# Story 3.4: The "Host-Pays" Session Logic

Status: review

<!-- Ultimate context engine analysis completed - comprehensive developer guide created -->

## Story

As a host,
I want my guests' AI actions inside my Canvas session to be billed to my account,
so that my guests never hit a paywall while collaborating with me.

**Epic:** Epic 3 — The SaaS Commercial Engine (Monetization & API Cost Protection)  
**FR coverage:** FR15 (Host-Pays Session Billing Logic), NFR-P3 (routing must evaluate Host-Pays within 50ms — satisfied if billing resolution is sync DB read, no extra HTTP).

---

## Acceptance Criteria

1. [AC1] **Billing owner resolution:** For any collaborative Brainstorm session, a single function `getBillingOwner(sessionId, requestingUserId)` returns `BrainstormSession.userId` (the session host). Guests never become the billing owner.
2. [AC2] **Guest AI → host quota:** When a guest (participant with `role !== 'host'` or access via share/public link) triggers an AI-backed Brainstorm action, `checkEntitlementOrThrow` and `incrementUsageAsync` use the **host's** `userId`, not the guest's.
3. [AC3] **Host AI → host quota:** When the host triggers the same actions, billing still uses the host's quota (same code path — no special case).
4. [AC4] **Covered AI surfaces (minimum):**
   - `POST /api/brainstorm` — initial wave generation (session create)
   - `POST /api/brainstorm/[sessionId]/expand` — wave expansion
   - `POST /api/brainstorm/[sessionId]/manual-idea` — vector context search + async title/description enrichment
5. [AC5] **HTTP 402 on host exhaustion:** If the **host's** quota is exhausted, the API returns **402** with body compatible with existing `QuotaExceededError.toJSON()` plus:
   - `billingOwnerId` (host user id)
   - `triggeredByUserId` (actor who clicked)
   - `isGuestActor: boolean`
   Guests must **not** receive a 402 implying *their* personal quota is exhausted.
6. [AC6] **Guest quota untouched:** A guest on BASIC tier with personal Discovery Pack remaining can collaborate freely until the **host's** quota is hit; verify with test: guest triggers expand → host Redis key increments, guest keys unchanged.
7. [AC7] **Ordering:** Entitlement check runs **before** any paid AI call (LLM `generateText`, embedding generation used for brainstorm context). Quota increment runs **after** successful AI completion (same fire-and-forget pattern as Story 3.1).
8. [AC8] **Alignment with 3.3 fallback:** Provider fallback (`withAiProviderFallback`) must attribute usage to `billingOwnerId`, not `requestingUserId`. Do not add fallback inside routes — wrap provider execution only after entitlement passes.
9. [AC9] **Regression:** Stories 3.1–3.3 behavior for non-brainstorm routes (`/api/chat`, `/api/ai/tags`, etc.) unchanged. Existing `resolveAiContextUserId` guest note scoping remains intact.

---

## Tasks / Subtasks

- [x] Task 1: Billing owner API (AC: #1)
  - [x] Subtask 1.1: Add `getBillingOwner(sessionId, requestingUserId): Promise<string>` in `lib/brainstorm-collab.ts` — load session, throw if missing, return `session.userId`
  - [x] Subtask 1.2: Unit test: host actor → returns host id; guest actor → returns host id (not guest id)
- [x] Task 2: Brainstorm feature keys in entitlements (AC: #2, #6)
  - [x] Subtask 2.1: Extend `VALID_FEATURES` in `lib/quota-utils.ts`: `brainstorm_create`, `brainstorm_expand`, `brainstorm_enrich`
  - [x] Subtask 2.2: Add tier limits in `lib/entitlements.ts` `TIER_LIMITS` (suggested MVP — tune with product):
    - BASIC: `brainstorm_create: 1` (lifetime/month per existing product intent), `brainstorm_expand: 10`, `brainstorm_enrich: 20` per period
    - PRO/BUSINESS: generous monthly limits or map `brainstorm_*` to shared `chat` pool — **document chosen mapping in Dev Agent Record**
  - [x] Subtask 2.3: Extend `QuotaExceededError` optional fields: `billingOwnerId?`, `triggeredByUserId?`, `isGuestActor?` — include in `toJSON()` for frontend
- [x] Task 3: Wire API routes (AC: #4, #5, #7)
  - [x] Subtask 3.1: `app/api/brainstorm/route.ts` — before `getTagsProvider` / `generateText`, `checkEntitlementOrThrow(userId, 'brainstorm_create')`; on success `incrementUsageAsync(userId, 'brainstorm_create')`
  - [x] Subtask 3.2: `expand/route.ts` — resolve `billingOwnerId = await getBillingOwner(...)`; check/increment `brainstorm_expand`; wrap LLM with `withAiProviderFallback('tags', config, ...)` if not already; catch `QuotaExceededError` → 402 with guest metadata
  - [x] Subtask 3.3: `manual-idea/route.ts` — check `brainstorm_enrich` **before** embedding + before scheduling `enrichAsync`; increment once per successful enrichment (not on idea create alone); 402 on host exhaustion **before** returning 201 if enrichment is required — **product choice:** block create vs allow raw idea without AI (recommend: check before async enrich; if fail, still return 201 but emit `idea:ai_failed` with `quota_exceeded` reason — document in implementation)
  - [x] Subtask 3.4: Map `QuotaExceededError` in all three routes to `NextResponse.json({ ...err.toJSON(), billingOwnerId, triggeredByUserId, isGuestActor }, { status: 402 })`
- [x] Task 4: Frontend guest/host messaging (AC: #5)
  - [x] Subtask 4.1: In `hooks/use-brainstorm.ts` — on 402 from expand/manual-idea, surface toast/modal: guest sees "Session host has reached their AI limit"; host sees standard upgrade CTA
  - [x] Subtask 4.2: i18n keys in `locales/en.json` + `fr.json` — no hardcoded UI strings
- [x] Task 5: Tests (AC: #6, #9)
  - [x] Subtask 5.1: `tests/unit/brainstorm-billing.test.ts` — mock prisma session + redis; guest expand bills host
  - [x] Subtask 5.2: Run `npm run test:unit -- tests/unit/brainstorm-billing.test.ts tests/unit/entitlements.test.ts`

---

## Dev Notes

### Epic context

| Story | Relevance to 3.4 |
|-------|------------------|
| 3.1 | Redis `checkEntitlementOrThrow` / `incrementUsageAsync` — reuse; change **which userId** is passed |
| 3.2 | Provider routing — unchanged; billing user ≠ routing config user |
| 3.3 | Fallback must not mis-attribute usage — pass `billingOwnerId` into increment after fallback success |
| 3.5 | BYOK bypass on quota — **out of scope**; add `// Story 3.5: skip quota when host has active BYOK` seam only |
| 3.6 | Stripe tier upgrades — host tier drives limits |

### Critical brownfield reality (READ BEFORE CODING)

**Partial host-pays already exists for note context, NOT for billing:**

```47:78:memento-note/lib/brainstorm-collab.ts
// resolveAiContextUserId — guests use host's notes for RAG
export async function resolveAiContextUserId(sessionId, requestingUserId) {
  // ...
  if (isHost) return { aiUserId: requestingUserId, isGuest: false, publicNoteIds: null }
  return { aiUserId: session.userId, isGuest: true, publicNoteIds: ... }
}
```

**Gap:** Brainstorm AI routes call **no** `checkEntitlementOrThrow` today:

| Route | AI operations today | Bills |
|-------|---------------------|-------|
| `POST /api/brainstorm` | `getTagsProvider` + `generateText` | Nobody |
| `POST .../expand` | embedding (host only) + `generateText` | Nobody |
| `POST .../manual-idea` | embedding + async `generateText` enrich | Nobody |

`getBillingOwner` does **not** exist in codebase — only in `memento-note/docs/byok-billing-patch-v3.md`.

### Data model

- **Host** = `BrainstormSession.userId` (creator), NOT `BrainstormParticipant.role === 'host'` alone — always use session owner for billing even if participant table is inconsistent.
- **Guest** = any `requestingUserId !== session.userId` with valid participant/share access (already enforced by `verifyParticipant` / `resolveAccessRole`).

### Recommended implementation shape

```typescript
// lib/brainstorm-collab.ts
export async function getBillingOwner(
  sessionId: string,
  requestingUserId: string,
): Promise<{ billingOwnerId: string; isGuestActor: boolean }> {
  const session = await prisma.brainstormSession.findUnique({
    where: { id: sessionId },
    select: { userId: true },
  })
  if (!session) throw new Error('Session not found')
  return {
    billingOwnerId: session.userId,
    isGuestActor: session.userId !== requestingUserId,
  }
}

// Usage in expand/route.ts (before LLM)
const { billingOwnerId, isGuestActor } = await getBillingOwner(sessionId, session.user.id)
await checkEntitlementOrThrow(billingOwnerId, 'brainstorm_expand')
// ... run AI with withAiProviderFallback ...
incrementUsageAsync(billingOwnerId, 'brainstorm_expand')
```

**Do not conflate** `aiUserId` (note/RAG scope) with `billingOwnerId` (quota). Today both equal `session.userId` for guests — keep two functions for clarity and 3.5 BYOK.

### Files — expected touch list

**NEW**

- `memento-note/tests/unit/brainstorm-billing.test.ts`

**UPDATE**

- `memento-note/lib/brainstorm-collab.ts` — `getBillingOwner`
- `memento-note/lib/quota-utils.ts` — feature keys
- `memento-note/lib/entitlements.ts` — tier limits + extended `QuotaExceededError`
- `memento-note/app/api/brainstorm/route.ts`
- `memento-note/app/api/brainstorm/[sessionId]/expand/route.ts`
- `memento-note/app/api/brainstorm/[sessionId]/manual-idea/route.ts`
- `memento-note/hooks/use-brainstorm.ts`
- `memento-note/locales/en.json`, `fr.json`

**READ BEFORE MODIFY**

- `memento-note/lib/brainstorm-collab.ts` — `resolveAiContextUserId`, `verifyParticipant`
- `memento-note/lib/entitlements.ts` — `checkEntitlementOrThrow`, fail-open Redis behavior
- `memento-note/lib/ai/fallback.ts` — Story 3.3 wrapper (no quota logic inside)
- `memento-note/app/api/chat/route.ts` — reference 402 handling pattern

**DO NOT MODIFY (unless broken by typing)**

- `socket-server.ts` — no AI billing today; socket quota events are optional follow-up
- `finalize`, `dismiss`, `convert`, `export` routes — no LLM

### Product / scope boundaries

**In scope for 3.4:** Host-pays **quota attribution** on Brainstorm AI paths.

**Explicitly out of scope (later stories / docs):**

- `BrainstormContextPool` and “1 brainstorm/month on BASIC” session caps (`byok-billing-patch-v3.md` §5) — do not implement full pool unless PM confirms; minimal `brainstorm_create` Redis counter is enough for AC
- BYOK quota bypass — Story **3.5**
- Stripe tier sync — Story **3.6**
- Real-time socket `error:quota_exceeded` — optional; HTTP 402 is sufficient for MVP
- Billing LLM token $ cost to `LLMCallLog` — aspirational in patch doc

### UX requirements (FR15 / PRD journey)

From PRD: *"Collaborator (Guest): Can interact and generate ideas, but AI queries are routed through the Host's quota."*

- Guest must never see “Upgrade your plan” for **their** BASIC quota when the host still has credits.
- When host is exhausted, guest sees collaborative framing: host must upgrade or add BYOK (3.5).
- Reuse existing paywall modal component if present (`usage-meter` / upgrade flow from 3.1) with `billingOwnerId` prop when `isGuestActor`.

### Testing standards

- Vitest; mock `prisma.brainstormSession.findUnique` and Redis via entitlements mocks.
- Assert Redis key `usage:{hostId}:brainstorm_expand:YYYY-MM` increments when guest calls expand.
- Assert guest's `usage:{guestId}:brainstorm_expand:*` unchanged.

---

## Dev Agent Guardrails

### Technical requirements

- **NFR-SC2:** `getBillingOwner` is one Prisma `findUnique` by primary key — keep sync with session fetch already done in routes (avoid duplicate query: compute billing owner from loaded `brainstormSession.userId` when session is already loaded).
- **Fail-open:** If Redis down, entitlements fail-open allows AI (3.1 behavior) — do not change global policy in this story.
- **402 body:** Must remain parseable by existing chat/tags clients; only add fields.
- **Async manual-idea enrich:** Race: two guests enqueue enrich — both bill host per successful enrich (acceptable); use one increment per completed enrich.

### Architecture compliance

- Brownfield Next.js App Router under `memento-note/`.
- Host-pays is a **billing concern** — lives in `brainstorm-collab.ts` + route glue, not in `router.ts`.
- i18n: all new UI strings via `t('brainstorm.hostQuotaExceeded')` etc.

### Library / framework requirements

- Reuse Story 3.1 entitlements — no new Redis client.
- Reuse Story 3.3 `withAiProviderFallback` for LLM calls in brainstorm routes (lane `'tags'` matches current `getTagsProvider` usage).

### File structure requirements

- `getBillingOwner` next to `resolveAiContextUserId` in `lib/brainstorm-collab.ts`.
- Tests under `tests/unit/`.

---

## Previous Story Intelligence

**Source:** `docs/3-3-smart-routing-fallback.md`

- Fallback on 429/5xx; `withAiProviderFallback` integrated in chat, tags, embeddings.
- Explicit note: *"3.4 — Host-pays billing context; fallback must not mis-attribute token usage."*
- `skipSystemFallback` stub for BYOK (3.5).
- Brainstorm routes **deferred** for fallback in 3.3 — 3.4 implementer should add fallback + billing together on expand/manual-idea.

**Source:** `docs/3-1-freemium-quota-tracking.md`

- `checkEntitlementOrThrow` before AI; `incrementUsageAsync` after.
- 402 / `QuotaExceededError` pattern established.
- Feature keys today: `semantic_search`, `auto_tag`, `auto_title`, `reformulate`, `chat` only.

**Source:** `docs/3-2-custom-llm-router.md`

- `getTagsProvider` used for brainstorm text generation — keep lane `tags` for fallback wrapper.

---

## Git Intelligence Summary

| Commit | Insight |
|--------|---------|
| `1fcea6e` | Recent brainstorm + embedding work — expand/manual-idea paths active |
| `41596c2` | OpenRouter key resolution — billing owner irrelevant to provider keys (system config) |
| `195e845` | Security patterns — guest note scoping already hardened |

---

## Latest Technical Information

- **Entitlements pattern (2026-05):** Monthly Redis keys `usage:{userId}:{feature}:{YYYY-MM}`; extend with `brainstorm_*` features rather than overloading `chat` to preserve Discovery Pack semantics.
- **Prisma:** `BrainstormSession.userId` is canonical host; participant `role: 'host'` should match but do not rely on it for billing.

---

## Project Context Reference

| Document | Use |
|----------|-----|
| `docs/epics.md` | Story 3.4 AC + FR15 |
| `docs/prd.md` | Host-Pays journey, RBAC matrix |
| `memento-note/docs/byok-billing-patch-v3.md` | §3 Host-Pays (aspirational — implement AC scope only) |
| `docs/3-1-freemium-quota-tracking.md` | Entitlements API |
| `docs/3-3-smart-routing-fallback.md` | Fallback + usage attribution |
| `docs/sprint-status.yaml` | Tracking |

---

## Dev Agent Record

### Agent Model Used

Composer (Cursor)

### Debug Log References

- `npm run test:unit -- tests/unit/brainstorm-billing.test.ts tests/unit/entitlements.test.ts` — 23 passed

### Completion Notes List

- Added `getBillingOwner`, `billingOwnerFromSession`, and `checkSessionEntitlementOrThrow` for host-pays quota attribution.
- Tier limits: BASIC `brainstorm_create: 1`, `brainstorm_expand: 10`, `brainstorm_enrich: 20`; PRO/BUSINESS/ENTERPRISE dedicated monthly caps (not mapped to `chat` pool).
- Routes: create/expand return 402 on host exhaustion; manual-idea returns 201 with raw idea and emits `idea:ai_failed` + `quota_exceeded` when host enrich quota is exhausted (no embedding).
- `withAiProviderFallback('tags', …)` on create, expand, and async enrich paths.
- Frontend: `BrainstormQuotaError` + `brainstormQuotaMessageKey`; toast in canvas; i18n `quotaGuest` / `quotaHost` (en, fr).

### File List

- `memento-note/lib/brainstorm-collab.ts`
- `memento-note/lib/brainstorm-quota-client.ts` (new)
- `memento-note/lib/quota-utils.ts`
- `memento-note/lib/entitlements.ts`
- `memento-note/app/api/brainstorm/route.ts`
- `memento-note/app/api/brainstorm/[sessionId]/expand/route.ts`
- `memento-note/app/api/brainstorm/[sessionId]/manual-idea/route.ts`
- `memento-note/hooks/use-brainstorm.ts`
- `memento-note/components/brainstorm/brainstorm-canvas.tsx`
- `memento-note/locales/en.json`
- `memento-note/locales/fr.json`
- `memento-note/tests/unit/brainstorm-billing.test.ts` (new)

### Change Log

- 2026-05-15: Implemented host-pays session billing for brainstorm AI (FR15).

---

## Story Completion Status

- Story ID: 3.4
- Story Key: `3-4-host-pays-session-logic`
- File: `docs/3-4-host-pays-session-logic.md`
- Status: **review**
- Completion Note: Host-pays quota wired on brainstorm create/expand/enrich; unit tests passing.