only public surface anyone was thinking about was a landing page. Three of them already existed on the other host.

So: which hostname serves /, /explore, /docs, /legal and /p/*, and by what mechanism?

The options, and the one to record

• A — one origin per hostname; motir.co is served by motir-core, and the landing moves into it. RECOMMENDED.
• B — an edge router (Cloudflare, or Next rewrites in motir-marketing) path-splits motir.co across two Fly origins.
• C — keep the split; make motir.co a better hub and accept that content lives on app.motir.co.
• D — move /explore · /docs · /legal INTO motir-marketing.

Rung-2 evidence (the shipped code — grep origin/main, not a local checkout)

• motir-marketing/app/ holds ONE route. app/sitemap.ts returns one URL and says "⚠️ ONE ENTRY, AND THAT IS THE HONEST ANSWER."
• motir-marketing/lib/destinations.ts — EXPLORE, DOCS, LEGAL_PRIVACY, LEGAL_TERMS, LEGAL_INDEX are all built from APP_ORIGIN.
• motir-marketing/app/_components/RootJsonLd.tsx — the WebSite SearchAction targets the other origin, under its own ⚠️.
• motir-core/app/(public)/ — /explore, /docs, /legal make no getSession() call; /p/* does, in twelve files.
• motir-core/lib/baseUrl.ts — ONE variable, MOTIR_BASE_URL, currently answers both "where is the app?" and "what origin do public canonicals use?". This decision splits those two questions.
• motir-core/tests/navigation/proxy-matcher.test.ts — the evidence against option B: "the matcher was a COMMENT asking future authors to remember, and thirteen of sixteen segments were never added."
• D is refuted by measurement, not preference: there is no app/api/public/p/[identifier]/route.ts, six of the public API routes read the session, /api/public/* is outside the v1 contract, and the surface is ~25 components plus Server Actions over RLS-bound reads.

Rung-1 evidence (the mirrors — observe, do not recall)

Vercel and Notion serve marketing plus first-party content from one origin each (vercel.com/docs, vercel.com/templates; notion.com/help, notion.com/blog). Stripe splits reference documentation to docs.stripe.com — a hostname per origin, not a path-split across origins. No mirror in this set stitches one hostname across two origins with an edge router, which is what makes B the deviation rather than the default.

What the record must decide, explicitly

1. Which host answers each of the five surfaces, in a table.
2. Whether the session cookie's scope changes. It does not — and the record says why, citing GitHub's "Yummy cookies across domains" and the Public Suffix List.
3. What becomes of motir-marketing: the repository, the Fly app, its CI, and whether the landing is ported or packaged (@motir/design-system and @motir/brand are the precedent for packaging — MOTIR-3724).
4. Which owner writes robots.txt and sitemap.xml for each host after the change.
5. The COSTS of A, stated rather than glossed — marketing copy rides motir-core's CI, migrations and fly-deploy concurrency group (which marketing-site-hosting.md §2's option D raised and this record must answer, not ignore); the en/zh catalogue-parity gate the landing inherits; and the external registrations that move with MOTIR_BASE_URL.
6. The reversal condition, concretely.

Acceptance criteria

• motir-core/docs/decisions/public-surface-hosts.md exists, in the directory's Status → Context → Decision → Consequences shape with a numbered-Q section and a per-Q rejected-alternatives table, matching marketing-site-hosting.md's own convention.
• It carries a per-surface host table for /, /explore, /explore/topic/*, /docs, /legal, /p/* — and marks /p/* as DEFERRED to its own story, citing that story by key, so no reader takes silence for a decision.
• Every rung-2 claim above appears with the file it was read from; every rung-1 claim names the product and what was observed.
• Options B, C and D each have a rejected-alternatives row giving the reason, and B's row states the routing-manifest cost in the terms proxy-matcher.test.ts sets.
• §5's costs are written as costs, not as a case for the decision.
• marketing-site-hosting.md gains a dated AMENDMENT pointing at this record and naming which of its own answers now stand (Q3 CI, Q4 subprocessor) and which are superseded (Q1's origin for the apex, Q2's A/AAAA target).
• The record does not decide /p/*'s namespace — that is the next story's ADR, and this one says so.

Context refs

• motir-core/docs/decisions/marketing-site-hosting.md — the record amended, especially §2's option D and its stated objection
• motir-core/docs/decisions/application-hosting.md — the Fly shape both records build on
• motir-core/docs/decisions/work-item-type-taxonomy.md — the decision-record convention
• motir-core/lib/baseUrl.ts · lib/publicProjects/urls.ts · tests/navigation/proxy-matcher.test.ts
• motir-marketing/lib/destinations.ts · lib/appOrigin.ts · lib/siteOrigin.ts · app/sitemap.ts · app/robots.ts · app/_components/RootJsonLd.tsx
• MOTIR-2854 — the ADR this one follows; MOTIR-3724 — the packaging precedent
• GitHub, Yummy cookies across domains; the Public Suffix List