# 🤖 Instance contexts — one engine, different social defaults

Status: **design** (not built). Builds on `docs/threads-and-dms.md` and
`docs/invite-redesign.md`. Nothing here weakens the E2EE invariant: the server
still only ever stores ciphertext and key-blobs it cannot open.

## The idea

Different kinds of instances — a family, a friend group, a community — share
~80% of what they need and differ in ~20%. The shared 80% is the **messaging
core**: threads, per-thread AES-GCM keys, the instance key, DMs, attachments,
mini-apps, the whole crypto model. None of it changes across kinds.

The 20% that differs is entirely **social structure**: who may cross the trust
boundary, who can see whom, and which channels exist by default. So "context" is
**not a mode the engine branches on** — it's a *named preset* over a handful of
orthogonal knobs, plus tailored onboarding copy and channel scaffolding.

- **A context is a template chosen at instance creation.** It expands into
  concrete values on the knobs below, seeds a channel or two, and sets the
  onboarding language.
- **We store the resolved knobs, not the context.** Real instances drift — a
  family later wants a General; a friend group wants restricted DMs for the one
  kid. The admin console exposes the dials directly; the context only seeds them.
- **"Other / Custom" always exists** — the dials with no preset. Future contexts
  (Team, Club, Business) are new template rows, not engine changes.

## The knobs

| Knob | Options | Backing |
|---|---|---|
| **1. Admission** — who adds a *person* to the instance | admin-only · members-can-invite · open join-link | `usersCanInvite` / `user_invites_enabled` (exists); open join-link is new |
| **2. Default channels** | none · one "General" · several topic channels | instance-key–encrypted threads, auto-created |
| **3. Member-created open channels** | off · on | deferred *"group join links, instance-members-only"* (`threads-and-dms.md`) |
| **4. Discovery / roster visibility** | full-open · restricted (e.g. kids ↔ parents only) | deferred *"restricted discovery"* |
| **5. Who can DM whom** | anyone ↔ anyone · restricted | same restricted-discovery lever |

Everything else (composing, reactions, attachments, mini-apps) is the shared
core and is identical in every context.

## Contexts as presets

| | Admission | Default channels | Member open channels | Discovery / DMs |
|---|---|---|---|---|
| **Family** | admin-only (parents) | none | off | restricted (kids ↔ parents), else open |
| **Friends** | members-can-invite | one General | maybe | full-open |
| **Community** | members-invite or open-link | several topic channels | on | full-open (role-gated later) |
| **Other** | fully custom | — | — | — |
| *Team (later)* | domain/SSO-gated | per-project | on | open within org |
| *Business (later)* | admin/SSO | departmental | on | + retention/audit ⚠️ (see below) |
| *Club (later)* | invite or open-link | few | on | open |

Each row is a starting point on those five dials, nothing more.

## Two knobs that aren't cosmetic

Most knobs are defaults you can flip freely. Two change the **trust model** and
must be designed as first-class features, not preset side effects.

### Admission is the security posture

Family's "admin-only" *is* the safety model: a parent vets everyone who can read
anything. Community's "open join-link" means unvetted people get keys wrapped to
them. That is not a preference — it changes who you are trusting. The open
join-link is its own carefully-gated feature (expiring links, optional
admin-approval step), never something a preset silently turns on.

### Open channels: subscription vs. access

An "open" channel (General, topic channels) is cleanest **encrypted with the
instance key**: every member already holds it, and new members get it
auto-granted via the existing nudge-and-wrap flow (`instance-key/nudge` →
`grantInstanceKeyTo`), so the channel is instantly readable with zero per-member
re-wrap. That gives a clean split:

- **Open channels** → instance-key encrypted. *Any member can read* (even browse
  before joining). Membership here is a **subscription** — it controls what's in
  your list and what notifies you, not access.
- **Private groups / DMs** → per-thread key. Membership is **access** — you
  cannot decrypt without being wrapped in.

Honest limitation: an open channel is cryptographically readable by every
current and future instance member, forever. "Leaving" one is unsubscribing, not
losing the ability to decrypt. For a community that is usually the intended
meaning of "open." A channel that is open-to-join but revocable-on-leave needs a
per-channel key + rekey-on-leave, which costs an online key-holder and O(members)
re-wraps — a bill that grows with size.

## Cross-cutting concerns (not knobs)

- **Scale is the silent axis.** Family (~5) and Friends (~20) sit in the model's
  sweet spot. Community (100s) strains two mechanics: the nudge-and-grant
  instance-key handshake assumes a holder is reachable, and remove-a-member rekey
  is O(members) and needs someone online. Decide how far up the size curve each
  context is meant to go before designing it.
- **Business bends the model.** Team and Club are basically Community + roles.
  Business drags in retention, compliance, and admin-visible audit — several of
  which *directly conflict* with E2EE (you cannot hand a compliance officer
  plaintext the server never sees). Business needs its own trust-model
  conversation; it is not a preset.

## Status / next steps

Design only — nothing here is built. Natural build order once we commit:

1. Land the knobs as instance config + admin-console dials (extends the existing
   `user_invites_enabled` pattern).
2. Instance-key–encrypted "General" / open channels + the subscription-vs-access
   split.
3. Member-created open channels (the deferred group-join-links design).
4. Restricted discovery (kids ↔ parents).
5. Context templates + onboarding copy on top, once the dials exist.