wip (#256)

foundry/packages/backend/CLAUDE.md

@@ -16,6 +16,47 @@ OrganizationActor
 └─ SandboxInstanceActor(sandboxProviderId, sandboxId) × N
 ```
 
+## Coordinator Pattern
+
+Actors follow a coordinator pattern where each coordinator is responsible for:
+1. **Index tables** — keeping a local SQLite index/summary of its child actors' data
+2. **Create/destroy** — handling lifecycle of child actors
+3. **Routing** — resolving lookups to the correct child actor
+
+Children push updates **up** to their direct coordinator only. Coordinators broadcast changes to connected clients. This keeps the read path local (no fan-out to children).
+
+### Coordinator hierarchy and index tables
+
+```text
+OrganizationActor (coordinator for repos + auth users)
+│
+│  Index tables:
+│  ├─ repos              → RepositoryActor index (repo catalog)
+│  ├─ taskLookup         → TaskActor index (taskId → repoId routing)
+│  ├─ taskSummaries      → TaskActor index (materialized sidebar projection)
+│  ├─ authSessionIndex   → AuthUserActor index (session token → userId)
+│  ├─ authEmailIndex     → AuthUserActor index (email → userId)
+│  └─ authAccountIndex   → AuthUserActor index (OAuth account → userId)
+│
+├─ RepositoryActor (coordinator for tasks)
+│  │
+│  │  Index tables:
+│  │  └─ taskIndex       → TaskActor index (taskId → branchName)
+│  │
+│  └─ TaskActor (coordinator for sessions + sandboxes)
+│     │
+│     │  Index tables:
+│     │  ├─ taskWorkbenchSessions → Session index (session metadata, transcript, draft)
+│     │  └─ taskSandboxes         → SandboxInstanceActor index (sandbox history)
+│     │
+│     └─ SandboxInstanceActor (leaf)
+│
+├─ HistoryActor (organization-scoped audit log, not a coordinator)
+└─ GithubDataActor (GitHub API cache, not a coordinator)
+```
+
+When adding a new index table, annotate it in the schema file with a doc comment identifying it as a coordinator index and which child actor it indexes (see existing examples).
+
 ## Ownership Rules
 
 - `OrganizationActor` is the organization coordinator and lookup/index owner.
@@ -29,8 +70,24 @@ OrganizationActor
 - `SandboxInstanceActor` stays separate from `TaskActor`; tasks/sessions reference it by identity.
 - The backend stores no local git state. No clones, no refs, no working trees, and no git-spice. Repository metadata comes from GitHub API data and webhook events. Any working-tree git operation runs inside a sandbox via `executeInSandbox()`.
 - When a backend request path must aggregate multiple independent actor calls or reads, prefer bounded parallelism over sequential fan-out when correctness permits. Do not serialize independent work by default.
+- Only a coordinator creates/destroys its children. Do not create child actors from outside the coordinator.
+- Children push state changes up to their direct coordinator only — never skip levels (e.g., task pushes to repo, not directly to org, unless org is the direct coordinator for that index).
+- Read paths must use the coordinator's local index tables. Do not fan out to child actors on the hot read path.
+- Never build "enriched" read actions that chain through multiple actors (e.g., coordinator → child actor → sibling actor). If data from multiple actors is needed for a read, it should already be materialized in the coordinator's index tables via push updates. If it's not there, fix the write path to push it — do not add a fan-out read path.
+
+## Multiplayer Correctness
+
+Per-user UI state must live on the user actor, not on shared task/session actors. This is critical for multiplayer — multiple users may view the same task simultaneously with different active sessions, unread states, and in-progress drafts.
+
+**Per-user state (user actor):** active session tab, unread counts, draft text, draft attachments. Keyed by `(userId, taskId, sessionId)`.
+
+**Task-global state (task actor):** session transcript, session model, session runtime status, sandbox identity, task status, branch name, PR state. These are shared across all users viewing the task — that is correct behavior.
+
+Do not store per-user preferences, selections, or ephemeral UI state on shared actors. If a field's value should differ between two users looking at the same task, it belongs on the user actor.
 
 ## Maintenance
 
 - Keep this file up to date whenever actor ownership, hierarchy, or lifecycle responsibilities change.
 - If the real actor tree diverges from this document, update this document in the same change.
+- When adding, removing, or renaming coordinator index tables, update the hierarchy diagram above in the same change.
+- When adding a new coordinator index table in a schema file, add a doc comment identifying which child actor it indexes (pattern: `/** Coordinator index of {ChildActor} instances. ... */`).